Moved synchronization to its own module

22 files changed, 1677 insertions, 0 deletions

diff --git a/net/sync/README.md b/net/sync/README.md
new file mode 100644
index 000000000..ec3a5d3a7
--- /dev/null
+++ b/net/sync/README.md
@@ -0,0 +1,3 @@
+# :net:sync
+
+This folder contains modules related to external services for synchronization. The module `model` provides the basic interfaces for implementing a synchronization backend. The other modules contains backends for specific synchronization services.
diff --git a/net/sync/gpoddernet/README.md b/net/sync/gpoddernet/README.md
new file mode 100644
index 000000000..1f8bb95bd
--- /dev/null
+++ b/net/sync/gpoddernet/README.md
@@ -0,0 +1,3 @@
+# :net:sync:gpoddernet
+
+This module contains the sync backend for the open-source podcast synchronization service "Gpodder.net".
diff --git a/net/sync/gpoddernet/build.gradle b/net/sync/gpoddernet/build.gradle
new file mode 100644
index 000000000..cd6d8d04c
--- /dev/null
+++ b/net/sync/gpoddernet/build.gradle
@@ -0,0 +1,57 @@
+apply plugin: "com.android.library"
+
+android {
+    compileSdkVersion rootProject.ext.compileSdkVersion
+
+    defaultConfig {
+        minSdkVersion rootProject.ext.minSdkVersion
+        targetSdkVersion rootProject.ext.targetSdkVersion
+
+        multiDexEnabled false
+
+        testApplicationId "de.danoeh.antennapod.core.tests"
+        testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"
+    }
+    buildTypes {
+        release {
+            minifyEnabled false
+            proguardFiles getDefaultProguardFile("proguard-android.txt")
+        }
+        debug {
+            // debug build has method count over 64k single-dex threshold.
+            // For building debug build to use on Android < 21 (pre-Android 5) devices,
+            // you need to manually change class
+            // de.danoeh.antennapod.PodcastApp to extend MultiDexApplication .
+            // See Issue #2813
+            multiDexEnabled true
+        }
+    }
+
+    compileOptions {
+        sourceCompatibility JavaVersion.VERSION_1_8
+        targetCompatibility JavaVersion.VERSION_1_8
+    }
+
+    testOptions {
+        unitTests {
+            includeAndroidResources = true
+        }
+    }
+
+    lintOptions {
+        disable 'GradleDependency'
+        checkDependencies true
+        warningsAsErrors true
+        abortOnError true
+    }
+}
+
+dependencies {
+    implementation project(':net:sync:model')
+
+    annotationProcessor "androidx.annotation:annotation:$annotationVersion"
+    implementation "androidx.appcompat:appcompat:$appcompatVersion"
+
+    implementation "com.squareup.okhttp3:okhttp:$okhttpVersion"
+    implementation "org.apache.commons:commons-lang3:$commonslangVersion"
+}
diff --git a/net/sync/gpoddernet/src/main/AndroidManifest.xml b/net/sync/gpoddernet/src/main/AndroidManifest.xml
new file mode 100644
index 000000000..df37bed3c
--- /dev/null
+++ b/net/sync/gpoddernet/src/main/AndroidManifest.xml
@@ -0,0 +1 @@
+<manifest package="de.danoeh.antennapod.net.sync.gpoddernet" />
diff --git a/net/sync/gpoddernet/src/main/java/de/danoeh/antennapod/net/sync/gpoddernet/GpodnetService.java b/net/sync/gpoddernet/src/main/java/de/danoeh/antennapod/net/sync/gpoddernet/GpodnetService.java
new file mode 100644
index 000000000..d41f613a7
--- /dev/null
+++ b/net/sync/gpoddernet/src/main/java/de/danoeh/antennapod/net/sync/gpoddernet/GpodnetService.java
@@ -0,0 +1,818 @@
+package de.danoeh.antennapod.net.sync.gpoddernet;
+
+import android.util.Log;
+import androidx.annotation.NonNull;
+import de.danoeh.antennapod.net.sync.gpoddernet.model.GpodnetDevice;
+import de.danoeh.antennapod.net.sync.model.EpisodeAction;
+import de.danoeh.antennapod.net.sync.model.EpisodeActionChanges;
+import de.danoeh.antennapod.net.sync.gpoddernet.model.GpodnetEpisodeActionPostResponse;
+import de.danoeh.antennapod.net.sync.gpoddernet.model.GpodnetPodcast;
+import de.danoeh.antennapod.net.sync.model.ISyncService;
+import de.danoeh.antennapod.net.sync.model.SubscriptionChanges;
+import de.danoeh.antennapod.net.sync.gpoddernet.model.GpodnetTag;
+import de.danoeh.antennapod.net.sync.gpoddernet.model.GpodnetUploadChangesResponse;
+import de.danoeh.antennapod.net.sync.model.SyncServiceException;
+import de.danoeh.antennapod.net.sync.model.UploadChangesResponse;
+import okhttp3.Credentials;
+import okhttp3.MediaType;
+import okhttp3.OkHttpClient;
+import okhttp3.Request;
+import okhttp3.RequestBody;
+import okhttp3.Response;
+import okhttp3.ResponseBody;
+import org.json.JSONArray;
+import org.json.JSONException;
+import org.json.JSONObject;
+
+import java.io.ByteArrayOutputStream;
+import java.io.IOException;
+import java.io.InputStream;
+import java.net.HttpURLConnection;
+import java.net.MalformedURLException;
+import java.net.URI;
+import java.net.URISyntaxException;
+import java.net.URL;
+import java.nio.charset.Charset;
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.LinkedList;
+import java.util.List;
+import java.util.Locale;
+
+import java.util.regex.Matcher;
+import java.util.regex.Pattern;
+
+/**
+ * Communicates with the gpodder.net service.
+ */
+public class GpodnetService implements ISyncService {
+    public static final String TAG = "GpodnetService";
+    public static final String DEFAULT_BASE_HOST = "gpodder.net";
+    private static final int UPLOAD_BULK_SIZE = 30;
+    private static final MediaType TEXT = MediaType.parse("plain/text; charset=utf-8");
+    private static final MediaType JSON = MediaType.parse("application/json; charset=utf-8");
+    private String baseScheme;
+    private int basePort;
+    private final String baseHost;
+    private final String deviceId;
+    private String username;
+    private String password;
+    private boolean loggedIn = false;
+
+    private final OkHttpClient httpClient;
+
+    // split into schema, host and port - missing parts are null
+    private static Pattern urlsplit_regex = Pattern.compile("(?:(https?)://)?([^:]+)(?::(\\d+))?");
+
+    public GpodnetService(OkHttpClient httpClient, String baseHosturl,
+                          String deviceId, String username, String password)  {
+        this.httpClient = httpClient;
+        this.deviceId = deviceId;
+        this.username = username;
+        this.password = password;
+
+        Matcher m = urlsplit_regex.matcher(baseHosturl);
+        if (m.matches()) {
+            this.baseScheme = m.group(1);
+            this.baseHost = m.group(2);
+            if (m.group(3) == null) {
+                this.basePort = -1;
+            } else {
+                this.basePort = Integer.parseInt(m.group(3));    // regex -> can only be digits
+            }
+        } else {
+            // URL does not match regex: use it anyway -> this will cause an exception on connect
+            this.baseScheme = "https";
+            this.baseHost = baseHosturl;
+            this.basePort = 443;
+        }
+
+        if (this.baseScheme == null) {      // assume https
+            this.baseScheme = "https";
+        }
+
+        if (this.baseScheme.equals("https") && this.basePort == -1) {
+            this.basePort = 443;
+        }
+
+        if (this.baseScheme.equals("http") && this.basePort == -1) {
+            this.basePort = 80;
+        }
+    }
+
+    private void requireLoggedIn() {
+        if (!loggedIn) {
+            throw new IllegalStateException("Not logged in");
+        }
+    }
+
+    /**
+     * Returns the [count] most used tags.
+     */
+    public List<GpodnetTag> getTopTags(int count) throws GpodnetServiceException {
+        URL url;
+        try {
+            url = new URI(baseScheme, null, baseHost, basePort,
+                    String.format(Locale.US, "/api/2/tags/%d.json", count), null, null).toURL();
+        } catch (MalformedURLException | URISyntaxException e) {
+            e.printStackTrace();
+            throw new GpodnetServiceException(e);
+        }
+
+        Request.Builder request = new Request.Builder().url(url);
+        String response = executeRequest(request);
+        try {
+            JSONArray jsonTagList = new JSONArray(response);
+            List<GpodnetTag> tagList = new ArrayList<>(jsonTagList.length());
+            for (int i = 0; i < jsonTagList.length(); i++) {
+                JSONObject jsonObject = jsonTagList.getJSONObject(i);
+                String title = jsonObject.getString("title");
+                String tag = jsonObject.getString("tag");
+                int usage = jsonObject.getInt("usage");
+                tagList.add(new GpodnetTag(title, tag, usage));
+            }
+            return tagList;
+        } catch (JSONException e) {
+            e.printStackTrace();
+            throw new GpodnetServiceException(e);
+        }
+    }
+
+    /**
+     * Returns the [count] most subscribed podcasts for the given tag.
+     *
+     * @throws IllegalArgumentException if tag is null
+     */
+    public List<GpodnetPodcast> getPodcastsForTag(@NonNull GpodnetTag tag, int count)
+            throws GpodnetServiceException {
+        try {
+            URL url = new URI(baseScheme, null, baseHost, basePort,
+                    String.format(Locale.US, "/api/2/tag/%s/%d.json", tag.getTag(), count), null, null).toURL();
+            Request.Builder request = new Request.Builder().url(url);
+            String response = executeRequest(request);
+
+            JSONArray jsonArray = new JSONArray(response);
+            return readPodcastListFromJsonArray(jsonArray);
+
+        } catch (JSONException | MalformedURLException | URISyntaxException e) {
+            e.printStackTrace();
+            throw new GpodnetServiceException(e);
+        }
+    }
+
+    /**
+     * Returns the toplist of podcast.
+     *
+     * @param count of elements that should be returned. Must be in range 1..100.
+     * @throws IllegalArgumentException if count is out of range.
+     */
+    public List<GpodnetPodcast> getPodcastToplist(int count) throws GpodnetServiceException {
+        if (count < 1 || count > 100) {
+            throw new IllegalArgumentException("Count must be in range 1..100");
+        }
+
+        try {
+            URL url = new URI(baseScheme, null, baseHost, basePort,
+                    String.format(Locale.US, "/toplist/%d.json", count), null, null).toURL();
+            Request.Builder request = new Request.Builder().url(url);
+            String response = executeRequest(request);
+
+            JSONArray jsonArray = new JSONArray(response);
+            return readPodcastListFromJsonArray(jsonArray);
+
+        } catch (JSONException | MalformedURLException | URISyntaxException e) {
+            e.printStackTrace();
+            throw new GpodnetServiceException(e);
+        }
+    }
+
+    /**
+     * Returns a list of suggested podcasts for the user that is currently
+     * logged in.
+     * <p/>
+     * This method requires authentication.
+     *
+     * @param count The
+     *              number of elements that should be returned. Must be in range
+     *              1..100.
+     * @throws IllegalArgumentException              if count is out of range.
+     * @throws GpodnetServiceAuthenticationException If there is an authentication error.
+     */
+    public List<GpodnetPodcast> getSuggestions(int count) throws GpodnetServiceException {
+        if (count < 1 || count > 100) {
+            throw new IllegalArgumentException("Count must be in range 1..100");
+        }
+
+        try {
+            URL url = new URI(baseScheme, null, baseHost, basePort,
+                    String.format(Locale.US, "/suggestions/%d.json", count), null, null).toURL();
+            Request.Builder request = new Request.Builder().url(url);
+            String response = executeRequest(request);
+
+            JSONArray jsonArray = new JSONArray(response);
+            return readPodcastListFromJsonArray(jsonArray);
+        } catch (JSONException | MalformedURLException | URISyntaxException e) {
+            e.printStackTrace();
+            throw new GpodnetServiceException(e);
+        }
+    }
+
+    /**
+     * Searches the podcast directory for a given string.
+     *
+     * @param query          The search query
+     * @param scaledLogoSize The size of the logos that are returned by the search query.
+     *                       Must be in range 1..256. If the value is out of range, the
+     *                       default value defined by the gpodder.net API will be used.
+     */
+    public List<GpodnetPodcast> searchPodcasts(String query, int scaledLogoSize) throws GpodnetServiceException {
+        String parameters = (scaledLogoSize > 0 && scaledLogoSize <= 256) ? String
+                .format(Locale.US, "q=%s&scale_logo=%d", query, scaledLogoSize) : String
+                .format("q=%s", query);
+        try {
+            URL url = new URI(baseScheme, null, baseHost, basePort, "/search.json",
+                    parameters, null).toURL();
+            Request.Builder request = new Request.Builder().url(url);
+            String response = executeRequest(request);
+
+            JSONArray jsonArray = new JSONArray(response);
+            return readPodcastListFromJsonArray(jsonArray);
+
+        } catch (JSONException | MalformedURLException e) {
+            e.printStackTrace();
+            throw new GpodnetServiceException(e);
+        } catch (URISyntaxException e) {
+            e.printStackTrace();
+            throw new IllegalStateException(e);
+        }
+    }
+
+    /**
+     * Returns all devices of a given user.
+     * <p/>
+     * This method requires authentication.
+     *
+     * @throws GpodnetServiceAuthenticationException If there is an authentication error.
+     */
+    public List<GpodnetDevice> getDevices() throws GpodnetServiceException {
+        requireLoggedIn();
+        try {
+            URL url = new URI(baseScheme, null, baseHost, basePort,
+                    String.format("/api/2/devices/%s.json", username), null, null).toURL();
+            Request.Builder request = new Request.Builder().url(url);
+            String response = executeRequest(request);
+            JSONArray devicesArray = new JSONArray(response);
+            return readDeviceListFromJsonArray(devicesArray);
+        } catch (JSONException | MalformedURLException | URISyntaxException e) {
+            e.printStackTrace();
+            throw new GpodnetServiceException(e);
+        }
+    }
+
+    /**
+     * Returns synchronization status of devices.
+     * <p/>
+     * This method requires authentication.
+     *
+     * @throws GpodnetServiceAuthenticationException If there is an authentication error.
+     */
+    public List<List<String>> getSynchronizedDevices() throws GpodnetServiceException {
+        requireLoggedIn();
+        try {
+            URL url = new URI(baseScheme, null, baseHost, basePort,
+                    String.format("/api/2/sync-devices/%s.json", username), null, null).toURL();
+            Request.Builder request = new Request.Builder().url(url);
+            String response = executeRequest(request);
+            JSONObject syncStatus = new JSONObject(response);
+            List<List<String>> result = new ArrayList<>();
+
+            JSONArray synchronizedDevices = syncStatus.getJSONArray("synchronized");
+            for (int i = 0; i < synchronizedDevices.length(); i++) {
+                JSONArray groupDevices = synchronizedDevices.getJSONArray(i);
+                List<String> group = new ArrayList<>();
+                for (int j = 0; j < groupDevices.length(); j++) {
+                    group.add(groupDevices.getString(j));
+                }
+                result.add(group);
+            }
+
+            JSONArray notSynchronizedDevices = syncStatus.getJSONArray("not-synchronized");
+            for (int i = 0; i < notSynchronizedDevices.length(); i++) {
+                result.add(Collections.singletonList(notSynchronizedDevices.getString(i)));
+            }
+
+            return result;
+        } catch (JSONException | MalformedURLException | URISyntaxException e) {
+            e.printStackTrace();
+            throw new GpodnetServiceException(e);
+        }
+    }
+
+    /**
+     * Configures the device of a given user.
+     * <p/>
+     * This method requires authentication.
+     *
+     * @param deviceId The ID of the device that should be configured.
+     * @throws GpodnetServiceAuthenticationException If there is an authentication error.
+     */
+    public void configureDevice(@NonNull String deviceId, String caption, GpodnetDevice.DeviceType type)
+            throws GpodnetServiceException {
+        requireLoggedIn();
+        try {
+            URL url = new URI(baseScheme, null, baseHost, basePort,
+                    String.format("/api/2/devices/%s/%s.json", username, deviceId), null, null).toURL();
+            String content;
+            if (caption != null || type != null) {
+                JSONObject jsonContent = new JSONObject();
+                if (caption != null) {
+                    jsonContent.put("caption", caption);
+                }
+                if (type != null) {
+                    jsonContent.put("type", type.toString());
+                }
+                content = jsonContent.toString();
+            } else {
+                content = "";
+            }
+            RequestBody body = RequestBody.create(JSON, content);
+            Request.Builder request = new Request.Builder().post(body).url(url);
+            executeRequest(request);
+        } catch (JSONException | MalformedURLException | URISyntaxException e) {
+            e.printStackTrace();
+            throw new GpodnetServiceException(e);
+        }
+    }
+
+    /**
+     * Links devices for synchronization.
+     * <p/>
+     * This method requires authentication.
+     *
+     * @throws GpodnetServiceAuthenticationException If there is an authentication error.
+     */
+    public void linkDevices(@NonNull List<String> deviceIds) throws GpodnetServiceException {
+        requireLoggedIn();
+        try {
+            final URL url = new URI(baseScheme, null, baseHost, basePort,
+                    String.format("/api/2/sync-devices/%s.json", username), null, null).toURL();
+            JSONObject jsonContent = new JSONObject();
+            JSONArray group = new JSONArray();
+            for (String deviceId : deviceIds) {
+                group.put(deviceId);
+            }
+
+            JSONArray synchronizedGroups = new JSONArray();
+            synchronizedGroups.put(group);
+            jsonContent.put("synchronize", synchronizedGroups);
+            jsonContent.put("stop-synchronize", new JSONArray());
+
+            Log.d("aaaa", jsonContent.toString());
+            RequestBody body = RequestBody.create(JSON, jsonContent.toString());
+            Request.Builder request = new Request.Builder().post(body).url(url);
+            executeRequest(request);
+        } catch (JSONException | MalformedURLException | URISyntaxException e) {
+            e.printStackTrace();
+            throw new GpodnetServiceException(e);
+        }
+    }
+
+    /**
+     * Returns the subscriptions of a specific device.
+     * <p/>
+     * This method requires authentication.
+     *
+     * @param deviceId The ID of the device whose subscriptions should be returned.
+     * @return A list of subscriptions in OPML format.
+     * @throws GpodnetServiceAuthenticationException If there is an authentication error.
+     */
+    public String getSubscriptionsOfDevice(@NonNull String deviceId) throws GpodnetServiceException {
+        requireLoggedIn();
+        try {
+            URL url = new URI(baseScheme, null, baseHost, basePort,
+                    String.format("/subscriptions/%s/%s.opml", username, deviceId), null, null).toURL();
+            Request.Builder request = new Request.Builder().url(url);
+            return executeRequest(request);
+        } catch (MalformedURLException | URISyntaxException e) {
+            e.printStackTrace();
+            throw new GpodnetServiceException(e);
+        }
+    }
+
+    /**
+     * Returns all subscriptions of a specific user.
+     * <p/>
+     * This method requires authentication.
+     *
+     * @return A list of subscriptions in OPML format.
+     * @throws IllegalArgumentException              If username is null.
+     * @throws GpodnetServiceAuthenticationException If there is an authentication error.
+     */
+    public String getSubscriptionsOfUser() throws GpodnetServiceException {
+        requireLoggedIn();
+        try {
+            URL url = new URI(baseScheme, null, baseHost, basePort,
+                    String.format("/subscriptions/%s.opml", username), null, null).toURL();
+            Request.Builder request = new Request.Builder().url(url);
+            return executeRequest(request);
+        } catch (MalformedURLException | URISyntaxException e) {
+            e.printStackTrace();
+            throw new GpodnetServiceException(e);
+        }
+    }
+
+    /**
+     * Uploads the subscriptions of a specific device.
+     * <p/>
+     * This method requires authentication.
+     *
+     * @param deviceId      The ID of the device whose subscriptions should be updated.
+     * @param subscriptions A list of feed URLs containing all subscriptions of the
+     *                      device.
+     * @throws IllegalArgumentException              If username, deviceId or subscriptions is null.
+     * @throws GpodnetServiceAuthenticationException If there is an authentication error.
+     */
+    public void uploadSubscriptions(@NonNull String deviceId, @NonNull List<String> subscriptions)
+            throws GpodnetServiceException {
+        requireLoggedIn();
+        try {
+            URL url = new URI(baseScheme, null, baseHost, basePort,
+                    String.format("/subscriptions/%s/%s.txt", username, deviceId), null, null).toURL();
+            StringBuilder builder = new StringBuilder();
+            for (String s : subscriptions) {
+                builder.append(s);
+                builder.append("\n");
+            }
+            RequestBody body = RequestBody.create(TEXT, builder.toString());
+            Request.Builder request = new Request.Builder().put(body).url(url);
+            executeRequest(request);
+        } catch (MalformedURLException | URISyntaxException e) {
+            e.printStackTrace();
+            throw new GpodnetServiceException(e);
+        }
+
+    }
+
+    /**
+     * Updates the subscription list of a specific device.
+     * <p/>
+     * This method requires authentication.
+     *
+     * @param added    Collection of feed URLs of added feeds. This Collection MUST NOT contain any duplicates
+     * @param removed  Collection of feed URLs of removed feeds. This Collection MUST NOT contain any duplicates
+     * @return a GpodnetUploadChangesResponse. See {@link GpodnetUploadChangesResponse}
+     * for details.
+     * @throws GpodnetServiceException            if added or removed contain duplicates or if there
+     *                                            is an authentication error.
+     */
+    @Override
+    public UploadChangesResponse uploadSubscriptionChanges(List<String> added, List<String> removed)
+            throws GpodnetServiceException {
+        requireLoggedIn();
+        try {
+            URL url = new URI(baseScheme, null, baseHost, basePort,
+                    String.format("/api/2/subscriptions/%s/%s.json", username, deviceId), null, null).toURL();
+
+            final JSONObject requestObject = new JSONObject();
+            requestObject.put("add", new JSONArray(added));
+            requestObject.put("remove", new JSONArray(removed));
+
+            RequestBody body = RequestBody.create(JSON, requestObject.toString());
+            Request.Builder request = new Request.Builder().post(body).url(url);
+
+            final String response = executeRequest(request);
+            return GpodnetUploadChangesResponse.fromJSONObject(response);
+        } catch (JSONException | MalformedURLException | URISyntaxException e) {
+            e.printStackTrace();
+            throw new GpodnetServiceException(e);
+        }
+
+    }
+
+    /**
+     * Returns all subscription changes of a specific device.
+     * <p/>
+     * This method requires authentication.
+     *
+     * @param timestamp A timestamp that can be used to receive all changes since a
+     *                  specific point in time.
+     * @throws GpodnetServiceAuthenticationException If there is an authentication error.
+     */
+    @Override
+    public SubscriptionChanges getSubscriptionChanges(long timestamp) throws GpodnetServiceException {
+        requireLoggedIn();
+        String params = String.format(Locale.US, "since=%d", timestamp);
+        String path = String.format("/api/2/subscriptions/%s/%s.json", username, deviceId);
+        try {
+            URL url = new URI(baseScheme, null, baseHost, basePort, path, params, null).toURL();
+            Request.Builder request = new Request.Builder().url(url);
+
+            String response = executeRequest(request);
+            JSONObject changes = new JSONObject(response);
+            return readSubscriptionChangesFromJsonObject(changes);
+        } catch (URISyntaxException e) {
+            e.printStackTrace();
+            throw new IllegalStateException(e);
+        } catch (JSONException | MalformedURLException e) {
+            e.printStackTrace();
+            throw new GpodnetServiceException(e);
+        }
+
+    }
+
+    /**
+     * Updates the episode actions
+     * <p/>
+     * This method requires authentication.
+     *
+     * @param episodeActions Collection of episode actions.
+     * @return a GpodnetUploadChangesResponse. See {@link GpodnetUploadChangesResponse}
+     * for details.
+     * @throws GpodnetServiceException            if added or removed contain duplicates or if there
+     *                                            is an authentication error.
+     */
+    @Override
+    public UploadChangesResponse uploadEpisodeActions(List<EpisodeAction> episodeActions) throws SyncServiceException {
+        requireLoggedIn();
+        UploadChangesResponse response = null;
+        for (int i = 0; i < episodeActions.size(); i += UPLOAD_BULK_SIZE) {
+            response = uploadEpisodeActionsPartial(episodeActions,
+                    i, Math.min(episodeActions.size(), i + UPLOAD_BULK_SIZE));
+        }
+        return response;
+    }
+
+    private UploadChangesResponse uploadEpisodeActionsPartial(List<EpisodeAction> episodeActions, int from, int to)
+            throws SyncServiceException {
+        try {
+            Log.d(TAG, "Uploading partial actions " + from + " to " + to + " of " + episodeActions.size());
+            URL url = new URI(baseScheme, null, baseHost, basePort,
+                    String.format("/api/2/episodes/%s.json", username), null, null).toURL();
+
+            final JSONArray list = new JSONArray();
+            for (int i = from; i < to; i++) {
+                EpisodeAction episodeAction = episodeActions.get(i);
+                JSONObject obj = episodeAction.writeToJsonObject();
+                if (obj != null) {
+                    obj.put("device", deviceId);
+                    list.put(obj);
+                }
+            }
+
+            RequestBody body = RequestBody.create(JSON, list.toString());
+            Request.Builder request = new Request.Builder().post(body).url(url);
+
+            final String response = executeRequest(request);
+            return GpodnetEpisodeActionPostResponse.fromJSONObject(response);
+        } catch (JSONException | MalformedURLException | URISyntaxException e) {
+            e.printStackTrace();
+            throw new SyncServiceException(e);
+        }
+    }
+
+    /**
+     * Returns all subscription changes of a specific device.
+     * <p/>
+     * This method requires authentication.
+     *
+     * @param timestamp A timestamp that can be used to receive all changes since a
+     *                  specific point in time.
+     * @throws SyncServiceException If there is an authentication error.
+     */
+    @Override
+    public EpisodeActionChanges getEpisodeActionChanges(long timestamp) throws SyncServiceException {
+        requireLoggedIn();
+        String params = String.format(Locale.US, "since=%d", timestamp);
+        String path = String.format("/api/2/episodes/%s.json", username);
+        try {
+            URL url = new URI(baseScheme, null, baseHost, basePort, path, params, null).toURL();
+            Request.Builder request = new Request.Builder().url(url);
+
+            String response = executeRequest(request);
+            JSONObject json = new JSONObject(response);
+            return readEpisodeActionsFromJsonObject(json);
+        } catch (URISyntaxException e) {
+            e.printStackTrace();
+            throw new IllegalStateException(e);
+        } catch (JSONException | MalformedURLException e) {
+            e.printStackTrace();
+            throw new SyncServiceException(e);
+        }
+
+    }
+
+
+    /**
+     * Logs in a specific user. This method must be called if any of the methods
+     * that require authentication is used.
+     *
+     * @throws IllegalArgumentException If username or password is null.
+     */
+    @Override
+    public void login() throws GpodnetServiceException {
+        URL url;
+        try {
+            url = new URI(baseScheme, null, baseHost, basePort,
+                    String.format("/api/2/auth/%s/login.json", username), null, null).toURL();
+        } catch (MalformedURLException | URISyntaxException e) {
+            e.printStackTrace();
+            throw new GpodnetServiceException(e);
+        }
+        RequestBody requestBody = RequestBody.create(TEXT, "");
+        Request request = new Request.Builder().url(url).post(requestBody).build();
+        try {
+            String credential = Credentials.basic(username, password, Charset.forName("UTF-8"));
+            Request authRequest = request.newBuilder().header("Authorization", credential).build();
+            Response response = httpClient.newCall(authRequest).execute();
+            checkStatusCode(response);
+            response.body().close();
+            this.loggedIn = true;
+        } catch (Exception e) {
+            e.printStackTrace();
+            throw new GpodnetServiceException(e);
+        }
+    }
+
+    private String executeRequest(@NonNull Request.Builder requestB) throws GpodnetServiceException {
+        Request request = requestB.build();
+        String responseString;
+        Response response;
+        ResponseBody body = null;
+        try {
+
+            response = httpClient.newCall(request).execute();
+            checkStatusCode(response);
+            body = response.body();
+            responseString = getStringFromResponseBody(body);
+        } catch (IOException e) {
+            e.printStackTrace();
+            throw new GpodnetServiceException(e);
+        } finally {
+            if (body != null) {
+                body.close();
+            }
+        }
+        return responseString;
+    }
+
+    private String getStringFromResponseBody(@NonNull ResponseBody body) throws GpodnetServiceException {
+        ByteArrayOutputStream outputStream;
+        int contentLength = (int) body.contentLength();
+        if (contentLength > 0) {
+            outputStream = new ByteArrayOutputStream(contentLength);
+        } else {
+            outputStream = new ByteArrayOutputStream();
+        }
+        try {
+            byte[] buffer = new byte[8 * 1024];
+            InputStream in = body.byteStream();
+            int count;
+            while ((count = in.read(buffer)) > 0) {
+                outputStream.write(buffer, 0, count);
+            }
+            return outputStream.toString("UTF-8");
+        } catch (IOException e) {
+            e.printStackTrace();
+            throw new GpodnetServiceException(e);
+        }
+    }
+
+    private void checkStatusCode(@NonNull Response response) throws GpodnetServiceException {
+        int responseCode = response.code();
+        if (responseCode != HttpURLConnection.HTTP_OK) {
+            if (responseCode == HttpURLConnection.HTTP_UNAUTHORIZED) {
+                throw new GpodnetServiceAuthenticationException("Wrong username or password");
+            } else {
+                if (BuildConfig.DEBUG) {
+                    try {
+                        Log.d(TAG, response.body().string());
+                    } catch (IOException e) {
+                        e.printStackTrace();
+                    }
+                }
+                throw new GpodnetServiceBadStatusCodeException("Bad response code: " + responseCode, responseCode);
+            }
+        }
+    }
+
+    private List<GpodnetPodcast> readPodcastListFromJsonArray(@NonNull JSONArray array) throws JSONException {
+        List<GpodnetPodcast> result = new ArrayList<>(array.length());
+        for (int i = 0; i < array.length(); i++) {
+            result.add(readPodcastFromJsonObject(array.getJSONObject(i)));
+        }
+        return result;
+    }
+
+    private GpodnetPodcast readPodcastFromJsonObject(JSONObject object) throws JSONException {
+        String url = object.getString("url");
+
+        String title;
+        Object titleObj = object.opt("title");
+        if (titleObj instanceof String) {
+            title = (String) titleObj;
+        } else {
+            title = url;
+        }
+
+        String description;
+        Object descriptionObj = object.opt("description");
+        if (descriptionObj instanceof String) {
+            description = (String) descriptionObj;
+        } else {
+            description = "";
+        }
+
+        int subscribers = object.getInt("subscribers");
+
+        Object logoUrlObj = object.opt("logo_url");
+        String logoUrl = (logoUrlObj instanceof String) ? (String) logoUrlObj : null;
+        if (logoUrl == null) {
+            Object scaledLogoUrl = object.opt("scaled_logo_url");
+            if (scaledLogoUrl instanceof String) {
+                logoUrl = (String) scaledLogoUrl;
+            }
+        }
+
+        String website = null;
+        Object websiteObj = object.opt("website");
+        if (websiteObj instanceof String) {
+            website = (String) websiteObj;
+        }
+        String mygpoLink = object.getString("mygpo_link");
+
+        String author = null;
+        Object authorObj = object.opt("author");
+        if (authorObj instanceof String) {
+            author = (String) authorObj;
+        }
+        return new GpodnetPodcast(url, title, description, subscribers, logoUrl, website, mygpoLink, author);
+    }
+
+    private List<GpodnetDevice> readDeviceListFromJsonArray(@NonNull JSONArray array) throws JSONException {
+        List<GpodnetDevice> result = new ArrayList<>(array.length());
+        for (int i = 0; i < array.length(); i++) {
+            result.add(readDeviceFromJsonObject(array.getJSONObject(i)));
+        }
+        return result;
+    }
+
+    private GpodnetDevice readDeviceFromJsonObject(JSONObject object) throws JSONException {
+        String id = object.getString("id");
+        String caption = object.getString("caption");
+        String type = object.getString("type");
+        int subscriptions = object.getInt("subscriptions");
+        return new GpodnetDevice(id, caption, type, subscriptions);
+    }
+
+    private SubscriptionChanges readSubscriptionChangesFromJsonObject(@NonNull JSONObject object)
+            throws JSONException {
+
+        List<String> added = new LinkedList<>();
+        JSONArray jsonAdded = object.getJSONArray("add");
+        for (int i = 0; i < jsonAdded.length(); i++) {
+            String addedUrl = jsonAdded.getString(i);
+            // gpodder escapes colons unnecessarily
+            addedUrl = addedUrl.replace("%3A", ":");
+            added.add(addedUrl);
+        }
+
+        List<String> removed = new LinkedList<>();
+        JSONArray jsonRemoved = object.getJSONArray("remove");
+        for (int i = 0; i < jsonRemoved.length(); i++) {
+            String removedUrl = jsonRemoved.getString(i);
+            // gpodder escapes colons unnecessarily
+            removedUrl = removedUrl.replace("%3A", ":");
+            removed.add(removedUrl);
+        }
+
+        long timestamp = object.getLong("timestamp");
+        return new SubscriptionChanges(added, removed, timestamp);
+    }
+
+    private EpisodeActionChanges readEpisodeActionsFromJsonObject(@NonNull JSONObject object)
+            throws JSONException {
+
+        List<EpisodeAction> episodeActions = new ArrayList<>();
+
+        long timestamp = object.getLong("timestamp");
+        JSONArray jsonActions = object.getJSONArray("actions");
+        for (int i = 0; i < jsonActions.length(); i++) {
+            JSONObject jsonAction = jsonActions.getJSONObject(i);
+            EpisodeAction episodeAction = EpisodeAction.readFromJsonObject(jsonAction);
+            if (episodeAction != null) {
+                episodeActions.add(episodeAction);
+            }
+        }
+        return new EpisodeActionChanges(episodeActions, timestamp);
+    }
+
+    @Override
+    public void logout() {
+
+    }
+
+    public void setCredentials(String username, String password) {
+        this.username = username;
+        this.password = password;
+    }
+}
diff --git a/net/sync/gpoddernet/src/main/java/de/danoeh/antennapod/net/sync/gpoddernet/GpodnetServiceAuthenticationException.java b/net/sync/gpoddernet/src/main/java/de/danoeh/antennapod/net/sync/gpoddernet/GpodnetServiceAuthenticationException.java
new file mode 100644
index 000000000..d0a9c2dd6
--- /dev/null
+++ b/net/sync/gpoddernet/src/main/java/de/danoeh/antennapod/net/sync/gpoddernet/GpodnetServiceAuthenticationException.java
@@ -0,0 +1,9 @@
+package de.danoeh.antennapod.net.sync.gpoddernet;
+
+public class GpodnetServiceAuthenticationException extends GpodnetServiceException {
+    private static final long serialVersionUID = 1L;
+
+    public GpodnetServiceAuthenticationException(String message) {
+        super(message);
+    }
+}
diff --git a/net/sync/gpoddernet/src/main/java/de/danoeh/antennapod/net/sync/gpoddernet/GpodnetServiceBadStatusCodeException.java b/net/sync/gpoddernet/src/main/java/de/danoeh/antennapod/net/sync/gpoddernet/GpodnetServiceBadStatusCodeException.java
new file mode 100644
index 000000000..e403d8fff
--- /dev/null
+++ b/net/sync/gpoddernet/src/main/java/de/danoeh/antennapod/net/sync/gpoddernet/GpodnetServiceBadStatusCodeException.java
@@ -0,0 +1,12 @@
+package de.danoeh.antennapod.net.sync.gpoddernet;
+
+class GpodnetServiceBadStatusCodeException extends GpodnetServiceException {
+    private static final long serialVersionUID = 1L;
+
+    private final int statusCode;
+
+    public GpodnetServiceBadStatusCodeException(String message, int statusCode) {
+        super(message);
+        this.statusCode = statusCode;
+    }
+}
diff --git a/net/sync/gpoddernet/src/main/java/de/danoeh/antennapod/net/sync/gpoddernet/GpodnetServiceException.java b/net/sync/gpoddernet/src/main/java/de/danoeh/antennapod/net/sync/gpoddernet/GpodnetServiceException.java
new file mode 100644
index 000000000..298c59073
--- /dev/null
+++ b/net/sync/gpoddernet/src/main/java/de/danoeh/antennapod/net/sync/gpoddernet/GpodnetServiceException.java
@@ -0,0 +1,15 @@
+package de.danoeh.antennapod.net.sync.gpoddernet;
+
+import de.danoeh.antennapod.net.sync.model.SyncServiceException;
+
+public class GpodnetServiceException extends SyncServiceException {
+    private static final long serialVersionUID = 1L;
+
+    public GpodnetServiceException(String message) {
+        super(message);
+    }
+
+    public GpodnetServiceException(Throwable e) {
+        super(e);
+    }
+}
diff --git a/net/sync/gpoddernet/src/main/java/de/danoeh/antennapod/net/sync/gpoddernet/model/GpodnetDevice.java b/net/sync/gpoddernet/src/main/java/de/danoeh/antennapod/net/sync/gpoddernet/model/GpodnetDevice.java
new file mode 100644
index 000000000..ed7d80ee0
--- /dev/null
+++ b/net/sync/gpoddernet/src/main/java/de/danoeh/antennapod/net/sync/gpoddernet/model/GpodnetDevice.java
@@ -0,0 +1,75 @@
+package de.danoeh.antennapod.net.sync.gpoddernet.model;
+
+import androidx.annotation.NonNull;
+
+import java.util.Locale;
+
+public class GpodnetDevice {
+
+    private final String id;
+    private final String caption;
+    private final DeviceType type;
+    private final int subscriptions;
+
+    public GpodnetDevice(@NonNull String id,
+                         String caption,
+                         String type,
+                         int subscriptions) {
+        this.id = id;
+        this.caption = caption;
+        this.type = DeviceType.fromString(type);
+        this.subscriptions = subscriptions;
+    }
+
+    @Override
+    public String toString() {
+        return "GpodnetDevice [id=" + id + ", caption=" + caption + ", type="
+                + type + ", subscriptions=" + subscriptions + "]";
+    }
+
+    public enum DeviceType {
+        DESKTOP, LAPTOP, MOBILE, SERVER, OTHER;
+
+        static DeviceType fromString(String s) {
+            if (s == null) {
+                return OTHER;
+            }
+
+            switch (s) {
+                case "desktop":
+                    return DESKTOP;
+                case "laptop":
+                    return LAPTOP;
+                case "mobile":
+                    return MOBILE;
+                case "server":
+                    return SERVER;
+                default:
+                    return OTHER;
+            }
+        }
+
+        @Override
+        public String toString() {
+            return super.toString().toLowerCase(Locale.US);
+        }
+
+    }
+
+    public String getId() {
+        return id;
+    }
+
+    public String getCaption() {
+        return caption;
+    }
+
+    public DeviceType getType() {
+        return type;
+    }
+
+    public int getSubscriptions() {
+        return subscriptions;
+    }
+
+}
diff --git a/net/sync/gpoddernet/src/main/java/de/danoeh/antennapod/net/sync/gpoddernet/model/GpodnetEpisodeActionPostResponse.java b/net/sync/gpoddernet/src/main/java/de/danoeh/antennapod/net/sync/gpoddernet/model/GpodnetEpisodeActionPostResponse.java
new file mode 100644
index 000000000..6d573ebfc
--- /dev/null
+++ b/net/sync/gpoddernet/src/main/java/de/danoeh/antennapod/net/sync/gpoddernet/model/GpodnetEpisodeActionPostResponse.java
@@ -0,0 +1,49 @@
+package de.danoeh.antennapod.net.sync.gpoddernet.model;
+
+import androidx.collection.ArrayMap;
+
+import de.danoeh.antennapod.net.sync.model.UploadChangesResponse;
+import org.apache.commons.lang3.builder.ToStringBuilder;
+import org.apache.commons.lang3.builder.ToStringStyle;
+import org.json.JSONArray;
+import org.json.JSONException;
+import org.json.JSONObject;
+
+import java.util.Map;
+
+public class GpodnetEpisodeActionPostResponse extends UploadChangesResponse {
+    /**
+     * URLs that should be updated. The key of the map is the original URL, the value of the map
+     * is the sanitized URL.
+     */
+    private final Map<String, String> updatedUrls;
+
+    private GpodnetEpisodeActionPostResponse(long timestamp, Map<String, String> updatedUrls) {
+        super(timestamp);
+        this.updatedUrls = updatedUrls;
+    }
+
+    /**
+     * Creates a new GpodnetUploadChangesResponse-object from a JSON object that was
+     * returned by an uploadChanges call.
+     *
+     * @throws org.json.JSONException If the method could not parse the JSONObject.
+     */
+    public static GpodnetEpisodeActionPostResponse fromJSONObject(String objectString) throws JSONException {
+        final JSONObject object = new JSONObject(objectString);
+        final long timestamp = object.getLong("timestamp");
+        JSONArray urls = object.getJSONArray("update_urls");
+        Map<String, String> updatedUrls = new ArrayMap<>(urls.length());
+        for (int i = 0; i < urls.length(); i++) {
+            JSONArray urlPair = urls.getJSONArray(i);
+            updatedUrls.put(urlPair.getString(0), urlPair.getString(1));
+        }
+        return new GpodnetEpisodeActionPostResponse(timestamp, updatedUrls);
+    }
+
+    @Override
+    public String toString() {
+        return ToStringBuilder.reflectionToString(this, ToStringStyle.SHORT_PREFIX_STYLE);
+    }
+}
+
diff --git a/net/sync/gpoddernet/src/main/java/de/danoeh/antennapod/net/sync/gpoddernet/model/GpodnetPodcast.java b/net/sync/gpoddernet/src/main/java/de/danoeh/antennapod/net/sync/gpoddernet/model/GpodnetPodcast.java
new file mode 100644
index 000000000..f09ab1244
--- /dev/null
+++ b/net/sync/gpoddernet/src/main/java/de/danoeh/antennapod/net/sync/gpoddernet/model/GpodnetPodcast.java
@@ -0,0 +1,72 @@
+package de.danoeh.antennapod.net.sync.gpoddernet.model;
+
+import androidx.annotation.NonNull;
+
+public class GpodnetPodcast {
+    private final String url;
+    private final String title;
+    private final String description;
+    private final int subscribers;
+    private final String logoUrl;
+    private final String website;
+    private final String mygpoLink;
+    private final String author;
+
+    public GpodnetPodcast(@NonNull String url,
+                          @NonNull String title,
+                          @NonNull String description,
+                          int subscribers,
+                          String logoUrl,
+                          String website,
+                          String mygpoLink,
+                          String author
+                          ) {
+        this.url = url;
+        this.title = title;
+        this.description = description;
+        this.subscribers = subscribers;
+        this.logoUrl = logoUrl;
+        this.website = website;
+        this.mygpoLink = mygpoLink;
+        this.author = author;
+    }
+
+    @Override
+    public String toString() {
+        return "GpodnetPodcast [url=" + url + ", title=" + title
+                + ", description=" + description + ", subscribers="
+                + subscribers + ", logoUrl=" + logoUrl + ", website=" + website
+                + ", mygpoLink=" + mygpoLink + "]";
+    }
+
+    public String getUrl() {
+        return url;
+    }
+
+    public String getTitle() {
+        return title;
+    }
+
+    public String getDescription() {
+        return description;
+    }
+
+    public int getSubscribers() {
+        return subscribers;
+    }
+
+    public String getLogoUrl() {
+        return logoUrl;
+    }
+
+    public String getWebsite() {
+        return website;
+    }
+
+    public String getAuthor() { return author; }
+
+    public String getMygpoLink() {
+        return mygpoLink;
+    }
+
+}
diff --git a/net/sync/gpoddernet/src/main/java/de/danoeh/antennapod/net/sync/gpoddernet/model/GpodnetTag.java b/net/sync/gpoddernet/src/main/java/de/danoeh/antennapod/net/sync/gpoddernet/model/GpodnetTag.java
new file mode 100644
index 000000000..d3d683598
--- /dev/null
+++ b/net/sync/gpoddernet/src/main/java/de/danoeh/antennapod/net/sync/gpoddernet/model/GpodnetTag.java
@@ -0,0 +1,66 @@
+package de.danoeh.antennapod.net.sync.gpoddernet.model;
+
+import android.os.Parcel;
+import android.os.Parcelable;
+import androidx.annotation.NonNull;
+
+public class GpodnetTag implements Parcelable  {
+
+    private final String title;
+    private final String tag;
+    private final int usage;
+
+    public GpodnetTag(@NonNull String title, @NonNull String tag, int usage) {
+        this.title = title;
+        this.tag = tag;
+        this.usage = usage;
+    }
+
+    private GpodnetTag(Parcel in) {
+        title = in.readString();
+        tag = in.readString();
+        usage = in.readInt();
+    }
+
+    @Override
+    public String toString() {
+        return "GpodnetTag [title="+title+", tag=" + tag + ", usage=" + usage + "]";
+    }
+
+    public String getTitle() {
+        return title;
+    }
+
+    public String getTag() {
+        return tag;
+    }
+
+    public int getUsage() {
+        return usage;
+    }
+
+    @Override
+    public int describeContents() {
+        return 0;
+    }
+
+    @Override
+    public void writeToParcel(Parcel dest, int flags) {
+        dest.writeString(title);
+        dest.writeString(tag);
+        dest.writeInt(usage);
+    }
+
+    public static final Creator<GpodnetTag> CREATOR = new Creator<GpodnetTag>() {
+        @Override
+        public GpodnetTag createFromParcel(Parcel in) {
+            return new GpodnetTag(in);
+        }
+
+        @Override
+        public GpodnetTag[] newArray(int size) {
+            return new GpodnetTag[size];
+        }
+    };
+
+}
diff --git a/net/sync/gpoddernet/src/main/java/de/danoeh/antennapod/net/sync/gpoddernet/model/GpodnetUploadChangesResponse.java b/net/sync/gpoddernet/src/main/java/de/danoeh/antennapod/net/sync/gpoddernet/model/GpodnetUploadChangesResponse.java
new file mode 100644
index 000000000..7b09531a5
--- /dev/null
+++ b/net/sync/gpoddernet/src/main/java/de/danoeh/antennapod/net/sync/gpoddernet/model/GpodnetUploadChangesResponse.java
@@ -0,0 +1,53 @@
+package de.danoeh.antennapod.net.sync.gpoddernet.model;
+
+import androidx.collection.ArrayMap;
+
+import de.danoeh.antennapod.net.sync.gpoddernet.GpodnetService;
+import de.danoeh.antennapod.net.sync.model.UploadChangesResponse;
+import org.json.JSONArray;
+import org.json.JSONException;
+import org.json.JSONObject;
+
+import java.util.Map;
+
+/**
+ * Object returned by {@link GpodnetService} in uploadChanges method.
+ */
+public class GpodnetUploadChangesResponse extends UploadChangesResponse {
+    /**
+     * URLs that should be updated. The key of the map is the original URL, the value of the map
+     * is the sanitized URL.
+     */
+    public final Map<String, String> updatedUrls;
+
+    public GpodnetUploadChangesResponse(long timestamp, Map<String, String> updatedUrls) {
+        super(timestamp);
+        this.updatedUrls = updatedUrls;
+    }
+
+    /**
+     * Creates a new GpodnetUploadChangesResponse-object from a JSON object that was
+     * returned by an uploadChanges call.
+     *
+     * @throws org.json.JSONException If the method could not parse the JSONObject.
+     */
+    public static GpodnetUploadChangesResponse fromJSONObject(String objectString) throws JSONException {
+        final JSONObject object = new JSONObject(objectString);
+        final long timestamp = object.getLong("timestamp");
+        Map<String, String> updatedUrls = new ArrayMap<>();
+        JSONArray urls = object.getJSONArray("update_urls");
+        for (int i = 0; i < urls.length(); i++) {
+            JSONArray urlPair = urls.getJSONArray(i);
+            updatedUrls.put(urlPair.getString(0), urlPair.getString(1));
+        }
+        return new GpodnetUploadChangesResponse(timestamp, updatedUrls);
+    }
+
+    @Override
+    public String toString() {
+        return "GpodnetUploadChangesResponse{" +
+                "timestamp=" + timestamp +
+                ", updatedUrls=" + updatedUrls +
+                '}';
+    }
+}
diff --git a/net/sync/model/README.md b/net/sync/model/README.md
new file mode 100644
index 000000000..21d842914
--- /dev/null
+++ b/net/sync/model/README.md
@@ -0,0 +1,3 @@
+# :net:sync:model
+
+This module contains the basic interfaces for implementing a sync backend.
diff --git a/net/sync/model/build.gradle b/net/sync/model/build.gradle
new file mode 100644
index 000000000..299359602
--- /dev/null
+++ b/net/sync/model/build.gradle
@@ -0,0 +1,54 @@
+apply plugin: "com.android.library"
+
+android {
+    compileSdkVersion rootProject.ext.compileSdkVersion
+
+    defaultConfig {
+        minSdkVersion rootProject.ext.minSdkVersion
+        targetSdkVersion rootProject.ext.targetSdkVersion
+
+        multiDexEnabled false
+
+        testApplicationId "de.danoeh.antennapod.core.tests"
+        testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"
+    }
+    buildTypes {
+        release {
+            minifyEnabled false
+            proguardFiles getDefaultProguardFile("proguard-android.txt")
+        }
+        debug {
+            // debug build has method count over 64k single-dex threshold.
+            // For building debug build to use on Android < 21 (pre-Android 5) devices,
+            // you need to manually change class
+            // de.danoeh.antennapod.PodcastApp to extend MultiDexApplication .
+            // See Issue #2813
+            multiDexEnabled true
+        }
+    }
+
+    compileOptions {
+        sourceCompatibility JavaVersion.VERSION_1_8
+        targetCompatibility JavaVersion.VERSION_1_8
+    }
+
+    testOptions {
+        unitTests {
+            includeAndroidResources = true
+        }
+    }
+
+    lintOptions {
+        disable 'GradleDependency'
+        checkDependencies true
+        warningsAsErrors true
+        abortOnError true
+    }
+}
+
+dependencies {
+    implementation project(':model')
+
+    annotationProcessor "androidx.annotation:annotation:$annotationVersion"
+    implementation "androidx.appcompat:appcompat:$appcompatVersion"
+}
diff --git a/net/sync/model/src/main/AndroidManifest.xml b/net/sync/model/src/main/AndroidManifest.xml
new file mode 100644
index 000000000..a7bd1e34b
--- /dev/null
+++ b/net/sync/model/src/main/AndroidManifest.xml
@@ -0,0 +1 @@
+<manifest package="de.danoeh.antennapod.net.sync.model" />
diff --git a/net/sync/model/src/main/java/de/danoeh/antennapod/net/sync/model/EpisodeAction.java b/net/sync/model/src/main/java/de/danoeh/antennapod/net/sync/model/EpisodeAction.java
new file mode 100644
index 000000000..1aae5c811
--- /dev/null
+++ b/net/sync/model/src/main/java/de/danoeh/antennapod/net/sync/model/EpisodeAction.java
@@ -0,0 +1,267 @@
+package de.danoeh.antennapod.net.sync.model;
+
+import android.text.TextUtils;
+import android.util.Log;
+
+import androidx.annotation.NonNull;
+import androidx.core.util.ObjectsCompat;
+import de.danoeh.antennapod.model.feed.FeedItem;
+import org.json.JSONException;
+import org.json.JSONObject;
+
+import java.text.ParseException;
+import java.text.SimpleDateFormat;
+import java.util.Date;
+import java.util.Locale;
+import java.util.TimeZone;
+
+public class EpisodeAction {
+    private static final String TAG = "EpisodeAction";
+    private static final String PATTERN_ISO_DATEFORMAT = "yyyy-MM-dd'T'HH:mm:ss";
+    public static final Action NEW = Action.NEW;
+    public static final Action DOWNLOAD = Action.DOWNLOAD;
+    public static final Action PLAY = Action.PLAY;
+    public static final Action DELETE = Action.DELETE;
+
+    private final String podcast;
+    private final String episode;
+    private final Action action;
+    private final Date timestamp;
+    private final int started;
+    private final int position;
+    private final int total;
+
+    private EpisodeAction(Builder builder) {
+        this.podcast = builder.podcast;
+        this.episode = builder.episode;
+        this.action = builder.action;
+        this.timestamp = builder.timestamp;
+        this.started = builder.started;
+        this.position = builder.position;
+        this.total = builder.total;
+    }
+
+    /**
+     * Create an episode action object from JSON representation. Mandatory fields are "podcast",
+     * "episode" and "action".
+     *
+     * @param object JSON representation
+     * @return episode action object, or null if mandatory values are missing
+     */
+    public static EpisodeAction readFromJsonObject(JSONObject object) {
+        String podcast = object.optString("podcast", null);
+        String episode = object.optString("episode", null);
+        String actionString = object.optString("action", null);
+        if (TextUtils.isEmpty(podcast) || TextUtils.isEmpty(episode) || TextUtils.isEmpty(actionString)) {
+            return null;
+        }
+        EpisodeAction.Action action;
+        try {
+            action = EpisodeAction.Action.valueOf(actionString.toUpperCase(Locale.US));
+        } catch (IllegalArgumentException e) {
+            return null;
+        }
+        EpisodeAction.Builder builder = new EpisodeAction.Builder(podcast, episode, action);
+        String utcTimestamp = object.optString("timestamp", null);
+        if (!TextUtils.isEmpty(utcTimestamp)) {
+            try {
+                SimpleDateFormat parser = new SimpleDateFormat(PATTERN_ISO_DATEFORMAT, Locale.US);
+                parser.setTimeZone(TimeZone.getTimeZone("UTC"));
+                builder.timestamp(parser.parse(utcTimestamp));
+            } catch (ParseException e) {
+                e.printStackTrace();
+            }
+        }
+        if (action == EpisodeAction.Action.PLAY) {
+            int started = object.optInt("started", -1);
+            int position = object.optInt("position", -1);
+            int total = object.optInt("total", -1);
+            if (started >= 0 && position > 0 && total > 0) {
+                builder
+                        .started(started)
+                        .position(position)
+                        .total(total);
+            }
+        }
+        return builder.build();
+    }
+
+    public String getPodcast() {
+        return this.podcast;
+    }
+
+    public String getEpisode() {
+        return this.episode;
+    }
+
+    public Action getAction() {
+        return this.action;
+    }
+
+    private String getActionString() {
+        return this.action.name().toLowerCase(Locale.US);
+    }
+
+    public Date getTimestamp() {
+        return this.timestamp;
+    }
+
+    /**
+     * Returns the position (in seconds) at which the client started playback.
+     *
+     * @return start position (in seconds)
+     */
+    public int getStarted() {
+        return this.started;
+    }
+
+    /**
+     * Returns the position (in seconds) at which the client stopped playback.
+     *
+     * @return stop position (in seconds)
+     */
+    public int getPosition() {
+        return this.position;
+    }
+
+    /**
+     * Returns the total length of the file in seconds.
+     *
+     * @return total length in seconds
+     */
+    public int getTotal() {
+        return this.total;
+    }
+
+    @Override
+    public boolean equals(Object o) {
+        if (this == o) {
+            return true;
+        }
+        if (!(o instanceof EpisodeAction)) {
+            return false;
+        }
+
+        EpisodeAction that = (EpisodeAction) o;
+        return started == that.started && position == that.position && total == that.total && action != that.action
+                && ObjectsCompat.equals(podcast, that.podcast)
+                && ObjectsCompat.equals(episode, that.episode)
+                && ObjectsCompat.equals(timestamp, that.timestamp);
+    }
+
+    @Override
+    public int hashCode() {
+        int result = podcast != null ? podcast.hashCode() : 0;
+        result = 31 * result + (episode != null ? episode.hashCode() : 0);
+        result = 31 * result + (action != null ? action.hashCode() : 0);
+        result = 31 * result + (timestamp != null ? timestamp.hashCode() : 0);
+        result = 31 * result + started;
+        result = 31 * result + position;
+        result = 31 * result + total;
+        return result;
+    }
+
+    /**
+     * Returns a JSON object representation of this object.
+     *
+     * @return JSON object representation, or null if the object is invalid
+     */
+    public JSONObject writeToJsonObject() {
+        JSONObject obj = new JSONObject();
+        try {
+            obj.putOpt("podcast", this.podcast);
+            obj.putOpt("episode", this.episode);
+            obj.put("action", this.getActionString());
+            SimpleDateFormat formatter = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss", Locale.US);
+            formatter.setTimeZone(TimeZone.getTimeZone("UTC"));
+            obj.put("timestamp", formatter.format(this.timestamp));
+            if (this.getAction() == Action.PLAY) {
+                obj.put("started", this.started);
+                obj.put("position", this.position);
+                obj.put("total", this.total);
+            }
+        } catch (JSONException e) {
+            Log.e(TAG, "writeToJSONObject(): " + e.getMessage());
+            return null;
+        }
+        return obj;
+    }
+
+    @NonNull
+    @Override
+    public String toString() {
+        return "EpisodeAction{"
+                + "podcast='" + podcast + '\''
+                + ", episode='" + episode + '\''
+                + ", action=" + action
+                + ", timestamp=" + timestamp
+                + ", started=" + started
+                + ", position=" + position
+                + ", total=" + total
+                + '}';
+    }
+
+    public enum Action {
+        NEW, DOWNLOAD, PLAY, DELETE
+    }
+
+    public static class Builder {
+
+        // mandatory
+        private final String podcast;
+        private final String episode;
+        private final Action action;
+
+        // optional
+        private Date timestamp;
+        private int started = -1;
+        private int position = -1;
+        private int total = -1;
+
+        public Builder(FeedItem item, Action action) {
+            this(item.getFeed().getDownload_url(), item.getMedia().getDownload_url(), action);
+        }
+
+        public Builder(String podcast, String episode, Action action) {
+            this.podcast = podcast;
+            this.episode = episode;
+            this.action = action;
+        }
+
+        public Builder timestamp(Date timestamp) {
+            this.timestamp = timestamp;
+            return this;
+        }
+
+        public Builder currentTimestamp() {
+            return timestamp(new Date());
+        }
+
+        public Builder started(int seconds) {
+            if (action == Action.PLAY) {
+                this.started = seconds;
+            }
+            return this;
+        }
+
+        public Builder position(int seconds) {
+            if (action == Action.PLAY) {
+                this.position = seconds;
+            }
+            return this;
+        }
+
+        public Builder total(int seconds) {
+            if (action == Action.PLAY) {
+                this.total = seconds;
+            }
+            return this;
+        }
+
+        public EpisodeAction build() {
+            return new EpisodeAction(this);
+        }
+
+    }
+
+}
diff --git a/net/sync/model/src/main/java/de/danoeh/antennapod/net/sync/model/EpisodeActionChanges.java b/net/sync/model/src/main/java/de/danoeh/antennapod/net/sync/model/EpisodeActionChanges.java
new file mode 100644
index 000000000..570e012c5
--- /dev/null
+++ b/net/sync/model/src/main/java/de/danoeh/antennapod/net/sync/model/EpisodeActionChanges.java
@@ -0,0 +1,34 @@
+package de.danoeh.antennapod.net.sync.model;
+
+
+import androidx.annotation.NonNull;
+
+import java.util.List;
+
+public class EpisodeActionChanges {
+
+    private final List<EpisodeAction> episodeActions;
+    private final long timestamp;
+
+    public EpisodeActionChanges(@NonNull List<EpisodeAction> episodeActions, long timestamp) {
+        this.episodeActions = episodeActions;
+        this.timestamp = timestamp;
+    }
+
+    public List<EpisodeAction> getEpisodeActions() {
+        return this.episodeActions;
+    }
+
+    public long getTimestamp() {
+        return this.timestamp;
+    }
+
+    @NonNull
+    @Override
+    public String toString() {
+        return "EpisodeActionGetResponse{"
+                + "episodeActions=" + episodeActions
+                + ", timestamp=" + timestamp
+                + '}';
+    }
+}
diff --git a/net/sync/model/src/main/java/de/danoeh/antennapod/net/sync/model/ISyncService.java b/net/sync/model/src/main/java/de/danoeh/antennapod/net/sync/model/ISyncService.java
new file mode 100644
index 000000000..9c75e5dac
--- /dev/null
+++ b/net/sync/model/src/main/java/de/danoeh/antennapod/net/sync/model/ISyncService.java
@@ -0,0 +1,20 @@
+package de.danoeh.antennapod.net.sync.model;
+
+import java.util.List;
+
+public interface ISyncService {
+
+    void login() throws SyncServiceException;
+
+    SubscriptionChanges getSubscriptionChanges(long lastSync) throws SyncServiceException;
+
+    UploadChangesResponse uploadSubscriptionChanges(
+            List<String> addedFeeds, List<String> removedFeeds) throws SyncServiceException;
+
+    EpisodeActionChanges getEpisodeActionChanges(long lastSync) throws SyncServiceException;
+
+    UploadChangesResponse uploadEpisodeActions(List<EpisodeAction> queuedEpisodeActions)
+            throws SyncServiceException;
+
+    void logout() throws SyncServiceException;
+}
diff --git a/net/sync/model/src/main/java/de/danoeh/antennapod/net/sync/model/SubscriptionChanges.java b/net/sync/model/src/main/java/de/danoeh/antennapod/net/sync/model/SubscriptionChanges.java
new file mode 100644
index 000000000..2fbc8b45e
--- /dev/null
+++ b/net/sync/model/src/main/java/de/danoeh/antennapod/net/sync/model/SubscriptionChanges.java
@@ -0,0 +1,39 @@
+package de.danoeh.antennapod.net.sync.model;
+
+import androidx.annotation.NonNull;
+
+import java.util.List;
+
+public class SubscriptionChanges {
+    private final List<String> added;
+    private final List<String> removed;
+    private final long timestamp;
+
+    public SubscriptionChanges(@NonNull List<String> added,
+                               @NonNull List<String> removed,
+                               long timestamp) {
+        this.added = added;
+        this.removed = removed;
+        this.timestamp = timestamp;
+    }
+
+    @Override
+    public String toString() {
+        return "SubscriptionChange [added=" + added.toString()
+                + ", removed=" + removed.toString() + ", timestamp="
+                + timestamp + "]";
+    }
+
+    public List<String> getAdded() {
+        return added;
+    }
+
+    public List<String> getRemoved() {
+        return removed;
+    }
+
+    public long getTimestamp() {
+        return timestamp;
+    }
+
+}
diff --git a/net/sync/model/src/main/java/de/danoeh/antennapod/net/sync/model/SyncServiceException.java b/net/sync/model/src/main/java/de/danoeh/antennapod/net/sync/model/SyncServiceException.java
new file mode 100644
index 000000000..57262db17
--- /dev/null
+++ b/net/sync/model/src/main/java/de/danoeh/antennapod/net/sync/model/SyncServiceException.java
@@ -0,0 +1,13 @@
+package de.danoeh.antennapod.net.sync.model;
+
+public class SyncServiceException extends Exception {
+    private static final long serialVersionUID = 1L;
+
+    public SyncServiceException(String message) {
+        super(message);
+    }
+
+    public SyncServiceException(Throwable cause) {
+        super(cause);
+    }
+}
diff --git a/net/sync/model/src/main/java/de/danoeh/antennapod/net/sync/model/UploadChangesResponse.java b/net/sync/model/src/main/java/de/danoeh/antennapod/net/sync/model/UploadChangesResponse.java
new file mode 100644
index 000000000..7503f429b
--- /dev/null
+++ b/net/sync/model/src/main/java/de/danoeh/antennapod/net/sync/model/UploadChangesResponse.java
@@ -0,0 +1,13 @@
+package de.danoeh.antennapod.net.sync.model;
+
+public abstract class UploadChangesResponse {
+
+    /**
+     * timestamp/ID that can be used for requesting changes since this upload.
+     */
+    public final long timestamp;
+
+    public UploadChangesResponse(long timestamp) {
+        this.timestamp = timestamp;
+    }
+}