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diff --git a/pse-server/src/main/java/org/psesquared/server/subscriptions/api/service/SubscriptionService.java b/pse-server/src/main/java/org/psesquared/server/subscriptions/api/service/SubscriptionService.java
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+++ b/pse-server/src/main/java/org/psesquared/server/subscriptions/api/service/SubscriptionService.java
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+package org.psesquared.server.subscriptions.api.service;
+
+import java.time.LocalDateTime;
+import java.time.ZoneOffset;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.NoSuchElementException;
+import java.util.Map;
+import java.util.HashMap;
+import java.util.logging.Level;
+import java.util.logging.Logger;
+import lombok.RequiredArgsConstructor;
+import org.psesquared.server.authentication.api.data.access.AuthenticationDao;
+import org.psesquared.server.episode.actions.api.controller.EpisodeActionPost;
+import org.psesquared.server.episode.actions.api.data.access.EpisodeActionDao;
+import org.psesquared.server.episode.actions.api.service.EpisodeActionService;
+import org.psesquared.server.model.Subscription;
+import org.psesquared.server.model.SubscriptionAction;
+import org.psesquared.server.model.User;
+import org.psesquared.server.subscriptions.api.controller.SubscriptionDelta;
+import org.psesquared.server.subscriptions.api.controller.SubscriptionTitles;
+import org.psesquared.server.subscriptions.api.data.access.SubscriptionActionDao;
+import org.psesquared.server.subscriptions.api.data.access.SubscriptionDao;
+import org.psesquared.server.util.RssParser;
+import org.springframework.http.HttpStatus;
+import org.springframework.stereotype.Service;
+import org.springframework.transaction.annotation.Transactional;
+
+/**
+ * This service class manages all business logic associated with the
+ * episode action API.
+ * <br>
+ * It is called from the
+ * {@link
+ * org.psesquared.server.subscriptions.api.controller.SubscriptionController}
+ * and passes on requests concerning data access mainly to the
+ * {@link SubscriptionDao} and {@link SubscriptionActionDao}.
+ */
+@Service
+@Transactional
+@RequiredArgsConstructor
+public class SubscriptionService {
+
+  /**
+   * The error message that is logged if no subscription exists
+   * for a remove action.
+   */
+  private static final String NO_SUB_WARNING
+      = "Subscription for remove action does not exist!";
+
+  /**
+   * The logger for logging some warnings.
+   */
+  private static final Logger LOGGER
+      = Logger.getLogger(SubscriptionService.class.getName());
+
+  /**
+   * The class for fetching data from RSS feeds.
+   */
+  private final RssParser rssParser;
+
+  /**
+   * The JPA repository that handles all user related database requests.
+   */
+  private final AuthenticationDao authenticationDao;
+
+  /**
+   * The JPA repository that handles all subscription related database requests.
+   */
+  private final SubscriptionDao subscriptionDao;
+
+  /**
+   * The JPA repository that handles all subscription action related database
+   * requests.
+   */
+  private final SubscriptionActionDao subscriptionActionDao;
+
+  /**
+   * The JPA repository that handles all episode action related database
+   * requests.
+   */
+  private final EpisodeActionDao episodeActionDao;
+
+  /**
+   * The service class of the episode action API.
+   */
+  private final EpisodeActionService episodeActionService;
+
+  /**
+   * It takes a list of podcast URLs in the form of strings and checks if they
+   * exist in the database.
+   * If they do not exist yet, it creates them.
+   * <br>
+   * Then it checks, if the user already has a subscription action for each
+   * subscription, separately.
+   * If not, it creates one.
+   * If yes, it updates the action.
+   *
+   * @param username            The username of the user
+   * @param subscriptionStrings List of Strings, each String is a URL of a
+   *                            podcast
+   * @return {@link HttpStatus#OK} on success,
+   *        <br>
+   *        {@link HttpStatus#NOT_FOUND} user not found
+   */
+  public HttpStatus uploadSubscriptions(
+      final String username,
+      final List<String> subscriptionStrings) {
+    User user;
+    try {
+      user = authenticationDao.findByUsername(username)
+          .orElseThrow();
+    } catch (NoSuchElementException e) {
+      return HttpStatus.NOT_FOUND;
+    }
+
+    Subscription subscription;
+    for (String subscriptionString : subscriptionStrings) {
+
+      try {
+        subscription
+            = subscriptionDao.findByUrl(subscriptionString).orElseThrow();
+      } catch (NoSuchElementException e) {
+        subscription = Subscription.builder()
+            .url(subscriptionString)
+            .timestamp(LocalDateTime.now().toEpochSecond(ZoneOffset.UTC))
+            .build();
+        subscriptionDao.save(subscription);
+        rssParser.validate(subscription);
+      }
+
+      try {
+        SubscriptionAction subscriptionAction
+            = subscriptionActionDao
+            .findByUserAndSubscription(user, subscription)
+            .orElseThrow();
+        subscriptionAction.setAdded(true);
+        subscriptionAction.setTimestamp(
+            LocalDateTime.now().toEpochSecond(ZoneOffset.UTC));
+        subscriptionActionDao.save(subscriptionAction);
+      } catch (NoSuchElementException e) {
+        SubscriptionAction subscriptionAction = SubscriptionAction.builder()
+            .user(user)
+            .added(true)
+            .subscription(subscription)
+            .timestamp(LocalDateTime.now().toEpochSecond(ZoneOffset.UTC))
+            .build();
+        subscriptionActionDao.save(subscriptionAction);
+      }
+    }
+
+    return HttpStatus.OK;
+  }
+
+  /**
+   * It returns a URL List of all podcasts the user is subscribed to in the form
+   * of a String list.
+   *
+   * @param username The username of the user whose subscriptions are being
+   *                 requested
+   * @return A list of RSS-Feed URLs of all subscribed podcasts
+   */
+  public List<String> getSubscriptions(final String username) {
+    List<SubscriptionAction> subscriptionActions
+        = subscriptionActionDao.findByUserUsernameAndAddedTrue(username);
+    List<String> subscriptionUrls = new ArrayList<>();
+    for (SubscriptionAction subscriptionAction : subscriptionActions) {
+      subscriptionUrls.add(subscriptionAction.getSubscription().getUrl());
+    }
+    return subscriptionUrls;
+  }
+
+  /**
+   * All subscription changes of the user are uploaded to the database.
+   *
+   * @param username The username of the user uploading their subscription
+   *                 changes
+   * @param delta    The subscription changes in the form of a SubscriptionDelta
+   *                 containing the added / removed subscriptions
+   */
+  public void applySubscriptionDelta(
+      final String username,
+      final SubscriptionDelta delta) {
+
+    SubscriptionDelta minimizedDelta = minimizeDelta(delta);
+
+    uploadSubscriptions(username, minimizedDelta.getAdd());
+    for (String removeSub : minimizedDelta.getRemove()) {
+      try {
+        SubscriptionAction subscriptionAction
+            = subscriptionActionDao
+            .findByUserUsernameAndSubscriptionUrl(username, removeSub)
+            .orElseThrow();
+        subscriptionAction.setAdded(false);
+        subscriptionAction.setTimestamp(
+            LocalDateTime.now().toEpochSecond(ZoneOffset.UTC));
+        subscriptionActionDao.save(subscriptionAction);
+        episodeActionDao
+            .deleteByUserUsernameAndEpisodeSubscriptionUrl(username, removeSub);
+      } catch (NoSuchElementException e) {
+        LOGGER.log(Level.WARNING, NO_SUB_WARNING);
+      }
+    }
+  }
+
+  private SubscriptionDelta minimizeDelta(SubscriptionDelta oldDelta){
+    SubscriptionDelta minimizedDelta = new SubscriptionDelta(new ArrayList<>(), new ArrayList<>());
+
+    Map<String, Integer> deltaMap = new HashMap<>();
+    for (String addString : oldDelta.getAdd()) {
+      if(deltaMap.containsKey(addString)) {
+        deltaMap.put(addString, deltaMap.get(addString) + 1);
+      }
+      else{
+        deltaMap.put(addString, 1);
+      }
+    }
+
+    for (String removeString : oldDelta.getRemove()) {
+      if(deltaMap.containsKey(removeString)) {
+        deltaMap.put(removeString, deltaMap.get(removeString) - 1);
+      }
+      else{
+        deltaMap.put(removeString, -1);
+      }
+    }
+
+    for(Map.Entry<String, Integer> deltaEntry : deltaMap.entrySet()) {
+      if(deltaEntry.getValue() > 0) {
+        minimizedDelta.getAdd().add(deltaEntry.getKey());
+      } else if(deltaEntry.getValue() < 0) {
+        minimizedDelta.getRemove().add(deltaEntry.getKey());
+      }
+    }
+
+    return minimizedDelta;
+  }
+
+  /**
+   * Returns a SubscriptionDelta of all changes made to a users subscriptions
+   * since a given point in time.
+   *
+   * @param username The username of the user whose subscription changes are
+   *                 being requested
+   * @param since    The timestamp signifying how old the changes are allowed to
+   *                 be
+   * @return The SubscriptionDelta of all changes made since the given timestamp
+   */
+  public SubscriptionDelta getSubscriptionDelta(
+      final String username,
+      final long since) {
+    List<String> added = new ArrayList<>();
+    List<String> removed = new ArrayList<>();
+
+    List<SubscriptionAction> subscriptionActions = subscriptionActionDao
+        .findByUserUsernameAndTimestampGreaterThanEqual(username, since);
+    for (SubscriptionAction subscriptionAction : subscriptionActions) {
+      if (subscriptionAction.isAdded()) {
+        added.add(subscriptionAction.getSubscription().getUrl());
+      } else {
+        removed.add(subscriptionAction.getSubscription().getUrl());
+      }
+    }
+
+    return new SubscriptionDelta(added, removed);
+  }
+
+  /**
+   * Returns all Subscriptions and their 20 latest episodes of a given user as a
+   * List of SubscriptionTitles.
+   *
+   * @param username The username of the user whose subscriptions are being
+   *                 requested
+   * @return A list of SubscriptionTitles containing each Subscription and their
+   *        20 latest Episodes
+   */
+  public List<SubscriptionTitles> getTitles(final String username) {
+    List<SubscriptionAction> subscriptionActions
+        = subscriptionActionDao.findByUserUsernameAndAddedTrue(username);
+    List<Subscription> subscriptions = new ArrayList<>();
+    List<SubscriptionTitles> subscriptionTitlesList = new ArrayList<>();
+
+    for (SubscriptionAction subscriptionAction : subscriptionActions) {
+      subscriptions.add(subscriptionAction.getSubscription());
+    }
+
+    for (Subscription subscription : subscriptions) {
+      List<EpisodeActionPost> episodeActionPosts
+          = episodeActionService
+          .getEpisodeActionsOfPodcast(username, subscription.getUrl());
+      SubscriptionTitles subscriptionTitles
+          = new SubscriptionTitles(subscription, episodeActionPosts);
+      subscriptionTitlesList.add(subscriptionTitles);
+    }
+
+    return subscriptionTitlesList;
+  }
+
+}
diff --git a/pse-server/src/main/java/org/psesquared/server/subscriptions/api/service/package-info.java b/pse-server/src/main/java/org/psesquared/server/subscriptions/api/service/package-info.java
new file mode 100644
index 0000000..9e2a598
--- /dev/null
+++ b/pse-server/src/main/java/org/psesquared/server/subscriptions/api/service/package-info.java
@@ -0,0 +1,13 @@
+/**
+ * This package represents the logical middle layer of the subscription API
+ * ({@link org.psesquared.server.subscriptions.api}) - the service layer.
+ * <br>
+ * All business logic is handled here with the
+ * {@link
+ * org.psesquared.server.subscriptions.api.service.SubscriptionService}
+ * class.
+ *
+ * @author PSE-Squared Team
+ * @version 1.0
+ */
+package org.psesquared.server.subscriptions.api.service;