Create a service class that is a singleton that keeps track of likes on posts, comments. Periodically flush the data to the database.

class LikeService {
   // format: <pk, nlikes>
   HashMap<Integer, Integer> cachePostLikes = new HashMap<Integer,Integer>();  
   HashMap<Integer, Integer> cacheCommentLikes = new HashMap<Integer,Integer>(); 

  void incrementPostLikes() {}
  void incrementCommentLikes() {}

  Long getPostLikes(Integer postId) {}
  Long getCommentLikes(Integer commentId) {}

  void flushToDB() {}
}

Create a service class that is a singleton that keeps track of likes on posts, comments. Periodically flush the data to the database.

class LikeService {
   // format: <pk, nlikes>
   HashMap<Integer, Integer> cachePostLikes = new HashMap<Integer,Integer>();  
   HashMap<Integer, Integer> cacheCommentLikes = new HashMap<Integer,Integer>(); 
   HashMap<Integer, Integer> dbPostLikes = new HashMap<Integer,Integer>();  
   HashMap<Integer, Integer> dbPostLikes = new HashMap<Integer,Integer>();  


  void incrementPostLikes() {}
  void incrementCommentLikes() {}

  Long getPostLikes(Integer postId) {
      if(!cachePostLikes.containsKey(postId)) {
          // load from db
      }
      return cachePostLikes.get(postId) + dbPostLikes.get(postId)
  }
  Long getCommentLikes(Integer commentId) {}
  
  // Always reset counts in cache hashmaps to zero
  void flushToDB() {}
}

Create a service class that is a singleton that keeps track of likes on posts, comments. Periodically flush the data to the database.

class LikeService {
   // format: <pk, nlikes>
   HashMap<Integer, Integer> cachePostLikes = new HashMap<Integer,Integer>();  
   HashMap<Integer, Integer> cacheCommentLikes = new HashMap<Integer,Integer>(); 

  void incrementPostLikes() {}
  void incrementCommentLikes() {}

  Long getPostLikes(Long postId) {}
  Long getCommentLikes(Integer commentId) {}

  void flushToDB() {}
}

Create a service class that is a singleton that keeps track of likes on posts, comments. Periodically flush the data to the database.

class LikeService {
   // format: <pk, nlikes>
   HashMap<Integer, Integer> cachePostLikes = new HashMap<Integer,Integer>();  
   HashMap<Integer, Integer> cacheCommentLikes = new HashMap<Integer,Integer>(); 

  void incrementPostLikes() {}
  void incrementCommentLikes() {}

  Long getPostLikes(Integer postId) {}
  Long getCommentLikes(Integer commentId) {}

  void flushToDB() {}
}