• Chat servers facilitate message sending/receiving.
• Presence servers manage online/offline status.
• API servers handle everything including user login, signup, change profile, etc.
• Notification servers send push notifications.
• Finally, the key-value store is used to store chat history. When an offline user comes online, she will see all her previous chat history.

Chat history data storage:

KV store

Key-value stores are adopted by other proven reliable chat applications. For example, both Facebook messenger and Discord use key-value stores. Facebook messenger uses HBase [4], and Discord uses Cassandra

Service Discovery

recommend the best chat server for a client based on the criteria like geographical location, server capacity, etc

ZooKeeper (Yahoo/Apache)