Storage-Message roaming

• Goal

• Differences between message roaming and offline message pulling

  • Offline message pulling

    • Trigger

    • Storage key

    • Retention period

  • Message roaming

    • Trigger

    • Storage key

• Storage model

• Flowchart

  • Offline message implementation

  • History message implementation

• Timeline model in DingTalk (钉钉)

• TODO

Goal

• Whatever device a user logs in, he could access past messaging history.

• For example, Telegram/QQ could support it but WeChat does not support it.

Differences between message roaming and offline message pulling

• Offline msgs should not be stored together with normal online msgs because

  • Offline msgs will contain operation instructions which will not be persisted in online cases. For example, client A deletes a message in device A. When syncing from Client A's device B, the message should still be deleted.

Offline message pulling

Trigger

• Whenever a user opens app, the app will pull the offline messags. After network jittery, app will pull the offline messages. Pulling offline message is a high frequency operation.

Storage key

• Offline message includes different types of messages including 1-on-1 chat, group chat, system notifications, etc.

• So the offline messages should be keyed on each user's identity, such as UserID.

Retention period

• The offline messages only have a certain retention period (1 week) or upper limit (1000 messages). Since the number of users' devices is unknown, offline messages could not stored forever. It should be stored in a FIFO basis.

Message roaming

Trigger

• Whenever a user opens a conversation and scroll down, app will pull all history conversations. Message roaming is a low frequent operation.

• Typical scenarios:

  • User installs a new IM app and will be able to see history messages.

  • When user chats on PC device, the history should also be available on mobile app.

Storage key

• Historical message should be keyed on a per conversation id.

Storage model

Flowchart

• Differences:

  • Move all offline message storage to cache, keyed by per user.

  • Move all history message storage separate, keyed by per thread.

Offline message implementation

• For each user, created a SortedSet structure keyed by UserID.

• Within the SortedSet, offline messages are sorted by their sequence order.

• Complexity for adding an entry: O(logN)

• Complexity for reading an entry: O(logN + M)

History message implementation

• Store history messages according to the group id.

• For each user in the group, store a timestamp when the user joined the group.

Timeline model in DingTalk (钉钉)

• References: https://www.alibabacloud.com/blog/implementation-of-message-push-and-storage-architectures-of-modern-im-systems_594780

• Chinese references:

  • TimeLine模型下确保消息有序不丢
  • 基于TimeLine模型的消息同步机制

TODO

• http://www.52im.net/forum.php?mod=collection&action=view&ctid=29&fromop=all