Garbage Collection

One of the most important issues in CRDT systems is handling tombstones. Tombstones are used to properly synchronize the document even when remote peer’s operations refer to the elements that have already been locally deleted. This causes the problem that the document keeps growing even if elements are deleted.

Yorkie provides Garbage Collection to solve this problem.

How it works

Garbage collection checks that deleted nodes are no longer referenced remotely and purges them completely.

Server records the logical timestamp of the last change pulled by the client whenever the client requests PushPull. And Server returns the smallest logical timestamp, min_synced_seq of all clients in response PushPull to the client. min_synced_seq is used to check whether deleted nodes are no longer to be referenced remotely or not.

An example of garbage collection:

State 1:

In the initial state, both clients have "ab".

State 2:

Client c1 deletes "b", which is recorded as a change with logical timestamp 3. The text node of "b" can be referenced by remote, so it is only marked as tombstone. And the client c1 sends change 3 to server through PushPull API and receives as a response that min_synced_seq is 2. Since all clients did not receive the deletion change 3, the text node is not purged by garbage collection.

Meanwhile, client c2 inserts "c" after textnode "b".

State 3:

Client c2 pushes change 4 to server and receives as a response that min_synced_seq is 3. After the client applies change 4, the contents of document are changed to ac. This time, all clients have received change 3, so textnode "b" is completely removed.

State 4:

Finally, after client c1 receives change 4 from server, purges textnode "b" because it is no longer referenced remotely.

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