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Ok, replace "P2P restriction" with "idempotent, commutative restriction". > For example, OT has a tradeoff where you can discard old operations. The cost of doing so is that you can no longer merge old changes. Why wouldn’t you be able to? My server receives operations, applies them to the document, and discards them. It can receive operations as old as it wants. ___ > Can you give some examples of this metadata? Yes, it depends on the CRDT, but if we're talking about lists or tree structures with insert and delete operations, these can come in the form of thombstones, or operation logs, or originRight or originLeft, or DAG. Even with a garbage collector, the CRDT needs to retain some of this metadata. Yes, you can optimize by not bringing it into memory when it’s not needed. But they’re still there, even though they could be avoided entirely if you assume a central server that guarantees a deterministic ordering of operations. |
OT needs to do operation transformation. If you get passed old operations, you need to transform them before they can be applied. This requires keeping extra data around. I mean, this is entirely the point of the metadata you describe in the second part of your comment:
> these can come in the form of thombstones, or operation logs, or originRight or originLeft, or DAG. Even with a garbage collector, the CRDT needs to retain some of this metadata.
> But they’re still there, even though they could be avoided entirely if you assume a central server that guarantees a deterministic ordering of operations.
A central server doesn't remove the need for this data though! Lets assume a completely deterministic ordering of operations on the server, like Jupiter. If the server is at version 1000, and I send the server an operation at version 900, the server needs to use information about operations 900-1000 to be able to apply the change. This is true in Jupiter OT - which uses the actual operations. Or in Yjs or Diamond Types or any other collaborative text editing system. We need some of that information to figure out how to transform the incoming change and order it correctly.
At least, I've never seen or heard of any scheme which doesn't need some data like this.