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Dgraph supports distributed ACID transactions through snapshot isolation.

Can we do pre-writes only on leaders?

Seems like a good idea, but has bad implications. If we only do a prewrite in-memory, only on leader, then this prewrite wouldn’t make it to the Raft log, or disk; but would be considered successful.

Then zero could mark the transaction as committed; but this leader could go down, or leadership could change. In such a case, we’d end up losing the transaction altogether despite it having been considered committed.

Therefore, pre-writes do have to make it to disk. And if so, better to propose them in a Raft group.