Scene 06

Replication is async — acked writes can vanish

Redis replication is asynchronous primary-backup: writes ack on the master before reaching replicas, so failover can lose acked writes; the backlog buys partial resyncs, WAIT is best-effort, and only min-replicas-to-write converts loss into unavailability.

Previously

One node, however well-tuned, is a single point of failure. The first answer is to keep a copy on another node — but that copy lags behind, and that lag is the whole story of this scene.

Scene 06

Replication is async — acked writes can vanish

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What to watch for

Master accepts writes; the replication backlog ring fills; PSYNC streams keep both replicas' offsets within a tick of the master. This is the steady state — and the trap.

Implementation

Highlighted lines are the ones running in the diagram right now.

Replica.psync

(replication-id, offset) handshake; partial vs full

1def psync():
2    repl_id, offset = self.replicationId, self.offset
3    resp = master.PSYNC(repl_id, offset)
4    if resp == CONTINUE:
5        # backlog still covers our gap
6        partial_resync()       # stream missed bytes
7    else:  # FULLRESYNC <new_id> <new_offset>
8        self.replicationId = resp.new_id
9        full_resync_via_rdb()  # fork + ship RDB
10    stream_replication_link()

Master.handleWrite

append locally, ack the client, then replicate

1def handleWrite(cmd):
2    self.applyToDataset(cmd)
3    self.offset += len(cmd)
4    self.appendToBacklog(cmd)
5    replyOK(client)            # ack now
6    for r in self.replicas:
7        r.send(cmd)            # fire-and-forget

Sources

Where this sits in Build Redis

Scene 06 of 10, in the HA act — Replication and Sentinel — async by design.. PSYNC, replication backlog, and the AP-not-CP gotcha: WAIT doesn't fix it; min-replicas-to-write does (at the cost of unavailability).

Up next. Sentinel — replicas keep a copy. But who decides the master is dead, and who promotes a replica into its place?

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