fix(wal): gate flush feedback on durable server acknowledgements - #124
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The WAL streaming client reported a flush position to PostgreSQL as soon as WAL data was handed to the gRPC send buffer, before the Klio server confirmed it durable. That made it unsafe to use Klio as a synchronous replication target for zero RPO: PostgreSQL could acknowledge commits, or advance the replication slot's restart_lsn, for data not yet guaranteed to survive a server or network failure. Convert the Put RPC to bidirectional streaming so the server acknowledges each block once fsynced, and add a require_durable_ack client option (default off) that advances the flush position only up to durably acknowledged data. The default preserves the previous optimistic behavior. Assisted-by: Claude Signed-off-by: Gabriele Quaresima <gabriele.quaresima@enterprisedb.com>
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Add a requireDurableAck field to the PluginConfiguration spec and wire it into the generated WAL client configuration, so synchronous replication targets can require durable acknowledgements. Document how to enable it. Assisted-by: Claude Signed-off-by: Gabriele Quaresima <gabriele.quaresima@enterprisedb.com>
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Problem
The WAL streaming client reported a flush position to PostgreSQL as soon as WAL
data was handed to the gRPC send buffer, before the Klio server confirmed it
durable. This made it unsafe to use Klio as a synchronous replication target for
zero RPO: PostgreSQL could acknowledge commits, or advance the physical slot's
restart_lsn, for data not yet guaranteed to survive a Klio server or network
failure.
Change
PutRPC to bidirectional streaming so the serveracknowledges each block once it has been fsynced.
require_durable_ackclient option (default off) that advances thereported flush position only up to durably acknowledged data. The default
preserves the previous optimistic behavior.
PluginConfiguration.spec.requireDurableAckand wire it into thegenerated client config.
claims.
Testing
on commit latency or throughput. With Klio as the sole synchronous standby it
gates commits as intended: in the single-client worst case, per-commit latency
went from ~200 ms to ~400 ms (one extra flush cycle for the durable round
trip); async runs were unchanged.
Closes #98