Semi-synchronous Replication

Giriş

Açıklaması şöyle. Master tüm replica'ların da transaction'ı commit etmesini beklemez. Sadece bazı replica'ların relay log'a yazmasını bekler. Yani aslında replica'da halen commit yapmamıştır ama relay log'a başarıyla yazdığımı bildirmiştir.

Semisynchronous replication falls between asynchronous and fully synchronous replication. The source waits until at least one replica has received and logged the events (the required number of replicas is configurable), and then commits the transaction. The source does not wait for all replicas to acknowledge receipt, and it requires only an acknowledgement from the replicas, not that the events have been fully executed and committed on the replica side. Semisynchronous replication therefore guarantees that if the source crashes, all the transactions that it has committed have been transmitted to at least one replica.

Compared to asynchronous replication, semisynchronous replication provides improved data integrity, because when a commit returns successfully, it is known that the data exists in at least two places. Until a semisynchronous source receives acknowledgment from the required number of replicas, the transaction is on hold and not committed.

Compared to fully synchronous replication, semisynchronous replication is faster, because it can be configured to balance your requirements for data integrity (the number of replicas acknowledging receipt of the transaction) with the speed of commits, which are slower due to the need to wait for replicas.

Şekle şöyle

Semi-synchronous Replication da veri kaybedebilir. Açıklaması şöyle

Semisynchronous replication between a source and its replicas operates as follows:

- A replica indicates whether it is semisynchronous-capable when it connects to the source.

- If semisynchronous replication is enabled on the source side and there is at least one semisynchronous replica, a thread that performs a transaction commit on the source blocks and waits until at least one semisynchronous replica acknowledges that it has received all events for the transaction, or until a timeout occurs.

- The replica acknowledges receipt of a transaction's events only after the events have been written to its relay log and flushed to disk.

- If a timeout occurs without any replica having acknowledged the transaction, the source reverts to asynchronous replication. When at least one semisynchronous replica catches up, the source returns to semisynchronous replication.

- Semisynchronous replication must be enabled on both the source and replica sides. If semisynchronous replication is disabled on the source, or enabled on the source but on no replicas, the source uses asynchronous replication.

Vitess Açısından

Vitess açısından şöyle. Yani Vitess veri kaybetmez, veri kaybedeceğine transaction 'ı commit' lemez.

Vitess strongly recommends the use of Semi-synchronous replication for High Availability. When enabled in Vitess, semi-sync has the following characteristics:

- The primary will only accept writes if it has at least one replica connected, and configured correctly to send semi-sync ACKs. Vitess configures the semi-sync timeout to essentially an unlimited number so that it will never fallback to asyncronous replication. This is important to prevent split brain (or alternate futures) in case of a network partition. If we can verify all replicas have stopped replicating, we know the old primary is not accepting writes, even if we are unable to contact the old primary itself.

- Tablets of type rdonly will not send semi-sync ACKs. This is intentional because rdonly tablets are not eligible to be promoted to primary, so Vitess avoids the case where a rdonly tablet is the single best candidate for election at the time of primary failure.

rpl_semi_sync_master_wait_for_slave_count Ayarı

Bu ayar Primary tarafında kullanılır. Açıklaması şöyle

The primary waits up to rpl_semi_sync_master_timeout, after which it falls back to asynchronous replication mode, committing and responding to the user even if not all expected replicas have acknowledged receipt.