MySQL的semi-synchronous replication通過確保至少一個副本接收事務(wù)來平衡性能與數(shù)據(jù)安全。 1. 在事務(wù)提交時，主服務(wù)器等待至少一個副本確認(rèn)接收並寫入中繼日誌；2. 一旦確認(rèn)，主服務(wù)器才提交事務(wù)並向客戶端返回成功；3. 若超時未收到響應(yīng)，則自動回退到異步模式以維持系統(tǒng)運(yùn)行；4. 啟用該功能需在主從服務(wù)器安裝插件並設(shè)置相應(yīng)參數(shù)；5. 其優(yōu)勢在於提供比異步複製更強(qiáng)的數(shù)據(jù)完整性，但存在輕微性能損耗及網(wǎng)絡(luò)延遲影響。這種複制方式適用於需要較高數(shù)據(jù)一致性但無法接受全同步性能開銷的場景。

Semi-synchronous replication in MySQL is a type of replication setup that strikes a balance between performance and data safety. Unlike fully synchronous replication, which waits for all replicas to confirm receipt of data before committing a transaction (and can be slow), and asynchronous replication, which doesn't wait for any replica confirmation (and risks data loss), semi-synchronous replication ensures that at least one replica has received the transaction before the master commits it.

This means you get better data integrity without paying the full performance cost of full synchronization.

What happens during semi-synchronous replication

In a semi-synchronous setup, when a transaction is committed on the master server, it waits for at least one replica to acknowledge that it has received and written the transaction to its relay log. Once that acknowledgment comes back, the transaction is considered successful and gets committed on the master.

If no replica responds within a defined timeout period, MySQL automatically falls back to asynchronous mode to keep the system running smoothly. This fallback behavior helps maintain performance during network issues or replica failures.

Here's what typically happens:

• Master writes the transaction to its binary log.
• Master sends the transaction to connected replicas.
• At least one replica acknowledges receipt.
• Master commits the transaction and returns success to the client.

How to enable semi-synchronous replication

Enabling semi-synchronous replication requires both the master and at least one replica to support it. It's available in MySQL 5.5 and later versions (with plugin support added in 5.6 ).

Here are the basic steps:

• Install the semi-synchronous plugins on both the master and replica servers.
• Enable the master-side plugin using SET GLOBAL rpl_semi_sync_master_enabled = 1; .
• Enable the replica-side plugin with SET GLOBAL rpl_semi_sync_slave_enabled = 1; (note: in newer versions, this variable may be called rpl_semi_sync_replica_enabled ).
• Restart replication threads if needed.

You can check whether semi-sync is active by querying the status variables:

 SHOW STATUS LIKE 'Rpl_semi_sync_%_status';

Benefits and trade-offs

Using semi-synchronous replication gives you stronger guarantees than asynchronous replication — specifically, that each committed transaction has been received by at least one replica. That reduces the risk of losing data if the master crashes right after a commit.

However, there are some downsides:

• There's a slight performance hit compared to asynchronous replication, since the master waits for at least one response.
• The fallback to async mode under stress or failure conditions means it's not fully safe in all scenarios.
• Network latency can affect how quickly replicas respond, potentially slowing down the master.

Still, for most applications where data consistency matters but full synchronous replication is too strict, semi-synchronous replication is a solid choice.

When to use it (and when not to)

Semi-synchronous replication works well in environments where:

• You need better data protection than async offers.
• Your replicas are geographically close to the master.
• You're okay with a small performance trade-off for increased reliability.

It might not be the best fit if:

• You require strict zero-data-loss guarantees (in which case, consider full synchronous replication or other consensus-based systems like Galera or InnoDB Cluster).
• Your network is unstable or high-latency.
• Performance is your top priority and data loss risk is acceptable.

Basically, semi-synchronous replication adds a layer of safety over asynchronous replication without locking everything up like full sync does. It's a good middle ground for many production setups — just make sure your infrastructure supports it and you understand how it behaves under different conditions.

以上是半同步複製如何在MySQL中起作用？的詳細(xì)內(nèi)容。更多資訊請關(guān)注PHP中文網(wǎng)其他相關(guān)文章！