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Table of Contents
What happens during semi-synchronous replication
How to enable semi-synchronous replication
Benefits and trade-offs
When to use it (and when not to)
Home Database Mysql Tutorial How does semi-synchronous replication work in MySQL?

How does semi-synchronous replication work in MySQL?

Jun 18, 2025 am 12:24 AM
mysql semisynchronous replication

MySQL's semi-synchronous replication balances performance with data security by ensuring at least one replica receives transactions. 1. When the transaction is submitted, the master server waits for at least one replica to confirm receipt and writes the relay log; 2. Once confirmed, the master server submits the transaction and returns it to the client successfully; 3. If the timeout does not receive a response, it will automatically fall back to asynchronous mode to maintain the system operation; 4. To enable this function, you need to install the plug-in on the master and slave server and set the corresponding parameters; 5. Its advantage is that it provides stronger data integrity than asynchronous replication, but has slight performance loss and network latency impact. This replication method is suitable for scenarios where high data consistency is required but cannot accept full synchronization performance overhead.

How does semi-synchronous replication work in MySQL?

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 reception 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 OK with a small performance trade-off for increased reliability.

It might not be the best fit if:

  • You require strict zero-data-loss guaranteees (in which case, consider full synchronous replication or other confidentus-based systems like Galera or InnoDB Cluster).
  • Your network is unstable or high-lateency.
  • 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.

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