How does master-replica (master-slave) replication work in Redis?
Jul 13, 2025 am 12:10 AM
Redis master-slave replication achieves data consistency through full synchronization and incremental synchronization. During the first connection, the slave node sends a PSYNC command, the master node generates an RDB file and sends it, and then sends the write command in the cache to complete the initialization; subsequently, incremental synchronization is performed by copying the backlog buffer to reduce resource consumption. Its common uses include read and write separation, failover preparation and data backup analysis. Notes include: ensuring network stability, reasonably configuring timeout parameters, enabling the min-slaves-to-write option according to needs, and combining Sentinel or Cluster to achieve high availability.
The Master-Replica (Master-Slave) mechanism of Redis is the basis for realizing data redundancy, read-write separation and high availability. Its core logic is actually not complicated: copy the data of the master node from the slave node and maintain consistency . Let’s take a look at how it works from several key perspectives.
Full synchronization during the first connection
When a slave node connects to the master node for the first time, full Resynchronization is triggered. This process is roughly divided into several steps:
- The slave node sends a
PSYNCcommand to the master node and requests synchronization. - The master node executes
BGSAVEto create an RDB snapshot file and caches subsequent write commands. - The master node sends the generated RDB file to the slave node.
- Load RDB data from the node and complete initialization.
- The master node then sends the write command in the cache to the slave node to ensure that the two are consistent.
This step usually takes a long time, especially when the data volume is large. Therefore, if you frequently disconnect and reconnect, you have to synchronize it in full every time, which will affect performance.
Incremental synchronization of subsequent updates
Once the initial synchronization is completed, the master and slave will enter a "normal working state". At this time, the master node will not send full data every time, but will update the slave node through Partial Resynchronization .
Specifically:
- The master node maintains a replication backlog buffer (repl_backlog_buffer) to record the most recently sent write operations.
- The slave node periodically sends its own offset to the master node.
- The master node determines whether part of the data can be retrieved from the backlog buffer and sent to it based on the offset.
- If possible, only the difference part is sent; otherwise, the full synchronization must be done again.
This method is efficient, but depends on the size of the backlog buffer. If your data is updated very frequently, it is recommended to increase the buffer appropriately.
Common uses of master-slave replication
Master-slave replication is not just for backup, it has many uses in practical applications:
- Read and write separation : The master node processes write requests, and multiple slave nodes share read requests, improving overall performance.
- Failover preparation : In conjunction with sentinel or cluster mechanism, quickly switch when the master node goes down.
- Data backup and analysis : Slave nodes can be used for offline data analysis without affecting the performance of the master node.
For example, if you build an e-commerce system, users place orders and go to the main nodes, and browse products and go to the slave nodes, it can effectively alleviate the pressure on the main node.
Notes and configuration suggestions
Although the Redis master-slave replication mechanism is practical, there are some details that are easy to overlook:
- Ensure the network stability. Frequent disconnection will lead to repeated full synchronization.
- Adjust
repl-timeoutandrepl-ping-slave-periodappropriately to avoid misjudgment of timeout. - If the data consistency requirements are high, you can enable
min-slaves-to-writeconfiguration to prevent the master from continuing to write when there is no slave node available.
In addition, master-slave replication does not provide automatic failover function itself. To achieve true high availability, it also needs to be combined with Redis Sentinel or Cluster mode.
Basically that's it. Master-slave replication seems simple, but there are still many details that need to be paid attention to in actual deployment, especially the selection of network environment and configuration parameters.
The above is the detailed content of How does master-replica (master-slave) replication work in Redis?. For more information, please follow other related articles on the PHP Chinese website!
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