Setting up read replicas for scaling MySQL read operations
Jul 04, 2025 am 12:52 AM
Read replicas scale MySQL reads by offloading queries to secondary servers. To set up a basic read replica, enable binary logging on the primary server, create a replication user, take a snapshot with mysqldump, restore it on the replica, and start replication while ensuring unique server-ids and open firewall ports. For query routing, use separate connection strings for reads and writes in app code, or tools like ProxySQL. Monitor replication lag, binary log size, and connection errors regularly, and handle failures by restarting threads or rebuilding replicas as needed.
Setting up read replicas is one of the most straightforward and effective ways to scale MySQL for heavy read workloads. It helps reduce the load on your primary database server by offloading read queries to replica instances.
Here’s how to approach it in a practical, real-world way.
What a Read Replica Actually Does
A read replica is basically a copy of your main MySQL database that only handles SELECT queries (or other read operations). It works by replicating data from the primary database using binary logs. This replication can be asynchronous, semi-synchronous, or even synchronous depending on your setup and requirements.
The key point here is: you’re not making your database faster, you’re spreading the read load across multiple servers. That means if your application has more reads than writes — which is common — this can give you a big performance boost without changing your schema or code much.
How to Set Up a Basic Read Replica
Setting up a basic MySQL read replica involves a few core steps:
- Enable binary logging on the primary server
- Create a dedicated replication user
- Take a consistent snapshot of the primary DB
- Restore that snapshot on the replica
- Start the replication process
You don’t need fancy tools for this — just mysqldump and some config changes. One thing people often forget is to set server-id uniquely on each instance. If both servers have the same ID, replication won’t start and you’ll waste time debugging why.
Also, make sure your firewall allows traffic between the servers on port 3306 (or whatever port you're using).
Routing Queries to the Right Server
Once the replica is running, the next challenge is getting your app to use it. You have a few options:
- Use separate connection strings in your app code for reads and writes
- Use a proxy like ProxySQL or MaxScale to route queries automatically
- Implement a simple round-robin or read-from-replica logic in your ORM or DB layer
Most applications end up going with the first option because it's easy to understand and control. For example, you might have something like this in your config:
production: write_db: primary-host read_db: replica-host-1, replica-host-2
Then in your code, you send SELECTs to one of the read hosts, and everything else to the primary. Just remember: not all reads can go to replicas. If you need strong consistency, those queries should still hit the primary.
Monitoring and Handling Failures
Replication isn't perfect. Sometimes replicas fall behind or break entirely. So you need monitoring and a plan for when things go wrong.
At minimum, check these regularly:
- Replication lag (
SHOW SLAVE STATUS) - Binary log size and retention
- Connection errors between replica and primary
If a replica falls behind, sometimes restarting the IO thread helps. If it breaks completely, you may need to rebuild it from scratch. That sounds bad, but it's manageable if you automate the setup process.
Also, don’t ignore disk space. Replicas can run out of room if they fall behind and keep buffering logs. Make sure your monitoring system alerts you before that happens.
That’s basically it. Setting up read replicas doesn’t have to be complicated, but there are enough gotchas that you should test thoroughly before pushing to production. Once it's working, though, it’s a solid way to scale MySQL reads without a ton of overhead.
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