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MySQL “master-master” replication is two independent replication channels: node A replicates from node B, while node B replicates from node A. It is asynchronous source-to-source replication, not automatic conflict resolution. The safest production design is one active writer at a time, with the second VPS ready for an explicit failover. If both servers must accept writes concurrently, the application must prevent overlapping writes and define deterministic conflict handling.
This guide shows a MySQL 8.0 design using unique server IDs, binary logs, GTIDs, and SOURCE_AUTO_POSITION=1. It also covers initial data synchronization, generated-key collisions, validation, failover, rejoining a recovered node, and the failure modes that make circular replication unsafe when it is treated like a cluster.
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What this topology does—and what it does not do
Replication copies committed changes from a source to a replica. MySQL replication is asynchronous by default, so a commit on the writer can return before the peer has received and applied the event. Semisynchronous replication can make a commit wait for at least one replica to acknowledge receipt and logging, but it is still different from synchronous clustering. MySQL associates synchronous replication with NDB Cluster rather than ordinary source/replica channels.
In this article, node A has a channel reading from node B, and node B has a channel reading from node A. Each server therefore acts as both a source and a replica. A network partition can leave both nodes accepting writes, producing divergent transactions or duplicate keys. Replication will not merge those changes for you.
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Recommended operating model
- Keep one node writable for normal traffic.
- Keep the peer caught up and ready to become the writer after a controlled promotion.
- Use fencing, a load-balancer decision, or an external leader mechanism so the old writer cannot continue accepting writes during a partition or failover.
- Only use simultaneous writers after testing table ownership, conflict handling, and transaction ordering at the application layer.
Prerequisites for two VPS servers
Prepare both VPS instances before changing MySQL configuration. The servers should run supported, compatible MySQL installations and have stable private connectivity, a VPN, or firewall rules that expose port 3306 only to the peer. Use fixed private addresses or internal DNS names such as node-a.example.internal and node-b.example.internal.
- Assign a different positive
server_idto every server in the replication topology. - Enable binary logging on both nodes and reserve durable disk space for binary and relay logs.
- Synchronize system clocks; consistent timestamps make incident analysis and failover decisions easier.
- Use matching schemas and compatible storage engines. Existing data must begin from a consistent snapshot or logical backup.
- Create a dedicated replication account on each server. Limit its source address and grant only the replication privileges required by your installed MySQL version.
- Keep an independent backup. Replication copies mistakes, accidental deletes, and corrupted transactions as readily as valid changes.
- Write a rollback and promotion plan before enabling unrestricted application writes on both VPS instances.
Plan the identities, logs, and generated-key ranges
The following values are an example topology. The IDs only need to be unique within your replication setup; they are not required to match the VPS addresses.
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|---|---|---|
| Replication identity | server_id=101 |
server_id=102 |
| Example host name | node-a.example.internal |
node-b.example.internal |
| Auto-increment offset when both nodes insert | 1 |
2 |
| Auto-increment increment when both nodes insert | 2 |
2 |
Set log_bin and choose explicit locations for binary and relay logs in each server’s MySQL configuration. The paths below are examples; use filesystems with sufficient capacity and backup characteristics for your VPS.
[mysqld]
server_id=101
log_bin=/var/lib/mysql/binlog
relay_log=/var/lib/mysql/relay-bin
Use the same configuration shape on node B, changing server_id to 102 and selecting its local log paths. Keep binary logs long enough to cover your recovery objective, but monitor disk usage and rotate or purge them according to that objective.
Enable GTIDs consistently
Global transaction identifiers eliminate manual binary-log file and position bookkeeping. With GTIDs enabled on both servers, the channel can request missing transactions automatically by using SOURCE_AUTO_POSITION=1. This substantially simplifies rejoining a recovered node, provided the node’s data and executed GTID set are trustworthy.
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Before starting replication, configure both servers with:
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[mysqld]
gtid_mode=ON
enforce_gtid_consistency=ON
Restart each server according to your distribution’s service procedure and verify that the settings are active. Both nodes must use GTID mode; enabling it on only one side does not create a usable GTID topology.
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Converting an existing topology
Do not switch a live, independently writable pair to GTIDs without a controlled cutover. Make both servers read-only for application traffic, allow current replication to catch up, stop the servers, and restart them with GTID mode and GTID consistency enforcement. Non-GTID transactions must be fully cleared from the topology first. After conversion, take a fresh backup.
Backups and binary logs containing transactions without GTIDs may not be usable on GTID-enabled servers. Treat the post-conversion backup as a new recovery baseline rather than assuming an older backup can be restored into the new configuration.
Create reciprocal replication accounts
Each node needs an account that the other node can use to connect. On node A, create an account restricted to node B’s private address; on node B, create the reciprocal account restricted to node A. Use a secret manager instead of placing a reusable password in shell history or source control, and enable TLS for the replication connection when supported by your deployment.
The exact replication privilege name and TLS options depend on the MySQL release and authentication policy. Follow the account-creation step for your installed version, grant only the privileges needed for a replica connection, and verify that firewall rules allow the peer’s address to reach port 3306. Do not expose the replication account or MySQL port to the public internet.
Synchronize the data before starting either channel
Replication assumes that the starting datasets are compatible. For an existing database, take a consistent backup or storage snapshot, restore it to the peer, and verify that schemas, table definitions, and storage engines match. A cold start can create the schema on both nodes before any application data is written.
Do not begin with two unrelated datasets and expect GTID auto-positioning to reconcile them. If the nodes already contain different rows, stop and choose a known-good source of truth. Provision the other node from that source, then configure its channel.
Configure the channel from node B to node A
On node A, stop any stale channel before replacing its source settings. MySQL 8.0.23 and later use CHANGE REPLICATION SOURCE TO and START REPLICA. Substitute the private host name and the account created on node B.
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STOP REPLICA;
CHANGE REPLICATION SOURCE TO
SOURCE_HOST='node-b.example.internal',
SOURCE_PORT=3306,
SOURCE_USER='repl_from_b',
SOURCE_PASSWORD='use-a-secret-manager',
SOURCE_AUTO_POSITION=1;
START REPLICA;
On releases older than MySQL 8.0.23, the equivalent statement names are CHANGE MASTER TO and START SLAVE. Do not mix old and new names arbitrarily; use the syntax supported by the server you are operating.
Configure the channel from node A to node B
On node B, configure the reciprocal source and account:
STOP REPLICA;
CHANGE REPLICATION SOURCE TO
SOURCE_HOST='node-a.example.internal',
SOURCE_PORT=3306,
SOURCE_USER='repl_from_a',
SOURCE_PASSWORD='use-a-secret-manager',
SOURCE_AUTO_POSITION=1;
START REPLICA;
Run the command only after node B has been provisioned with the consistent dataset described above. Starting a channel against an unrelated dataset can produce errors or conceal divergence until a later write exposes it.
Prevent AUTO_INCREMENT collisions
If both nodes can insert into the same AUTO_INCREMENT tables, configure disjoint generated-key sequences. Set auto_increment_increment=2 on both servers, with auto_increment_offset=1 on node A and auto_increment_offset=2 on node B.
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auto_increment_increment=2
auto_increment_offset=1
Use offset 2 in node B’s configuration. These are server-wide settings: they affect every AUTO_INCREMENT column on that server, not only one table. Review all such tables, migration scripts, bulk-load jobs, and session-level overrides before enabling the policy. The settings prevent collisions in generated numeric keys; they do not prevent two writers from updating the same logical row or violating another unique constraint.
Choose a write policy before production traffic
Single-writer failover
Route normal application writes to one active node. Keep the second node replicating and remove it from write traffic. During a failure, stop or fence the old writer, promote the peer, redirect clients, and verify that the application reconnects. This model is the simplest way to avoid circular-replication conflicts.
Multi-writer operation
Concurrent writes require more than different auto-increment offsets. Define which application or table owns each write, prevent two nodes from changing the same logical rows, and specify deterministic behavior for deletes, updates, retries, and transactions that cross ownership boundaries. Test network partitions and simultaneous failover; otherwise both nodes can accept valid-looking but incompatible transactions.
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Validate both replication threads
After starting the channels, inspect status on each node:
SHOW REPLICA STATUS extbackslash GOn older releases, use the release’s equivalent status statement. Confirm that the receiver and applier threads are running, no last-error fields contain a failure, and retrieved and executed GTID sets continue to advance. A running connection alone is not sufficient: the applier can be stopped while the receiver remains connected.
Run a disposable round-trip check
- Create a disposable test schema that is not used by production traffic.
- Insert a uniquely identifiable row on node A and record its transaction time and identifier.
- Confirm that the row appears on node B and that node B’s executed GTID set advances.
- Repeat in the reverse direction, inserting a different value on node B and checking node A.
- Remove the test rows and verify that the cleanup transactions also replicate.
Record the observed delay under normal load. This is an operator runbook check, not a guarantee that production transactions will never conflict.
Perform a controlled failover and rejoin
Promote the peer
- Stop application writes to the failed or suspect node, or fence it so clients cannot continue sending traffic.
- Confirm the promoted node has applied the transactions required by the application. If the old writer was unreachable, account for any transactions that had not reached the peer.
- Redirect the application’s database endpoint to the promoted node and test new reads and writes.
- Start the reverse replication channel only after deciding which node’s data is authoritative.
Promotion is an operational decision, not an automatic consequence of a thread failure. A node can be reachable while its data is stale, or both nodes can be reachable while the network between them is partitioned.
Rejoin the recovered node
Do not blindly start stale channels on a node that was writable during the outage. Provision it from a known-good backup or use GTID auto-positioning only after comparing its data and GTID history with the promoted node. If the old node contains transactions absent from the new writer, preserve logs and resolve the difference before making it a replica again.
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Duplicate-key or row conflicts
These commonly result from concurrent writes to the same logical rows, reused server_id values, or unsafe key generation. Stop one writer, preserve binary and relay logs, identify the conflicting transaction, and repair the data deliberately before restarting the channel.
Errant transactions
An errant transaction exists in a node’s GTID history but was not produced by the expected source. Do not skip an error casually. Identify the transaction, compare affected data, and repair from a known-good backup or controlled SQL. Apply any GTID correction procedure only after documenting why the transaction can be discarded or represented safely.
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Network partition
Both VPS instances may continue accepting writes while disconnected, and each can then replicate a different history when connectivity returns. Use fencing or an external leader decision so only one node remains writable during the partition. Reconcile any writes made on the isolated side before rejoining it.
Version mismatch
Use supported MySQL compatibility paths and test upgrades in a staging topology. Statement names and startup options changed over time: MySQL 8.0.23 introduced CHANGE REPLICATION SOURCE TO, and from 8.0.26 the startup option is skip_replica_start rather than the older slave spelling. Keep both nodes on a tested compatibility combination during upgrades.
Storage exhaustion
Binary logs and relay logs can fill a VPS even when replication threads appear healthy. Alert on filesystem usage, relay-log growth, and delayed purge. Retain logs long enough for the recovery and rejoin procedures you have promised, then rotate them according to that policy.
Monitoring and maintenance checklist
- Check receiver and applier thread state on both nodes.
- Alert when replication lag increases or retrieved and executed GTID sets stop advancing.
- Monitor relay-log and binary-log disk growth.
- Alert on certificate expiry and failed TLS connections.
- Compare row counts or checksums for important tables on a schedule appropriate to their size and write rate.
- Test client reconnect behavior and the promotion runbook before an outage.
- Take independent backups and periodically verify that they can be restored.
Replication is not a backup
A replicated delete, bad migration, or corrupted transaction is copied to the peer. Keep backups on separate storage and test restoration. After enabling GTIDs, create a fresh backup because backups and binary logs containing transactions without GTIDs may not be usable on GTID-enabled servers.
When another architecture is a better fit
Evaluate alternatives when your requirement is true multi-primary writing, automatic conflict resolution, or synchronous durability. Compare each option on write concurrency, consistency model, recovery time, conflict handling, operational complexity, backup and rejoin workflow, and cost.
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|---|---|---|
| One writer with a standby | Appropriate when failover is controlled and conflicts are avoided. | Automation for fencing, promotion, and health checks. |
| Concurrent writes on both nodes | Possible only with tested application-level ownership and conflict rules. | A multi-primary design or clustering product built for that consistency model. |
| Commit acknowledgment from another node | Asynchronous by default; semisynchronous mode can wait for receipt and logging. | A design with explicit synchronous durability requirements. |
| Automatic conflict resolution | Not provided by ordinary source-to-source replication. | Application conflict handling or a system designed for coordinated writes. |
For many two-VPS deployments, circular replication is most defensible as a single-writer setup with a rehearsed promotion and rejoin procedure. Treat the channels as data-copy mechanisms, not as a substitute for leader election, fencing, backups, or application-level consistency.
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