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To configure MySQL Group Replication across servers, prepare every instance with a unique identity and compatible replication settings, make the Group Replication network reachable, bootstrap exactly one seed server, then start the remaining members and verify that each reaches ONLINE. Group Replication keeps membership and replication state; it does not move existing client sessions, so production deployments also need MySQL Router, a connector, a load balancer, or equivalent routing middleware.
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The procedure below is written for current MySQL releases. Variable names, recovery-account privileges, and a few required settings differ between releases, so check the manual for the exact version installed on every server before applying the configuration.
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What you are building
A Group Replication deployment is a set of MySQL instances that communicate over a dedicated group channel, exchange membership information, and replicate transactions using GTIDs and row-based binary logging. Each server still has its own SQL listener for applications and administration. The group channel is controlled by group_replication_local_address; SQL or recovery connections use the addresses configured for those purposes.
MySQL documents a maximum of nine servers per group. A three-member group is a common minimum for tolerating one member failure while retaining a majority, but the correct number depends on your write load, failure-domain design, and operational budget.
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| Design item | What must be true |
|---|---|
| Server identity | Every member has a unique server_id and a distinct, reachable Group Replication local address. |
| Replication compatibility | Members use compatible MySQL releases and identical values for Group Replication variables that require consistency. |
| Network | Every member can resolve or reach the hostnames and ports used for group communication, seed discovery, SQL access, and distributed recovery. |
| State recovery | A dedicated recovery account and a configured group_replication_recovery channel allow a joining member to copy missing state from a donor. |
| Client routing | Applications connect through a connector, Router, load balancer, or another middleware layer that can select an available member. |
Choose the topology before changing MySQL
Single-primary mode
Single-primary mode presents one writable primary at a time. It is usually the least disruptive choice for applications that assume one writer, use ordinary primary-key allocation, or do not have conflict-handling logic. Other members serve as secondaries and can take over when membership and failure detection trigger a primary change.
Multi-primary mode
Multi-primary mode permits writes on multiple members. That can reduce dependence on one writer, but it also makes workload placement and conflict avoidance your responsibility. Transactions that touch the same rows or related keys can conflict, and applications must tolerate the resulting certification failures and retry behavior. Use this mode only after reviewing write patterns, key generation, transaction size, and retry logic.
| Decision axis | Single-primary | Multi-primary |
|---|---|---|
| Writer topology | One active writer at a time. | Several members can accept writes. |
| Application behavior | Fits clients that expect a conventional primary and replicas. | Requires writes to be safe on any member and able to retry certification failures. |
| Conflict risk | Lower because writes are concentrated on one primary. | Higher when concurrent transactions affect overlapping data. |
| Operational complexity | Simpler workload placement and troubleshooting. | More demanding schema, key, and workload design. |
| Failover | A new primary can be selected, but clients still need routing middleware. | Membership can continue with surviving writers, but clients still need routing middleware. |
Prepare every server
1. Inventory names, addresses, and failure domains
Assign stable names such as db1.example.net, db2.example.net, and db3.example.net. Every member must resolve the names used in the seed list and for SQL or recovery connections. If DNS is not available, use reachable IP addresses consistently. Do not use a loopback address, an address available only on one host, or a hostname that resolves differently on different members.
Choose a distinct Group Replication port for each host, commonly an internal port separate from the SQL port. The exact port is your design choice; what matters is that firewalls permit peer-to-peer TCP connections between all members. Keep internal replication traffic separate from client traffic when segmentation or security policy requires it.
2. Install and load the plugin
Install the Group Replication component supplied with your MySQL distribution. If it is not already loaded, the standard SQL installation form is:
INSTALL PLUGIN group_replication SONAME 'group_replication.so';Confirm that the plugin is present before continuing:
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FROM INFORMATION_SCHEMA.PLUGINS
WHERE PLUGIN_NAME = 'group_replication';The status should show that the plugin is active. Package layouts and plugin-loading methods vary by release, so use the distribution’s documented package or startup option when the shared object is installed elsewhere.
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3. Set unique identities and compatible replication options
Each instance needs a unique server_id, binary logging, GTID-based replication, and the row format required by Group Replication. Settings that the release identifies as group-wide must match on every member. Some of those variables cannot be changed while Group Replication is running; changing them requires a full group reboot and a new controlled bootstrap.
The following fragment illustrates the shape of a configuration. Values such as hostnames, the group UUID, and the communication port are examples; use your own inventory and the requirements for the installed release.
[mysqld]
server_id=101
log_bin=binlog
binlog_format=ROW
gtid_mode=ON
enforce_gtid_consistency=ON
plugin_load_add='group_replication.so'
group_replication_group_name='aaaaaaaa-bbbb-cccc-dddd-eeeeeeeeeeee'
group_replication_local_address='db1.example.net:33061'
group_replication_group_seeds='db1.example.net:33061,db2.example.net:33061,db3.example.net:33061'
group_replication_start_on_boot=OFF
group_replication_bootstrap_group=OFF
group_replication_single_primary_mode=ONOn the second and third servers, change server_id and group_replication_local_address to their unique values. Keep the group name, seed list, mode, and every other setting that the manual marks as requiring consistency identical. Do not enable automatic startup until you have tested a controlled restart procedure.
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4. Open and test the network paths
Test name resolution from every member to every other member. Then test TCP reachability to the Group Replication local address, the SQL port used for recovery, and any port used by your routing layer. A successful DNS lookup alone is not sufficient: firewalls, security groups, host policies, and bind-address settings can still block the connection.
- Allow member-to-member TCP traffic on the Group Replication port.
- Allow a joining member to reach the donor’s SQL endpoint for distributed recovery.
- Ensure the MySQL instance listens on an address reachable from the other members.
- Restrict these ports to the required hosts or network segments rather than exposing them broadly.
Create distributed-recovery credentials
Group Replication uses the group_replication_recovery channel to obtain missing transactions from a donor. Create a dedicated account for this purpose, grant only the privileges required by your MySQL release, and use a password policy suitable for an internal replication credential. Do not reuse an application account.
One supported approach is to provide credentials when starting Group Replication:
START GROUP_REPLICATION USER='rpl_recovery', PASSWORD='use-a-secret-managed-value';Another is to configure the recovery channel with the account and then start Group Replication without placing the password in the start command. The exact CHANGE REPLICATION SOURCE TO syntax depends on the MySQL release; versions that renamed the former master terminology use the SOURCE_ form. Whichever method you select, make it consistent with your secret-management and automatic-start policy, and verify that the account can connect from every member to a donor.
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Bootstrap the first member exactly once
Bootstrap is the operation that creates the initial group view. Perform it on one server only, normally the server you have chosen as the initial seed. Stop and investigate if another member is already running Group Replication; never bootstrap a second group merely because the first member is temporarily unreachable.
- Confirm that the seed has the final group name, local address, seed list, replication settings, and recovery credentials.
- Set the bootstrap flag for the current runtime session:
SET GLOBAL group_replication_bootstrap_group=ON; - Start Group Replication:
START GROUP_REPLICATION; - Immediately turn the flag off:
SET GLOBAL group_replication_bootstrap_group=OFF;
Oracle’s manual states: “The bootstrap should only be done by a single server, the one that starts the group and only once.” Never leave group_replication_bootstrap_group=ON in a persistent option file. If it remains enabled, a restart can create a second, independent group and lead to a split-brain incident.
Verify the seed before admitting traffic
Inspect the membership table on the seed:
SELECT *
FROM performance_schema.replication_group_members;Confirm that the row for the seed reports MEMBER_STATE as ONLINE, the expected host and port, the intended group communication details, and the expected role. The table’s columns vary slightly by MySQL release; newer releases expose additional information such as member version and communication stack. Query the columns available in your version rather than assuming every release has the same schema.
An ONLINE state means the member is participating in the group. It does not prove that your application can connect, that recovery credentials are safely stored, or that the routing layer will fail over. Test those paths separately.
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1. Prepare the joining instance
Apply the same group name and compatible group-wide settings to the second server. Give it its own server_id and local communication address. Configure the seed list so it includes at least one currently available member, and ensure that the joining host can resolve and reach that seed.
2. Configure recovery
Set the recovery account using the method selected earlier. The joining member must be able to establish the recovery connection and receive state from a donor. If your security policy uses TLS or a restricted internal network, apply the corresponding release-specific recovery options before starting.
3. Start Group Replication
START GROUP_REPLICATION;Do not set group_replication_bootstrap_group=ON on a joining member. It should discover the existing group through the seed list, obtain membership information, and enter distributed state recovery.
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4. Watch recovery and membership
Run the membership query on an existing member and on the joining member. A new server can appear as RECOVERING while it copies missing transactions; direct application traffic to it only after it reaches ONLINE. Check the Group Replication connection and applier status tables for errors if recovery does not progress.
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Group Replication maintains membership and detects member failures, but it does not redirect an existing client session. A TCP connection pointed directly at a failed server remains failed, and a new connection aimed at an offline member will not be moved automatically by Group Replication itself.
Place a routing component between applications and the group. Depending on your environment, that can be MySQL Router, a driver or connector with topology awareness, a load balancer, or equivalent middleware. Configure health checks that distinguish a member that accepts TCP from one that is actually ONLINE and suitable for the application’s read or write role.
- In single-primary mode, route writes to the current primary and remove an old primary after a role change.
- In multi-primary mode, route writes only to members your application is prepared to use and retry safely after certification errors.
- Set connection and retry timeouts so a dead member does not stall request threads indefinitely.
- Test new connections, in-flight transactions, and pooled connections during a planned member failure; each behaves differently.
Observe health after the group is running
Make membership state part of routine monitoring. At minimum, collect the member identifier, host, port, state, role, version, and communication stack from performance_schema.replication_group_members. Alert when a member is not ONLINE, when the group has lost its expected majority, or when a primary role changes unexpectedly.
Use the other Performance Schema Group Replication tables and status variables supplied by your release to inspect:
- Distributed-recovery connection state and the selected donor.
- Applier errors, certification failures, and transactions waiting to be applied.
- Member statistics, flow-control activity, and message delivery health.
- Whether a member is lagging enough that routing traffic to it would surprise users.
Keep server error logs alongside these tables. Membership state tells you what the group believes; the error log often explains why a transition occurred.
Troubleshoot common failures
The member stays OFFLINE after startup
- Check that the Group Replication plugin is installed and active.
- Confirm that the group name, local address, and seed list are present in the running configuration, not only in an unprocessed file.
- Verify that the member’s
server_idis unique and that the local address is not already claimed by another server. - Read the error log for a rejected replication or GTID setting, then compare every group-wide variable with a working member.
The member cannot reach a seed
Resolve each seed hostname from the joining host, test the Group Replication TCP port, and inspect firewalls and bind-address settings on both ends. A SQL connection test to port 3306 does not prove that the internal group port is reachable, and the reverse is also true.
Recovery remains stuck or fails authentication
Verify the recovery username, password, host permissions, and the channel configuration. Confirm that a donor is available and that the joining member can reach its SQL endpoint. Check the recovery connection status and error log for TLS, privilege, or GTID errors. Do not repeatedly bootstrap while trying to fix recovery; bootstrap is unrelated to ordinary joining.
A member is RECOVERING for an unexpectedly long time
Inspect donor load, network throughput, transaction backlog, and applier errors. Keep the member out of application traffic until it is ONLINE. If the donor is unhealthy, follow your release’s documented procedure for selecting another donor or restarting recovery rather than deleting replication metadata at random.
Two independent groups appear after a restart
This is usually caused by leaving group_replication_bootstrap_group=ON in a persistent configuration or bootstrapping an isolated server. Stop writes to the isolated group, preserve logs and transaction metadata, and use a controlled recovery plan to rejoin the intended group. Never merge two groups by simply turning both on and hoping membership converges.
Multi-primary transactions conflict
Certification conflicts are a workload-design issue, not a network failure. Identify which members are writing the same keys, reduce overlapping write ownership, keep transactions short where practical, and implement bounded retries for transactions rejected during certification. If the application cannot do that safely, single-primary mode is usually the better topology.
Restart, maintenance, and planned failure tests
Because automatic startup is commonly left disabled during initial rollout, document the exact order for restarting members. Keep at least the required majority available, restart one member at a time, and verify that it returns to ONLINE before touching the next. Confirm that the routing layer removes a member during shutdown and restores it only after its role and state are healthy.
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- Stopping the current primary and observing the replacement role.
- Opening new application connections through the routing layer.
- Handling an in-flight transaction interrupted by the failure.
- Rejoining the stopped member and waiting for
ONLINE. - Restoring the original topology without enabling bootstrap on any member.
Re-test bootstrap and recovery during planned maintenance. The bootstrap flag is a controlled runtime operation, not a permanent high-availability setting.
Quick Recap
Final deployment checklist
- Every server has a unique
server_idand Group Replication local address. - Binary logging, row-based logging, GTIDs, and release-required replication settings are enabled.
- Group-wide variables are identical where the manual requires consistency.
- All member and seed hostnames resolve identically, or reachable IP addresses are used.
- Peer, SQL, and recovery traffic are allowed through the intended firewall boundaries.
- The recovery account has only the privileges required for the installed MySQL release.
- Exactly one server was bootstrapped, and its bootstrap flag was turned off immediately.
- Every joining server was started without bootstrapping and reached
ONLINE. - The membership, recovery, applier, and member-statistics tables are monitored.
- Applications use routing middleware and have tested connection and transaction failover.
- The group remains within the documented nine-server maximum.
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