High availability reference queries

This guide provides a complete reference for all commands used to manage a Memgraph High Availability (HA) cluster.

Cluster registration commands

ADD COORDINATOR

Adds a coordinator to the cluster.

ADD COORDINATOR coordinatorId WITH CONFIG {
  "bolt_server": boltServer, 
  "coordinator_server": coordinatorServer, 
  "management_server": managementServer
}; 

Parameters

• coordinatorId (int) Unique ID for each coordinator. Typically incremented sequentially.
• boltServer (string) External Bolt endpoint: "IP_OR_DNS:PORT". Usually port 7687. Must be reachable by external applications.
• coordinatorServer (string) Internal raft endpoint: "HOSTNAME_OR_DNS:COORDINATOR_PORT".
• managementServer (string) Coordinator management endpoint: "HOSTNAME_OR_DNS:MANAGEMENT_PORT".

DNS/FQDN is recommended when IPs are ephemeral (e.g., Kubernetes).

Behavior & implications

• Can be run before or after registering data instances.
• Must be executed once for each coordinator.
• External Bolt server must be reachable; Raft and management endpoints may be internal.
• Writes cluster configuration to Raft log.

Example

ADD COORDINATOR 1 WITH CONFIG {
  "bolt_server": "my_outside_coordinator_1_IP:7687",
  "coordinator_server": "memgraph-coordinator-1.default.svc.cluster.local:12000",
  "management_server": "memgraph-coordinator-1.default.svc.cluster.local:10000"
};

REMOVE COORDINATOR

Removes a follower coordinator from the cluster.

REMOVE COORDINATOR coordinatorId;

Parameters

• coordinatorId (int) — previously registered coordinator ID.

Behavior & implications

• Must be executed on the leader coordinator.
• Leader coordinator cannot remove itself. To remove the leader, first trigger a leadership change.

Example

REMOVE COORDINATOR 2;

REGISTER INSTANCE

Registers a data instance in the cluster.

REGISTER INSTANCE instanceName ( AS ASYNC | AS STRICT_SYNC ) ? WITH CONFIG {
  "bolt_server": boltServer, 
  "management_server": managementServer, 
  "replication_server": replicationServer
};

Parameters

• instanceName Unique symbolic name of the data instance.
• AS ASYNC / AS STRICT_SYNC (optional) Selects replication mode. Default: SYNC.
• boltServer External Bolt endpoint: "IP_OR_DNS:PORT".
• managementServer Coordinator → data instance health check endpoint.
• replicationServer Replication endpoint, typically using port 20000.

Behavior

• Coordinator connects via management_server to verify liveness.
• Coordinator begins periodic health checks.
• Instance is automatically demoted to REPLICA.
• Replication server is started on the data instance.
• Operation is persisted in Raft.

Replication mode rules

• Allowed combinations:

  • STRICT_SYNC + ASYNC
  • SYNC + ASYNC

• Forbidden combination:

  • STRICT_SYNC + SYNC

Notes

• In Kubernetes, use service DNS names (e.g. memgraph-data-1.default.svc.cluster.local).
• Local development uses localhost.

Example

REGISTER INSTANCE instance1 WITH CONFIG {
  "bolt_server": "my_outside_instance1_IP:7687",
  "management_server": "memgraph-data-1.default.svc.cluster.local:10000",
  "replication_server": "memgraph-data-1.default.svc.cluster.local:20000"
};

UNREGISTER INSTANCE

Removes a data instance from the cluster.

UNREGISTER INSTANCE instanceName;

Parameters

• instanceName — name of the data instance.

Implications

• Do not unregister the MAIN instance; this may corrupt cluster state.
• A healthy MAIN must exist during the operation.
• The instance is also removed from MAIN’s replica set.

Example

UNREGISTER INSTANCE instance_1;

Replication role management

SET INSTANCE ... TO MAIN

Promotes a replica to MAIN.

SET INSTANCE instanceName TO MAIN;

Behavior

• All other registered instances become replicas of the new MAIN.
• Written to Raft log.

Implications

• Fails if a MAIN already exists.
• Fails if any instance is unavailable.

Example

SET INSTANCE instance_0 TO MAIN;

DEMOTE INSTANCE

Demotes the current MAIN to a REPLICA.

DEMOTE INSTANCE instanceName;

Behavior

• MAIN becomes REPLICA.
• Written to Raft log.

Implications

• Failover is not automatic after demotion. You must manually promote another instance using SET INSTANCE ... TO MAIN.

Tip: Combine DEMOTE INSTANCE + SET INSTANCE ... TO MAIN for manual failover, useful during maintenance.

Example

DEMOTE INSTANCE instance1;

Monitoring Commands

SHOW INSTANCES

Displays the state of all servers in the cluster.

SHOW INSTANCES;

Output includes

1. Network endpoints (bolt, coordinator, management)
2. Health state
3. Role: MAIN, REPLICA, LEADER, FOLLOWER, or UNKNOWN
4. Time since last health ping

Behavior on followers

1. Follower attempts to query the leader for accurate state.
2. If leader unavailable, follower reports all servers as "down".

SHOW INSTANCE

Displays information about the coordinator you’re connected to.

SHOW INSTANCE;

Output includes

1. Instance name
2. External Bolt server
3. Coordinator (Raft) endpoint
4. Management server endpoint
5. Cluster role (LEADER/FOLLOWER)

If ADD COORDINATOR has not been run, bolt_server will be empty.

SHOW REPLICATION LAG

Shows replication lag (in committed transactions) for all instances. Must be run on the leader.

SHOW REPLICATION LAG;

Implications

• Lag values survive restarts (stored in snapshots + WAL).
• Useful during manual failover to evaluate risk of data loss.

Troubleshooting commands

FORCE RESET CLUSTER STATE

Resets cluster state when the cluster cannot reach a healthy configuration.

FORCE RESET CLUSTER STATE;

Behavior

1. All alive instances are demoted to REPLICA.
2. A new MAIN is selected from alive instances.
3. Down instances are demoted after they come back online.
4. Writes changes to Raft.

Implications

• Must be executed on the leader coordinator.