Monitor replication status

Neo4j 5.24 introduces the dbms.cluster.statusCheck() procedure, which can be used to monitor the ability to replicate in clustered databases. In most cases this means a clustered database is write available. The procedure identifies which members of a clustered database are up-to-date and can participate in successful replication. Therefore, it is useful in determining the fault tolerance of a clustered database. Additionally, you can use the procedure to identify the leader of a clustered database within the cluster.

The procedure replicates a dummy transaction within the cluster and verifies that it can be replicated and applied. Since the status check does not replicate an actual transaction, it does not guarantee write availability, as other factors in the write path (e.g., database issues) may block transactions. However, a healthy status typically indicates write availability in most cases.

Cluster status check

Syntax

CALL dbms.cluster.statusCheck(databases :: LIST<STRING>, timeoutMilliseconds = null :: INTEGER)

Input arguments

Name Type Description

databases

List<String>

Databases for which the status check should run. Providing an empty list runs the status check for all clustered databases on that server, i.e. it does not run on singles or secondaries.

timeoutMilliseconds

Integer

How long to allow for replication, before returning it was unsuccessful. Default value is 1000 milliseconds.

Return arguments

The procedure returns a row for all primary members of all the requested databases where each row consists of:

Name Type Description

database

String

The database for which a status check entry was replicated.

serverId

String

The UUID of the server, which did or did not participate in a successful replication of the status check entry.

serverName

String

The friendly name of the server, or its UUID if no name is set.

address

String

The address of the Bolt port for the server.

replicationSuccessful

Boolean

Indicates if the server (on which the procedure is run) can replicate a transaction.

memberStatus

String

The status of each primary member.

recognisedLeader

String

The server id of the perceived leader of each primary member.

recognisedLeaderTerm

Integer

The term of the perceived leader of each primary member. If the members report different leaders, the one with the highest term should be trusted.

requester

Boolean

Whether a server is the requester or not.

error

String

Contains the error message if one is present. An example of an error is that one or more of the requested databases do not exist on the requester.

Possible values of replicationSuccessful

  • TRUE — if this server managed to replicate the dummy transaction to a majority of cluster members within the given timeout.

  • FALSE — if it failed to replicate within the timeout. The value is the same column-wise. A failed replication can either indicate a real issue in the cluster (e.g., no leader) or that this server is too far behind in applying updates and can’t replicate.

Possible values of memberStatus

  • APPLYING means that the member can replicate and is actively applying transactions.

  • REPLICATING means that the member can participate in replicating but cannot apply. This state is uncommon, but may happen while waiting for the database to start and accept transactions.

  • UNAVAILABLE means that the member is either too far behind the leader or unreachable. They are unhealthy and cannot add to the fault-tolerance.

Possible values of requester

  • TRUE — for the server on which the procedure is run.

  • FALSE — on the remaining servers.

In general, you can use the replicationSuccessful field to determine overall write-availability, whereas the memberStatus field can be checked in order to see whether the database is fault-tolerant or not.

Example

Running the status check

When running the cluster status check against a server, expect similar output to the following:

+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| database | serverId                               | serverName                             | address          | replicationSuccessful | memberStatus | recognisedLeader                       | recognisedLeaderTerm | requester | error |
+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| "neo4j"  | "d3fe2e6a-494d-4ab8-81b1-7de2ce31ce11" | "d3fe2e6a-494d-4ab8-81b1-7de2ce31ce11" | "localhost:7682" | TRUE                  | "APPLYING"   | "565130e8-b8f0-41ad-8f9d-c660bd8d5519" | 4                    | FALSE     | NULL  |
| "neo4j"  | "565130e8-b8f0-41ad-8f9d-c660bd8d5519" | "565130e8-b8f0-41ad-8f9d-c660bd8d5519" | "localhost:7681" | TRUE                  | "APPLYING"   | "565130e8-b8f0-41ad-8f9d-c660bd8d5519" | 4                    | TRUE      | NULL  |
| "neo4j"  | "58c70f4b-910d-4d0e-b0f2-3084554079ec" | "58c70f4b-910d-4d0e-b0f2-3084554079ec" | "localhost:7683" | TRUE                  | "APPLYING"   | "565130e8-b8f0-41ad-8f9d-c660bd8d5519" | 4                    | FALSE     | NULL  |
| "system" | "565130e8-b8f0-41ad-8f9d-c660bd8d5519" | "565130e8-b8f0-41ad-8f9d-c660bd8d5519" | "localhost:7681" | TRUE                  | "APPLYING"   | "d3fe2e6a-494d-4ab8-81b1-7de2ce31ce11" | 1                    | TRUE      | NULL  |
| "system" | "58c70f4b-910d-4d0e-b0f2-3084554079ec" | "58c70f4b-910d-4d0e-b0f2-3084554079ec" | "localhost:7683" | TRUE                  | "APPLYING"   | "d3fe2e6a-494d-4ab8-81b1-7de2ce31ce11" | 1                    | FALSE     | NULL  |
| "system" | "d3fe2e6a-494d-4ab8-81b1-7de2ce31ce11" | "d3fe2e6a-494d-4ab8-81b1-7de2ce31ce11" | "localhost:7682" | TRUE                  | "APPLYING"   | "d3fe2e6a-494d-4ab8-81b1-7de2ce31ce11" | 1                    | FALSE     | NULL  |
+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+