Managing databases in a cluster

When creating a database or altering it after creation, you can set the number of both primary and secondary servers to host your database. To specify the initial topology, use the command CREATE DATABASE. To change the topology once the database is created, run the ALTER DATABASE command. If a database is no longer needed, the command DROP DATABASE deletes the database from the cluster.

`CREATE DATABASE`

The command to create a database in a cluster is not significantly different from the command to create a database in a non-clustered environment (see Create, start, and stop databases for more information on database management on single servers). The difference in a clustered environment is that the topology can be specified, i.e. how many primaries and secondaries are desired for the database. To create a database foo with 3 servers hosting the database in primary mode and 2 servers in secondary mode, run the following command:

CREATE DATABASE foo TOPOLOGY 3 PRIMARIES 2 SECONDARIES

Alternatively, you can use parameters to provide the number of primaries and secondaries:

Parameters

{
  "dbname": "foo",
  "primary": 3,
  "secondary": 2
}

Query

CREATE DATABASE $dbname TOPOLOGY $primary PRIMARIES $secondary SECONDARIES

The command can only be executed successfully if the cluster’s servers are able to satisfy the specified topology. If they are not, the command results in an error. For example, if the cluster’s servers are set up with mode constraints to contain two primaries and three secondaries, or if only four servers exist, the command fails with an error.

If TOPOLOGY is not specified, the database is created according to initial.dbms.default_primaries_count and initial.dbms.default_secondaries_count specified in neo4j.conf. After cluster startup, you can overwrite these values using the dbms.setDefaultAllocationNumbers procedure.

A CREATE DATABASE command allocates the database, therefore there is no requirement to execute REALLOCATE DATABASES (described in Hosting databases on added servers).

However, over time, or after several CREATE DATABASE commands have been issued, the distribution of databases can become unbalanced. At this point you can run REALLOCATE DATABASES to make the cluster re-balance databases across all servers that are part of the cluster.

`ALTER DATABASE`

To alter the topology of or read/write access to a database after it has been created, use the command ALTER DATABASE.

Alter database topology

To change the topology of the database foo from the previous example, run the following command:

ALTER DATABASE foo SET TOPOLOGY 2 PRIMARIES 1 SECONDARY

Alternatively, you can use parameters to provide the number of primaries and secondaries:

Parameters

{
  "dbname": "foo",
  "primary": 2,
  "secondary": 1
}

Query

ALTER DATABASE $dbname SET TOPOLOGY $primary PRIMARIES $secondary SECONDARIES

Like the CREATE DATABASE command, this command results in an error if the cluster does not contain sufficient servers to satisfy the requested topology.

Additionally, ALTER DATABASE is optionally idempotent and also results in an error if the database does not exist. It is possible to append the command with IF EXISTS to make sure that no error is returned if the database does not exist.

When there is more than one possible permutation of the specified topology, Neo4j uses an allocator to decide how to spread the database across the cluster. Note, like CREATE DATABASE, the ALTER DATABASE command allocates the database and there is no requirement to execute REALLOCATE DATABASES unless there is a desire to re-balance databases across all servers that are part of the cluster.

This normally happens when the cluster is configured with more servers than the sum of the number of primaries and secondaries for any one database.

It is not possible to automatically transition to a topology with a single primary host. Attempting to do so results in an error.

However, it is possible to manually do this transition:

The first step is to back up the database, see Backup and restore for more information.
Once the database is backed up, the next step is to drop the database. See Delete databases for more details.
The last step is to either seed a cluster from the backup with the new topology, or to restore the backup on a single server. See Seed a cluster further on for information on seeding.

Also, it is possible to automatically transition from a topology with a single primary host to multiple primary hosts. Keep in mind that during such a transition, the database is unavailable for a short period of time.

If the ALTER DATABASE command decreases the number of allocations of a database, allocations on cordoned servers are removed first.

Query

ALTER DATABASE nonExisting IF EXISTS SET TOPOLOGY 1 PRIMARY 0 SECONDARY

Alter database access

To alter the access to the database foo, the syntax looks like this:

ALTER DATABASE foo SET ACCESS {READ ONLY | READ WRITE}

By default, a newly created database has both read and write access.

Deallocate databases

To relieve the load of a specific server(s), you can use one of the following procedures to deallocate databases causing the pressure from the server(s):

You must have the SERVER MANAGEMENT privilege to execute these procedures.

For example, server01 hosts two small databases, foo and bar, and one very large database baz, while other servers contain fewer or smaller databases, and server01 is under pressure.

You can use one of the following approaches to deallocate baz from server01 or to deallocate a number of databases from server01:

Deallocating a database from a server

// With dry run
neo4j@system> CALL dbms.cluster.deallocateDatabaseFromServer("server01", "baz", true);

// Without dry run
neo4j@system> CALL dbms.cluster.deallocateDatabaseFromServer("server01", "baz");

Deallocating a database from multiple servers

// With dry run
neo4j@system> CALL dbms.cluster.deallocateDatabaseFromServers(["server01", "server02"], "baz", true);

// Without dry run
neo4j@system> CALL dbms.cluster.deallocateDatabaseFromServers(["server01", "server02"], "baz");

Deallocating three databases from a server

// With dry run
neo4j@system> CALL dbms.cluster.deallocateNumberOfDatabases("server01", 3, true);

// Without dry run
neo4j@system> CALL dbms.cluster.deallocateNumberOfDatabases("server01", 3);

Reallocate databases

To rebalance all database allocations across the cluster, for example, because you added new servers, use either procedures or Cypher commands to reallocate databases onto the new servers.

Reallocate databases using a procedure

You can use the procedure dbms.cluster.reallocateDatabase to rebalance a specific database across the cluster, or dbms.cluster.reallocateNumberOfDatabases to rebalance a number of database allocations across the cluster and relieve overloaded servers. Note that if the cluster is already balanced, no reallocations will happen when running these procedures. These procedures do not require a server name and can be executed with or without a dry run.

You must have the SERVER MANAGEMENT privilege to execute these procedures.

For example, you add three new servers and want to move a very large database, baz, from all the servers containing it to the new servers.

Reallocate one database to new servers

// With dry run
neo4j@system> CALL dbms.cluster.reallocateDatabase("baz", true);

// Without dry run
neo4j@system> CALL dbms.cluster.reallocateDatabase("baz");

Reallocating a number of databases to new servers

// With dry run
neo4j@system> CALL dbms.cluster.reallocateNumberOfDatabases(3, true);

// Without dry run
neo4j@system> CALL dbms.cluster.reallocateNumberOfDatabases(3);

Reallocate databases using a Cypher command

You can use the Cypher command REALLOCATE DATABASES to rebalance all database allocations across the cluster and relieve overloaded servers. This command can also be used with DRYRUN to preview the new allocation of databases.

REALLOCATE DATABASES on a large cluster with many databases has the potential to move a lot of allocations at once, which might stress the cluster. Consider starting with more limited reallocations, such as dbms.cluster.reallocateNumberOfDatabases with a small number, and let the databases complete their reallocation before calling it again, until no more reallocations are necessary.

neo4j@neo4j> DRYRUN REALLOCATE DATABASES;
+----------------------------------------------------------------------------------------------------------------------------------------+
| database | fromServerName | fromServerId                           | toServerName | toServerId                             | mode      |
+----------------------------------------------------------------------------------------------------------------------------------------+
| "bar"    | "server-1"     | "00000000-27e1-402b-be79-d28047a9418a" | "server-5"   | "00000003-b76c-483f-b2ca-935a1a28f3db" | "primary" |
| "bar"    | "server-3"     | "00000001-7a21-4780-bb83-cee4726cb318" | "server-4"   | "00000002-14b5-4d4c-ae62-56845797661a" | "primary" |
+----------------------------------------------------------------------------------------------------------------------------------------+

Seed a cluster

There are two different ways to seed a cluster with data:

The first option is to use a designated seeder, where a designated server is used to create a backed-up database on other servers in the cluster.
The other option is to seed the cluster from a URI, where all servers to host the database are seeded with an identical seed from an external source specified by that URI. For more details, see Create a database from a URI.

Keep in mind that using a designated seeder can be problematic in some situations as it is not known in advance how a database is going to be allocated to the servers in a cluster. Also, this method relies on the seed already existing on one of the servers.

Designated seeder

In order to designate a server in the cluster as a seeder, a database backup is transferred to that server using the neo4j-admin database restore command. Subsequently, that server is used as the source for other cluster members to copy the backed-up database from.

This example creates a user database called foo, hosted on three servers in primary mode. The foo database should not previously exist on any of the servers in the cluster.

If a database with the same name as your backup already exists, use the command DROP DATABASE to delete it and all users and roles associated with it.

Restore the foo database on one server. In this example, the server01 member is used.
```
bin/neo4j-admin database restore --from-path=/path/to/foo-backup-dir foo
```

Find the server ID of server01 by logging in to Cypher Shell and running SHOW SERVERS. Cross-reference the address to find the server ID. Use any database to connect.

SHOW SERVERS YIELD serverId, name, address, state, health, hosting;

+-----------------------------------------------------------------------------------------------------------------------------------------------------+
| serverId                               | name                                   | address          | state     | health      | hosting              |
+-----------------------------------------------------------------------------------------------------------------------------------------------------+
| "25a7efc7-d063-44b8-bdee-f23357f89f01" | "25a7efc7-d063-44b8-bdee-f23357f89f01" | "localhost:7689" | "Enabled" | "Available" | ["system",  "neo4j"] |
| "782f0ee2-5474-4250-b905-4cd8b8f586ba" | "782f0ee2-5474-4250-b905-4cd8b8f586ba" | "localhost:7688" | "Enabled" | "Available" | ["system",  "neo4j"] |
| "8512c9b9-d9e8-48e6-b037-b15b0004ca18" | "8512c9b9-d9e8-48e6-b037-b15b0004ca18" | "localhost:7687" | "Enabled" | "Available" | ["system",  "neo4j"] |
+-----------------------------------------------------------------------------------------------------------------------------------------------------+

In this case, the address for server01 is localhost:7687 and thus, the server ID is 8512c9b9-d9e8-48e6-b037-b15b0004ca18.

On one of the servers, use the system database and create the database foo using the server ID of server01. The topology of foo is stored in the system database and when you create it, it is allocated according to the default topology (which can be shown with CALL dbms.showTopologyGraphConfig). This may be different from the topology of foo when it was backed up. If you want to ensure a certain allocation across the cluster, you can specify the desired topology with the TOPOLOGY clause in the CREATE DATABASE command. See CREATE DATABASE for more information.
```
CREATE DATABASE foo
TOPOLOGY [desired number of primaries] PRIMARIES [desired number of secondaries] SECONDARIES
OPTIONS {existingData: 'use', existingDataSeedServer: '8512c9b9-d9e8-48e6-b037-b15b0004ca18'};
```

Verify that the foo database is online on the desired number of servers, in the desired roles. If the foo database is of considerable size, the execution of the command can take some time.

SHOW DATABASE foo;

+------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| name  | type       | aliases | access       | address          | role      | writer | requestedStatus | currentStatus | statusMessage | default | home  | constituents |
+------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| "foo" | "standard" | []      | "read-write" | "localhost:7687" | "primary" | FALSE  | "online"        | "online"      | ""            | FALSE   | FALSE | []           |
| "foo" | "standard" | []      | "read-write" | "localhost:7688" | "primary" | FALSE  | "online"        | "online"      | ""            | FALSE   | FALSE | []           |
| "foo" | "standard" | []      | "read-write" | "localhost:7689" | "primary" | TRUE   | "online"        | "online"      | ""            | FALSE   | FALSE | []           |
+------------------------------------------------------------------------------------------------------------------------------------------------------------------------+

9 rows available after 3 ms, consumed after another 1 ms

Controlling locations with allowed/denied databases

A database can by default be allocated to run on any server in a cluster. However, it is possible to constrain the servers that specific databases are hosted on. This is done with ENABLE SERVER and ALTER SERVER, described in Managing servers in a cluster. The following options are available:

allowedDatabases - a set of databases that are allowed to be hosted on a server.
deniedDatabases - a set of databases that are denied to be hosted on a server. Allowed and denied are mutually exclusive.
modeConstraint - controls in what mode (primary, secondary, or none) databases can be hosted on a server. If not set, there are no mode constraints on the server.

Change the default database

You can use the procedure dbms.setDefaultDatabase("newDefaultDatabaseName") to change the default database for a DBMS. Starting with Neo4j 2025.04, the default database can also be set to a local or remote database alias.

Ensure that the database to be set as default exists, otherwise create it using the command CREATE DATABASE <database-name>.
Show the name and status of the current default database by using the command SHOW DEFAULT DATABASE.
Stop the current default database using the command STOP DATABASE <database-name>.
Run CALL dbms.setDefaultDatabase("newDefaultDatabaseName") against the system database to set the new default database.
Optionally, you can start the previous default database as non-default by using START DATABASE <database-name>.

Be aware that the automatically created initial default database may have a different topology to the default configuration values. See Default database in a cluster for more information.

Handling errors

Databases can get into error states. Typically you can observe this with the SHOW DATABASES command, and use the error handling guidance to help.

In more serious cases you may be dealing with a disaster situation, where the whole DBMS may not be responding correctly, or some specific databases cannot be restored without downtime. Refer to the disaster recovery guide for those situations.