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:
{
"dbname": "foo",
"primary": 3,
"secondary": 2
}
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 |
A However, over time, or after several |
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:
{
"dbname": "foo",
"primary": 2,
"secondary": 1
}
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.
ALTER DATABASE nonExisting IF EXISTS SET TOPOLOGY 1 PRIMARY 0 SECONDARY
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 |
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
:
// With dry run
neo4j@system> CALL dbms.cluster.deallocateDatabaseFromServer("server01", "baz", true);
// Without dry run
neo4j@system> CALL dbms.cluster.deallocateDatabaseFromServer("server01", "baz");
// 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");
// 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 |
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.
// With dry run
neo4j@system> CALL dbms.cluster.reallocateDatabase("baz", true);
// Without dry run
neo4j@system> CALL dbms.cluster.reallocateDatabase("baz");
// 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.
|
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, theserver01
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 runningSHOW 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
islocalhost:7687
and thus, the server ID is8512c9b9-d9e8-48e6-b037-b15b0004ca18
. -
On one of the servers, use the
system
database and create the databasefoo
using the server ID ofserver01
. The topology offoo
is stored in thesystem
database and when you create it, it is allocated according to the default topology (which can be shown withCALL dbms.showTopologyGraphConfig
). This may be different from the topology offoo
when it was backed up. If you want to ensure a certain allocation across the cluster, you can specify the desired topology with theTOPOLOGY
clause in theCREATE DATABASE
command. SeeCREATE 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 thefoo
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 thesystem
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.