25.6. High Availability setup tutorial

This guide will help you understand how to configure and deploy a Neo4j High Availability cluster. Two scenarios will be considered:

  • Configuring 3 instances to be deployed on 3 separate machines, in a setting similar to what might be encountered in a production environment.
  • Modifying the former to make it possible to run a cluster of 3 instances on the same physical machine, which is particularly useful during development.

Background

Each instance in a Neo4j HA cluster must be assigned an integer ID, which serves as its unique identifier. At startup, a Neo4j instance contacts the other instances specified in the ha.initial_hosts configuration option.

When an instance establishes a connection to any other, it determines the current state of the cluster and ensures that it is eligible to join. To be eligible the Neo4j instance must host the same database store as other members of the cluster (although it is allowed to be in an older state), or be a new deployment without a database store.

[Warning]Explicitly configure IP Addresses/Hostnames for a cluster

Neo4j will attempt to configure IP addresses for itself in the absence of explicit configuration. However in typical operational environments where machines have multiple network cards and support IPv4 and IPv6 it is strongly recommended that the operator explicitly sets the IP address/hostname configuration for each machine in the cluster.

Let’s examine the available settings and the values they accept.

ha.server_id

ha.server_id is the cluster identifier for each instance. It must be a positive integer and must be unique among all Neo4j instances in the cluster.

For example, ha.server_id=1.

ha.cluster_server

ha.cluster_server is an address/port setting that specifies where the Neo4j instance will listen for cluster communications (like hearbeat messages). The default port is 5001. In the absence of a specified IP address, Neo4j will attempt to find a valid interface for binding. While this behavior typically results in a well-behaved server, it is strongly recommended that users explicitly choose an IP address bound to the network interface of their choosing to ensure a coherent cluster deployment.

For example, ha.cluster_server=192.168.33.22:5001 will listen for cluster communications on the network interface bound to the 192.168.33.0 subnet on port 5001.

ha.initial_hosts

ha.initial_hosts is a comma separated list of address/port pairs, which specify how to reach other Neo4j instances in the cluster (as configured via their ha.cluster_server option). These hostname/ports will be used when the Neo4j instances starts, to allow it up to find and join the cluster. Specifying an instance’s own address is permitted.

[Warning]Warning

Do not use any whitespace in this configuration option.

For example, ha.initial_hosts=192.168.33.22:5001,192.168.33.21:5001 will attempt to reach Neo4j instances listening on 192.168.33.22 on port 5001 and 192.168.33.21 on port 5001 on the 192.168.33.0 subnet.

ha.server

ha.server is an address/port setting that specifies where the Neo4j instance will listen for transactions (changes to the graph data) from the cluster master. The default port is 6001. In the absence of a specified IP address, Neo4j will attempt to find a valid interface for binding. While this behavior typically results in a well-behaved server, it is strongly recommended that users explicitly choose an IP address bound to the network interface of their choosing to ensure a coherent cluster topology.

ha.server must user a different port to ha.cluster_server.

For example, ha.server=192.168.33.22:6001 will listen for cluster communications on the network interface bound to the 192.168.33.0 subnet on port 6001.

[Tip]Address/port format

The ha.cluster_server and ha.server configuration options are specified as <IP address>:<port>.

For ha.server the IP address MUST be the address assigned to one of the host’s network interfaces.

For ha.cluster_server the IP address MUST be the address assigned to one of the host’s network interfaces, or the value 0.0.0.0, which will cause Neo4j to listen on every network interface.

Either the address or the port can be omitted, in which case the default for that part will be used. If the address is omitted, then the port must be preceded with a colon (eg. :5001).

The syntax for setting the port range is: <hostname>:<first port>[-<second port>]. In this case, Neo4j will test each port in sequence, and select the first that is unused. Note that this usage is not permitted when the hostname is specified as 0.0.0.0 (the "all interfaces" address).

Getting started: Setting up a production cluster

Download and unpack Neo4j Enterprise

Download Neo4j Enterprise from the Neo4j download site, and unpack on 3 separate machines.

Start the Neo4j Servers

Start the Neo4j servers as normal. Note the startup order does not matter.

neo4j-01$ ./bin/neo4j start
neo4j-02$ ./bin/neo4j start
neo4j-03$ ./bin/neo4j start
[Tip]Startup Time

When running in HA mode, the startup script returns immediately instead of waiting for the server to become available. This is because the instance does not accept any requests until a cluster has been formed. In the example above this happens when you startup the second instance. To keep track of the startup state you can follow the messages in console.log - the path to that is printed before the startup script returns.

Now, you should be able to access the 3 servers and check their HA status:

http://neo4j-01.local:7474/webadmin/#/info/org.neo4j/High%20Availability/

http://neo4j-02.local:7474/webadmin/#/info/org.neo4j/High%20Availability/

http://neo4j-03.local:7474/webadmin/#/info/org.neo4j/High%20Availability/

[Tip]Tip

You can replace database #3 with an arbiter instance, see Arbiter Instances.

That’s it! You now have a Neo4j HA cluster of 3 instances running. You can start by making a change on any instance and those changes will be propagated between them. For more HA related configuration options take a look at HA Configuration.

Alternative setup: Creating a local cluster for testing

If you want to start a cluster similar to the one described above, but for development and testing purposes, it is convenient to run all Neo4j instances on the same machine. This is easy to achieve, although it requires some additional configuration as the defaults will conflict with each other.

Download and unpack Neo4j Enterprise

Download Neo4j Enterprise from the Neo4j download site, and unpack into 3 separate directories on your test machine.

Start the Neo4j Servers

Start the Neo4j servers as normal. Note the startup order does not matter.

localhost:~/neo4j-01$ ./bin/neo4j start
localhost:~/neo4j-02$ ./bin/neo4j start
localhost:~/neo4j-03$ ./bin/neo4j start

Now, you should be able to access the 3 servers and check their HA status:

http://127.0.0.1:7474/webadmin/#/info/org.neo4j/High%20Availability/

http://127.0.0.1:7475/webadmin/#/info/org.neo4j/High%20Availability/

http://127.0.0.1:7476/webadmin/#/info/org.neo4j/High%20Availability/