Set up ArangoDB clusters for Datacenter-to-Datacenter Replication

There are several ways to start an ArangoDB cluster. This section focuses on the recommended way to start ArangoDB: the ArangoDB Starter.

For other possibilities to deploy an ArangoDB cluster see Cluster Deployment.

The Starter simplifies things for the operator and coordinates a distributed cluster startup across several machines and assign cluster roles automatically.

When started on several machines and enough machines have joined, the Starters start Agents, Coordinators and DB-Servers on these machines.

The Starter supervises its child tasks (namely Coordinators, DB-Servers and Agents) and restarts them in case of failures.

Datacenter-to-Datacenter Replication (DC2DC) requires a normal ArangoDB cluster in both datacenters and one or more (arangosync) syncmasters & syncworkers in both datacenters. The Starter enables you to run these syncmasters & syncworkers in combination with your normal cluster.

To run a starter with DC2DC support, you set the following arguments in addition to the commands to start a cluster with the Starter:

--auth.jwt-secret=<path of file containing JWT secret for communication in local cluster>
--starter.address=<publicly visible address of this machine>
--starter.sync
--sync.master.jwt-secret=<path of file containing JWT secret used for communication between local syncmaster & workers>
--sync.server.keyfile=<path of keyfile containing TLS certificate & key for local syncmaster>
--sync.server.client-cafile=<path of file containing CA certificate for syncmaster client authentication>

See Securing Datacenter-to-Datacenter Replication for instructions how to create all certificates and keyfiles.

To start the cluster using a systemd unit file (and start DC2DC replication later manually), use the following:

[Unit]
Description=Run the ArangoDB Starter
After=network.target

[Service]
Restart=on-failure
EnvironmentFile=/etc/arangodb.env
EnvironmentFile=/etc/arangodb.env.local
Environment=DATADIR=/var/lib/arangodb/cluster
ExecStartPre=/usr/bin/sh -c "mkdir -p ${DATADIR}"
ExecStart=/usr/bin/arangodb \
    --starter.address=${PRIVATEIP} \
    --starter.data-dir=${DATADIR} \
    --starter.join=${STARTERENDPOINTS} \
    --auth.jwt-secret=${CLUSTERSECRETPATH}
TimeoutStopSec=60
LimitNOFILE=1048576

[Install]
WantedBy=multi-user.target

Cluster authentication

The communication between the cluster nodes use a token (JWT) to authenticate. This must be shared between cluster nodes.

Sharing secrets is obviously a very delicate topic. The above workflow assumes that the operator puts a secret in a file named ${CLUSTERSECRETPATH}.

We recommend to use a dedicated system for managing secrets like HashiCorp’s Vault.

Required ports

As soon as enough machines have joined, the Starter begins starting Agents, Coordinators and DB-Servers.

Each of these tasks needs a port to communicate. Please make sure that the following ports are available on all machines:

  • 8529 for Coordinators
  • 8530 for DB-Servers
  • 8531 for Agents

The Starter itself uses port 8528.

Since the Agents are so critical to the availability of both the ArangoDB and the ArangoSync cluster, it is recommended to run Agents on dedicated machines. They run a real-time system for the elections and bad performance can negatively affect the availability of the whole cluster.

DB-Servers are also important and you do not want to lose them, but depending on your replication factor, the system can tolerate some loss and bad performance slows things down but not stop things from working.

Coordinators can be deployed on other machines, since they do not hold persistent state. They might have some in-memory state about running transactions or queries, but losing a Coordinator does not lose any persisted data. Furthermore, new Coordinators can be added to a cluster without much effort.

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