Server-level Setup

This page describes the replication process based on a complete ArangoDB instance. That means that all included databases will be replicated.

Setting up a working Leader/Follower replication requires two ArangoDB instances:

• Leader: this is the instance where all data-modification operations should be directed to
• Follower: this is the instance that replicates the data from the Leader. We will start a replication applier on it, and it will fetch data from the Leader write-ahead log and apply its operations locally

For the following example setup, we will use the instance tcp://leader.domain.org:8529 as the Leader, and the instance tcp://follower.domain.org:8530 as a Follower.

The goal is to have all data of all databases on Leader tcp://leader.domain.org:8529 be replicated to the Follower instance tcp://follower.domain.org:8530.

On the Leader, nothing special needs to be done, as all write operations will automatically be logged in the Leader’s write-ahead log (WAL).

All-in-one setup

To make the replication copy the initial data from the Leader to the Follower and start the continuous replication on the Follower, there is an all-in-one command:

require("@arangodb/replication").setupReplicationGlobal(configuration);

The following example demonstrates how to use the command for setting up replication for the complete ArangoDB instance. Note that it should be run on the Follower and not the Leader:

db._useDatabase("_system");
require("@arangodb/replication").setupReplicationGlobal({
  endpoint: "tcp://127.0.0.1:8529",
  username: "root",
  password: "",
  autoStart: true
});

The command will return when the initial synchronization is finished and the continuous replication has been started, or in case the initial synchronization has failed.

If the initial synchronization is successful, the command will store the given configuration on the Follower. It also configures the continuous replication to start automatically if the Follower is restarted, i.e. autoStart is set to true.

If the command is run while the Follower’s replication applier is already running, it will first stop the running applier, drop its configuration and do a resynchronization of data with the Leader. It will then use the provided configration, overwriting any previously existing replication configuration on the Follower.

Stopping synchronization

The initial synchronization and continuous replication applier can also be started separately. To start replication on the Follower, make sure there currently is no replication applier running.

The following commands stop a running applier in the Follower’s instance:

db._useDatabase("_system");
require("@arangodb/replication").globalApplier.stop();

The stop operation will terminate any replication activity in the ArangoDB instance on the Follower.

After that, the initial synchronization can be run. It will copy the collections from the Leader to the Follower, overwriting existing data. To run the initial synchronization, execute the following commands on the Follower:

db._useDatabase("_system");
require("@arangodb/replication").syncGlobal({
  endpoint: "tcp://leader.domain.org:8529",
  username: "myuser",
  password: "mypasswd",
  verbose: false
});

Username and password only need to be specified when the Leader requires authentication. To check what the synchronization is currently doing, supply set the verbose option to true. If set, the synchronization will create log messages with the current synchronization status.

Warning: The syncGlobal command will replace data in the Follower database with data from the Leader database! Only execute these commands if you have verified you are on the correct server, in the correct database!

The sync operation will return an attribute named lastLogTick which we’ll need to note. The last log tick will be used as the starting point for subsequent replication activity. Let’s assume we got the following last log tick:

{ 
  "lastLogTick" : "40694126", 
  ...
}

Initial synchronization from the ArangoShell

The initial synchronization via the syncGlobal command may take a long time to complete. The shell will block until the Follower has completed the initial synchronization or until an error occurs. By default, the syncGlobal command in the ArangoShell will poll the Follower for a status update every 10 seconds.

Optionally the syncGlobal command can be made non-blocking by setting its async option to true. In this case, the syncGlobal command will return instantly with an id string, and the initial synchronization will run detached on the Leader. To fetch the current status of the syncGlobal progress from the ArangoShell, the getSyncResult function can be used as follows:

db._useDatabase("_system");
var replication = require("@arangodb/replication");

/* run command in async mode */
var id = replication.syncGlobal({
  endpoint: "tcp://leader.domain.org:8529",
  username: "myuser",
  password: "mypasswd",
  async: true
});

/* now query the status of our operation */
print(replication.getSyncResult(id));

getSyncResult will return false as long as the synchronization is not complete, and return the synchronization result otherwise.

Continuous synchronization

When the initial synchronization is finished, the continuous replication applier can be started using the last log tick provided by the syncGlobal command. Before starting it, there is at least one configuration option to consider: replication on the Follower will be running until the Follower gets shut down. When the Follower server gets restarted, replication will be turned off again. To change this, we first need to configure the Follower’s replication applier and set its autoStart attribute.

Here’s the command to configure the replication applier with several options, including the autoStart attribute:

db._useDatabase("_system");
require("@arangodb/replication").globalApplier.properties({
  endpoint: "tcp://leader.domain.org:8529",
  username: "myuser",
  password: "mypasswd",
  autoStart: true,
  autoResync: true,
  autoResyncRetries: 2,
  adaptivePolling: true,
  includeSystem: false,
  requireFromPresent: false,
  idleMinWaitTime: 0.5,
  idleMaxWaitTime: 1.5,
  verbose: false
});

An important consideration for replication is whether data from system collections (such as _graphs or _users) should be applied. The includeSystem option controls that. If set to true, changes in system collections will be replicated. Otherwise, they will not be replicated. It is often not necessary to replicate data from system collections, especially because it may lead to confusion on the Follower because the Follower needs to have its own system collections in order to start and keep operational.

The requireFromPresent attribute controls whether the applier will start synchronizing in case it detects that the Leader cannot provide data for the initial tick value provided by the Follower. This may be the case if the Leader does not have a big enough backlog of historic WAL logfiles, and when the replication is re-started after a longer pause. When requireFromPresent is set to true, then the replication applier will check at start whether the start tick from which it starts or resumes replication is still present on the Leader. If not, then there would be data loss. If requireFromPresent is true, the replication applier will abort with an appropriate error message. If set to false, then the replication applier will still start, and ignore the data loss.

The autoResync option can be used in conjunction with the requireFromPresent option as follows: when both requireFromPresent and autoResync are set to true and the Leader cannot provide the log data the Follower had requested, the replication applier will stop as usual. But due to the fact that autoResync is set to true, the Follower will automatically trigger a full resync of all data with the Leader. After that, the replication applier will go into continuous replication mode again. Additionally, setting autoResync to true will trigger a full re-synchronization of data when the continuous replication is started and detects that there is no start tick value.

Note that automatic re-synchronization (autoResync option set to true) may transfer a lot of data from the Leader to the Follower and can therefore be expensive. Still it’s turned on here so there’s less need for manual intervention.

The autoResyncRetries option can be used to control the number of resynchronization retries that will be performed in a row when automatic resynchronization is enabled and kicks in. Setting this to 0 will effectively disable autoResync. Setting it to some other value will limit the number of retries that are performed. This helps preventing endless retries in case resynchronizations always fail.

Now it’s time to start the replication applier on the Follower using the last log tick we got before:

db._useDatabase("_system");
require("@arangodb/replication").globalApplier.start("40694126");

This will replicate all operations happening in the Leader’s system database and apply them on the Follower, too.

After that, you should be able to monitor the state and progress of the replication applier by executing the state command on the Follower server:

db._useDatabase("_system");
require("@arangodb/replication").globalApplier.state();

Please note that stopping the replication applier on the Follower using the stop command should be avoided. The reason is that currently ongoing transactions (that have partly been replicated to the Follower) will be need to be restarted after a restart of the replication applier. Stopping and restarting the replication applier on the Follower should thus only be performed if there is certainty that the Leader is currently fully idle and all transactions have been replicated fully.

Note that while a Follower has only partly executed a transaction from the Leader, it might keep a write lock on the collections involved in the transaction.

You may also want to check the Leader and Follower states via the HTTP APIs (see HTTP Interface for Replication).