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Example Setup

Setting up a working master-slave replication requires two ArangoDB instances:

  • master: this is the instance that all data-modification operations should be directed to
  • slave: on this instance, we'll start a replication applier, and this will fetch data from the master database's write-ahead log and apply its operations locally

For the following example setup, we'll use the instance tcp://master.domain.org:8529 as the master, and the instance tcp://slave.domain.org:8530 as a slave.

The goal is to have all data from the database _system on master tcp://master.domain.org:8529 be replicated to the database _system on the slave tcp://slave.domain.org:8530.

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

All-in-one setup

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

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

The following example demonstrates how to use the command for setting up replication for the _system database. Note that it should be run on the slave and not the master:

db._useDatabase("_system");
require("@arangodb/replication").setupReplication({
  endpoint: "tcp://master.domain.org:8529",
  username: "myuser",
  password: "mypasswd",
  verbose: false,
  includeSystem: false,
  incremental: true,
  autoResync: 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 slave. It also configures the continuous replication to start automatically if the slave is restarted, i.e. autoStart is set to true.

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

Initial synchronization

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

The following commands stop a running applier in the slave's _system database:

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

The stop operation will terminate any replication activity in the _system database on the slave.

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

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

Username and password only need to be specified when the master 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 sync command will replace data in the slave database with data from the master 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 sync command may take a long time to complete. The shell will block until the slave has completed the initial synchronization or until an error occurs. By default, the sync command in the ArangoShell will poll the slave for a status update every 10 seconds.

Optionally the sync command can be made non-blocking by setting its async option to true. In this case, the sync command will return instantly with an id string, and the initial synchronization will run detached on the master. To fetch the current status of the sync 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.sync({
  endpoint: "tcp://master.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 sync command. Before starting it, there is at least one configuration option to consider: replication on the slave will be running until the slave gets shut down. When the slave server gets restarted, replication will be turned off again. To change this, we first need to configure the slave'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").applier.properties({
  endpoint: "tcp://master.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 slave because the slave 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 master cannot provide data for the initial tick value provided by the slave. This may be the case if the master 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 master. 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 master cannot provide the log data the slave had requested, the replication applier will stop as usual. But due to the fact that autoResync is set to true, the slave will automatically trigger a full resync of all data with the master. 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 master to the slave 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 slave using the last log tick we got before:

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

This will replicate all operations happening in the master's system database and apply them on the slave, too.

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

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

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

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

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