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How to set up replication on a small cluster?

In the replication process, the data is replicated from one storage (MAIN instance) to another (REPLICA instances), thus providing a combination of consistency, availability and partition tolerance when distributing data over several instances.

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This example demonstrates how to create a simple cluster of nodes running Memgraph instances, and set up replication using various replication modes.

Cluster topology

The cluster will consist of four nodes, one MAIN instance and two REPLICA instances. In order to showcase the creation of REPLICA instances with different replication modes, we will create:

  • The MAIN instance - contains the original data that will be replicated to REPLICA instances
  • REPLICA instance 1 - replication in the SYNC mode
  • REPLICA instance 2 - replication in the ASYNC mode

How to run multiple instances?

If you are running multiple instances, each on its own machine, run Memgraph as you usually would.

If you are exploring replication and running multiple instances on one machine, please install Memgraph with Docker, and run the following docker run commands:

The MAIN instance:

docker run -it -p 7687:7687 -p 3000:3000 -p 7444:7444 -v mg_lib:/var/lib/memgraph memgraph/memgraph-platform

REPLICA instance 1:

docker run -it -p 7688:7687 -p 3001:3000 -p 7445:7444 -v mg_lib2:/var/lib/memgraph memgraph/memgraph-platform

REPLICA instance 2:

docker run -it -p 7689:7687 -p 3002:3000 -p 7446:7444 -v mg_lib3:/var/lib/memgraph memgraph/memgraph-platform

You can connect to each instance with the Memgraph Lab in-browser application at:

  • the MAIN instance - localhost:3000
  • REPLICA instance 1 - localhost:3001
  • REPLICA instance 2 - localhost:3002

How to demote an instance to a REPLICA role?

Run the following query in both REPLICA instances to demote them to the REPLICA role:

SET REPLICATION ROLE TO REPLICA WITH PORT 10000;

If you set the port of each REPLICA instance to 10000, it will be easier to register replicas later on because the query for registering replicas uses port 10000 as the default one.

Otherwise, you can use any unassigned port between 1000 and 10000.

How to register REPLICA instances?

To register a REPLICA instance, you need to find out the IP address of each instance.

The IP addresses will probably be:

  • the MAIN instance - 172.17.0.2
  • REPLICA instance 1 - 172.17.0.3
  • REPLICA instance 2 - 172.17.0.4

If they are not, please change the IP addresses in the following queries to match the IP addresses on your cluster.

Then, run the following queries from the MAIN instance to register REPLICA instances:

  1. REPLICA instance 1 at 172.17.0.3

    REGISTER REPLICA REP1 SYNC TO "172.17.0.3";

    REPLICA instance 1 is called REP1, its replication mode is SYNC, and it is located at IP address 172.17.0.3. with port 10000.

    Once the MAIN instance commits a transaction, it will communicate the changes to all REPLICA instances running in SYNC mode and wait until it receives a response that the changes have been applied to the REPLICAs or that a timeout has been reached.
    In the case of a timeout, the MAIN instance will return an error to the user proposing a check of the REPLICAs' statuses as there might be network or hardware issues.

    If you used any port other than 10000 while demoting a REPLICA instance, you will need to specify it like this: "172.17.0.3:5000"

  2. REPLICA instance 2 at 172.17.0.4

    REGISTER REPLICA REP2 ASYNC TO "172.17.0.4";

    REPLICA instance 2 is called REP2, its replication mode is ASYNC, and it is located at IP address 172.17.0.4. with port 10000.

    When the REPLICA instance is running in ASYNC mode the MAIN instance will commit a transaction without receiving confirmation from REPLICA instances that they have received the same transaction. ASYNC mode ensures system availability and partition tolerance.

    If you used any port other than 10000 while demoting a REPLICA instance, you will need to specify it like this: "172.17.0.4:5000"

How to check info about registered REPLICA instances?

Check REPLICA instances by running the following query from the MAIN instance:

SHOW REPLICAS;

How to drop a REPLICA instance?

To drop a replica, run the following query:

DROP REPLICA <name>;

How to promote a REPLICA instance to MAIN?

To promote a REPLICA instance to MAIN, run the following query:

SET REPLICATION ROLE TO MAIN;