This is a tool which allows you to set up multi-master replication with MongoDB. It is emphatically not supported by 10gen, the makers of MongoDB.

It works by querying the oplog on one replica set and applying its operations to another replica set. It supports bidirectional replication by tagging each document replicated with its source.

To install, use pip:

pip install MongoMultiMaster

MMM needs access to the replica set oplog for each master. This means it doesn't work with servers that are configured as standalone servers. To convert a standalone server to a singleton replica set, first you need to tell it about the set it's in at launch time:

$ mongod --replSet foo

Then, to start the replica set, you need to execute the following at the mongo shell:

> rs.initiate()

Once you've created the replica set master, you'll need to tell MMM where the servers are. This is done through a YAML config file. A sample config file with two servers is included below:

server_a:
  id: '2c88ae84-7cb9-40f7-835d-c05e981f564d'
  uri: 'mongodb://localhost:27019'
server_b:
  id: '0d9c284b-b47c-40b5-932c-547b8685edd0'
  uri: 'mongodb://localhost:27017'

First, let's verify that there's no configuration and that we can connect to all the servers in the config file:

$ mmm -c test.yml clear-config
About to clear config on servers: ['server_a', 'server_b'], are you sure? (yN) y
Clear config for server_a
Clear config for server_b
$ mmm -c test.yml dump-config
=== Server Config ===
server_a (2c88ae84-7cb9-40f7-835d-c05e981f564d) => mongodb://localhost:27019
server_b (0d9c284b-b47c-40b5-932c-547b8685edd0) => mongodb://localhost:27017

=== server_a Replication Config
=== server_b Replication Config

Next, we'll set up two replicated collections:

$ mmm -c test.yml replicate --src=server_a/test.foo --dst=server_b/test.foo
$ mmm -c test.yml replicate --src=server_a/test.bar --dst=server_b/test.bar

And confirm they're configured correctly:

$ mmm -c test.yml dump-config
=== Server Config ===
server_a (2c88ae84-7cb9-40f7-835d-c05e981f564d) => mongodb://localhost:27019
server_b (0d9c284b-b47c-40b5-932c-547b8685edd0) => mongodb://localhost:27017

=== server_a Replication Config
=== server_b Replication Config
     - test.foo <= server_a/test.foo
     - test.bar <= server_a/test.bar

Now, let's make the replication bidirectional:

$ mmm -c test.yml replicate --src=server_b/test.foo --dst=server_a/test.foo
$ mmm -c test.yml replicate --src=server_b/test.bar --dst=server_a/test.bar

And verify that it's correct...

$ mmm -c test.yml dump-config
=== Server Config ===
server_a (2c88ae84-7cb9-40f7-835d-c05e981f564d) => mongodb://localhost:27019
server_b (0d9c284b-b47c-40b5-932c-547b8685edd0) => mongodb://localhost:27017

=== server_a Replication Config
     - test.foo <= server_b/test.foo
     - test.bar <= server_b/test.bar
=== server_b Replication Config
     - test.foo <= server_a/test.foo
     - test.bar <= server_a/test.bar

Now we can run the replicator:

$ mmm -c test.yml run

• Replication can fall behind if you're writing a lot. This is not handled at all.
• Replication begins at the time when mmm run was first called. You should be able to stop/start mmm and have it pick up where it left off.
• Conflicts between masters aren't handled; if you're writing to the same document on both heads frequently, you can get out of sync.
• Replication inserts a bookkeeping field into each document to signify the server UUID that last wrote the document. This expands the size of each document slightly.

There are probably sharp edges, other missed bugs, and various nasty things waiting for you if you use MMM in a production system without thorough testing. But if you like running with scissors and otherwise living dangerously, feel free to try it out.