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Replica Sets in Ruby

Here follow a few considerations for those using the MongoDB Ruby driver with replica sets.

Setup

First, make sure that you've configured and initialized a replica set.

Use ReplSetConnection.new to connect to a replica set:

@connection = ReplSetConnection.new(['n1.mydb.net', 27017], ['n2.mydb.net', 27017], ['n3.mydb.net', 27017])

The driver will attempt to connect to a master node and, when found, will replace all seed nodes with known members of the replica set.

Read slaves

If you want to read from a seconday node, you can pass :read_secondary => true to ReplSetConnection#new.

@connection = ReplSetConnection.new(['n1.mydb.net', 27017], ['n2.mydb.net', 27017], ['n3.mydb.net', 27017],
              :read_secondary => true)

A random secondary will be chosen to be read from. In a typical multi-process Ruby application, you'll have a good distribution of reads across secondary nodes.

Connection Failures

Imagine that either the master node or one of the read nodes goes offline. How will the driver respond?

If any read operation fails, the driver will raise a ConnectionFailure exception. It then becomes the client's responsibility to decide how to handle this.

If the client decides to retry, it's not guaranteed that another member of the replica set will have been promoted to master right away, so it's still possible that the driver will raise another ConnectionFailure. However, once a member has been promoted to master, typically within a few seconds, subsequent operations will succeed.

The driver will essentially cycle through all known seed addresses until a node identifies itself as master.

Recovery

Driver users may wish to wrap their database calls with failure recovery code. Here's one possibility, which will attempt to connection every half second and time out after thirty seconds.

# Ensure retry upon failure
def rescue_connection_failure(max_retries=60)
    success = false
    retries = 0
    while !success
      begin
        yield
        success = true
      rescue Mongo::ConnectionFailure => ex
        retries += 1
        raise ex if retries >= max_retries
        sleep(0.5)
      end
    end
  end
end

# Wrapping a call to #count()
rescue_connection_failure do
  @db.collection('users').count()
end

Of course, the proper way to handle connection failures will always depend on the individual application. We encourage object-mapper and application developers to publish any promising results.

Testing

The Ruby driver (>= 1.1.5) includes unit tests for verifying replica set behavior. They reside in tests/replica_sets. You can run them as a group with the following rake task:

rake test:rs

The suite will set up a five-node replica set by itself and ensure that driver behaves correctly even in the face of individual node failures. Node that the mongod executable must be in the search path for this to work.

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