HA MongoDB services on The Platform use Replica Sets to provide automated failover in the event of a MongoDB node failure. The Platform handles the replica set configuration within your environment, and expose a single connection URI in an environment variable, DATABASE_URL.

Note: The variable may not always be named DATABASE_URL if the environment also contains more than one database service. There will always also be another variable named after the service, such as MONGO01_URL or DB01_URL.

The environment variable’s value follows the format defined by MongoDB. An example connection string:

mongodb://127.0.1.1:27017,127.0.1.2:27017/myDB?replicaSet=myReplSet

In the above example, the primary MongoDb node is at 127.0.1.1:27017 and the secondary node is at 127.0.1.2:27017. myDB will be the default database to connect to. The replica set to connect to is named myReplSet.

You will need to write your application using a client that supports replica sets - official driver list here.

To test your application locally using a replica set, we recommend that you follow the instructions in this blog post. This will give you a local testing environment that is configured very similarly to your HA MongoDB on The Platform.

See also

• Local Testing
• Express (Node) & MongoDB