Travis is a fantastic resource for developers building open source projects since they provide free build servers to run your tests automatically. Every time you push new code to your repos on Github Travis can pull down the changes, run the tests, and update you when they fail. It also works with just about every imaginable language and provides quite a few databases out of the box.

When setting your project up for Travis a basic config can often work pretty well but sometimes you need something a little more custom. I like to use Travis for most of my projects so while I was building oplog-transform-tail (a node module that keeps your MongoDB data in sync with a secondary data store in real time) I went ahead and created a somewhat standard Travis config file for a node project using MongoDB.

That looks something like this:

#.travis.yml
language: node_js
node_js:
  - "0.11"
  - "0.10"
services:
  - mongodb

This config just says that we’re using node, we want to test on version 0.10 and 0.11, and under services we say we want access to a MongoDB database. The problem with this setup is oplog-transform-tail requires access to MongoDB’s oplog. By default MongoDB starts as a single server with no replica set and as such does not utilize the oplog.

To turn this feature on we just need to guide Travis a little bit and tell it to set our MongoDB up as a replica set. Luckily Travis has a few lifecycle entrypoints that we can take advantage of to do just that. Our use case just needs to happen before the script event (when the tests get run) so we’ll use the before_script hook to make the changes ensuring our database is set up properly for testing.

Here’s our new config for Travis:

#.travis.yml
language: node_js
node_js:
  - "0.11"
  - "0.10"
services:
  - mongodb
# make mongodb a replica set
before_script:
  - echo "replSet = myReplSetName" | sudo tee -a /etc/mongodb.conf
  - sudo service mongodb restart
  - sleep 20
  - mongo --eval 'rs.initiate()'
  - sleep 15

Let’s talk about what this does.

1. echo "replSet = myReplSetName" | sudo tee -a /etc/mongodb.conf Here we append a string to the MongoDB config file, telling it set the replSet name to myReplSetName. This is all that’s required for basic replica set initialization.

2. sudo service mongodb restart Now we restart MongoDB so it can use the new config

3. sleep 20 Sometimes the database can take a while to reboot, we sleep 20 seconds here to make sure it’s ready for the next step

4. mongo --eval 'rs.initiate()' Here we tell MongoDB to evaluate our command, in this case rs.initiate() which tells MongoDB to start up the replica set functionality.

5. sleep 15 Here we sleep for another 15 seconds to give MongoDB a chance to properly set up the replica set.

And there you have it, now the MongoDB instance is running as a single server replica set and will write operations to the oplog allowing the tests a chance to run properly.