Twelve Factor nodejs app

This is a work in progress.

This app demonstrates the twelve-factor methodology in Node.js on cloud.gov.

The goal is to walk a user through the twelve-factor methodology with specific examples of how you would implement them in Node.js on cloud.gov. The companion application that demonstrates the principles is not implemented yet, but the idea is that you could link to specific line numbers to see a working app, with working code implementing best practices.

The twelve-factor methodology is not specific to Node.js and much of these tips are already general enough for any cloud.gov application.

The Twelve Factors

I. Codebase

One codebase tracked in revision control, many deploys.

How we do it

This code is tracked on github. Git flow can be used to manage branches for releases.

II. Dependencies

Explicitly declare and isolate dependencies.

How we do it

package.json declare and lock dependencies to specific versions. npm installs modules to a local node_modules dir so each application's dependencies are isolated from the rest of the system.

III. Config

Store config in the environment.

How we do it

Configuration is stored in enviornment variables and supplied through the manifest.yml.

Secrets are also stored in environment variables but supplied through a Cloud Foundry User Provided Service. When setting up the app, they are created with a one-time command cf create-user-provided-service tfn-secrets -p '{"SECRET_KEY": "your-secret-key"}'.

Connection configuration to Cloud Foundry Services, like our database, are provided through the VCAP_SERVICES environment variable.

IV. Backing services

Treat backing services as attached resources.

How we do it

We connect to the database via a connection url provided by the VCAP_SERVICES environment variable. If we needed to setup a new database, we would simply create a new database with cf create-service and bind the database to our application. After restaging with cf restage, the VCAP_SERVICES environment will be updated with the new connection url and our app would be talking to the new database.

We use a library which handles the database connection. This library abstracts away the differences between different SQL-based databases. This makes it easier to migrate from one database provider to another.

We expect to be using a database hosted on Cloud Foundry, but using this strategy we could store the connection url in a separate environment variable which could point to a database outside of the Cloud Foundry environment and this strategy would work fine.

Of course, how you handle migrating your data from one database to another can be complicated and is out of scope with regard to the twelve factor app.

V. Build, release, run

Strictly separate build and run stages.

How we do it

package.json allows to configure "scripts" so that we can codify various tasks. npm run build is used to build this application and produces minified javascript and css files to be served as static assets.

npm start is used to start the application. The nodejs_buildpack runs this command by default to start your application.

VI. Processes

Execute the app as one or more stateless processes.

How we do it

We listen to SIGTERM and SIGINT to know it's time to shutdown. The platform is constantly being updated even if our application is not. Machines die, security patches cause reboots. Server resources become consumed. Any of these things could cause the platform to kill your application. Don't worry though, Cloud Foundry makes sure to start a new process on the new freshly patched host before killing your old process.

By listening to process signals, we know when to stop serving requests, flush database connections, and close any open resources.

VII. Port binding

Export services via port binding.

How we do it

Cloud Foundry assigns your application instance a port on the host machine and exposes it through the PORT environment variable.

VIII. Concurrency

Scale out via the process model

How we do it

Our app keeps no state on it's own. Configuration is stored in the environment and read at startup. User sessions are stored as cookies on the client. Any other state is kept in the database. This allows our application to scale simply by adding more processes.

We are running two application instances on Cloud Foundry. Each application instance represents a running process of our application. The two instances are likely running on different host machines and have no way of communicating with each other. By making our processes stateless, the two application instances have no need to communicate because all state is stored in our backing service (the database in our case).

Here are the steps the app takes when a request comes in:

1. A user request comes in.
2. We parse their session cookie to figure out who they are.
3. We lookup their user information in the database.
4. Process their request, possibly with additional database lookups.

Notice how if a user's request comes in on instance 1, the same user's second request could be served by any instance. The steps to process subsequent requests are the same.

If you wanted to add some kind of session caching, that would be a job for another backing service like Memcached or Redis. That way all instances of your application could use a shared cache.

IX. Disposability

Maximize robustness with fast startup and graceful shutdown.

How we do it

SIGTERM SIGINT above?

X. Dev/prod parity

Keep development, staging, and production as similar as possible

How we do it

Choosing your environments is up to you, but it's probably good to have at least two for development. For us, development is our local laptop. Staging is a Cloud Foundry environment we use to preview the application to our partners. Production is a Cloud Foundry environment once it has been accepted by our partners.

As much as possible, the differences between development, staging, and production is simply the configuration which is stored in the environment.

Occasionally we use NODE_ENV, NODE_CONFIG to produce slightly different behavior. Specifically, anything we

Environment variable	Description
NODE_ENV	How the app is running.
NODE_CONFIG	Where the app is running.

Environment variable	development	staging	production
NODE_ENV	<unset>	production†	production
NODE_CONFIG	<unset>	staging	production

† That's not a typo, remeber NODE_ENV is how the app is running. Both staging and production are Cloud Foundry environments and warrant a production setup.

For nodejs, NODE_ENV=production has special meaning. npm install will only install dependencies listed in your package.json and will omit any devDependencies. We also use NODE_ENV to condition on how we build our static assets. NODE_ENV=production will include some extra optimizations.

NODE_CONFIG is used sparingly, and only to load environment specific configuration files.

XI. Logs

Treat logs as event streams.

How we do it

We use winston as our logger. We use logging levels to provide feedback about how the application is working. Some of this feedback could warrant a bug fix.

Warnings are conditions that are unexpected and might hint that a bug exists in the code.

XII. Admin processes

Run admin/management tasks as one-off processes.

How we do it

Any one-off tasks are added as npm scripts. The meat of these tasks is added to the tasks directory. Some take inputs which can be specified when running the task npm run script -- arguments. Note that by default, we avoid writing interactive scripts. If configuration is complex, the task can accept a configuration file or read a configuration from stdin.

Beyond Twelve Factor

Blue/green deploy

By default, cf push will stop your application while it rebuilds the new one. That means that a cf push results in a service disruption. If your application fails to build, the old application is gone so you are left with no running application at all. Blue/green is a strategy that creates a new application living side-by-side your old application. When the new application is known to be good, the old application is removed and you are left with a working new version of your application.

The autopilot zero-downtime-push plugin will do this for you automatically.

Continuous Integration and Delivery with Git Flow

Git Flow is a useful process for managing relases using git branches. The idea is that branches map to different deployment environments.

master represents well-vetted and partner accepted work and deploys to the production environment.

release-x.y is on track for release and deploys to your staging environment. This allows partners to review the work before it is released. Each release branch is based on development. Once created, this allows developers to commit bug fixes directly to the release branch without hindering development on the next release.

development is an integration branch. This allows developers on your team to test their work-in-progress features with features from other developers.

feature-\* branches represent a single feature. One or more developers may be committing to this branch. When the feature is tested and reviewed it can be merged to development.