Conditionally detecting whether a Node server is running inside a Docker Container

Question

I have my node.js code where I establish mongodb connections like this: mongodb://localhost:27017/mycollection

Now, I put my server in one container and db in another container and I am able to connect to my db from the server like this: mongodb://mycontainer:27017/mycollection

I have this connection string configured in my server code/config.

Now, how do I detect whether a person is running the server in a container or not and accordingly take the connection string for the db?

If he is running it in the host machine, I want to use the first connection string with localhost and connect to the db in the host machine and if he connects through a container, I want to use the container link name to connect as mentioned in the second case.

Is there any way to do this?

Answer 1

Personally, when I want to accomplish that, I set an ENV variable in the Dockerfile like the following:

ENV DATABASE_HOST db

You can have the full documentation on the Dockerfile reference.

Then, in your Node.js code source, you need to know whether the DATABASE_HOST is set or not (I can redirect you to this Stack Overflow Jayesh's post: Read environment variables in Node.js):

var dbHost = 'localhost';

if (process.env.DATABASE_HOST) {
    dbHost = process.env.DATABASE_HOST;
}

or in one line:

var dbHost = process.env.DATABASE_HOST || 'localhost';

Then, for MongoDB connection:

var mongodbConnection = 'mongodb://' + dbHost + ':27017/mycollection'

Now, when you run the container, you must link the container in the docker run command with --link <your mongodb container>:db (since db is the value set in the ENV variable).

But, you can also use the option -e DATABASE_HOST=<somthing else> (again with the docker run command) and use a MongoDB container under another name: -e DATABASE_HOST=anotherOne --link mongo:anotherOne.

And again, you can use an external MongoDB without linking any container if you want (which is not in another container maybe): -e DATABASE_HOST=www.mymongo.com.

EDIT: This solution is maybe better than just identifying if the application is run in a Docker container because with this one your code is usable anywhere.