domain-context¶

Getting started¶

Install domain-context with npm:

% npm install domain-context

And require('domain-context') in your code.

Rationale¶

You need to have an access to some data inside your functions — usually you would pass such data as an argument.

But when you need to have an access “everywhere”, e.g. access to a currently active database connection through the duration of HTTP request, you probably don’t want to “poison” your function definitions and function calls with boilerplate arguments.

That the place where domain-context library comes handy. It allows basically to database connection in the currently active domain and automatically perform cleanup actions on the domain’s disposal. Your code could have access to the stored database connection as long as it is running inside the domain.

Connect/express middleware¶

The middleware can only be used inside an active domain, use connect-domain middleware to create one:

var connectDomain = require('connect-domain'),
    domainContext = require('domain-context'),
    express = require('express');

var lifecycle = {
  context: function() {
    return {db: new pg.Client(...)}
  },
  cleanup: function(context) {
    context.db.query('commit');
    context.db.end();
  },
  onError: function(err, context) {
    context.db.query('rollback');
    context.db.end();
  }
};

app = express();
app.use(connectDomain());
app.use(domainContext.middleware(lifecycle));
// your applicaiton's middleware goes here
app.use(domainContext.middlewareOnError(lifecycle));

Note that because of connect/express design you are required to place two middlewares around your application — domainContext.middleware() and domainContext.middlewareOnError().

Now you can use domainContext.get() to query data from the currently active domain:

var domainContext = require('connect-reqcontext');

function getUserById(id, cb) {
  domainContext.get('db').query("select ...", cb);
}