loopback-context

Current context for LoopBack applications, based on cls-hooked.

WARNING

cls-hooked module uses undocumented AsyncWrap API that was introduced to Node.js relatively recently. While this new API seems to be more reliable than the old async-listener used by continuation-local-storage, there are still cases where the context (local storage) is not preserved correctly. Please consider this risk before using loopback-context.

Known issues

• when, a popular Promise implementation, breaks context propagation. Please consider using the built-in Promise implementation provided by Node.js or Bluebird instead.

• Express middleware chains which contain a "bad" middleware (i.e. one which breaks context propagation inside its function body, in a way mentioned in this doc) especially if called before other "good" ones needs refactoring, in order to prevent the context from getting mixed up among HTTP requests. See usage below for details.

  Discussion: https://github.com/strongloop/loopback-context/issues/17

In general, any module that implements a custom task queue or a connection pool is prone to break context storage. This is an inherent problem of continuation local storage that needs to be fixed at lower level - first in Node.js core and then in modules implementing task queues and connection pools.

Installation

$ npm install --save loopback-context cls-hooked

Make sure you are running on a Node.js version supported by this module (^4.5, ^5.10 or ^6.0). When installing, check the output of npm install and make sure there are no engine related warnings.

Usage

Setup cls-hooked

To minimize the likelihood of loosing context in your application, you should ensure that cls-hooked is loaded as the first module of your application, so that it can wrap certain Node.js APIs before any other modules start using these APIs.

Our recommended approach is to add -r cls-hooked to node's list of arguments when starting your LoopBack application.

$ node -r cls-hooked .

If you are using a process manager like strong-pm or pm2, then consult their documentation whether it's possible to configure the arguments used to spawn worker processes. Note that slc run does not support this feature yet, see strong-supervisor#56.

Alternatively, you can add the following line as the first line of your main application file:

require('cls-hooked');

This approach should be compatible with all process managers, including strong-pm. However, we feel that relying on the order of require statements is error-prone.

Configure context propagation

To setup your LoopBack application to create a new context for each incoming HTTP request, configure per-context middleware in your server/middleware.json as follows:

{
  "initial": {
    "loopback-context#per-request": {
    }
  }
}

IMPORTANT: By default, the HTTP req/res objects are not set onto the current context. You need to set enableHttpContext to true to enable automatic population of req/res objects.

{
  "initial": {
    "loopback-context#per-request": {
      "params": {
        "enableHttpContext": true
      }
    }
  }
}

Use the current context

Once you’ve enabled context propagation, you can access the current context object using LoopBackContext.getCurrentContext(). The context will be available in middleware (if it is loaded after the context middleware), remoting hooks, model hooks, and custom methods.

var LoopBackContext = require('loopback-context');

// ...

MyModel.myMethod = function(cb) {
  var ctx = LoopBackContext.getCurrentContext();
  ctx.get('key');
  ctx.set('key', { foo: 'bar' });
});

Bind for concurrency

In order to workaround the aforementioned concurrency issue with when (and similar Promise-like and other libraries implementing custom queues and/or connection pools), it's recommended to activate context binding inside each HTTP request or concurrent runInContext() call, by using the bind option, as in this example:

var ctx = LoopBackContext.getCurrentContext({ bind: true });

With the option enabled, this both creates the context, and binds the access methods of the context (i.e. get and set), at once.

Warning: this only works if it's the first expression evaluated in every middleware/operation hook/runInContext() call etc. that uses getCurrentContext. (It must be the first expression; it may not be enough if it's at the first line). Explanation: you must bind the context while it's still correct, i.e. before it gets mixed up between concurrent operations affected by bugs. Therefore, to be sure, you must bind it before any operation.

Also, with respect to the "bad", context-breaking middleware use case mentioned in "Known issues" before, the following 2 lines need to be present at the beginning of the middleware body. At least the "bad" one; but, as a preventive measure, they can be present in every other middleware of every chain as well, being backward-compatible:

 var badMiddleware = function(req, res, next) {
   // these 2 lines below are needed
   var ctx = LoopBackContext.getCurrentContext({bind: true});
   next = ctx.bind(next);
   ...

The bind option defaults to false. This is only in order to prevent breaking legacy apps; but if your app doesn't have such issue, then you can safely use bind: true everywhere in your app (e.g. with a codemod, or by monkey-patching getCurrentContext() globally, if you prefer an automated fashion).

Warning: this only applies to application modules. In fact, if the module affected by the concurrency issue is of this kind, you can easily refactor/write your own code so to enable bind. Not if it's a 3rd-party module, nor a Loopback non-core module, unless you fork and fix it.

Use current authenticated user in remote methods

In advanced use cases, for example when you want to add custom middleware, you have to add the context middleware at the right position in the middleware chain (before the middleware that depends on LoopBackContext.getCurrentContext).

IMPORTANT: LoopBackContext.perRequest() detects the situation when it is invoked multiple times on the same request and returns immediately in subsequent runs.

Here is a snippet using a middleware function to place the currently authenticated user into the context so that remote methods may use it:

server/middleware/store-current-user.js

module.exports = function(options) {
  return function storeCurrentUser(req, res, next) {
    if (!req.accessToken) {
      return next();
    }

    app.models.UserModel.findById(req.accessToken.userId, function(err, user) {
      if (err) {
        return next(err);
      }
      if (!user) {
        return next(new Error('No user with this access token was found.'));
      }
      var loopbackContext = LoopBackContext.getCurrentContext();
      if (loopbackContext) {
        loopbackContext.set('currentUser', user);
      }
      next();
    });
  };
};

server/middleware.json

{
  "initial": {
    "loopback-context#per-request": {}
  },
  "auth": {
    "loopback#token": {}
  },
  "auth:after": {
    "./middleware/store-current-user": {}
  }
}

common/models/YourModel.json

var LoopBackContext = require('loopback-context');
module.exports = function(YourModel) {
  ...
  //remote method
  YourModel.someRemoteMethod = function(arg1, arg2, cb) {
    var ctx = LoopBackContext.getCurrentContext();
    var currentUser = ctx && ctx.get('currentUser');
    console.log('currentUser.username: ', currentUser.username); // voila!
    ...
    cb(null);
  };
  ...
};