hapi-pioc

A hapi.js plugin for the pioc dependency injection container

What?

When developing node.js applications, it can sometimes be hard to get the information you need to the place where you need it. It can also be hard to create mocks for testing when you rely on the native require function as you've just hard-coded the path to the service.

By using the Inversion of Control pattern, you decouple a service from the places where it is used.

hapi-pioc extends the hapi server object by adding a lookup method that can be used to fetch a service. Thus it follows the Service Locator pattern.

Within your services however, you are free to rely on Dependency Injection through Constructor Injection, Property Injection or Lazy Property Injection.

Why?

Because I honestly believe that Inversion of Control is the right way to implement an application.

The best possible case would've been to use Dependency Injection within the route handlers as well but that didn't work well. Using a Service Locator is still better than fetching services yourself so it seems like a good compromise.

How?

Using the standard hapi API, of course.

When you register your plugins, just also register the hapi-pioc plugin. Personally, I prefer to use glue and make this part of a manifest.js and it might even make sense to separate the services option into a file of its own. You should also be able to use this with confidence for ultimate flexibility.

server.register({
    register: require('hapi-pioc'),
    options: {
        // relative paths are resolved against this path
        baseUrl: process.cwd(),
        // the name of the exposed method
        methodName: 'lookup',
        services: {
            // use the "value!" prefix to load as value
            'value!UserEncryptionPrivateKeyProvider': './lib/services/UserEncryptionPrivateKeyProvider',
            // without supports node_modules path resolution
            'value!Promise': 'bluebird',
            // the value can be any kind of pioc service,
            // only strings are resolved as paths
            'value!MongoConfiguration': {
                url: 'mongodb://localhost:27017/demo',
                settings: {
                    db: {
                        native_parser: false
                    }
                }
            },
            'MongoConnection': './lib/services/MongoConnection',
            'Post': './lib/services/models/Post',
            'User': './lib/services/models/User'
        }
    }
}, function(err, server) {
    // ...
})

And within your route handlers, you can now use the method to lookup services:

server.route({
    path: '/{slug}',
    method: 'GET',
    handler(request, reply) {
        var Post = server.methods.lookup('Post');
        Post.getBySlug(request.params.slug)
            .then(result => createSinglePostAppData(request.path, result))
            .then(appData => renderApp(server, appData))
            .then(render(reply))
            .catch(reply);
    }
});

The above Post service is defined like this:

var Joi = require('joi'),
    MongoQuery = require('lib/MongoQueryMixin'),
    ValidateMixin = require('lib/ValidateMixin');
 
exports[MongoQuery.collection] = 'posts';
exports[MongoQuery.indexes] = [
    [{slug: 1}, {unique: true}],
    [{tags: 1}]
];
Object.assign(exports, MongoQuery.Mixin, ValidateMixin, {
    schema: Joi.object().keys({
        slug: Joi.string().required(),
        title: Joi.string().required(),
        content: Joi.string().required(),
        html: Joi.string().optional(),
        tags: Joi.array().includes(Joi.string()).optional(),
        createdAt: Joi.date().required(),
        updatedAt: Joi.date().required()
    }),
 
    getBySlug(slug) {
        return this.findOne({ slug });
    },
 
    getPage(query, options = { sort: '-created', limit: 20, page: 1 }, fields = '') {
        var { sort, limit, page } = options;
        return this.pagedFind(query, fields, sort, limit, page);
    },
 
    updateById(id, data) {
        var update = { $set: data };
        update.$currentDate = { updatedAt: true };
        return this.findByIdAndUpdate(id, update);
    }
});

You'll probably notice that I'm still using the require statement and that is just fine. The two mixins that I'm importing are utilities, not services. Being mixins however, they can naturally depend on services themself, like this:

import { inject } from 'pioc';
import Mongo from 'mongodb';
import Promise from 'bluebird';
 
/**
* A mixin for objects that expose the following properties:
* - [collection]: String => The collection name
* - [indexes]: Array<IndexDefinition> => the indexes
*/
export var collection = Symbol();
export var indexes = Symbol();
export var Mixin = {
    connection: inject('MongoConnection'),
 
    collection() {
        return this.connection.get().then(db => db.collection(this[collection]));
    },
    // ...
};

This pattern is well supported by pioc and when the Post service is resolved, the MongoConnection service is injected into the Post service.

If, at some point, I were to decide that I'd like to use a cache in front of the Post service, I'd just define it as the service implementation by updating the configuration. If I were using confidence and multiple servers, I could even A/B test it that way as long as the cache implementation offered the same interface.

Summary

By using the Inversion of Control pattern, you can start to decouple your application. Route handlers become the simple controllers they were meant to be while your service layer is independend, easily testable and reusable.

pioc in particular supports many styles of module definitions and injections and can be an important part in any modern application, especially for isomorphic applications.

Changelog

• 0.1.1 Include pioc as dependency