Laconic Asynchronous Dependency Injection container for JavaScript
- Declarative configuration and Lazy Loading of modules
- Asynchronous resolution of services
- Tiny size and flexible clear API
- Ability to inject the same dependencies concurrently
- Lifetime Management: Singleton, Transient and Scoped lifetime
Imagine a web server that has dozens of services. If its startup speed is critical, then asynchronous Lazy Loading of services solves that problem. It will allow the application to launch faster, with the minimal count of necessary services. Even while loading dependencies graph the server stays still responsive. Lazy loading Express middleware
- Installation
- Usage
- Dependencies graph
- Dependency Injection pattern
- Registering modules
- Dependency Injection Container
Install with npm
npm install di-async --saveDI-Async has a simple clear API. You need to do few things:
- Refactor your modules leveraging the DI pattern
- Create a DI Container
- Register some modules in it
- Resolve services and use
// simple-sync.js
// create container
const DI = require('di-async');
const di = DI();
// register services
di.join(require('./modules.json'));
di.set('logger', console );
// synchronous services resolving and use
try {
const a1 = di.getSync('accumulator');
a1.add(1);
a1.add(4);
console.info('Amount is %d', a1.tot);
const a2 = di.getSync('accumulator');
a2.add(10);
a2.add(40);
console.info('Amount is %d', a2.tot);
const a3 = di.getSync('accumulator');
a3.add(100);
a3.add(400);
console.info('Amount is %d', a3.tot);
const s = di.getSync('storage');
console.info('Total amount is %d', s.tot);
const D = di.getSync('DerivedA');
const d = new D('Den');
console.info(d.name, d.sum(8, 2));
}
catch (err) {
console.error(`Catch: ${err}`)
};
// modules.json
{
"storage": { "path": "./services/storage" },
"thresshold": { "path": "./services/thresshold" },
"accumulator": { "path": "./services/accumulator", "transient": true },
"ClassA": { "path": "./services/class-a" },
"DerivedA": { "path": "./services/derived-a" }
}Application stays responsive, resolving services with asynchronous approach.
// simple.js
// create container
const DI = require('di-async');
const di = DI();
// register services
di.join(require('./modules.json'));
di.set('logger', console );
// sequential services resolving and use
const run = async () => {
const a1 = await di.get('accumulator');
a1.add(1);
a1.add(4);
console.info('Amount is %d', a1.tot);
const a2 = await di.get('accumulator');
a2.add(10);
a2.add(40);
console.info('Amount is %d', a2.tot);
const a3 = await di.get('accumulator');
a3.add(100);
a3.add(400);
console.info('Amount is %d', a3.tot);
const s = await di.get('storage');
console.info('Total amount is %d', s.tot);
const D = await di.get('DerivedA');
const d = new D('Den');
console.info(d.name, d.sum(8, 2));
};
run().catch(err => console.error(`Catch: ${err}`));Both outputs are the same.
1 added to 0
4 added to 1
Amount is 5
10 added to 0
40 added to 10
Amount is 50
100 added to 0
400 added to 100
Storage limit 500 exceeded by 55 !
Amount is 500
Total amount is 555
Den saccessfully created
Den 10
You can try services resolution concurrently.
// concurrent.js
// create container
const DI = require('di-async');
const di = DI();
// register services
di.join(require('./modules.json'));
di.set('logger', console);
// concurrently services resolving and use
const p1 = di.get('accumulator').then(a1 => {
a1.add(1);
a1.add(4);
logger.info('Amount is %d', a1.tot);
});
const p2 = di.get('accumulator').then(a2 => {
a2.add(10);
a2.add(40);
logger.info('Amount is %d', a2.tot);
});
const p3 = di.get('accumulator').then(a3 => {
a3.add(100);
a3.add(400);
logger.info('Amount is %d', a3.tot);
});
const p0 = di.get('storage');
Promise.all([p0, p1, p2, p3]).then(r => {
logger.info('Total amount is %d', r[0].tot)
}).catch(err => logger.error(`Catch on Total amount: ${err}`));
di.get('DerivedA').then(D => {
const d = new D('Den');
logger.info(d.name, d.sum(8, 2));
}).catch(err => logger.error(`Catch on DerivedA: ${err}`));Output shows that Den shifted forward by three positions.
1 added to 0
4 added to 1
Amount is 5
Den saccessfully created
Den 10
10 added to 0
40 added to 10
Amount is 50
100 added to 0
400 added to 100
Storage limit 500 exceeded by 55 !
Amount is 500
Total amount is 555
The example of the Dependencies graph represent dependencies of several services towards each other. Directions of graph links assumed from bottom to top.
Dependency Injection (DI) is a pattern where the dependencies of a service component are provided as input by the external entity often called the injector.
Refactor your modules to accomplish DI patterns as shown below. Every service module exports a fabric function with deps property set to an array of dependencies names.
// accumulator.js
// fabric function of accumulator service
module.exports = (storage, logger) => {
let acc = 0;
// returned instance
return {
add(a) {
acc += a;
logger.info(`${a} added to ${acc}`);
storage.add(a);
if (storage.exceeded)
logger.info(`Storage limit ${storage.limit} exceeded by ${storage.tot - storage.limit} !`);
},
get tot() {
return acc;
},
}
}
module.exports.sname = "Accumulator service";
module.exports.deps = ["storage", "logger"];
// storage.js
// fabric function of storage service
module.exports = (thresshold) => {
let acc = 0;
return {
add(a) {
acc += a;
},
get exceeded() {
return acc > thresshold.val;
},
get limit() {
return thresshold.val;
},
get tot() {
return acc;
}
}
};
module.exports.sname = "Storage service";
module.exports.deps = ["thresshold"];
// thresshold.js
// fabric function without dependencies
module.exports = () => {
let limit = 500;
return {
set val(a) {
limit = a;
},
get val() {
return limit;
},
}
};
module.exports.sname = "Thresshold";As said in my favorite book (Node.js Design Patterns ISBN 978-1-83921-411-0):
The nature of JavaScript affects traditional design patterns. There are so many ways in which traditional design patterns can be implemented in JavaScript that the traditional, strongly object-oriented implementation stops being relevant.
Class as a service is a more flexible approach than using a constructor as a dependency injection function. Constructor arguments stay free from dependencies. There is no need to register constants as input services.
// class-a.js
// class defined inside factory function
module.exports = (logger) => {
class ClassA {
constructor(name) {
this._name = name;
logger.info(`${this.name} saccessfully created`);
}
get name() {
return this._name;
}
}
return ClassA;
};
module.exports.sname = "class A";
module.exports.deps = ["logger"];
// derived-a.js
// derived class defined inside factory function
module.exports = (ClassA) => {
class DerivedA extends ClassA {
constructor(name) {
super(name);
}
sum(a, b) {
return a + b;
}
}
return DerivedA;
};
module.exports.sname = "derived A";
module.exports.deps = ["ClassA"];Just for example, DerivedA service was rewritten in asynchronous manner.
module.exports = (ClassA) => {
class DerivedA extends ClassA {
constructor(name) {
super(name);
}
sum(a, b) {
return a + b;
}
}
return new Promise(resolve => setTimeout(resolve, 2000, DerivedA));
};
module.exports.sname = "derived A";
module.exports.deps = ["ClassA"];Therefore, service would be available two seconds later.
Lifetime Management is the concept of regulating the number of returned instances and the duration of the lifetime of those instances.
By default, container returns Singleton instance and holds on reference to it. The container always returns that same instance.
Registering service as Transient lead to another outcome. Returned instance is not cached in the container. A new instance of the component will be created each time the service is requested from the container.
Registering Singleton service instance.
di.set('logger', console);
// or equivalently
di.setEntry('logger', { inst: console });Registering multiple services. The accumulator service registered as Transient.
di.join({
"storage": { "path": "./services/storage" },
"thresshold": { "path": "./services/thresshold" },
"accumulator": { "path": "./services/accumulator", "transient": true },
"ClassA": { "path": "./services/class-a" },
"DerivedA": { "path": "./services/derived-a" }
});
// or from external JSON-file
di.join(require('./modules.json'));The Scoped service behaves much like the Singleton service within a single, well-defined scope.
To obtain Scoped service behavior create new container as well-defined scope and register appropriate Singleton services. To optimize execution performance should share service factories and instances within a dedicated container.
const DI = require('di-async');
const shared = DI();
shared.join(require('./modules.json'));
// singleton instance shared with scoped containers
shared.set('logger', console );Then create new scoped containers through method newScope of shared container and do some actions. And now, the storage dependency of the accumulator service behaves as Scoped.
const run = async () => {
// singleton instance shared with scoped containers
const lim = await shared.get('thresshold');
lim.val = 50;
const scope1 = shared.newScope();
const a11 = await scope1.get('accumulator');
a11.add(1);
a11.add(4);
console.info('Amount is %d', a11.tot);
const a12 = await scope1.get('accumulator');
a12.add(10);
a12.add(40);
console.info('Amount is %d', a12.tot);
// Singleton instance within scope1
const s1 = await scope1.get('storage');
console.info('Total amount is %d', s1.tot);
lim.val = 100;
const scope2 = shared.newScope();
const a21 = await scope2.get('accumulator');
a21.add(1);
a21.add(9);
console.info('Amount is %d', a21.tot);
const a22 = await scope2.get('accumulator');
a22.add(10);
a22.add(90);
console.info('Amount is %d', a22.tot);
// Singleton instance within scope2
const s2 = await scope2.get('storage');
console.info('Total amount is %d', s2.tot);
};
run().catch(err => console.error(`Catch: ${err}`));Each instances of the storage service have they own scope. Both share the same instance of threshold.
1 added to 0
4 added to 1
Amount is 5
10 added to 0
40 added to 10
Storage limit 50 exceeded by 5 !
Amount is 50
Total amount is 55
1 added to 0
9 added to 1
Amount is 10
10 added to 0
90 added to 10
Storage limit 100 exceeded by 10 !
Amount is 100
Total amount is 110
Simplified example of scoped service used in middleware.
const DI = require('di-async');
const shared = DI();
// register services
shared.join({
"ClassA": { "path": "./services/class-a" },
"student": { "path": "./services/student" }
});
shared.set('logger', console );
// register middleware function
// in which create the scoped container
app.use((req, res, next) => {
shared.get('ClassA').then(A => {
// new scope per request
req.scope = shared.newScope();
// provide constructor arguments
const person = new A(req.user.name);
// register scoped service instance
req.scope.set('person', person);
next();
}).catch(next);
});
// somewhere obtain from scope and use it
app.get('/api/me', (req, res) => {
const person = req.scope.getSync('person');
// Do some work with that person
res.end(person.name);
});
// or obtain from scope and use dependent service
app.get('/api/student', (req, res) => {
const student = req.scope.getSync('student');
res.end(student.ask());
});
// student.js - used as scoped service
module.exports = (person) => {
return {
ask() {
return `My name is ${person.name}`;
}
}
};
module.exports.sname = "student service";
module.exports.deps = ["person"];The person service implicitly depends on ClassA service, there is no module with service definition, but you can explicitly define dependents chain of that service.
DI-Async loads service modules in an asynchronous manner only when needed, therefore performs Lazy Loading strategy. No module loading before service have been requested.
Container implementation allows concurrently load the same Singleton dependency. In example program concurrent.js, shown above, there are three implicit and one explicit concurrent requests to storage dependency. Only first request loads factory and create service instance within method get, the other three awaits the same promise, that will return resolved instance.