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What is Context?

  • An abstraction of all state and dependencies in your application
  • LoopBack uses context to manage everything
  • A global registry for anything/everything in your app (all configs, state, dependencies, classes, etc)
  • An inversion of control container used to inject dependencies into your code

Why is it important?

  • You can use the context as a way to give loopback more “info” so that other dependencies in your app may retrieve it. It works as a centralized place/ global built-in/in-memory storage mechanism.
  • LoopBack can help “manage” your resources automatically (through Dependency Injection and decorators).
  • You have full access to updated/real-time application and request state at all times.

How to create a context?

A context can be created with an optional parent and an optional name. If the name is not provided, a UUID will be generated as the value. Context instances can be chained using the parent to form a hierarchy. For example, the code below creates a chain of three contexts: reqCtx -> serverCtx -> rootCtx.

import {Context} from '@loopback/context';

const rootCtx = new Context('root-ctx'); // No parent
const serverCtx = new Context(rootCtx, 'server-ctx'); // rootCtx as the parent
const reqCtx = new Context(serverCtx); // No explicit name, a UUID will be generated

LoopBack’s context system allows an unlimited amount of Context instances, each of which may have a parent Context.

An application typically has three “levels” of context: application-level, server-level, and request-level.

Application-level context (global)

  • Stores all the initial and modified app states throughout the entire life of the app (while the process is alive)
  • Generally configured when the application is created (though the context may be modified while running)

Here is a simple example:

import {Application} from '@loopback/core';

// Please note `Application` extends from `Context`
const app = new Application(); // `app` is a "Context"
class MyController {}
app.controller(MyController);

In this case, you are using the .controller helper method to register a new controller. The important point to note is MyController is actually registered into the Application Context (app is a Context).

Server-level context

Server-level context:

  • Is a child of application-level context
  • Holds configuration specific to a particular server instance

Your application will typically contain one or more server instances, each of which will have the application-level context as its parent. This means that any bindings that are defined on the application will also be available to the server(s), unless you replace these bindings on the server instance(s) directly.

For example, @loopback/rest has the RestServer class, which sets up a running HTTP/S server on a port, as well as defining routes on that server for a REST API. To set the port binding for the RestServer, you would bind the RestBindings.PORT key to a number.

We can selectively re-bind this value for certain server instances to change what port they use:

// src/application.ts
async start() {
  // publicApi will use port 443, since it inherits this binding from the app.
  app.bind(RestBindings.PORT).to(443);
  const publicApi = await app.getServer<RestServer>('public');
  const privateApi = await app.getServer<RestServer>('private');
  // privateApi will be bound to 8080 instead.
  privateApi.bind(RestBindings.PORT).to(8080);
  await super.start();
}

Request-level context (request)

Using @loopback/rest as an example, we can create custom sequences that:

  • are dynamically created for each incoming server request
  • extend the application level context to give you access to application-level dependencies during the request/response lifecycle
  • are garbage-collected once the response is sent for memory management

Let’s see this in action:

import {DefaultSequence, RestBindings, RequestContext} from '@loopback/rest';

class MySequence extends DefaultSequence {
  async handle(context: RequestContext) {
    // RequestContext provides request/response properties for convenience
    // and performance, but they are still available in the context too
    const req = await this.ctx.get(RestBindings.Http.REQUEST);
    const res = await this.ctx.get(RestBindings.Http.RESPONSE);
    this.send(res, `hello ${req.query.name}`);
  }
}
  • this.ctx is available to your sequence
  • allows you to craft your response using resources from the app in addition to the resources available to the request in real-time (right when you need it)

Storing and retrieving items from a Context

Items in the Context are indexed via a key and bound to a BoundValue. A BindingKey is simply a string value and is used to look up whatever you store along with the key. For example:

// app level
const app = new Application();
app.bind('hello').to('world'); // BindingKey='hello', BoundValue='world'
console.log(app.getSync<string>('hello')); // => 'world'

In this case, we bind the ‘world’ string BoundValue to the ‘hello’ BindingKey. When we fetch the BoundValue via getSync, we give it the BindingKey and it returns the BoundValue that was initially bound (we can do other fancy things too – ie. instantiate your classes, etc)

The process of registering a BoundValue into the Context is known as binding. Please find more details at Binding.

For a list of the available functions you can use for binding, visit the Context API Docs.

Dependency injection

  • Many configs are adding to the Context during app instantiation/boot time by you/developer.
  • When things are registered, the Context provides a way to use your dependencies during runtime.

How you access these things is via low level helpers like app.getSync or the sequence class that is provided to you as shown in the example in the previous section.

However, when using classes, LoopBack provides a better way to get at stuff in the context via the @inject decorator:

import {inject} from '@loopback/context';
import {Application} from '@loopback/core';

const app = new Application();
app.bind('defaultName').to('John');

export class HelloController {
  constructor(@inject('defaultName') private name: string) {}

  greet(name?: string) {
    return `Hello ${name || this.name}`;
  }
}

Notice we just use the default name as though it were available to the constructor. Context allows LoopBack to give you the necessary information at runtime even if you do not know the value when writing up the Controller. The above will print Hello John at run time.

Please refer to Dependency injection for further details.

Context metadata and sugar decorators

Other interesting decorators can be used to help give LoopBack hints to additional metadata you may want to provide in order to automatically set things up. For example, let’s take the previous example and make it available on the GET /greet route using decorators provided by @loopback/rest:

class HelloController {
  // tell LoopBack you want this controller method
  // to be available at the GET /greet route
  @get('/greet')
  greet(
    // tell LoopBack you want to accept
    // the name parameter as a string from
    // the query string
    @param.query.string('name') name: string,
  ) {
    return `Hello ${name}`;
  }
}

These “sugar” decorators allow you to quickly build up your application without having to code up all the additional logic by simply giving LoopBack hints (in the form of metadata) to your intent.

Context events

The Context emits the following events:

  • bind: Emitted when a new binding is added to the context.
    • binding: the newly added binding object
    • context: Owner context of the binding object
  • unbind: Emitted when an existing binding is removed from the context
    • binding: the newly removed binding object
    • context: Owner context of the binding object
  • error: Emitted when an observer throws an error during the notification process
    • err: the error object thrown

When an existing binding key is replaced with a new one, an unbind event is emitted for the existing binding followed by a bind event for the new binding.

If a context has a parent, binding events from the parent are re-emitted on the context when the binding key does not exist within the current context.

Context observers

Bindings can be added or removed to a context object. With emitted context events, we can add listeners to a context object to be invoked when bindings come and go. There are a few caveats associated with that:

  1. The binding object might not be fully configured when a bind event is emitted.

    For example:

    const ctx = new Context();
    ctx
      .bind('foo')
      .to('foo-value')
      .tag('foo-tag');
    ctx.on('bind', binding => {
      console.log(binding.tagNames); // returns an empty array `[]`
    });
    

    The context object emits a bind event when ctx.bind method is called. It does not control the fluent apis .to('foo-value').tag('foo-tag'), which happens on the newly created binding object. As a result, the bind event listener receives a binding object which only has the binding key populated.

    A workaround is to create the binding first before add it to a context:

    const ctx = new Context();
    const binding = Binding.create('foo')
      .to('foo-value')
      .tag('foo-tag');
    ctx.add(binding);
    ctx.on('bind', binding => {
      console.log(binding.tagMap); // returns `['foo-tag']`
    });
    
  2. It’s hard for event listeners to perform asynchronous operations.

To make it easy to support asynchronous event processing, we introduce ContextObserver and corresponding APIs on Context:

  1. ContextObserverFn type and ContextObserver interface
/**
 * Listen on `bind`, `unbind`, or other events
 * @param eventType Context event type
 * @param binding The binding as event source
 * @param context Context object for the binding event
 */
export type ContextObserverFn = (
  eventType: ContextEventType,
  binding: Readonly<Binding<unknown>>,
  context: Context,
) => ValueOrPromise<void>;

/**
 * Observers of context bind/unbind events
 */
export interface ContextObserver {
  /**
   * An optional filter function to match bindings. If not present, the listener
   * will be notified of all binding events.
   */
  filter?: BindingFilter;

  /**
   * Listen on `bind`, `unbind`, or other events
   * @param eventType Context event type
   * @param binding The binding as event source
   */
  observe: ContextObserverFn;
}

/**
 * Context event observer type - An instance of `ContextObserver` or a function
 */
export type ContextEventObserver = ContextObserver | ContextObserverFn;

If filter is not required, we can simply use ContextObserverFn.

  1. Context APIs
  • subscribe(observer: ContextEventObserver)

    Add a context event observer to the context chain, including its ancestors

  • unsubscribe(observer: ContextEventObserver)

    Remove the context event observer from the context chain

  • close()

    Close the context and release references to other objects in the context chain. Please note a child context registers event listeners with its parent context. As a result, the close method must be called to avoid memory leak if the child context is to be recycled.

To react on context events asynchronously, we need to implement the ContextObserver interface or provide a ContextObserverFn and register it with the context.

For example:

const app = new Context('app');
server = new Context(app, 'server');

const observer: ContextObserver = {
  // Only interested in bindings tagged with `foo`
  filter: binding => binding.tagMap.foo != null,

  observe(event: ContextEventType, binding: Readonly<Binding<unknown>>) {
    if (event === 'bind') {
      console.log('bind: %s', binding.key);
      // ... perform async operation
    } else if (event === 'unbind') {
      console.log('unbind: %s', binding.key);
      // ... perform async operation
    }
  },
};

server.subscribe(observer);
server
  .bind('foo-server')
  .to('foo-value')
  .tag('foo');
app
  .bind('foo-app')
  .to('foo-value')
  .tag('foo');

// The following messages will be printed:
// bind: foo-server
// bind: foo-app

Please note when an observer subscribes to a context, it will be registered with all contexts on the chain. In the example above, the observer is added to both server and app contexts so that it can be notified when bindings are added or removed from any of the context on the chain.

  • Observers are called in the next turn of Promise micro-task queue

  • When there are multiple async observers registered, they are notified in series for an event.

  • When multiple binding events are emitted in the same event loop tick and there are async observers registered, such events are queued and observers are notified by the order of events.

Observer error handling

It’s recommended that ContextEventObserver implementations should not throw errors in their code. Errors thrown by context event observers are reported as follows over the context chain.

  1. Check if the current context object has error listeners, if yes, emit an error event on the context and we’re done. if not, try its parent context by repeating step 1.

  2. If no context object of the chain has error listeners, emit an error event on the current context. As a result, the process exits abnormally. See https://nodejs.org/api/events.html#events_error_events for more details.

Context view

Bindings in a context can come and go. It’s often desirable for an artifact (especially an extension point) to keep track of other artifacts (extensions). For example, the RestServer needs to know routes contributed by controller classes or other handlers. Such routes can be added or removed after the RestServer starts. When a controller is added after the application starts, new routes are bound into the application context. Ideally, the RestServer should be able to pick up these new routes without restarting.

To support the dynamic tracking of such artifacts registered within a context chain, we introduce ContextObserver interface and ContextView class that can be used to watch a list of bindings matching certain criteria depicted by a BindingFilter function and an optional BindingComparator function to sort matched bindings.

import {Context, ContextView} from '@loopback/context';

// Set up a context chain
const appCtx = new Context('app');
const serverCtx = new Context(appCtx, 'server'); // server -> app

// Define a binding filter to select bindings with tag `controller`
const controllerFilter = binding => binding.tagMap.controller != null;

// Watch for bindings with tag `controller`
const view = serverCtx.createView(controllerFilter);

// No controllers yet
await view.values(); // returns []

// Bind Controller1 to server context
serverCtx
  .bind('controllers.Controller1')
  .toClass(Controller1)
  .tag('controller');

// Resolve to an instance of Controller1
await view.values(); // returns [an instance of Controller1];

// Bind Controller2 to app context
appCtx
  .bind('controllers.Controller2')
  .toClass(Controller2)
  .tag('controller');

// Resolve to an instance of Controller1 and an instance of Controller2
await view.values(); // returns [an instance of Controller1, an instance of Controller2];

// Unbind Controller2
appCtx.unbind('controllers.Controller2');

// No more instance of Controller2
await view.values(); // returns [an instance of Controller1];

The key benefit of ContextView is that it caches resolved values until context bindings matching the filter function are added/removed. For most cases, we don’t have to pay the penalty to find/resolve per request.

To fully leverage the live list of extensions, an extension point such as RoutingTable should either keep a pointer to an instance of ContextView corresponding to all routes (extensions) in the context chain and use the values() function to match again the live routes per request or implement itself as a ContextObserver to rebuild the routes upon changes of routes in the context with listen().

If your dependency needs to follow the context for values from bindings matching a filter, use @inject.view for dependency injection.

ContextView events

A ContextView object can emit one of the following events:

  • ‘refresh’: when the view is refreshed as bindings are added/removed
  • ‘resolve’: when the cached values are resolved and updated
  • ‘close’: when the view is closed (stopped observing context events)

Such as events can be used to update other states/cached values other than the values watched by the ContextView object itself. For example:

class MyController {
  private _total: number | undefined = undefined;
  constructor(
    @inject.view(filterByTag('counter'))
    private taggedAsFoo: ContextView<Counter>,
  ) {
    // Invalidate cached `_total` if the view is refreshed
    taggedAsFoo.on('refresh', () => {
      this._total = undefined;
    });
  }

  async total() {
    if (this._total != null) return this._total;
    // Calculate the total of all counters
    const counters = await this.taggedAsFoo.values();
    let result = 0;
    for (const c of counters) {
      result += c.value;
    }
    this._total = result;
    return this._total;
  }
}