Http

friend of the web since ‘91

Authors

By Jeff Cross with much input from Caitlin Potter, Ben Lesh, Igor Minar, and other Angular team members. All are responsible for bad ideas, Jeff is responsible for errors.

Overview

Apps need a way to retrieve and change and assets from other places. Http is one of many reasonable ways to accomplish this, and the most widely-available option for data access in the wild world wide web. This doc is concerned with the Http collection of modules that will be part of Angular2, heretofore referred to as Http.

The primary use cases for this library are:

Transactional data access, as opposed to continuous access provided by other means like WebSockets and Server-Sent Events
Facilitating fetching and updating of remote application models
Loading media into an application
Uploading files to servers

Design Goals

Simplicity

The tools in the Http toolbelt should be easy to comprehend from the start, easy to comprehend when implementing advanced use cases, and easy to comprehend when reviewing someone else’s code.

Platform Flexibility

Http should work as well in node.js as it works in a browser. In fact, it should be easy to make work in any JavaScript environment that’s capable of http communication, with minimal work. It should also be possible to use multiple underlying mechanisms in a single platform, like using JSONP instead of XHR. And on a server, if an Angular2 app is being pre-rendered before being served, perhaps the underlying Http mechanism would actually be connecting to the local filesystem, database, or cache, to allow the app to re-use code without making unnecessary network calls.

Testability

It should be easy to mock responses, verify requests made in order (and expressed in logical order in tests), and execute tests synchronously.

Performance

Inasmuch as is possible without introducing unreasonable magic or surprise to users, the Http lib should be performance-conscious. Most of this will be accomplished by providing guidance or APIs to tune performance, such as request cancellation, caching of resources, etc.

Security

Since the library will target modern browsers, which have implemented many safeguards to prevent XSRF and JSON vulnerabilities, Http will focus on exposing primitives to allow developers to add security measures required for their application. The documentation for the library should make note of what precautions users of the library should keep in mind to secure their applications.

Feels Like Angular

Idioms and semantics should feel natural for users who are familiar with other Angular 2 APIs. Users should expect the same design considerations and thoughtful implementation that characterizes the rest of the framework. This may be the most appealing design goal, compared to other libraries.

Love It or Leave It

Http is focused on accomplishing the established goals, and may not be the right fit for all applications. So the rest of the core Angular2 framework should not give special treatment to Http, so that developers can easily use the http library of their choice. Perhaps other parts of a broader Angular2 data framework would be built with this Http implementation in mind, but it should be possible to bridge other http libraries.

Design Anti-Goals

Only Understand Response Types

Http will not concern itself with the structure of data being sent or returned; it will only care about the expected response type, as specified by the user-provided configuration, and when necessary, confirmed by the response content-type header.

Low Magic, Limited Scope

It should always be clear what the Http library is doing from looking at calls in code, without worrying about how it has been configured somewhere else. Http should do as few things as possible, and should not try to be too smart. Features like connection-retries should be made easy for developers to manage, but should not be an automatic, opt-out feature of the library.

Don’t Reinvent Wheels

Modern browsers have improved security features, increasingly improving caching ability, and baked- in http caching (presuming servers send the correct headers). Http should defer as much work as possible to browsers, and should provide logical hooks for developers to manage caching and headers themselves.

Composable, Flat API Surface

Http may be accompanied by other libraries that provide features like caching, data serialization, transformation, and validation. These tools should be used by developers in conjunction with the Http library, and the Http library should not know or care how they’re being used. In other words, Http can’t be configured to automatically leverage these tools as part of a request/response lifecycle, though developers may define their own wrapper functions to make certain processes seamless. Consequently, it’s up to developers to first fetch from a proprietary cache before requesting from Http, if not relying on implicit browser cache mechanisms.

Proposed Design

The primary building block of the Http implementation is a Connection object, which is created by calling the http factory function, or another factory of the application's choosing. A Connection instance represents a single request and response, and implements the Observable interface.

Here is a simple example of loading a repository from Github's API:

import {Http} from 'http';

class MyComponent {
  @Inject(Http);
  constructor(http:Http) {
    http('https://api.github.com/repos/angular/angular.js')
      .subscribe(function(res) {
        console.log(res.body);
      });
  }
}

The Http library will include the following Connection factories, and will provide an interface to ensure third-party Connection implementations conform to the expected API.

Http
HttpJsonp
HttpNode
HttpMock
HttpFetch TBD

Connection

Connections are constructed with a single argument, either a url string, or a ConnectionConfig object.

Using a callable function that accepts a single argument instead of instantiating connections via "new" keyword enables creating connections in part of a functional chain without having to wrap the instantiation in a closure. The requirement of using a factory function is only imposed on the officially-supported Connection factories. Other conforming implementations may instantiate Connections in whatever way suits the Connection, but the returned value should conform to the Observable interface.

`ConnectionConfig`

Connections implement the Observable interface, with the returned observable representing the response. The ConnectionConfig can pass in optional observers to observe connection properties "state", and "uploadProgress". The "progress" event of requests can be used to trigger calls to onNext of the "downloadProgress" Observer.

import {Http} from 'http';

class MyComponent {
  @Inject(Http);
  constructor(http:Http) {
    http({
      url: 'https://'
    })
  }
}

Since subscribing to the Connection Observable returns a disposable object, if all subscriptions to the observable are disposed prior to the underlying connection completing, the connection should attempt to abort the connection, only for the sake of releasing the resources necessary to fulfill the request. Aborting should not be relied upon to prevent an RPC from being called. The ConnectionConfig can specify that a connection should persist even after subscriptions have been disposed by setting the persistBeyondDisposal property to true.

`BaseConnectionConfig`

The officially-supported connections will ship with an immutable BaseConnectionConfig object, which can be used as a basis to create new ConnectionConfig objects, such as a shareable, application-specific config. The BaseConnectionConfig will be immutable to prevent accidental global state, such as one component in an application making changes to defaults, without another component knowing about it. If an application needs a common default configuration different from the provided base, it is encouraged as a best practice to create an injectable class derived from the base config that can be shared across the app, and further copied for with Connection-instance- specific properties.

The BaseConnectionConfig could also be replaced with a custom implementation in the Injector during application bootstrap by creating a new binding.

import {bootstrap} from 'angular2/angular2';
import {bind} from 'angular2/di';
import {MyAppConfig, MyAppComponent} from 'myapp';
import {BaseConnectionConfig} from 'http';

bootstrap(MyAppComponent, [
  bind(BaseConnectionConfig).toValue(MyAppConfig);
]);

Passed-in ConnectionConfig objects should be merged with the BaseConnectionConfig when creating the Connection. Values from the BaseConnectionConfig object should replace undefined values in the provided ConnectionConfig object. A Connection should throw an exception if the provided config does not contain necessary values to execute a request.

Annotations

Types

`Connection` Interface

`ConnectionConfig` Interface

`BaseConnectionConfig` Class

`Response` Interface

`Observable` Interface

This will be a minimal subset of the RxJS [Observable interface](https://github.com/bor isyankov/DefinitelyTyped/blob/8fea426543e19e19db32eb827a02d11bdab30383/rx/rx-lite.d.ts#L217). Connections should all be able to upgrade to a compatible Observable interface if provided via injection into the Connection class. This would be accomplished by binding the compatible Observable class to the Observable token provided by the Http library prior to the Connection being instantiated, such as at the App Component level or in the config passed to bootstrap.

import {bind} from 'angular2/di';
import {bootstrap} from 'angular2/angular2';

boostrap(AppComponent, [
  bind(Observable).toValue(Rx.Observable)
]);

IDisposable Interface

found here

Testing with MockConnection

describe(‘MyComponent’, function() {
  beforeEach(function() {

  });
});

Performance Considerations

Connections in separate modules, load what you need
Abortable requests
Optionally cold connections

Accessibility Considerations

End User Experience Considerations

More insight into connection state to provide realtime status to users
Better support of streaming data
Better resilience in network flakes via retry()

Mobile Considerations

Caching

The cache requirements are different for Http in Angular 2 vs $http in Angular 1, since there is a separate fetching mechanism for template loading. In Angular 1, every request for a component’s template would make an http request that would try to grab the template from the $templateCache service before making a network request. And even though Angular 2 supports fetching of modules, which AngularJS 1 did not, these fetches will rely on ES6 System.import (or traceur equivalent), not the Http library.

So Http will not be depended on by the core framework, and only needs to meet the needs of end-users (developers), and other data access libraries that will be part of angular 2.

JS/Dart-Specific Features

Extensibility (Connection-swapping API, first-class JSONP, XHR, MockConnection) (Could use

annotations? @Connection(JSONP))

Testability (Connection mocking)

In-band response (no "respondWhen" declared before tests)
Progressive response support

Security

Request/Response Transformation and Serialization

Use cases:

Append a session token to url for all requests to a certain host
Flatten a response from a single request, ie response "{data: []}" -> "[]"

(If using observable idioms, old way of an array of functions with same signature will not work) (since a function could not be certain the structure after other functions have transformed it) (Should requestTransforms and responseTransforms be part of the request config?)

Authentication/Authorization

Browser Support

Recipes

Request / Response Transformations

Serialization

Caching Responses and Reading from Cache

Connection Retry / Backoff

(should be dynamic to not retry for specific auth codes, like 4xx)

Synchronize to Local Database

Transforming Requests

// Strip all headers if connection config contains arbitrary property // Adding an XSRF token to requests // Changing URLs

Fetching in ServiceWorker

Authentication

Integration with Router Pipeline

Integration with Templates

Integration with Change Detection

Short-polling

Long-polling

//Use Observable.retryWhen?

Making Requests within Observable Chain

Bridging Request to ES6 Promise

Incorporating into a Promise Chain

Uploading a File

Composing with EventSource

Pre-Fetching Data

with Annotation+Router Single Callback and Continuous Callback

jeffbcross/http-design

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