middleware.ts

import { Logger } from "./logger";
import { UserAgent } from "./util";

export interface InitializeHandlerArguments<Input extends object> {
  /**
   * User input to a command. Reflects the userland representation of the
   * union of data types the command can effectively handle.
   */
  input: Input;
}

export interface InitializeHandlerOutput<Output extends object> extends DeserializeHandlerOutput<Output> {
  output: Output;
}

export interface SerializeHandlerArguments<Input extends object> extends InitializeHandlerArguments<Input> {
  /**
   * The user input serialized as a request object. The request object is unknown,
   * so you cannot modify it directly. When work with request, you need to guard its
   * type to e.g. HttpRequest with 'instanceof' operand
   *
   * During the build phase of the execution of a middleware stack, a built
   * request may or may not be available.
   */
  request?: unknown;
}

export interface SerializeHandlerOutput<Output extends object> extends InitializeHandlerOutput<Output> {}

export interface BuildHandlerArguments<Input extends object> extends FinalizeHandlerArguments<Input> {}

export interface BuildHandlerOutput<Output extends object> extends InitializeHandlerOutput<Output> {}

export interface FinalizeHandlerArguments<Input extends object> extends SerializeHandlerArguments<Input> {
  /**
   * The user input serialized as a request.
   */
  request: unknown;
}

export interface FinalizeHandlerOutput<Output extends object> extends InitializeHandlerOutput<Output> {}

export interface DeserializeHandlerArguments<Input extends object> extends FinalizeHandlerArguments<Input> {}

export interface DeserializeHandlerOutput<Output extends object> {
  /**
   * The raw response object from runtime is deserialized to structured output object.
   * The response object is unknown so you cannot modify it directly. When work with
   * response, you need to guard its type to e.g. HttpResponse with 'instanceof' operand.
   *
   * During the deserialize phase of the execution of a middleware stack, a deserialized
   * response may or may not be available
   */
  response: unknown;
  output?: Output;
}

export interface InitializeHandler<Input extends object, Output extends object> {
  /**
   * Asynchronously converts an input object into an output object.
   *
   * @param args  An object containing a input to the command as well as any
   *              associated or previously generated execution artifacts.
   */
  (args: InitializeHandlerArguments<Input>): Promise<InitializeHandlerOutput<Output>>;
}

export type Handler<Input extends object, Output extends object> = InitializeHandler<Input, Output>;

export interface SerializeHandler<Input extends object, Output extends object> {
  /**
   * Asynchronously converts an input object into an output object.
   *
   * @param args  An object containing a input to the command as well as any
   *              associated or previously generated execution artifacts.
   */
  (args: SerializeHandlerArguments<Input>): Promise<SerializeHandlerOutput<Output>>;
}

export interface FinalizeHandler<Input extends object, Output extends object> {
  /**
   * Asynchronously converts an input object into an output object.
   *
   * @param args  An object containing a input to the command as well as any
   *              associated or previously generated execution artifacts.
   */
  (args: FinalizeHandlerArguments<Input>): Promise<FinalizeHandlerOutput<Output>>;
}

export interface BuildHandler<Input extends object, Output extends object> {
  (args: BuildHandlerArguments<Input>): Promise<BuildHandlerOutput<Output>>;
}

export interface DeserializeHandler<Input extends object, Output extends object> {
  (args: DeserializeHandlerArguments<Input>): Promise<DeserializeHandlerOutput<Output>>;
}

/**
 * A factory function that creates functions implementing the {Handler}
 * interface.
 */
export interface InitializeMiddleware<Input extends object, Output extends object> {
  /**
   * @param next The handler to invoke after this middleware has operated on
   * the user input and before this middleware operates on the output.
   *
   * @param context Invariant data and functions for use by the handler.
   */
  (next: InitializeHandler<Input, Output>, context: HandlerExecutionContext): InitializeHandler<Input, Output>;
}

/**
 * A factory function that creates functions implementing the {BuildHandler}
 * interface.
 */
export interface SerializeMiddleware<Input extends object, Output extends object> {
  /**
   * @param next The handler to invoke after this middleware has operated on
   * the user input and before this middleware operates on the output.
   *
   * @param context Invariant data and functions for use by the handler.
   */
  (next: SerializeHandler<Input, Output>, context: HandlerExecutionContext): SerializeHandler<Input, Output>;
}

/**
 * A factory function that creates functions implementing the {FinalizeHandler}
 * interface.
 */
export interface FinalizeRequestMiddleware<Input extends object, Output extends object> {
  /**
   * @param next The handler to invoke after this middleware has operated on
   * the user input and before this middleware operates on the output.
   *
   * @param context Invariant data and functions for use by the handler.
   */
  (next: FinalizeHandler<Input, Output>, context: HandlerExecutionContext): FinalizeHandler<Input, Output>;
}

export interface BuildMiddleware<Input extends object, Output extends object> {
  (next: BuildHandler<Input, Output>, context: HandlerExecutionContext): BuildHandler<Input, Output>;
}

export interface DeserializeMiddleware<Input extends object, Output extends object> {
  (next: DeserializeHandler<Input, Output>, context: HandlerExecutionContext): DeserializeHandler<Input, Output>;
}

export type MiddlewareType<Input extends object, Output extends object> =
  | InitializeMiddleware<Input, Output>
  | SerializeMiddleware<Input, Output>
  | BuildMiddleware<Input, Output>
  | FinalizeRequestMiddleware<Input, Output>
  | DeserializeMiddleware<Input, Output>;

/**
 * A factory function that creates the terminal handler atop which a middleware
 * stack sits.
 */
export interface Terminalware {
  <Input extends object, Output extends object>(context: HandlerExecutionContext): DeserializeHandler<Input, Output>;
}

export type Step = "initialize" | "serialize" | "build" | "finalizeRequest" | "deserialize";

export type Priority = "high" | "normal" | "low";

export interface HandlerOptions {
  /**
   * Handlers are ordered using a "step" that describes the stage of command
   * execution at which the handler will be executed. The available steps are:
   *
   * - initialize: The input is being prepared. Examples of typical
   *      initialization tasks include injecting default options computing
   *      derived parameters.
   * - serialize: The input is complete and ready to be serialized. Examples
   *      of typical serialization tasks include input validation and building
   *      an HTTP request from user input.
   * - build: The input has been serialized into an HTTP request, but that
   *      request may require further modification. Any request alterations
   *      will be applied to all retries. Examples of typical build tasks
   *      include injecting HTTP headers that describe a stable aspect of the
   *      request, such as `Content-Length` or a body checksum.
   * - finalizeRequest: The request is being prepared to be sent over the wire. The
   *      request in this stage should already be semantically complete and
   *      should therefore only be altered as match the recipient's
   *      expectations. Examples of typical finalization tasks include request
   *      signing and injecting hop-by-hop headers.
   * - deserialize: The response has arrived, the middleware here will deserialize
   *      the raw response object to structured response
   *
   *      Unlike initialization and build handlers, which are executed once
   *      per operation execution, finalization and deserialize handlers will be
   *      executed foreach HTTP request sent.
   *
   * @default 'initialize'
   */
  step?: Step;

  /**
   * A list of strings to any that identify the general purpose or important
   * characteristics of a given handler.
   */
  tags?: Array<string>;

  /**
   * A unique name to refer to a middleware
   */
  name?: string;

  /**
   * A flag to override the existing middleware with the same name. Without
   * setting it, adding middleware with duplicated name will throw an exception.
   * @internal
   */
  override?: boolean;
}
export interface AbsoluteLocation {
  /**
   * By default middleware will be added to individual step in un-guaranteed order.
   * In the case that
   *
   * @default 'normal'
   */
  priority?: Priority;
}

export type Relation = "before" | "after";

export interface RelativeLocation {
  /**
   * Specify the relation to be before or after a know middleware.
   */
  relation: Relation;

  /**
   * A known middleware name to indicate inserting middleware's location.
   */
  toMiddleware: string;
}

export type RelativeMiddlewareOptions = RelativeLocation & Omit<HandlerOptions, "step">;

export interface InitializeHandlerOptions extends HandlerOptions {
  step?: "initialize";
}

export interface SerializeHandlerOptions extends HandlerOptions {
  step: "serialize";
}

export interface BuildHandlerOptions extends HandlerOptions {
  step: "build";
}

export interface FinalizeRequestHandlerOptions extends HandlerOptions {
  step: "finalizeRequest";
}

export interface DeserializeHandlerOptions extends HandlerOptions {
  step: "deserialize";
}

/**
 * A stack storing middleware. It can be resolved into a handler. It supports 2
 * approaches for adding middleware:
 * 1. Adding middleware to specific step with `add()`. The order of middleware
 *    added into same step is determined by order of adding them. If one middleware
 *    needs to be executed at the front of the step or at the end of step, set
 *    `priority` options to `high` or `low`.
 * 2. Adding middleware to location relative to known middleware with `addRelativeTo()`.
 *    This is useful when given middleware must be executed before or after specific
 *    middleware(`toMiddleware`). You can add a middleware relatively to another
 *    middleware which also added relatively. But eventually, this relative middleware
 *    chain **must** be 'anchored' by a middleware that added using `add()` API
 *    with absolute `step` and `priority`. This mothod will throw if specified
 *    `toMiddleware` is not found.
 */
export interface MiddlewareStack<Input extends object, Output extends object> extends Pluggable<Input, Output> {
  /**
   * Add middleware to the stack to be executed during the "initialize" step,
   * optionally specifying a priority, tags and name
   */
  add(middleware: InitializeMiddleware<Input, Output>, options?: InitializeHandlerOptions & AbsoluteLocation): void;

  /**
   * Add middleware to the stack to be executed during the "serialize" step,
   * optionally specifying a priority, tags and name
   */
  add(middleware: SerializeMiddleware<Input, Output>, options: SerializeHandlerOptions & AbsoluteLocation): void;

  /**
   * Add middleware to the stack to be executed during the "build" step,
   * optionally specifying a priority, tags and name
   */
  add(middleware: BuildMiddleware<Input, Output>, options: BuildHandlerOptions & AbsoluteLocation): void;

  /**
   * Add middleware to the stack to be executed during the "finalizeRequest" step,
   * optionally specifying a priority, tags and name
   */
  add(
    middleware: FinalizeRequestMiddleware<Input, Output>,
    options: FinalizeRequestHandlerOptions & AbsoluteLocation
  ): void;

  /**
   * Add middleware to the stack to be executed during the "deserialize" step,
   * optionally specifying a priority, tags and name
   */
  add(middleware: DeserializeMiddleware<Input, Output>, options: DeserializeHandlerOptions & AbsoluteLocation): void;

  /**
   * Add middleware to a stack position before or after a known middleware，optionally
   * specifying name and tags.
   */
  addRelativeTo(middleware: MiddlewareType<Input, Output>, options: RelativeMiddlewareOptions): void;

  /**
   * Apply a customization function to mutate the middleware stack, often
   * used for customizations that requires mutating multiple middleware.
   */
  use(pluggable: Pluggable<Input, Output>): void;

  /**
   * Create a shallow clone of this stack. Step bindings and handler priorities
   * and tags are preserved in the copy.
   */
  clone(): MiddlewareStack<Input, Output>;

  /**
   * Removes middleware from the stack.
   *
   * If a string is provided, it will be treated as middleware name. If a middleware
   * is inserted with the given name, it will be removed.
   *
   * If a middleware class is provided, all usages thereof will be removed.
   */
  remove(toRemove: MiddlewareType<Input, Output> | string): boolean;

  /**
   * Removes middleware that contains given tag
   *
   * Multiple middleware will potentially be removed
   */
  removeByTag(toRemove: string): boolean;

  /**
   * Create a stack containing the middlewares in this stack as well as the
   * middlewares in the `from` stack. Neither source is modified, and step
   * bindings and handler priorities and tags are preserved in the copy.
   */
  concat<InputType extends Input, OutputType extends Output>(
    from: MiddlewareStack<InputType, OutputType>
  ): MiddlewareStack<InputType, OutputType>;

  /**
   * Builds a single handler function from zero or more middleware classes and
   * a core handler. The core handler is meant to send command objects to AWS
   * services and return promises that will resolve with the operation result
   * or be rejected with an error.
   *
   * When a composed handler is invoked, the arguments will pass through all
   * middleware in a defined order, and the return from the innermost handler
   * will pass through all middleware in the reverse of that order.
   */
  resolve<InputType extends Input, OutputType extends Output>(
    handler: DeserializeHandler<InputType, OutputType>,
    context: HandlerExecutionContext
  ): InitializeHandler<InputType, OutputType>;
}

/**
 * Data and helper objects that are not expected to change from one execution of
 * a composed handler to another.
 */
export interface HandlerExecutionContext {
  /**
   * A logger that may be invoked by any handler during execution of an
   * operation.
   */
  logger?: Logger;

  /**
   * Additional user agent that inferred by middleware. It can be used to save
   * the internal user agent sections without overriding the `customUserAgent`
   * config in clients.
   */
  userAgent?: UserAgent;

  [key: string]: any;
}

export interface Pluggable<Input extends object, Output extends object> {
  /**
   * A function that mutate the passed in middleware stack. Functions implementing
   * this interface can add, remove, modify existing middleware stack from clients
   * or commands
   */
  applyToStack: (stack: MiddlewareStack<Input, Output>) => void;
}