typed-request-stack
v2.0.0
Published
Middleware stack runner for typed HTTP requests
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typed-request-stack
Middleware stack runner for typed HTTP requests
Example
var stack = require("typed-request-stack");
var middleware = require('./my-service-middleware');
var requestValidation = require('./request-validation');
var responseValidtion = require('./response-validation');
// -> Exports function (typedRequest, opts, handle)
module.exports = stack([
// User-implemented role-based security middleware,
middleware.secure(['Admin']),
// Validation for this endpoint
middleware.validate(requestValidation, responseValidation)
], dummyEndpoint)
// The main body of the endpoint implemetation
function dummyEndpoint(typedRequest, opts, callback) {
callback(null, {
statusCode: 200,
body: "Hello world"
});
}
Docs
var endpoint = stack([/*middleware*/], endpointHandler)
typed-request-stack := (
stack: Array<TypedHandler>,
endpoint?: TypedRequestHandler
) => (
typedRequest: TypedRequest,
opts: Object,
callback?: (err?: error, value: Any) => void
) => void
TypedRequestStack
allows you to to compose a collection of "middleware"
functions and apply them in order. The stack is descended from the first
handler for the request, and then in reverse order for the response.
Start End
| ^
V |
+----------------------+ +----------------------+
A - | Handle request | +---> | Handle response |
+----------------------+ | +----------------------+
| | ^
V | |
middleware A calls handle.request | middleware B calls handle.reponse
| | |
v | |
+----------------------+ | +----------------------+
B - | Handle request | ?--+ | Handle response |
+----------------------+ +----------------------+
| middleware B could abort ^
| early (if there was an |
| error for example) by |
| calling handle.response |
| inside handle request |
| |
+------+ +------+ Endpoint calls
| | callback
V |
+----------------------+
| Endpoint |
+----------------------+
The callback passed into the function returned by typed-request-stack
will
receive the last result (err, value)
in the response phase.
When a handleResponse function aborts the request by calling handle.response, the parent response handler is first to receive the value. This behaves as if the parent response function is a callback passed into the request handler.
Defining middleware TypedHandler
type TypedHandler : {
handleRequest: TypedRequestHandler,
handleResponse: TypedResponseHandler
}
Middleware in the stack should implement handleRequest
or handleResponse
.
There is no obligation to implement both, but one of these functions must be
implemented. If the next middleware does not implement a handler for the
request or response phase, it will simply be skipped, and the next handler
used.
Example: Implementing middleware
'use strict';
module.exports = Logger;
function Logger(logger) {
if (!(this instanceof Logger)) {
return new Logger(logger);
}
// We can configure our middleware in the constructor
this.logger = logger || console.log.bind(console);
}
Logger.prototype.handleRequest = handleRequest;
Logger.prototpye.handleResponse = handleResponse;
function handleRequest(typedRequest, opts, handle) {
this.logger.log(typedRequest);
// Handle the next request in the middleware stack
handle.request(opts);
}
function handleResponse(err, value, handle) {
if (err) {
this.logger.error(err);
} else {
this.logger.log(value);
}
// Continue back up the response chain
handle.response(err, value);
}
function typedRequestHandler(typedRequest, opts, handle)
type TypedRequestHandler : (
typedRequest: TypedRequest,
opts: Object,
handle: Handle
) => void
type Handle : {
request: (opts: Object) => void,
response: (err?: Error, value: Any) => void,
sharedState?: Any
}
When calling handle.request(opts)
- the opts passed in here is passed on to
the next request handler (below the current) in the stack.
If a typed request handler wishes to abort and start returning a value through
the response phase, it can do so by calling handle.reponse(opts)
A typed request handler must call either handle.request
or handle.reponse
at
some point. Precisely one of these functions must be called exactly one time.
function typedResponseHandler(err, value, handle)
type TypedResponseHandler : (
err?: Error,
value: Any,
handle: Handle
)
When calling handle.reponse(err)
or handle.response(null, value)
, the
next response handler (above the current) in the stack will receive these
values.
Eventually, after fully ascending the stack, the final error or value will be passed into a callback function supplied by the stack caller.
A TypedResponseHandler
must call handle.response
exactly once, and must
never call handle.request
.
handle.sharedState
When request and response handlers are paired together for a single unit of
middleware, they often wish to share some state per request. It's important
to stress that the properties on the middleware instance itself are global to
all requests, and so you must not use this
to store per-request state.
handle.sharedState
provides a mechanism for sharing state for this middleware
between the request and response handler. handle.sharedState
may be set to
anything inside handleRequest
, and it will be available on the handle
instance inside handleResponse
.
Example: computing and sharing state for each request
function RequestTimer(logger) {
this.logger = logger || console.log.bind(console);
}
RequestTimer.prototype.handleRequest = handleTimedRequest;
RequestTimer.prototype.handleResponse = handleTimedResponse;
function handleTimedRequest(typedRequest, opts, handle) {
// Write the shared state by setting handle.sharedState
handle.sharedState = {
startTime: Date.now()
};
handle.request(opts);
}
function handleTimedResponse(err, value, handle) {
// Read the shared state from `handle`, set in handleTimedRequest
var sharedState = handle.sharedState;
var requestTime = Date.now() - sharedState.startTime;
this.logger.log('Request took ' + requestTime + 'ms');
handle.response(err, value);
}
Motivation
The implementation of an HTTP endpoint should be
- Debuggable
- Efficient
- Modular
- Safe
Debuggable
typed-request-stack
aids with debugging by unwraping the closures that would
otherwise be relied on to implement middleware stacks. This allows us to
inspect, or even modify the stack of handlers that will run for a given
endpoint.
Inspecting handle._stack
will allow you to see which middleware has been
applied to the stack, and this can be inspected at any stage.
Furthermore, in the case of a core/heap dump, the configuration of your server's
endpoints becomes easier to debug. You can look for the instances of your
endpoints on the heap and inspect them. You can look for instances of
TypedRequestHandle
to see all of the in-flight requests at the time of a
crash, and know exactly which endpoint stack the request was executing through.
You can also rely on the handle._handlerIndex
to derive which specific
handler was executing at that point in time, and handle._typedRequest
to
inspect the incoming request.
Efficient
When relying on closure based stacks, you will often be creating closures at runtime for each request you serve.
This middleware stack approach completely removes the need to rely on closures, which means no more on-the-fly function generation. Using constructors also provides minor V8 efficiencies through the use of hidden classes.
Modular
The middleware pattern allows us to create shared units of funtionality and apply them easily. By implementing these shared units with the same consistent interface, we can easily combine them together in a stack.
This further promotes indepedent testing of these units with full coverage, reducing likely copy/paste errors and errors caused by not understanding a new interface.
Safe
One of the biggest problems with chaining modules together is understanding when a module will call the callback. There is always the question of whether the module calls the callback more than once, what the impact would be and how we would debug it if it did happen.
typed-request-stack
ensures that the callback is called only once and that
middleware act in-order. Out-of-order middleware could easily corrupt state or
behave in ways that are hard to reason about. The handle
passed into each
function is wrapped with a SafeHandle
type to ensure the correct calling
conventions are met.
Installation
npm install typed-request-stack
Tests
npm test
Contributors
- Matt Esch