Middleware

What is a Middleware

In general terms a "middleware" is that logic executed as a pipe which acts whenever a Request is received and also when a Response is given. In other words, it pre-processes a Request before being processed and also post-processes a response.

Middleware in Astro

In Astro the middleware must be declared as a Module called middleware and must be located in the src folder.
This condition is satisfied by one of both:

1. by having a src/middleware.[js|ts] file
2. by having a src/middleware/index.[js|ts] file

Astro allows middleware adding behaviours on Requests interception and also on Responses interception.
First ones are accomplished by placing that behaviour logic before calling the next() function, and following ones after that call.

Furthermore, those logics can be divided into pieces, which consists in a sequence of MiddlewareHandlers where the request and produced response will be "piped-through".

Middleware in this Project

In our case the middleware resides in the folder middleware/, and is exported with the file middleware/index.ts

If you take a look at the code of this file, you will see a sequence piping three middlewares called routes, auth, and response.

flowchart TB;
    evt(( )) 
    stp((( )))   
    subgraph REQUEST PROCESSING
      direction LR
      subgraph Astro Server
        direction TB
        Request(Request received)
        Response[/Response/]
      end
      evt-->Request
      subgraph Middleware
        direction TB
        Routes{{RoutesMiddleware}}
        Auth[AuthorizationMiddleware]        
        Slzr{{ResponseMiddleware}}
      end
      Request-- event interception ---Routes
      Routes-->Auth
      subgraph Pages/Endpoints
          Processing[[Request processing]]
      end
    end
    Response-->stp
    Slzr-->Response    
    Processing-->Slzr
    Auth-->Processing & Slzr

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Request Interception Middleware

Routes Middleware: Loads context.locals.routes

During the interception of request, first middleware (provided by the routes file loads the routes configured in the private routes.json file into the context.locals.
This load makes those routes being available for the rest of middlewares and pages.

Authentication Middleware: Manages user access to content

Once private routes are available in the property routes at the context.locals global object, the requested route is matched against those private routes.
If user is not logged in, this middleware (provided by the [`routes` file](Middleware-(Code)#auth)) responds directly with a redirection.

Response Processing Middleware

Response Middleware: Adapts the response format to audience

This middleware helps in the serialization of the Response content to the appropiate format infered from the Request Accept header.
There are some endpoints that are called via fetch and its Accept header reports the expectation of a Response agree with it, usually application/json.