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x-middleware-composite

Build Status](https://travis-ci.org/x-component/x-middleware-composite)

• ./composite.js

./composite.js

• global_log

global_log

Concate middleware to function and handle errors

Composition

call composite(...) with a list of middlewares(req,res,next){..} or an array to compose them into a single composite middleware.

you cann add labels into the list by using a object with a single property:

  '''
  var pipeline=composite([,,,{ name: middleware },,,]);
  '''

The order is significant.

Nesting

Multiple composite pipelines can be nested: p.e. composite([,,,{ name : composite([...]) },,]), as each composite is just a normal middleware. It is also allowed to use an array directly: composite([,,,{ name : [...] },,,,]),

Injection

One can inject middlewares in an existing pipeline at the position 'name', before or after the current middleware at that position:

  '''
  pipeline.after(name:extra_middleware);
  '''

The modified pipeline then has a { name: [middleware,extra_middleware] } element.

To allow a more felixble namin / injection on can also access nested composites using before by passing as name in before /after a JSON selector:

Example:

  '''
  var pipeline=composite([,,,{ transform: [,,,{ dom: middlewares },,,] },,,,]);

  pipeline.after('.transform .dom':extra_middleware); // inserts it at the nested position
  '''

Design Rationale:

Often pipelines and request handlers can be reused. One can define generic functions seting up a url request handler with a default pipeline. or just returning a default pipeline. The caller then modifiy the returned pipeline by injection. For example to add some extra handling for a specific application/url without creating a complete copy of the generic pipeline.

Alternatives would have been:

• offer functions which return just building blocks of pipelines and and some function delivering some predefined ones

 positive: no declerative magic involded. Using code explicitly requests building blocks and composes them.
 negative: lots of array composition logic, lots of functions for each eventual possibility.
 This approach is still usable and usefull.

• event hooks

 one could mix middlewares with events, and prove hooks forcertain phases within the pipeline.
 For events how ever the order is conceptionally not defined and it results in a mixing of event concepts and middleware concepts.