ee-soa-transport-rewrite

#ee-soa-transport-rewrite Middleware to modify requests sent to a service. This module is under heavy development.

##Rewrites The module provides a basic set of rewrites. Rewrites are executable objects which modify a request based on a rule, to use the internal rewrites, the rules must at least have the following form. The middleware matches the domain and the path against the current http request to choose rules (see matching).

var rule = { name: ..., path: ..., field: ..., value: ..., domain: ... }

The name of the rule determines the corresponding rewrite class:

• append appends the specified value to the header specified in field.
• ensure checks if the header specified in field is present. If not it is set to field.
• override overrides the header field with value.
• path modifies the requested pathname path to value (use to map to api endpoints).
• template sets a template variable, basically request.template = rule.value
• parameter allows setting arbitrary values to an parameters hashtable called rewriteParameters ( request.rewriteParameters[field] = value)

Further planned but not implemented or tested yet are:

• extend extends an existing ruleset

Consider the following example:

var rule    = { domain: 'test.com', path: '/detail', name: 'ensure', field: 'select', value: '*' }
// is transformed to
var rewrite = new Ensure(rule, ...);
// and executed on the request
rewrite.execute(request, function(err){});

If the value field is of type 'function', it gets executed every time the rewrite rule is applied (e.g. a timing function)

Rewrites can be combined to a chain which then is executed sequentially (for development).

rewrite.then(new Ensure(...)).execute(request, function(err){});

###Matching The matching of the path of the rules is type based:

- if the path is `null` or '*' it is applied to all requests (which match the domain)
- if the path is of type string it is exactly matched e.g. `key == path`
- if the path is of type RegExp, a regexp matching is performed

##Loaders The rewrite module uses loaders to load rules from different sources. All loaders can be nested to combine loading/caching and transformation.

var loader =
{
    load: function(key, callback){
        var ruleset = //load your ruleset
        callback(err, ruleset);
    }
};

###FilterLoader A Loader that takes another loader and filters its results using the passed comparator. The comparator can be either a string which denotes the name of the rule property which is relevant for matching or a function. The later takes the key passed to the loader and a rule object to match to. The comparator has to adhere to the following contract:

1. Return true if the key can be matched with the rule
2. Return false if the key can not be matched with the rule
3. Throw an error if the rule has an unexpected form

If there is no comparator passed, the filter loader creates one itself: By default, the property which is taken into account is named key (rules are compared by their key property).

###TransformingLoader Takes a transformer (see transformers). The TransformingLoader passes the rule set returned by the loader to the transformer, which can transform the passed rules in any desired way before handing it back to the callback.

###InMemoryLoader A loader used to load rules from memory i.e. collections of rules. This loader is mainly for testing. The InMemoryLoader itself is a FilterLoader which reduces the full rule set to the rules which match a comparator (which can be passed or is created internally, see FilterLoader).

var rules  = [
    {domain: 'somewhere.com' ... }
    {domain: 'somewhere-else.com' ...}
    ...
];
var loader = new loaders.InMemoryLoader(rules, 'domain');
loader.load('somewhere.com', function(err, ruleset){ });

###CachedLoader A loader which takes a cache and another loader. All results returned by the inserted loader are written to the cache and taken from there if accessed again. This allows caching of slow rule sources such as a database, the network or the disk.

###DatabaseLoader Is currently in development, it probably does not make sense to have a default implementation.

###RewriteLoader Is a transforming loader which creates rewrite.Rewrite instances (or subclasses) from the rulesets to get executable rewrites and can be seen as a factory (internally uses the transformer.FactoryTransformer.

##Transformers Transformers are classes/objects to transform the loaded rulesets of a transformer that simply filters the rules based on the key value passed to the loader.

var transformer = {
    transform: function(key, resultset, callback){
        callback(null, resultset.filter(function(current){
            return current.key == key;
        }));
    }
}

var loader = new TransformingLoader(transformer, {load: function(err, cb){ cb(null, rules}; }};

##Caches Caches used with the cached loader must adhere to a simple interface:

var cache = {
    has:    function(key){}
    , get:  function(key){}
    , set:  function(key, value){}
}

#Changelog ##v0.1.2 - added Option rewrite rule - values which are of type function are evaluated now

Different caches are always injected into the loaders which makes them inherently testable.

• Todo: Add prepend rules
• Todo: Add templating rules... probably an extends rule...
• Todo: The rule chaining happens "in place" which means the initial rewrite is modified when chained. This should be changed.
• Todo: Add caching