# Chain Handlers

# Chaining handlers together with the ChainHandler

This project ships with a handler called the ChainHandler. It's purpose is to make it easier to wrap the functionality of one handler with additional functionality. For example using the relational data store may be ideal for storing resources, but what if we want to add an authentication layer on top of those resources? An authentication handler shouldn't know or care about the storage of the underlying resource, it should only deal with authentication - we need an abstraction layer to help us glue all of this together.

To get started with the ChainHandler, go ahead and instantiate one:

var jagql = require("@jagql/framework");
var testChainHandler = new jagql.ChainHandler();

From there it's a simple case of attaching new function handlers to testChainHandler, following the naming convention of ("before"|"after")+function - eg beforeSearch, beforeCreate, afterDelete.

These new functions need to match the signatures of the functions they're overriding and need to pass the data onwards via a callback. For example, here are two valid functions that don't add any functionality. Notice how the callback values match the function signatures:

testChainHandler.beforeSearch = function(request, callback) {
  return callback(null, request);
};

testChainHandler.afterSearch = function(request, results, pagination, callback) {
  return callback(null, results, pagination);
};

NOTE The only exception to the above is that all after function signatures are also passed the original request object in addition to the other objects. This enables us to implement clever features on a per-request basis by providing a mechanism to push state between before and after functions.

With a new ChainHandler ready to go, this is how we can stack them up:

jagql.define({
  resource: "articles",
  handlers: chainHandler1.chain(chainHandler2).chain(memoryHandler),
});

The call stack when chaining multiple ChainHandlers together looks like this:

chainHandler1.beforeSearch
  chainHandler2.beforeSearch
    MemoryHandler.search
  chainHandler2.afterSearch
chainHandler1.afterSearch