Chains

What Is a Chain?

In the context of Drash, a chain is a set of handlers connected together to form a variant of the Chain of Responsibility  pattern. Each handler receives an input and returns an output. Chains are responsible for:

• connecting handlers together;
• receiving inputs from clients;
• sending inputs to handlers; and
• returning outputs from handlers to clients.

In simplest form, their flow looks like:

When you build HTTP applications with Drash you do not assemble a chain yourself — you use the HTTP module’s Application class, which assembles one for you. This page covers what a chain is; that page covers the one you actually build.

What Data Does a Chain Process?

Chains process the data you give them. You are in charge of defining what goes into a chain and what comes out. Handlers pass along the input you give them, not a fixed request type — which is why one chain can take a Web Request on one runtime and a plain context object on another, and both work.

What a particular chain requires of that data is up to the handlers it holds. For the HTTP module’s Application, see the data it requires.

Building a Chain of Your Own

The HTTP module’s Application is the only chain Drash ships that serves a purpose out of the box. To compose a different one, extend AbstractChainBuilder or use BaseChain, which takes whatever handlers you give it in the order you give them.