The sample code demonstrates a Dedicated Web Worker. As the
name would indicate, a Dedicated Web Worker is targeted for applications
requiring point to point communication. The specification offers an alternative
called Shared Web Worker for communication with multiple producers and
consumers. A Shared Worker exposes more of the Messaging API components, but the
underlying infrastructure is the same as a Dedicated Web Worker.
As stated earlier WorkerGlobalScope provides a sort of
context for the Worker. Much of the infrastructure a developer leverages in a window/document
has been carried to a Web Worker. So, for example, a WorkerGlobalScope includes
Timer implementations. Each Web Worker implementation extends the
WorkerGlobalScope to suit its needs.
Web Workers leverage a lot of the existing Eventing
infrastructure. Web Workers also leverage a new Web Messaging Specification.
Web Messaging Infrastructure
Web Messaging more securely enables cross-document
communication. Enabling Cross-site scripting opens a security hole in a browser.
For security reasons cross-site scripting is disabled. Cross-document
communication is important to building Web Applications, so Web Messaging has
been architected for security as well as communication capability.
Web Messaging protocols pass around a MessageEvent object. In
the example, "data" is the attribute containing the message payload; "data"
is a string in the example, but can be any type.
Web Workers leverage the Web Messaging Channel Messaging
infrastructure. A MessageChannel connects two MessagePorts. The specification
refers to the setup as "entangling" the ports. A call to postMessage
on a MessagePort puts data across the channel. Each MessagePort maintains a
message queue. Messages posted on one port on the MessageChannel are set to the
other port on the MessageChannel and visa-versa. MessagePorts receive a message
via an "onmessage" function.
Web Workers extend the Web Messaging infrastructure
supporting posting to an Array of MessagePorts. MessagePort Arrays are handy
for multiple notifications.
background workers. Web Workers leverage the upcoming HTML5 Web Messaging API.