Undertow Concepts
Internal
Overview
Undertow is a web server written in Java. It provides both blocking and non-blocking APIs based on NIO. It has a composition-based architecture that allows assembling a web server combining small single purpose handlers.
Undertow is embeddable, its lifecycle is controlled by the embedding application and its configuration is controlled programmatically via API calls. Undertow is the default web server in WildFly since version 8, and in this case the configuration is exposed via XML in the server configuration file as:
...
<subsystem xmlns="urn:jboss:domain:undertow:3.0">
...
</subsystem>
...
It has support for Servlet 3.1.
Artifacts
- undertow-core
- undertow-servlet provides support for Servlet 3.1
- undertow-websockets-jsr provides support for the Java API for Websockets (JSR-356)
Container
There in no Undertow container. Undertow applications are assembled from Undertow handler classes and it is up to the embedding application to manage the lifecycle of all Undertow handlers.
Architecture
An Undertow server consists of one or more XNIO worker instances, one or more connectors and a handler chain.
XNIO Concepts
Connector
A connector (also known as a listener) is the Undertow part that handles incoming connections and the underlying wire protocol. The connector is tied to a XNIO Worker. By default, Undertow ships with 3 listeners: HTTP, HTTPS and AJP. If multiple connectors are set to invoke into the same handler chain, the may share a Worker, or they may have separate workers. Usually there will only be a single worker instance that is shared between listeners, however it is possible to create a new worker for each listener.
A connector comprises two XNIO Listeners: an open listener and a read listener:
Open Listener
The open listener is invoked when a connection is first received, and it will do any work necessary to set up the connection. Then, it will pass the connection to the read listener.
Read Listener
The read listener is responsible for parsing the incoming request into a HttpServerExchange instance, setting protocol specific state and passing it to the first handler in the handler chain.
If persistent connections are used, the same read listener is used for subsequent requests came on the same connections.
Listener Configuration
For details on how to configure listeners see Undertow Configuration.
HttpServerExchange
An exchange can be in blocking or non-blocking mode, and it can be put in blocking mode.
Handler
The Undertow functionality is provided by assembling together io.undertow.server.HttpHandler instances, which form a handler chain.
Handler Chain
Handlers are generally chained together by specifying the next handler at construction time. There is no pipeline concept. This means a handler can pick the next handler to invoke into at runtime based on the current request.
Buffer Pool
TODO: http://undertow.io/undertow-docs/undertow-docs-1.3.0/index.html#buffer-pool
Also see NIO ByteBuffer.