If you’re implementing a proxy using Undertow and its’ handlers mechanism and
you want to wrap your proxy call with a Hystrix command here’s a way how to do it.
The Hystrix Command
As the Undertow handlers are called asynchronously we’re implementing a
We create the command ① with some mandatory arguments:
the handler is the next HttpHandler in our handler chain
the exchange is the current Exchange processed by the server
commandConfiguration - optionally you can pass some configuration parameters
to properly configure the command itself. As shown in the example you can
do this by calling the parent constructor with appropriate Setter object.
In the example we use Semaphore as the isolation strategy. ② You can also use
threads but be aware that this creates another thread pool for you to manage and
might add additional overhead because of thread switching. Use this option with care.
For the command itself we need to override the construct() method which is
responsible for constructing the Observable for our command. The implementation does
adds an ExchangeComplete listener to our exchange - ③
and dispatches the request to the next handler in the chain on the current thread ⑦
The Listener is responsible for telling the subscriber whether our exchange was
finished as a success or an error - thus, potentially triggering Hystrix’s fallback and/or
circuit-breaker mechanisms. Without this the command would treat all responses as successes.
This is achieved by properly calling subscriber.onCompleted() (⑤) or subscriber.onError() (④).
It is important to make sure nextListener.proceed() ⑥ if we want to have other ExchangeComplete
listeners to do their work.
The handler which utilises the above command could like like this:
Calling command.observe() eagerly starts the execution of the command.
If you’d rather subscribe to the underlying Observable you can use something like this:
And that’s it. The chain of handlers that is added after your HystrixHandler
will be guarded by Hystrix.