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Timeout types

In Octoparts, there are 2 types of timeouts:

  1. HystrixCommand timeout, which allows Octoparts to fail fast and shed load in an isolated manner for each registered endpoint.
  2. Request timeout, which allows Octoparts (and subsequently the service that invokes Octoparts) to guarantee responsiveness.

Hystrix timeout

Octoparts and Hystrix

Octoparts wraps all requests to registered backend endpoints in a HystrixCommand object, which is a construct used to run a block of (usually blocking) code and return the result asynchronously in a resilient and responsive fashion (to learn more about Hystrix and all the benefits of using it, read the Hystrix wiki).

Part of the constructor arguments of a HystrixCommand is a timeout value in milliseconds, which defines how long the block of code inside the command has to finish processing.

HystrixCommand processing

Once a HystrixCommand is instantiated with our backend endpoint call as the block of code to be run, we invoke the command, getting back an asynchronousFuture result.

However, in the Hystrix world, the command is executed synchronously (using a thread from another threadpool). If the command takes longer than the timeout duration to finish, Hystrix will cause the Future return value to be a TimeoutException and returns the thread back to the threadpool.

Request timeout

Octoparts allows you to specify, on a per AggregateRequest level, the maximum duration you want the server to “wait” before replying with an AggregateResponse. This is done by specifying a value in the optional timeoutMs field in the requestMeta field of an AggregateResponse. However, by default, there is 5 second value specified for maximumAggReqTimeout, and the lower value between what is specified in timeoutMs and maximumAggReqTimeout is used as the timeout duration for a given request.

During the processing of an AggregateRequest, each PartRequest is processed asynchronously in parallel (as specified in Internals), and those that don’t complete within the aforementioned timeout duration are completed using a TimedoutException (those that finish processing within the duration are left alone).

Those familiar with Scala may want to checkout our RichFutureWithTimeout enrichment to see how this is done without blocking our request thread.

Motivation for having 2 types of timeouts

Asides from the two types serving 2 different functional requirements, the separation our timeouts allow us to do something cool: even if a request timeout has occured for a given PartRequest, so long as the corresponding HystrixCommand hasn’t timed out, we can cache the eventual result of the HystrixCommand so that the next time the same PartRequest is processed by Octoparts, we have a chance of getting a cache hit, which allows us to respond immediately. For more details, see Caching

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