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https://issues.apache.org/jira/browse/HTTPCORE-773?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17901976#comment-17901976
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Anithamahalakshmi commented on HTTPCORE-773:
--------------------------------------------
[~olegk] Thank you for the response. I agree that expensive operations should
not be performed on IO dispatcher threads. This suggestion is to prevent a
catastrophic effect in the unlikely situation when IO dispatcher thread gets
blocked on something due to a bug / design issue. Could you help me understand
why we follow round robin pattern in assigning requests to IO dispatcher
threads instead of letting the IO dispatcher poll for jobs as they become
available from the main queue of requests?
> Safeguard against blocked IOWorker thread
> -----------------------------------------
>
> Key: HTTPCORE-773
> URL: https://issues.apache.org/jira/browse/HTTPCORE-773
> Project: HttpComponents HttpCore
> Issue Type: Improvement
> Components: HttpCore NIO
> Reporter: Anithamahalakshmi
> Priority: Minor
>
> # During the initialisation of ApacheAsyncClient, a single main thread is
> created -
> [https://github.com/xSke/CoreServer/blob/master/src/org/apache/http/impl/nio/client/CloseableHttpAsyncClientBase.java#L40.]
> # This IO reactor thread does two key jobs: 1) It internally instantiate IO
> dispatcher threads once They never stop either. They keep polling for new IO
> event) and 2) It then works as a daemon (which never stops) to process new
> events. It worth to notice that all events IO reactor thread is connection
> related event per
> https://github.com/wso2/wso2-httpcore-nio/blob/master/modules/httpcore-nio/src/main/java/org/apache/http/impl/nio/reactor/DefaultConnectingIOReactor.java#L164
> # For http request which needs a new IO connection, IO reactor thread is
> invoked to create such IO connection asynchronously -
> [https://github.com/wso2/wso2-httpcore-nio/blob/master/modules/httpcore-nio/src/main/java/org/apache/http/nio/pool/AbstractNIOConnPool.java#L427]
> Connection pending to be created in such asynchronous process is reported as
> {{pending}} connection in Apache connection status.
> # In such asynchronous process, connection to be created is added into
> {{requestQueue}} -
> [https://github.com/wso2/wso2-httpcore-nio/blob/master/modules/httpcore-nio/src/main/java/org/apache/http/impl/nio/reactor/DefaultConnectingIOReactor.java#L251].
> {{requestQueue}} is later on polled by IO reactor thread as a daemon, and
> established channel should be handed off to IO dispatcher thread via
> following logic
> -[https://github.com/wso2/wso2-httpcore-nio/blob/master/modules/httpcore-nio/src/main/java/org/apache/http/impl/nio/reactor/DefaultConnectingIOReactor.java#L276].
> It is interesting to notice that which IO dispatcher thread such new channel
> should be handed off to is based on round-robin per
> [https://github.com/wso2/wso2-httpcore-nio/blob/master/modules/httpcore-nio/src/main/java/org/apache/http/impl/nio/reactor/AbstractMultiworkerIOReactor.java#L475.]
>
> # The IO dispatcher thread is expected to pull and process new IO event
> assigned to it by selector, and then process assigned new channel as well per
> [https://github.com/wso2/wso2-httpcore-nio/blob/master/modules/httpcore-nio/src/main/java/org/apache/http/impl/nio/reactor/AbstractIOReactor.java#L222]
> However if a certain IO dispatcher thread is blocked while some previous
> processing, it goes undetected and new requests+connection pair keep getting
> assigned to the blocked thread which is unable to process the request and
> release the connection back to the pool, eventually we exhaust the max
> connection possible from the pool and cause availability drop.
>
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