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From "Doug Cutting (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-255) Client Calls are not cancelled after a call timeout
Date Fri, 26 May 2006 15:55:30 GMT
    [ http://issues.apache.org/jira/browse/HADOOP-255?page=comments#action_12413490 ] 

Doug Cutting commented on HADOOP-255:

I think this is, in general, something that we won't fix.  It might be possible to improve
things, but we cannot, without elaborate handshake protocols, guarantee that RPC responses
are received by clients.  As Owen has indicated, we must instead make our applications tolerant
of that.

Note that this is generally a problem for HTTP-based services too.  When someone hits "stop"
in their browser for a slow request, the server generally continues to compute the request
and only discovers that the connection has been closed when it attempts to write the response,
if at all.  If the client times out after the server has flushed the response, then there's
no way for the server to know this.

You sugguest that we might queue requests on the client so that only a single request to a
particular server is outstanding at a time.  That would not work well for distributed search
(Nutch's original IPC application).  In distributed search a front end typically has many
queries outstanding.  Each query is broadcast to a number of back end servers.  Different
queries take different amounts of time.  We do not want to make fast queries wait for slow
queries, as that would make all queries slow and increase the burden on the front end servers.

Would folks object if I resolve this as a WONTFIX bug?

> Client Calls are not cancelled after a call timeout
> ---------------------------------------------------
>          Key: HADOOP-255
>          URL: http://issues.apache.org/jira/browse/HADOOP-255
>      Project: Hadoop
>         Type: Bug

>   Components: ipc
>     Versions: 0.2.1
>  Environment: Tested on Linux 2.6
>     Reporter: Naveen Nalam

> In ipc/Client.java, if a call times out, a SocketTimeoutException is thrown but the Call
object still exists on the queue.
> What I found was that when transferring very large amounts of data, it's common for queued
up calls to timeout. Yet even though the caller has is no longer waiting, the request is still
serviced on the server and the data is sent to the client. The client after receiving the
full response calls callComplete() which is a noop since nobody is waiting.
> The problem is that the calls that timeout will retry and the system gets into a situation
where data is being transferred around, but it's all data for timed out requests and no progress
is ever made.
> My quick solution to this was to add a "boolean timedout" to the Call object which I
set to true whenever the queued caller times out. And then when the client starts to pull
over the response data (in Connection::run) to first check if the Call is timedout and immediately
close the connection.
> I think a good fix for this is to queue requests on the client, and do a single sendParam
only when there is no outstanding request. This will allow closing the connection when receiving
a response for a request we no longer have pending, reopen the connection, and resend the
next queued request. I can provide a patch for this, but I've seen a lot of recent activity
in this area so I'd like to get some feedback first.

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