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From "Naveen Nalam (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-255) Client Calls are not cancelled after a call timeout
Date Fri, 26 May 2006 17:16:30 GMT
    [ http://issues.apache.org/jira/browse/HADOOP-255?page=comments#action_12413498 ] 

Naveen Nalam commented on HADOOP-255:

Well the problem I was seeing is that a getFile RPC request for say 1GB was issued, but then
the Call object timedout on the client. Yet the 1GB was still transferred fully to the client
and then discarded since there was no waiting Call object. My system got into a situation
where 1000s of getFile requests were queued up per node in the server's tcp receive buffers.
So you can see how no progress would have ever been made.

Are you suggesting that this is not going to be a problem because all the large response body
RPCs will now be done over HTTP?  I can't see how leaving the code as it is would be fine,
unless all RPCs are going to be very quick to service and have small response bodies.

You mentioned that search queries is an example of an RPC that could be broadcast out. What
would happen if the queries were taking too long to service and the client side rpc request
was already timing out. I could see it get into a similar situation where the servers would
become busy processing stale query requests.

So if all the RPCs will be small response bodies, then it seems fine to keep the connection
always open and just read in the response and throw it away. And then why not add a CANCEL-RPC
request type that can get sent over whenever the client request has timed out?

> 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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