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From "Chris Schneider (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (NUTCH-385) Server delay feature conflicts with maxThreadsPerHost
Date Sun, 06 Apr 2014 23:06:14 GMT

    [ https://issues.apache.org/jira/browse/NUTCH-385?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13961562#comment-13961562
] 

Chris Schneider commented on NUTCH-385:
---------------------------------------

Hi Julien,

Actually, I believe the original bug report made two basic requests for improvement:

1) The behavior of these two configuration parameters should be changed to make them more
consistent with one another.

2) The behavior of these two configuration parameters should be clearly documented in the
configuration file, including any interactions between them (such as who trumps whom).

Since then, Andrzej has attempted to justify the current behavior, though there seem to be
other opinions on how it really ought to work. Even if we decide not to change the current
implementation, I think it certainly deserves better documentation.

Chris

> Server delay feature conflicts with maxThreadsPerHost
> -----------------------------------------------------
>
>                 Key: NUTCH-385
>                 URL: https://issues.apache.org/jira/browse/NUTCH-385
>             Project: Nutch
>          Issue Type: Bug
>          Components: fetcher
>            Reporter: Chris Schneider
>
> For some time I've been puzzled by the interaction between two paramters that control
how often the fetcher can access a particular host:
> 1) The server delay, which comes back from the remote server during our processing of
the robots.txt file, and which can be limited by fetcher.max.crawl.delay.
> 2) The fetcher.threads.per.host value, particularly when this is greater than the default
of 1.
> According to my (limited) understanding of the code in HttpBase.java:
> Suppose that fetcher.threads.per.host is 2, and that (by chance) the fetcher ends up
keeping either 1 or 2 fetcher threads pointing at a particular host continuously. In other
words, it never tries to point 3 at the host, and it always points a second thread at the
host before the first thread finishes accessing it. Since HttpBase.unblockAddr never gets
called with (((Integer)THREADS_PER_HOST_COUNT.get(host)).intValue() == 1), it never puts System.currentTimeMillis()
+ crawlDelay into BLOCKED_ADDR_TO_TIME for the host. Thus, the server delay will never be
used at all. The fetcher will be continuously retrieving pages from the host, often with 2
fetchers accessing the host simultaneously.
> Suppose instead that the fetcher finally does allow the last thread to complete before
it gets around to pointing another thread at the target host. When the last fetcher thread
calls HttpBase.unblockAddr, it will now put System.currentTimeMillis() + crawlDelay into BLOCKED_ADDR_TO_TIME
for the host. This, in turn, will prevent any threads from accessing this host until the delay
is complete, even though zero threads are currently accessing the host.
> I see this behavior as inconsistent. More importantly, the current implementation certainly
doesn't seem to answer my original question about appropriate definitions for what appear
to be conflicting parameters. 
> In a nutshell, how could we possibly honor the server delay if we allow more than one
fetcher thread to simultaneously access the host?
> It would be one thing if whenever (fetcher.threads.per.host > 1), this trumped the
server delay, causing the latter to be ignored completely. That is certainly not the case
in the current implementation, as it will wait for server delay whenever the number of threads
accessing a given host drops to zero.



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