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From "Doug Cutting (JIRA)" <j...@apache.org>
Subject [jira] Commented: (NUTCH-385) Server delay feature conflicts with maxThreadsPerHost
Date Wed, 11 Oct 2006 20:51:37 GMT
    [ http://issues.apache.org/jira/browse/NUTCH-385?page=comments#action_12441552 ] 
            
Doug Cutting commented on NUTCH-385:
------------------------------------

> It would be one thing if whenever (fetcher.threads.per.host > 1), this trumped the
server delay [...]

Are any other alternatives useful?  I've always assumed that fetcher.threads.per.host was
only useful when server.delay is zero, to speed things when crawling servers that you control.
 The concerns are:

1. Being polite.  The standard interpretation of "crawl-delay" is the pause between successive,
single-threaded access.  So I don't think we could ever call ourselves polite when threads.per.host
> 1.

2. Crawling quickly.  For sites that are under the control of the crawl operator, or are known
not to care, one can ignore politeness.  In these cases multiple threads are warranted, and
delays are not.

I don't see an interesting middle ground.


> Server delay feature conflicts with maxThreadsPerHost
> -----------------------------------------------------
>
>                 Key: NUTCH-385
>                 URL: http://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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