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From "Oleg Kalnichevski (Updated) (JIRA)" <j...@apache.org>
Subject [jira] [Updated] (HTTPCLIENT-1165) Cache allows multipe requests to retrieve the same cacheable resource from the server in multithreaded environment due to lack of proper locking
Date Mon, 02 Apr 2012 18:57:23 GMT

     [ https://issues.apache.org/jira/browse/HTTPCLIENT-1165?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Oleg Kalnichevski updated HTTPCLIENT-1165:
------------------------------------------

    Fix Version/s: Future
    
> Cache allows multipe requests to retrieve the same cacheable resource from the server
in multithreaded environment due to lack of proper locking
> ------------------------------------------------------------------------------------------------------------------------------------------------
>
>                 Key: HTTPCLIENT-1165
>                 URL: https://issues.apache.org/jira/browse/HTTPCLIENT-1165
>             Project: HttpComponents HttpClient
>          Issue Type: Improvement
>          Components: Cache
>    Affects Versions: Snapshot
>            Reporter: Manish Tripathi
>             Fix For: Future
>
>
> Consider the following scenario:
> Two separate threads are launched at the same time with identical Http requests through
CachingHttpClient.
> Both threads look up the same URI in the cache at [almost] the same time and find no
cached response for that URI.
> Both threads fall back to backend HttpClient and make identical requests to the server.
> Both threads retrieve the resource and attempt to store it in the cache. 
> The same resource gets retrieved from the server twice and is stored in the cache twice.
> Obviously, the described algorithm is inefficient
> Suggested fix: introduce read-write locking mechanism which would block multiple requests
to retrieve the same URI until one of the concurrent requests has either received a response
header indicating that the response is not cacheable, or until cacheable response has been
fully retrieved and stored in the cache. The proposed pseudo-code follows:
> cachingClient.execute(url) {
>   if (lock_count(url)>0)
>     lock=lockingFactory.acquireReadLock(url);
>   else
>     lock=lockingFactory.acquireWriteLock(url);
>   response=satisfyFromCache(url);
>   if (response==null) {
>     if (lock.isReadLock()) { lock.release(); lock=lockingFactory.acquireWriteLock(url);
}
>     response=satisfyFromServerAndStoreInCache(url);
>   }
>   lock.release();
>   return response;
> }
> where lockingFactory instance is shared by multiple instances of CachingHttpClient.

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