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Date: Tue, 2 May 2017 13:20:04 +0000 (UTC)
From: "Oleg Kalnichevski (JIRA)" <jira@apache.org>
To: dev@hc.apache.org
Message-ID: <JIRA.12542505.1329213170000.102339.1493731204161@Atlassian.JIRA>
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References: <JIRA.12542505.1329213170000@Atlassian.JIRA> <JIRA.12542505.1329213170570@jira-lw-us.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
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archived-at: Tue, 02 May 2017 13:20:09 -0000


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

Oleg Kalnichevski updated HTTPCLIENT-1165:
------------------------------------------
           Labels: stuck volunteers-wanted  (was: )
    Fix Version/s:     (was: Future)
                   Stuck

> 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: HttpCache
>    Affects Versions: Snapshot
>            Reporter: Manish Tripathi
>              Labels: stuck, volunteers-wanted
>             Fix For: Stuck
>
>
> 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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