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Posted to dev@hc.apache.org by "Oleg Kalnichevski (Updated) (JIRA)" <ji...@apache.org> on 2012/04/02 20:57:23 UTC
[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
[ 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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