cache: single blocking request can block all other requests to cached content
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Key: MAGNOLIA-3167
URL: http://jira.magnolia-cms.com/browse/MAGNOLIA-3167
Project: Magnolia
Issue Type: Bug
Affects Versions: 4.3.1, 4.2.3, 4.1.4, 3.6.8
Reporter: Philipp Bärfuss
Assignee: Boris Kraft
Fix For: 4.3.x
the cache mechanism can block all requests:
* cache.get() will block if an other request is caching the same key (this is a
feature of the BlockingCache)
** mutex per key kept until cache.put() is called (either with an entry or
null value)
* this code is again in a synchronized block which synchronizes on the cache
object itself
** this blocks all other request trying to enter the synchronization block
The critical scenario which can prevent magnolia from responding any request
(all threads blocked) is the following
1) first request to a resource which is slow or never returns (request to a
service, poor database connection, ..)
2) second request to the same resource: --> thread is blocked at
EhCacheWrapper.get(key):56, but also keeps the lock on the cache
3) all other caching requests are blocked (no matter which url) due to the
synchronize block at Default.shouldCache(Cache, AggregationState,
FlushPolicy):89
Solution:
* don't synchronize on the cache (why are we doing this???)
* allow configuration of
[BlockingCache.timeoutMillis|http://ehcache.org/apidocs/net/sf/ehcache/constructs/blocking/BlockingCache.html#timeoutMillis]
** throw an exception if a request waits for to long
* uncomment finally block at doFilter(HttpServletRequest, HttpServletResponse,
FilterChain), this is a safety net and should log a FATAL ERROR message
** only relevant if the result is a store request
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