[ http://issues.apache.org/jira/browse/IBATISNET-134?page=comments#action_12360378 ]
Ron Grabowski commented on IBATISNET-134: ----------------------------------------- If there's a problem with items being in the cache when they shouldn't be, adding a check around the Add method isn't a solution to that problem. We should be Remove()ing items from the cache...not just overwriting them. > lock mechanism doesn't prevent duplicate cache keys from being inserted into > IList persistence > ---------------------------------------------------------------------------------------------- > > Key: IBATISNET-134 > URL: http://issues.apache.org/jira/browse/IBATISNET-134 > Project: iBatis for .NET > Type: Bug > Components: DataMapper > Versions: DataMapper 1.1 > Environment: .NET Framework 1.1, Windows XP Professional SP1, SQL Server > 2000. > Reporter: Jeff Ziolkowski > > Every so often, our application would encounter a "Cannot insert key. Cause: > Item has already been added." exception. > Upon further investigation, we found we were able to regularly reproduce this > exception by having two users concurrently perform an action in our > application that would call a SQL map to retrieve a list of items to populate > a drop down list. Previous to these actions, the caches for the SQL map were > cleared, meaning the map had to go to the SQL Server to retrieve a fresh > listing. > It seems the request generated by one user would find that the key for the > list wasn't present in the IList collection and would proceed to go to the > SQL Server to retrieve the list, but before the thread has the opportunity to > retrieve the data from the SQL Server and place it in the IList, the request > generated by the second user comes along and finds the key isn't present in > the IList collection and ALSO proceeds to retrieve the data from the SQL > Server. In the end, the requests from both users ends up retrieving data > from the SQL Server and attempts to place both entries in the IList which > causes the collection to throw the duplicate key not allowed exeception. > Here's the property in question. It comes from the LRUCacheController class. > /// <summary> > /// Adds an item with the specified key and value into cached > data. > /// Gets a cached object with the specified key. > /// </summary> > /// <value>The cached object or <c>null</c></value> > public object this [object key] > { > get > { > lock (this) > { > _keyList.Remove(key); > _keyList.Add(key); > return _cache[key]; > } > } > set > { > lock (this) > { > _cache.Add(key, value); > _keyList.Add(key); > if (_keyList.Count > > _cacheSize) > { > object oldestKey = > _keyList[0]; > _keyList.Remove(0); > > _cache.Remove(oldestKey); > } > } > } > } > Microsoft has noted that the lock(this) statement shouldn't be used on > non-static members, as multiple instances of the object can exist and > therefore cause synchronization issues. In a multi-threaded scenario, with > multiple requests for the same map coming in on different threads, this seems > like a problem. In addition, within the set section of this code... there > should be a check to see if the key already exists in the collection just to > be safe. -- This message is automatically generated by JIRA. - If you think it was sent incorrectly contact one of the administrators: http://issues.apache.org/jira/secure/Administrators.jspa - For more information on JIRA, see: http://www.atlassian.com/software/jira
