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From "Stirling Chow (JIRA)" <>
Subject [jira] Created: (AMQ-3021) HttpTunnelServlet leaks BlockingQueueTransport objects, causing eventual OOM on heap space
Date Fri, 05 Nov 2010 23:05:00 GMT
HttpTunnelServlet leaks BlockingQueueTransport objects, causing eventual OOM on heap space

                 Key: AMQ-3021
             Project: ActiveMQ
          Issue Type: Bug
          Components: Broker, Transport
    Affects Versions: 5.4.1
            Reporter: Stirling Chow
            Priority: Critical

We have a production system involving a network of 8 Brokers connected over HTTP.  The Brokers
discover each other using SimpleDiscoveryAgent.  Our network experienced a period of instability
during which time numerous Broker-to-Broker bridges were created and failed repeatedly.  Over
the course of about 7 hours, two of the Brokers crashed with OOM heap space errors.

We analyzed the heap dump and discovered several thousand instances of org.apache.activemq.transport.http.BlockingQueueTransport.
 These transports were associated with bridges that had failed, however, they were not being
garbage collected because HttpTunnelServlet was maintaining references to them.

This issue was easily replicated in a test environment were we repeatedly broke the connection
between a pair of Brokers connected over HTTP.  In each case, both Brokers maintained *indefinitely*
a number of instances of BlockingQueueTransport equal to the number of times the network was

When a bridge is first created over HTTP, the client broker's HttpClientTransport sends a
HEAD command to the server broker, which is processed by an instance of HttpTunnelServlet.
 In response,e HttpTunnelServlet creates an instance of BlockingQueueTransport to represent
the connection to the client broker.  This instance of BlockingQueueTransport is stored in
a private hash map managed by HttpTunnelServlet and indexed by the client's unique ID:

public class HttpTunnelServlet extends HttpServlet {
    private final Map<String, BlockingQueueTransport> clients = new HashMap<String,

    protected BlockingQueueTransport createTransportChannel(HttpServletRequest request, HttpServletResponse
response) throws IOException {
        String clientID = request.getHeader("clientID");
            answer = createTransportChannel();
            clients.put(clientID, answer);

Every time a client broker reestablishes a bridge, it generates a new clientID.  As a result,
the clients hash map accumulates instances of BlockingQueueTransport, one for each bridge
created.  Nowhere in the implementation of HttpTunnelServlet is there any code that removes
the instance when a client broker is no longer connected.  In an environment with multiple
brokers and an unreliable network, the client hash  map can accumulate thousands of instances
of BlockingQueueTransport.

HttpTunnelServlet needs to remove an instance of BlockingQueueTransport from the clients hash
map whenever that instance is no longer being used.  The addition of InactivityMonitor as
a default interceptor for the BlockingQueueTransport (see AMQ-2764) is a partial solution
in that it triggers the closure of unused BlockingQueueTransport instances; however, HttpTunnelServlet
does not detect these closures.

The solution is included a patch and involves the following changes to HttpTunnelServlet (not
all changes are directly related to the OOM):
1) The addition of a ServiceListener to the BlockingQueueTransport, which is triggered when
the transport is closed and causes the removal of the transport from the clients hash map
2) Refactoring of the access to the clients hash map to simplify thread safety (in particularly,
removal of explicit synchronization in lieue of ConcurrentHashMap)
3) An additional check on the BlockingQueueTransport to ensure that it was not prematurely
closed (the previous code ignored this possibility)

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