The ProxyConnectionPool closes a connection before it reaches its ttl. This causes performance issues and unnecessarily consumes cpu, memory and network resources.

In an Undertow setup with 1 IO thread, 2 cached connections and mod_cluster backend ttl set to 10s, Apache HTTP server benchmarking tool load tests reveal:

• 1 connection: all good, no connections closed
• 2 concurrent connections: all good, no connections closed
• 3 or more concurrent connections: connections closed before their ttl is reached (state TIME_WAIT)

I pinpointed the code responsible for this:

final int cachedConnectionCount = hostData.availableConnections.size();
if (cachedConnectionCount >= maxCachedConnections) {
    // Close the longest idle connection instead of the current one
    final ConnectionHolder holder = hostData.availableConnections.poll();
    if (holder != null) {
        IoUtils.safeClose(holder.clientConnection);
    }
}

The issue can be solved either by removing the code, or by putting it in an if statement that checks for ttl == 0. But that depends on the interpretation of ttl.
So does a ttl of 0s mean never expire, or expire immediately? Please let me know.