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Apache MPM event

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Description:A variant of the worker MPM with the goal of consuming threads only for connections with active processing
Module Identifier:mpm_event_module
Source File:event.c


The event Multi-Processing Module (MPM) is designed to allow more requests to be served simultaneously by passing off some processing work to the listeners threads, freeing up the worker threads to serve new requests.

To use the event MPM, add --with-mpm=event to the configure script's arguments when building the httpd.

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Relationship with the Worker MPM

event is based on the worker MPM, which implements a hybrid multi-process multi-threaded server. A single control process (the parent) is responsible for launching child processes. Each child process creates a fixed number of server threads as specified in the ThreadsPerChild directive, as well as a listener thread which listens for connections and passes them to a worker thread for processing when they arrive.

Run-time configuration directives are identical to those provided by worker, with the only addition of the AsyncRequestWorkerFactor.


How it Works

This MPM tries to fix the 'keep alive problem' in HTTP. After a client completes the first request, it can keep the connection open, sending further requests using the same socket and saving significant overhead in creating TCP connections. However, Apache HTTP Server traditionally keeps an entire child process/thread waiting for data from the client, which brings its own disadvantages. To solve this problem, this MPM uses a dedicated listener thread for each process to handle both the Listening sockets, all sockets that are in a Keep Alive state, sockets where the handler and protocol filters have done their work and the ones where the only remaining thing to do is send the data to the client.

This new architecture, leveraging non-blocking sockets and modern kernel features exposed by APR (like Linux's epoll), no longer requires the mpm-accept Mutex configured to avoid the thundering herd problem.

The total amount of connections that a single process/threads block can handle is regulated by the AsyncRequestWorkerFactor directive.

Async connections

Async connections would need a fixed dedicated worker thread with the previous MPMs but not with event. The status page of mod_status shows new columns under the Async connections section:

While sending the response to the client, it might happen that the TCP write buffer fills up because the connection is too slow. Usually in this case a write() to the socket returns EWOULDBLOCK or EAGAIN, to become writable again after an idle time. The worker holding the socket might be able to offload the waiting task to the listener thread, that in turn will re-assign it to the first idle worker thread available once an event will be raised for the socket (for example, "the socket is now writable"). Please check the Limitations section for more information.
Keep Alive handling is the most basic improvement from the worker MPM. Once a worker thread finishes to flush the response to the client, it can offload the socket handling to the listener thread, that in turns will wait for any event from the OS, like "the socket is readable". If any new request comes from the client, then the listener will forward it to the first worker thread available. Conversely, if the KeepAliveTimeout occurs then the socket will be closed by the listener. In this way the worker threads are not responsible for idle sockets and they can be re-used to serve other requests.
Sometimes the MPM needs to perform a lingering close, namely sending back an early error to the client while it is still transmitting data to httpd. Sending the response and then closing the connection immediately is not the correct thing to do since the client (still trying to send the rest of the request) would get a connection reset and could not read the httpd's response. The lingering close is time bounded but it can take relatively long time, so it's offloaded to a worker thread (including the shutdown hooks and real socket close). From 2.4.28 onward this is also the case when connections finally timeout (the listener thread never handles connections besides waiting for and dispatching their events).

These improvements are valid for both HTTP/HTTPS connections.

Graceful process termination and Scoreboard usage

This mpm showed some scalability bottlenecks in the past leading to the following error: "scoreboard is full, not at MaxRequestWorkers". MaxRequestWorkers limits the number of simultaneous requests that will be served at any given time and also the number of allowed processes (MaxRequestWorkers / ThreadsPerChild), meanwhile the Scoreboard is a representation of all the running processes and the status of their worker threads. If the scoreboard is full (so all the threads have a state that is not idle) but the number of active requests served is not MaxRequestWorkers, it means that some of them are blocking new requests that could be served but that are queued instead (up to the limit imposed by ListenBacklog). Most of the times the threads are stuck in the Graceful state, namely they are waiting to finish their work with a TCP connection to safely terminate and free up a scoreboard slot (for example handling long running requests, slow clients or connections with keep-alive enabled). Two scenarios are very common:

From 2.4.24 onward, mpm-event is smarter and it is able to handle graceful terminations in a much better way. Some of the improvements are:

The behavior described in the last point is completely observable via mod_status in the connection summary table through two new columns: "Slot" and "Stopping". The former indicates the PID and the latter if the process is stopping or not; the extra state "Yes (old gen)" indicates a process still running after a graceful restart.


The improved connection handling may not work for certain connection filters that have declared themselves as incompatible with event. In these cases, this MPM will fall back to the behavior of the worker MPM and reserve one worker thread per connection. All modules shipped with the server are compatible with the event MPM.

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