What is Difference between event, worker and prefork
Apache (HTPD) is very popular and widely deployed web server arround the world. A-Patchy server comes with multiple modules. The term MPM is used for multiprocessing module. We can check for default mpm by running this command “ httpd -l ”
Apache 2 is available with following 3 MPM modules.
PREFORK
WORKER
EVENT
(mpm_winnt This Multi-Processing Module is optimized for Windows NT.)
(mpm_netware Multi-Processing Module implementing an exclusively threaded web server optimized for Novell NetWare)
Prefork MPM
A prefork mpm handles http requests just like older Apache 1.3. As the name suggests it will pre-fork necessary child process while starting Apache. It is suitable for all those websites which don’t want threading for compatibility. i.e for non-thread-safe libraries . It is also known as the best MPM for isolating each incoming http request.
How it works
A single control (master) process is responsible for launching multiple child processes which serves incoming http requests. Apache always tries to maintain several spare (not-in-use) server processes, which stand ready to serve incoming requests. In this way, clients do not need to wait for a new child processes to be forked before their requests can be served.
We can adjust this spare process through the Apache configuration. Default settings are usually enough for small amount of traffic. One can always tune those Directives / Values as per their requirements.
Pre-Fork is the default module given by Apache.
#prefork MPM
#StartServers: number of server processes to start
#MinSpareServers: minimum number of server processes which are kept spare
#MaxSpareServers: maximum number of server processes which are kept spare
#ServerLimit: maximum value for MaxClients for the lifetime of the server
#MaxClients: maximum number of server processes allowed to start
#MaxRequestsPerChild: maximum number of requests a server process serves
<IfModule prefork.c>
StartServers 8
MinSpareServers 5
MaxSpareServers 20
ServerLimit 256
MaxClients 256
MaxRequestsPerChild 4000
</IfModule>
Worker MPM
A worker mpm is an Multi-Processing Module (MPM) which implements a hybrid multi-process multi-threaded server. By using threads to serve requests, it is able to serve a large number of requests with fewer system resources than a process-based server.
The most important directives used to control this MPM are ThreadsPerChild, which controls the number of threads deployed by each child process and MaxClients, which controls the maximum total number of threads that may be launched.
Strength : Memory usage and performance wise its better than prefork
Weakness : worker will not work properly with languages like php
How it works
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 server thread for processing when they arrive.
Apache always tries to maintain a group of spare or idle server threads, which stand ready to serve incoming requests. In this way, clients do not need to wait for a new threads or processes to be created before their requests can be served. The number of processes that will initially launched is set by the StartServers directive. During operation, Apache assesses the total number of idle threads in all processes, and forks or kills processes to keep this number within the boundaries specified by MinSpareThreads and MaxSpareThreads. Since this process is very self-regulating, it is rarely necessary to modify these directives from their default values. The maximum number of clients that may be served simultaneously (i.e., the maximum total number of threads in all processes) is determined by the MaxClients directive. The maximum number of active child processes is determined by the MaxClients directive divided by the ThreadsPerChild directive
#worker MPM
#StartServers: initial number of server processes to start
#MaxClients: maximum number of simultaneous client connections
#MinSpareThreads: minimum number of worker threads which are kept spare
#MaxSpareThreads: maximum number of worker threads which are kept spare
#ThreadsPerChild: constant number of worker threads in each server process
#MaxRequestsPerChild: maximum number of requests a server process serves
<IfModule worker.c>
StartServers 4
MaxClients 300
MinSpareThreads 25
MaxSpareThreads 75
ThreadsPerChild 25
MaxRequestsPerChild 0
</IfModule>
Event MPM
The event Multi-Processing Module (MPM) is designed to allow more requests to be served simultaneously by passing off some processing work to supporting threads, freeing up the main threads to work on new requests. Event has been released with stable in Apache 2.4. The Event MPM works the exact same way as the Worker MPM when it comes to processes and threads. The big difference is that an Event MPM will dedicate a thread to a request, not the whole HTTP connection.
How it works
This MPM tries to fix the ‘keep alive problem’ in HTTP. After a client completes the first request, the client can keep the connection open, and send further requests using the same socket. This can save 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 thread to handle both the Listening sockets, all sockets that are in a Keep Alive state, and sockets where the handler and protocol filters have done their work and the only remaining thing to do is send the data to the client. The status page of mod_status shows how many connections are in the mentioned states.
This is useful in a situation where you like the idea of threading, but have an application that uses rather long KeepAlive timeouts. With the Worker MPM, the thread would be bound to the connection, and stayed tied up regardless if a request was being processed or not.
With the Event MPM, the connection the thread is only used for requests and frees backup immediately after the request is fulfilled, regardless of the actual HTTP connection, which is handled by the parent process. Since the thread frees up immediately after the request is fulfilled, it can be used for other requests.
<IfModule event.c>
MinSpareThreads 64
MaxSpareThreads 128
ThreadsPerChild 64
ThreadLimit 64
MaxRequestsPerChild 20000
ListenBacklog 4096
</IfModule>
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