Complementary to what you will find in the on-line Apache documentation
at http://httpd.apache.org, and painted with a very wide brush :
When you start Apache, a single main Apache process is started.
It reads and checks the configuration, and loads a series of core and
add-on modules, as per your configuration.
When it is done with that phase, it starts either a number of child
processes (in the "prefork" type of setup), or a number of "children
threads" (in the "worker" type of setup).
Each of those is identical to the main Apache process that was built
according to the configuration, but from now on the "main" process and
the "children" will do different things.
The "main" process opens the listening TCP port (or ports), as described
in the "Listen" configuration directive(s), and starts waiting for
connections to that (or these) port(s).
When a connection comes in, it looks in the pool of available children
processes/threads to find one that is inactive. If it finds one, it
passes the opened TCP connection to that child/thread, and goes back to
listen for more new connections on the server listening port (typically,
port 80).
The selected child/thread now "has" this TCP connection, and it reads
and handles the HTTP request that came in on that connection. While it
is doing that, it is "busy", and the main process will not give it
another connection to handle.
In the meantime, if another client connection happens to the main
listening port, the main process will do the same again, this time
finding another "idle" (available, not doing anything at the moment)
child process/thread to pass this new connection to.
When the child/thread is done with processing a request, it lets the
main Apache process know that it is idle (free) again, ready to handle
another connection if one is pending.
The main Apache process does not handle requests. Its job is to listen
for connections on the listening server port, accept these connections,
and pass them on to children processes/threads for processing.
In addition, it manages the pool of children processes/threads. As the
load varies, the main Apache process may decide to start more children
processes/threads, or to kill off some that really have nothing to do.
This is controlled to some extent by a number of Apache configuration
directives, such as the ones you find here :
http://httpd.apache.org/docs/2.2/mod/directives.html
(look for the directives starting in general with Max.. or Min.., and
browse around)
When the main Apache process receives the command to shut down, it
normally lets each busy child finish their current request, then kills
them off, then exits itself.
The following is not specific to Apache, but relevant.
The listening TCP socket has a "wait queue". That means, if a client new
connection occurs before Apache has had time to accept the previous one
and hand it over to a child, the new connection will not be rejected
right away, it will wait a while. As soon as the main Apache process is
done with passing the previous onnection to a child, it will pick up the
first of these waiting connections, accept it, etc...
Then the next waiting one if any.
The maximum length of this queue varies; it is a TCP-level parameter.
If the rate of incoming connections is so high that Apache cannot cope,
the queue will get longer, up to this limit. If a new connection comes
in when the limit of waiting ones is already reached, it will be
rejected ("the server refused the connection").
This is how a webserver like Apache handles multiple requests
"simultaneously". It does not really accept new connections
simultaneously, but accepting a connection and passing it on to a child
is very fast. The child then can take much more time to actually
process this client request, while the main Apache is already accepting
the next one, and so on.
At any one point in time, there could thus be 100 Apache children
processes/threads in the process each of processing one client request
and/or sending a response, each one over a "private" TCP connection
between itself and the client browser.
There could also be 20 new connections in the queue for port 80, which
the main Apache process has not had time to accept yet.
These numbers vary constantly up and down, as the number of requests
(and the time to process each one of them) varies.
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