In any default installation, Apache 2 comes with keepAlive off, but looking at another server, the keepAlive module was turned on.
So, how do I know if keepAlive is right for me? Where can I find some good examples about configure this?
Server Fault is a question and answer site for system and network administrators. It only takes a minute to sign up.Sign up to join this community
There are 2 good answers already, but the perhaps most important real-life issue is not mentioned yet.
First off, the OP might want to read the 2 preceding answers and this little blog post to understand what keepalives are. (The author doesn't elaborate on the part about TCPI/IP getting "faster" the longer the connection is open. It is true, longer-lasting connections benefit from IP window scaling, but the effect isn't significant unless the files are large, or the bandwith-delay product is unusually large.)
The big argument against HTTP Keepalive when using Apache is that it blocks Apache processes. I.e. a client using keepalives will prevent 'his' Apache process from serving any other clients, until the client closes the connection or the timeout is reached. In the same span of time, this Apache instance could have served many other connections.
Now, a very common Apache configuration is the Prefork MPM and a PHP / Perl / Python interpreter, and application code in the mentioned language. In this case each Apache process is "heavy" in the sense that it occupies several megabytes of RAM (Apache linked with interpreter and application code). This, together with the blocking of each keepalive'd Apache instance, is inefficient.
A common workaround is to use 2 Apache servers (both on the same physical server, or on 2 servers, as needed) with different configurations:
You can then expand on this separation of dynamic and static content when needed, for example by:
Another approach with regards to avoid blocking Apache is to use a load balancer with smarter connection handling, such as Perlbal.
.. and much more. :-)
Keepalives can be good in some cases, they can be very bad in others. They reduce the time and effort of setting up a new connection, but they tie up server resources for the duration of keepalive timeout. Examples:
As you can see, KeepAliveTimeout will also play a large role in optimization of your server performance.
Look at your usage pattern and decide for yourself.
You should definitely use KeepAlive On.
A browser typically opens around 3 parallel connections to a domain. So let's say you have 18 objects in your site. The browser would open 3 connections, and it would download 6 objects in each connection - using the KeepAlive mode. Without KeepAlive, it would have to open 18 TCP connections, which is very slow.
Most, or all modern browsers are HTTP/1.1 compliant so this should just work.
Certain HTTP proxies like Squid are not HTTP/1.1 compliant, but they request the use of a KeepAlive connection anyway.