There menny mob farm out there.
This one is server friendly.
You can replace the glass if you like, Leave the spawner out and use "drop of evil" on the unther the "vector plates"
The "online detector" is to make it server friendly (when your offline the mobs drop into the voide)
You can turn off the mobfarm with the lever (when its off the mobs drop into the voide)
If your using a powerd spawner you can turn them off to with "redstone conduids" and a lever.
Replace the glass with a "vacum chest - hopper"
Im using this on a server and the Moderators like it. I made this my self and i have not look it up to see if some one els has it or not. My owner creation.