Are there explicit considerations about the latency of any single request in the node.js event loop? AFAICT every IO call returns an eventEmitter which emits an event. The processing of all the events is multiplexed through the use of a pipe. So it is possible that the event that needs to be processed for an important request may be placed too far back into the pipe. Is there some sort of priority queue that can be used to schedule the order of execution of eventHandlers ?
Here's why I asked this question in the first place. I decided to give a gist.github link because the reason is long and related to the technical question
It's not clear exactly what you're asking here. Your Javascript does not directly add things to the event queue (that is only done with native code). Instead, you call some async operation and the native code behind that operation adds something to the event queue when the async operation completes.
This article The Node.js Event Loop, Timers, and process.nextTick() gives you a lot of details about how the event queue is serviced and how it handles different types of events (timers, I/O, etc...).
In general things are FIFO (first in, first out) within an event type with some exceptions.
process.nextTick() will run its callback BEFORE waiting I/O events.
setImmediate() will runs its callback AFTER waiting I/O events.
More detail here: setImmediate vs. nextTick and nextTick vs setImmediate, visual explanation and setTimeout vs. setImmediate vs. nextTick
So it is possible that the event that needs to be processed for an important request may be placed too far back into the pipe.
You'd have to show us the specific situation you're concerned about. If you yourself are scheduling a callback with setTimeout(), setImmediate() or process.nextTick(), then you have some control over when it happens by which of these three you pick. If you aren't scheduling it yourself (e.g. it's the completion callback of some async operation), then you don't control it's scheduling in the event loop. It will go into the sub-queue that matches its type and be served FIFO from that phase or the event loop (as described in the above articles).
Is there some sort of priority queue that can be used to schedule the order of execution of eventHandlers ?
There is no exposed priority system. Within an event type, things are FIFO. Again, if you give us an actual coding example so we can see exactly what you're trying to do, we could offer some help on what your choices are. You may be able to use the setTimeout(), setImmediate() and process.nextTick() tools that are already available or you may want to implement your own task queuing and prioritization system that runs off some of the above three methods that allows you to prioritize things that are already queued yourself.