Event-based Concurrency

Posted 2023-03-09 11:00:08 ‐ 4 min read


Specifically, a different style of concurrent programming is often used in both GUI-based applications as well as some types of internet servers. This style, known as event-based concurrency.

The problem that event-based concurrency addresses is two-fold.

  • Manageing concurrency correctly in multi-threaded applications.
    • missing locks, deadlock, and so on.
  • In a multi-threaded application, the developer has little or no control over what is scheduled at a given moment in time
    • The programmer simply creates threads and then hopes that the underlying OS schedules them in a reasonable manner across avaiable CPUs.

how to build concurrent servers without threads

An Event Loop

You simple wait for something(i.e., an "event") to occur; when it does, you check what type of event it is and do the small amout of work it requires.

Canonical event-based server pseudocode:

while(1) {
  events = get_events();
  for (e in events) {

But this leaves us with a bigger question: how exactly does an event-based server determine which events are taking place?

Import API: select/poll/epoll

These interfaces enable a program to do is simple: check whether there is any incomming I/O that should be attended to.

Blocking vs. Non-Blocking Interfaces

Blocking(or synchronous) interface do all of their work before returning to the caller; non-blocking(or asynchronous) interfaces begin some work but return immediately, thus letting whatever work that needs to be doen get done in the background. Non-blocking interfces are essential in the event-based approach, as a call that blocks will halt all progress.

Why simpler? No locks needed

With a single CPU and an event-based application, the problems found in concurrent programs are no longer present. Specifically, because only one event is being handled at a time, there is no need to acquire or release locks.

A problem: blocking system calls

With an event-based approach ,however, there are no other threads to run: just the main event loop. And this implies that if an event handler issues a call that blocks, the entire server will do just that: block until the call completes. When the event loop blocks, the system sits idle, and thus is a huge potential waste of resources. We thus have a rule that must be obeyed in event-based systems: no blocking calls are allowed.

A solution: asynchronous I/O

loop check the asynchronous issue completed.

Another problem: state management

Thread-based code is more simpler. Packaging up some program state for the next event handler is not needed in thread-based programs, as the state the program need is on the stack of the thread.

For example, within a thread-based server:

int rc = read(fd, buffer, size);
rc = write(sd, buffer, size);

Doing this kind of work is trivial. But in an event-based system, life is not so easy. You must manually manage the state of program.

The solution sounds complicated, the idea is rahter simple: basically, record the needed information to finish processing this event in some data structure; when the event happens(i.e., when the disk I/O complates), look up the needed information and process the event.

What is still difficult with events

  • when systems moved from a single CPU to multiple CPUs, in order to utilize more than one CPU, the event server has to run multiple event handlers in parallel; when doing so, the usual syschronization problems(e.g., critical sections) arise.
  • implicit blcking. Event though the server has been structured to avoid explicit blocking, page faults is hard to avoid.
  • hard to manager over time
  • asynchronous disk I/O is never quite integrates with asynchronous network I/O in as simple and uniform a manaer. you maybe need some combination of select() for networking and the AIO calls for disk I/O