This post is a short summary of a presentation by Ulf Wiger called “Death by accidental complexity”.
Parallel problems, like a server distributing constant strings, are very simple and easily parallelizable. The complexity kicks in after the system has grown under the pressure of the requirements. Ulf Wiger has obviously dealt with such systems extensively. The presentations explores a number of issues that can (and do) bring highly concurrent and parallel systems to death.
You’ve got a mailbox and the whole world waiting to send you a message. You can either process the messages as they arrive, but then you must account for all of the possible orderings of events. Which should take approximately a lifetime.
The other option is called ‘selective message reception’. There are two option to process interleaving message streams selectively:
Actors in the system cannot be synchronized globally (side note: there is a paper back from 1977 called ‘Time, Clocks, and the Ordering of Events in a Distributed System’ which actually contains an approximation for the total ordering of events in a distributed system).
Ulf presents a toy application (called ‘POTS’ - Plain Ordinary Telephony System), basically a simulator for a simple telephony system with a GUI. It can be found @ github.
The original POTS implementation is done in FSM form. It’s pretty straightforward - you’ve got nodes and a set of states the node can be in at any given moment (like
waiting_for_signal). The code is quite clear, even if you’re not fluent in Erlang.
Then the rewrite using events is shown. Needless to say, the implementation which uses events is a lot more complicated due to the asynchronous nature of callbacks. It may not be obvious from the start, but the fact that callbacks cannot block is the source of complexity.
After doing the rewrite the number of states of the sample application has increased twofold (which is shown in a State-Event matrix (@~25:20)). The added states were hidden in the original FSM based implementation, but surfaced when examined in an asynchronous event-based perspective.
Ulf also gives an example of a large system implemented in an event-based manner. The complexity has actually killed it.
The goal of the presentation was to show that complexity in concurrency, states and events leads to unmanagable complexity in code which can bring death to projects. Concurrency based on events and callbacks which cannot block increases the size of a state-event matrix. As this matrix must be accounted for by every programmer, the entry to change gets higher and higher with each additional requirement while the codebase becomes unmaintainable. This complexity cannot be beaten by simple refactorings, but rather by going to a completely different concurrency model.
Which one? The answer to this question wasn’t in scope of this talk.