Node: Up and Running

Patterns

Event-driven programming is different from procedural programming. The easiest way to learn it is to practice routine patterns that have been discovered by previous generations of programmers. That is the purpose of this section.

Before we launch into patterns, we’ll take a look at what is really happening behind various programming styles to give the patterns some context. Most of this section will focus on I/O, because, as discussed in the previous section, event-driven programming is focused on solving problems with I/O. When it is working with data in memory that doesn’t require I/O, Node can be completely procedural.

The I/O Problem Space

We’ll start by looking at the types of I/O required in efficient systems. These will be the basis of our patterns.

The first obvious distinction to look at is serial versus parallel I/O. Serial is obvious: do this I/O, and after it is finished, do that I/O. Parallel is more complicated to implement but also easy to understand: do this I/O and that I/O at the same time. The important point here is that ordering is normally considered implicit in serial tasks, but parallel tasks could return in any order.

Groups of serial and parallel work can also be combined. For example, two groups of parallel requests could execute serially: do this and that together, then do other and another together.

In Node, we assume that all I/O has unbounded latency. This means that any I/O tasks could take from 0 to infinite time. We don’t know, and can’t assume, ...