Vol. 13 No. 3 – March 2015


There is No Now

Problems with simultaneity in distributed systems

"Now." The time elapsed between when I wrote that word and when you read it was at least a couple of weeks. That kind of delay is one that we take for granted and don't even think about in written media. "Now." If we were in the same room and instead I spoke aloud, you might have a greater sense of immediacy. You might intuitively feel as if you were hearing the word at exactly the same time that I spoke it. That intuition would be wrong. If, instead of trusting your intuition, you thought about the physics of sound, you would know that time must have elapsed between my speaking and your hearing. The motion of the air, carrying my word, would take time to get from my mouth to your ear.

by Justin Sheehy

Parallel Processing with Promises

A simple method of writing a collaborative system

In today's world, there are many reasons to write concurrent software. The desire to improve performance and increase throughput has led to many different asynchronous techniques. The techniques involved, however, are generally complex and the source of many subtle bugs, especially if they require shared mutable state. If shared state is not required, then these problems can be solved with a better abstraction called promises. These allow programmers to hook asynchronous function calls together, waiting for each to return success or failure before running the next appropriate function in the chain.

by Spencer Rathbun

Spicing Up Dart with Side Effects

A set of extensions to the Dart programming language, designed to support asynchrony and generator functions

The Dart programming language has recently incorporated a set of extensions designed to support asynchrony and generator functions. Because Dart is a language for Web programming, latency is an important concern. To avoid blocking, developers must make methods asynchronous when computing their results requires nontrivial time. Generator functions ease the task of computing iterable sequences.

by Erik Meijer, Kevin Millikin, Gilad Bracha

Reliable Cron across the Planet

...or How I stopped worrying and learned to love time

This article describes Google's implementation of a distributed Cron service, serving the vast majority of internal teams that need periodic scheduling of compute jobs. During its existence, we have learned many lessons on how to design and implement what might seem like a basic service. Here, we discuss the problems that distributed Crons face and outline some potential solutions.

by Štěpán Davidovi, Kavita Guliani