Month: April 2017

Living Without Atomic Clocks

“Living Without Atomic Clocks” is an interesting article that covers some design bits of distributed systems and CockroachDB (what a name!), especially those related to time precision.  This part in particular is the one I’m sure I’ll came back to at some point:

How does TrueTime provide linearizability?

OK, back to Spanner and TrueTime. It’s important to keep in mind that TrueTime does not guarantee perfectly synchronized clocks. Rather, TrueTime gives an upper bound for clock offsets between nodes in a cluster. Synchronization hardware helps minimize the upper bound. In Spanner’s case, Google mentions an upper bound of 7ms. That’s pretty tight; by contrast, using NTP for clock synchronization is likely to give somewhere between 100ms and 250ms.

So how does Spanner use TrueTime to provide linearizability given that there are still inaccuracies between clocks? It’s actually surprisingly simple. It waits. Before a node is allowed to report that a transaction has committed, it must wait 7ms. Because all clocks in the system are within 7ms of each other, waiting 7ms means that no subsequent transaction may commit at an earlier timestamp, even if the earlier transaction was committed on a node with a clock which was fast by the maximum 7ms. Pretty clever.

World’s Biggest Data Breaches

Here’s an interactive collection of the world’s biggest data breaches.  It goes back to 2004, where about 92,000,000 email addresses and screen names were stolen by an AOL employee, and covers most of the major events up until and including 2016.  There are a few ways to filter the data and change the representation.

Overall, should give you a pretty good idea of how safe and secure your online data is. Oh, and how private it is too.

Bashing up

Here are a couple of useful Bash resources that came upon my radar recently.

First one is Julia Evans’ blog post “Bash scripting quirks & safety tips“.  It’s quite introductory, but is has a few useful tips.  The one in particular I either didn’t know about or completely forgot  mentioned recently is on how to make Bash scripts safer by using “set -e“, “set -u“, and “set -o pipefail“.  These go well with another post of mine not so long ago.

The second is Sam Rowe’s blog post “Advancing in the Bash Shell“, which I found useful for all kinds of navigation and variable expansion in Bash command line.  Especially the bits on searching and reusing the history.

Bible Or Qur’an

Not being religious at all, I try to avoid the subject in this blog, but I think Bible Or Qur’an website deserves a mention. Mission statement from the front page:

The mission of BibleOrQuran is to educate people about the true nature and content of the Bible and Qur’an. False statements such as “81% of the Qur’an is about killing infidels!” and “The bible teaches only peace!” are commonly thrown around with little evidence to back them up. Despite their lack of credibility, however, they still continue to polarize and cause irrational hate and fear. We hope that by showing random Bible or Qur’an passages, we show how similar the two texts are and allay many of the fears of Islam and its teachings.

Basically, you get to see a short quote and then pick which of the two books it is from.  The website keeps the score, showing you how well you are doing on the guesswork.  In the meantime it drives the point of similarity in the depth and mood of both the Bible and the Qur’an.

Simple and effective, for those, of course, who can be convinced…