<antirez>

For a decade and half I contributed to open source regularly, and still it is relatively rare that I stop to think a bit more about what this means for me. Probably it is just because I like to write code, so this is how I use my time: writing code instead of thinking about what this means… however lately I'm starting to have a few recurring ideas about open source, its relationship with the IT industry, and my interpretation of what OSS is, for me, as a developer.

First of all, open source for me is not a way to contribute to the free software movement, but to contribute to humanity. This means a lot of things, for instance I don't care about what people do with my code, nor if they'll release back their modifications. I simply want people to use my code in one way or the other.

Dear Redis users, in the final part of 2012 I repeated many time that the focus, for 2013, is all about Redis Cluster and Redis Sentinel.

This is exactly what I'm going to do from the point of view of the big picture, however there are many smaller features that make a big difference from the point of view of the Redis user day to day operations. Such features can't be ignored as well. They are less shiny in a feature list, and they are not good to generate buzz and interest in new users, and sometimes boring to code, but they are very important from a practical point of view.

# Software defined radio is cool

About one week ago I received my RTLSDR dongle, entering the already copious crew of software defined radio enthusiasts.

It's really a lot of fun, for instance from my home that is at about 10 km from the Catania Airport I can listen the tower talking with the aircrafts in the 118.700 Mhz frequency with AM modulation, however because of lack of time I was not able to explore this further until the past Sunday.

My Sunday goal was to use the RTLSDR to see if I was able to capture some ADS-B message from the aircrafts lading or leaving from the airport. Basically ADS-B is a security device that is installed in most aircrafts that is used for collision avoidance and other stuff like this. Every aircraft broadcasts informations about heading, speed, altitude and so forth.

Currently I'm working on Redis partial resynchronization of slaves as I wrote in previous blog posts.

The idea is that we have a backlog of the replication stream, up to the specified amount of bytes (this will be in the order of a few megabytes by default).

If a slave lost the connection, it connects again, see if the master RUNID is the same, and asks to continue from a given offset. If this is possible, we continue, nothing is lost, and a full resynchronization is not needed. Otherwise if the offset is about data we no longer have in the backlog, we full resync.

While a big number of users use large farms of Redis nodes, from the point of view of the project itself currently Redis is a mostly single-instance business.

I've big plans about going distributed with the project, to the extent that I'm no longer evaluating any threaded version of Redis: for me from the point of view of Redis a core is like a computer, so that scaling multi core or on a cluster of computers is the same conceptually. Multiple instances is a share-nothing architecture. Everything makes sense AS LONG AS we have a *credible way* to shard :-)

Premise: a small rant about software reliability.
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I'm very serious about software reliability, and this is not just a good thing.
It is good in a sense, as I tend to work to ensure that the software I release is solid. At the same time I think I take this issue a bit too personally: I get upset if I receive a crash report that I can't investigate further for some reason, or that looks like almost impossible to me, or with an unhelpful stack trace.

Guess what? This is a bad attitude because to deliver bugs free software is simply impossible. We are used to think in terms of labels: "stable", "production ready", "beta quality". I think that these labels are actually pretty misleading if not put in the right perspective.

This is one of this trivial changes in Redis that can make a big difference for users. Basically in the unstable branch I added some code that has the following effect, when running Redis on Linux systems:

[32741] 19 Nov 12:00:55.019 * Background saving started by pid 391
[391] 19 Nov 12:01:00.663 * DB saved on disk
[391] 19 Nov 12:01:00.673 * RDB: 462 MB of memory used by copy-on-write

As you can see now the amount of additional memory used by the saving child is reported (it is also reported for AOF rewrite operations).

Memory errors in computers are so common that you can register domain names similar to very famous domain names, but altered in one bit, and get a number of requests per hour:

http://dinaburg.org/bitsquatting.html

On Twitter:

@War3zRub1 "Hahaha it's silly how people use Redis when they need a reverse proxy"
@C4ntc0de "ZOMG! Use a real message queue, Redis is not a queue!"
@L4m3tr00l "My face when Redis BLABLABLA..."

Meanwhile *at* Twitter:

OP1: "Hey guys, there is a spike in the number of lame messages today, load is increasing..."
OP2: "Yep noticed, it's the usual troll fiesta trowing shit at Redis, 59482 messages per second right now."
OP1: "Ok, no prob, let's spawn two additional Redis nodes to serve their timelines as smooth as usually".

This post by Peter Bailis is a must read. "Safety and Liveness: Eventual consistency is not safe" [1].

[1] http://www.bailis.org/blog/safety-and-liveness-eventual-consistency-is-not-safe/

An extreme TL;DR of this is.

1) In an eventually consistent system, when all the nodes will agree again after a partition?
2) In an eventually consistent system, HOW the nodes will agree about inconsistencies?
3) In immediately consistent systems, when I'm no longer able to write? When I'm no longer able to read?