Docker and Salt Redux

Recently I was digging into salt innards again; that meant it was time to dust off the old docker salt-cluster script and shoehorn a few more features in there.

Salt up close and personal.

NaCl up close and personal.

There are some couples that you just know ought to get themselves to a relationship counselor asap. Docker and SSHD fall smack dab into that category. [1]  When I was trying to get my base images for the various Ubuntu distros set up, I ran into issues with selinux, auditd and changed default config options for root, among others. The quickest way to deal with all these annoyances is to turn off selinux on the docker host and comment the heck out of a bunch of things in various pam configs and the sshd config.

The great thing about Docker though is that once you have your docker build files tested and have created your base images from those, starting up containers is relatively quick. If you need a configuration of several containers from different images with different things installed you can script that up and then with one command you bring up your test or development environment in almost no time.

Using this setup, I was able to test multiple combinations of salt and master versions on Ubuntu distros, bringing them up in a minute and then throwing them away when done, with no more concern than for tossing a bunch of temp files. I was also able to model our production cluster (running lucid, precise and trusty) with the two versions of salt in play, upgrade it, and poke at salt behavior after the upgrade.

A good dev-ops is a lazy dev-ops, or maybe it’s the other way around. Anyways, I can be as lazy as the best of ’em, and so when it came to setting up and testing the stock redis returner on these various salt and ubuntu versions, that needed to be scriptified too; changing salt configs on the fly is a drag to repeat manually. Expect, ssh, cp and docker ps are your best friends for something like this. [2]

In the course of getting the redis stuff to work, I ran across some annoying salt behavior, so before you run into it too, I’ll explain it here and maybe save you some aggravation.

The procedure for setting up the redis returner included the following bit:

– update the salt master config with the redis returner details
– restart the master
– copy the update script to the minions via salt

This failed more often than not, on trusty with 2014.1.10. After these steps, the master would be seen to be running, the minions were running, a test.ping on all the minions came back showing them all responsive, and yet… no script copy.

The first and most obvious thing is that the salt master restart returns right away, but the master is not yet ready to work. It has to read keys, spawn worker threads, each of those has to load a pile of modules, etc.  On my 8-core desktop, for 25 workers this could take up to 10 seconds.

Salt works on a sub/pub model [3], using ZMQ as the underlying transport mechanism for this. There’s no ack from the client; if the client gets the message, it runs the job if it’s one of the targets, and returns the results. If the client happens to be disconnected, it won’t get the message. Now salt minions do reconnect if their connection goes away but this takes time.

Salt (via ZMQ) also encrypts all messages. Upon restart, the master generates a new AES key, but the minions don’t learn about this til they receive their first message, typically with some job to run. They will try to use the key they had lying round from a minute ago to decrypt, fail, and then be forced to try again. But this retry takes time. And while the job will eventually be run and the results sent back to the master, your waiting script may have long since given up and gone away.

With the default salt config, the minion reconnect can take up to 5 seconds. And the minion re-auth retry can take up to 60 seconds. Why so long? Because in a production setting, if you restart the master and thousands of minions all try to connect at once, the thundering herd will kill you. So the 5 seconds is an upper limit, and each minion will wait a random amount of time up to that upper limit before reconnect. Likewise the 60 seconds is an upper limit for re-authentication. [4]

This means that after a master restart, it’s best to wait at least 15 seconds before running any job, 10 for master setup and 5 for the salt minion reconnect. This ensures that the salt minion will actually receive the job. (And after a minion restart, it’s best to wait at least 5 seconds before giving it any work to do, for the same reason.)

Then be sure to run your salt command with a nice long timeout of longer than 60 seconds. This ensures that the re-auth and the job run will get done, and the results returned to the master, before your salt command times out and gives up.

Now the truly annoying bit is that, in the name of perfect forward secrecy, an admittedly worthy goal, the salt master will regenerate its key after 24 hours of use, with the default config. And that means that if you happen to run a job within a few seconds of that regen, whenever it happens, you will hit this issue. Maybe it will be a puppet run that sets a grain, or some other automated task that trips the bug. Solution? Make sure all your scripts check for job returns allowing for the possibility that the minion had to re-auth.

Tune in next time for more docker-salt-zmq fun!

[1] Docker ssh issues on github
[2] Redis returner config automation
[3] ZMQ pub/sub docs
[4] Minion re-auth config and Running Salt at scale

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