Jepsen

Breaking distributed systems so you don't have to.

Jepsen is a Clojure library. A test is a Clojure program which uses the Jepsen library to set up a distributed system, run a bunch of operations against that system, and verify that the history of those operations makes sense. Jepsen has been used to verify everything from eventually-consistent commutative databases to linearizable coordination systems to distributed task schedulers. It can also generate graphs of performance and availability, helping you characterize how a system responds to different faults. See jepsen.io for examples of the sorts of analyses you can carry out with Jepsen.

Documentation

This tutorial walks you through writing a Jepsen test from scratch.

For reference, see the API documentation.

Design overview

A Jepsen test runs as a Clojure program on a control node. That program uses SSH to log into a bunch of db nodes, where it sets up the distributed system you're going to test using the test's pluggable os and db.

Once the system is running, the control node spins up a set of logically single-threaded processes, each with its own client for the distributed system. A generator generates new operations for each process to perform. Processes then apply those operations to the system using their clients. The start and end of each operation is recorded in a history. While performing operations, a special nemesis process introduces faults into the system--also scheduled by the generator.

Finally, the DB and OS are torn down. Jepsen uses a checker to analyze the test's history for correctness, and to generate reports, graphs, etc. The test, history, analysis, and any supplementary results are written to the filesystem under store/<test-name>/<date>/ for later review. Symlinks to the latest results are maintained at each level for convenience.

Setting up a Jepsen environment

Your control node needs a JVM and Leiningen 2 installed. Probably want JNA for SSH auth too.

sudo apt-get install openjdk-8-jre openjdk-8-jre-headless libjna-java

For your db nodes, you'll need some (I use five) debian boxes. Most of these tests are designed for Debian; either Jessie, for older versions of Jepsen, or Stretch, for more recent versions. Some DBs don't need the latest packages so you might get away with an older distribution, or possibly ubuntu. Each one should be accessible from the control node via SSH. By default they're named n1, n2, n3, n4, and n5, but that (along with SSH username, password, identity files, etc) is all definable in your test. The account you use on those boxes needs sudo access to set up DBs, control firewalls, etc.

Be advised that tests may mess with clocks, add apt repos, run killall -9 on processes, and generally break things, so you shouldn't, you know, point jepsen at your prod machines unless you like to live dangerously, or you wrote the test and know exactly what it's doing.

You can run your DB nodes as separate physical machines, VMs, LXC instances, or via Docker. Note that containers (LXC and Docker) can't change system clocks, so you won't be able to test anything that relies on clock skew.

You can launch a complete Jepsen cluster from the AWS Marketplace. Choose "5+1 node cluster" to get an entire cluster as a Cloudformation stack, with SSH keys and firewall rules preconfigured. Choose "Single AMI" if you'd just like a single node.
See lxc.md for some of my notes on setting up LXC instances.
You can also use Docker Compose for setting up Docker instances.

Running a test

Once you've got everything set up, you should be able to run cd aerospike; lein test, and it'll spit out something like

INFO  jepsen.core - Analysis invalid! (ﾉಥ益ಥ）ﾉ ┻━┻

{:valid? false,
 :counter
 {:valid? false,
  :reads
  [[190 193 194]
   [199 200 201]
   [253 255 256]
   ...}}

FAQ

JSCH auth errors

If you see com.jcraft.jsch.JSchException: Auth fail, this means something about your test's :ssh map is wrong, or your control node's SSH environment is a bit weird.

Confirm that you can ssh to the node that Jepsen failed to connect to. Try ssh -v for verbose information--pay special attention to whether it uses a password or private key.
If you intend to use a username and password, confirm that they're specified correctly in your test's :ssh map.
If you intend to log in with a private key, make sure your SSH agent is running.
- ssh-add -l should show the key you use to log in.
- If your agent isn't running, try launching one with ssh-agent.
- If your agent shows no keys, you might need to add it with ssh-add.
- If you're SSHing to a control node, SSH might be forwarding your local agent's keys rather than using those on the control node. Try ssh -a to disable agent forwarding.

If you've SSHed to a DB node already, you might also encounter a jsch bug which doesn't know how to read hashed known_hosts files. Remove all keys for the DB hosts from your known_hosts file, then:

ssh-keyscan -t rsa n1 >> ~/.ssh/known_hosts
ssh-keyscan -t rsa n2 >> ~/.ssh/known_hosts
ssh-keyscan -t rsa n3 >> ~/.ssh/known_hosts
ssh-keyscan -t rsa n4 >> ~/.ssh/known_hosts
ssh-keyscan -t rsa n5 >> ~/.ssh/known_hosts

to add unhashed versions of each node's hostkey to your ~/.ssh/known_hosts.