Scaling ARA to a million Ansible playbooks a month

The OpenStack community runs over 300 000 CI jobs with Ansible every month with the help of the awesome Zuul.

It even provides ARA reports for ARA’s integration test jobs in a sort-of nested way. Zuul’s Ansible ends up installing Ansible and ARA. It makes my brain hurt sometimes… but in an awesome way.

As a core contributor of the infrastructure team there, I get to witness issues and get a lot of feedback directly from the users.

Static HTML report generation in ARA is simple but didn’t scale very well for us. One day, I was randomly chatting with Ian Wienand and he pointed out an attempt at a WSGI middleware that would serve extracted logs.

That inspired me to write something similar but for dynamically loading ARA sqlite databases instead… This resulted in an awesome feature that I had not yet taken the time to explain very well… until now.

Excerpt from the documentation

To put this use case into perspective, it was “benchmarked” against a single job from the OpenStack-Ansible project:

• 4 playbooks
• 4760 results
• 53 hosts, of which 39 had gathered host facts
• 416 saved files

Generating a static report from that database takes ~1min30s on an average machine. The result contains 5321 files and 5243 directories for an aggregate size of 63MB (or 27MB recursively gzipped).

This middleware allows you to host the exact same report on your web server just by storing the sqlite database which is just one file and weighs 5.6MB.

This middleware can be useful if you’re not interested in aggregating data in a central database server like MySQL or PostgreSQL.

The OpenStack CI use case is decentralized: each of the >300 000 Zuul CI jobs have their own sqlite database uploaded as part of the log and artifact collection.

There’s a lot of benefits of doing things this way:

• There’s no network latency to a remote database server: the first bottleneck is your local disk speed.
• Even if it’s a 5ms road trip, this adds up over hundreds of hosts and thousands of tasks.
• Oh, and contrary to popular belief, sqlite is pretty damn fast.
• There’s no risk of a network interruption or central database server crash which would make ARA (and your sysadmins) panic.
• Instead of one large database with lots of rows, you have more databases (“shards”) with fewer rows.
• Instead of generating thousands of files and directories, you’re dealing with one small sqlite file.
• There’s no database cluster to maintain, just standard file servers with a web server in front.

Another benefit is that you can easily have as many individual reports as you’d like, all you have to do is to configure ARA to use a custom database location.

When I announced that we’d be switching to the sqlite middleware on openstack-dev, I mentioned that projects could leverage this within their jobs and OpenStack-Ansible was the first to take a stab at it: https://review.openstack.org/#/c/557921/.

Their job’s logs now look like this:

ara-report/ansible.sqlite   # ARA report for this Zuul job
logs/                       # Job's logs
└── ara-report/             # ARA report for this OpenStack-Ansible deployment
└── ansible.sqlite      # Database for this OpenStack-Ansible deployment


The performance improvements for the OpenStack community at large are significant.

Even if we’re spending 1 minute generating and transferring thousands of HTML files… That’s >300 000 minutes worth of compute that could be spent running other jobs.

How expensive are 300 000 minutes (or 208 days!) of compute ? What about bandwidth and storage ?

Unfreezing ARA’s stable release for development

The latest version of ARA is currently 0.14.6 and ARA was more or less in feature-freeze mode while all the work was focused on the next major release, “1.0”.

However, there is a growing amount of large scale users (me included!) that are really pushing the current limitations of ARA and 1.0 (or 2.0!) won’t be ready for a while still.

I couldn’t afford to leave performance issues and memory leaks ruin the experience of a tool that would otherwise be very useful to them.

These improvement opportunities have convinced me that there will be a 0.15.0 release for ARA.

Stay tuned for the 0.15.0 release notes and another update about 2.0 in the near future :)