10 May 2023

On Reliability

I read The Calculus of Service Availability: You’re only as available as the sum of your dependencies today and it summarizes some of the most salient wisdom in designing for reliability targets.

The takeaways are:

  1. Humans have enough imperfections in their nearby systems that 4 or 5 9s of reliability is the maximum value worth targeting
  2. Problems come from the the service itself or its critical dependencies.
  3. Availability = MTTF/(MTTF+MTTR)
  4. Rule of an extra 9 - Any service should rely on critical services that exceed their own SLA by 1x 9 of reliability, ie a 3x 9s service should only depend on 4x 9s services in its critical path. 5. When depending on services not meeting that threshold, it must be accounted for via resilient design 6. The math 7. Assuming a service has error budget of 0.01%. 8. They choose 0.005% error budget for service and the 0.005% for critical dependencies, then their 5 dependencies each get 1/5 of 0.005% or 0.001%, ie must be 99.999% available.
  5. Frequency * Detection * Recovery constitute the impact of outages and the feasibility of a given SLA. 7. Thus the levers for improvement are: reduce frequency of outage, blast radius (sharding, cellular architectures, or customer isolation), and MTTR.

I’ve been evolving our Online Database Platform at work and the themes of “rule of an extra 9” and how to move quickly as well as safely with limited blast radius are top of mind.

We’ve made some major changes (cluster topology, upgrades in nosql and sql, automation tooling) that are moving our stack to a point where I’m proud of the accomplishments.

Hundreds of TB of online data and hundreds of clusters managed by a team that I can count on one hand :).

12 Apr 2023

Deno for Shell Scripting a Pagerduty helper

Deno, a modern JS platform, worked well tonight as a scripting language and ecosystem for building a tiny cli interface for paging in teammates.

I will expand my use of it and replace usage of zx + js with vl + ts + Deno.

I prototyped the cli with a focus on reducing human cognitive overhead during stressful operations to provide sound defaults and reduced choice when choosing how many teammates to bring in, what to use as incident message, and details.

A few things made this easy:

  1. Confidence in deploying this as a standalone tool thanks to Deno without dependency management issues
  2. npx allowing for lazy loading of the pagerduty-cli tool to shell out to
  3. vl as a deno library that emulates behavior from Google’s SRE tooling of zx. These are javascript libraries that make for convenient bash-like scripting in a js/ts environment.

Prototype script below:

07 Apr 2023

Replacing inlined scripts with bundler inline

git smart-pull is a great tool for avoiding the messiness of git rebases when there’s changed content.

I long ago inlined the full ruby gem into a single executable file to avoid the hassle of installing it in various ruby environments. It’s worked well!

Ruby 3.2.0 broke my script in a tiny way and broke the underlying gem. The patch has sat unmerged for months and now with bundler/inline I have a better solution than keeping a spare inlined script… forking the project and pointing a wrapper script with bundler/inline at my own repo.

I’m applying patches from PRs in upstream (eg hub merge https://github.com/geelen/git-smart/pull/25).

It’s an elegant solution that I’ll re-use for other ruby scripts in my development environment.

06 Apr 2023

Automatically Warm EBS Volumes from Snapshots

We’re automating more of our cluster operations at work and here’s the procedure for warming an EBS volume created from a snapshot to avoid their first-read/write performance issues.

How it works

Follow instructions in gist for installing in root’s crontab as a @reboot run instruction. It uses an exclusive flock and a completion file to ensure idempotency.

06 Apr 2023

3x Faster Mongodb Controlled Failovers

I recently modified our failover protocol at work for MongoDB in a way that reduces the interruption from 14 seconds down to 3.5 seconds by altering election configurations ahead of controlled failovers. This was tested on a 3.4 cluster but should hold true up until modern versions. Starting in 4.0.2 it’s less valuable for controlled failovers but still useful as a tuning setting for uncontrolled failovers.

How it works

The premise is to make the shard call a new election as fast as possible by reducing electionTimeoutMillis and heartbeatIntervalMillis.


// on the primary
cfg = rs.conf()
cfg.settings["electionTimeoutMillis"] = 1000
cfg.settings["heartbeatIntervalMillis"] = 100

// wait 60 seconds for propagation

// wait for 60 seconds for election to settle
// connect to primary

cfg = rs.conf()
cfg.settings["electionTimeoutMillis"] = 10000
cfg.settings["heartbeatIntervalMillis"] = 1000

This is valuable to tune also if you’re on high quality, low latency networks. You’re missing faster failovers in non-controlled circumstances every time mongo insists on waiting 10 seconds before allowing an election, even when receiving failing heartbeats.

PS - While browsing the docs I found this ^_^ which is non-intuitive since I would expect no writes to one shard but no impact to other shards. Presumably it’s a typo and cluster means replicaset.

During the election process, the cluster cannot accept write operations until it elects the new primary.

28 Jan 2023

Use GEM_HOME for bundler inline

Ruby’s bundler inline installs to --system destination and does not respect BUNDLE_PATH (as I would expect).

Errors will be about permission errors and server requesting root access for the bundler install.

Digging around in github issues, this is desired behavior: https://github.com/rubygems/bundler/pull/7154


export GEM_HOME=vendor/bundle

05 Nov 2022

Hugo to 0.105.0

I’ve upgraded Hugo, ditched webpack for esbuild and removed most javascript from blog 🍋. The best part is that it now does syntax highlighting on the server side and builds in < 500ms 🐎.

While reading docs, I learned that Hugo allows for custom type blocks such as the mermaid diagramming in a code block below that’s rendered into an image. Unsure if this will have practical use in blog, but it’s a problem in search for an answer ;).

stateDiagram-v2 [*] --> Still Still --> [*] Still --> Moving Moving --> Still Moving --> Crash Crash --> [*]

02 Nov 2022

Export OSX Settings using `defaults` tool

Make your OSX setup reproducible by exporting settings using defaults cli tool:

$ defaults find com.apple.driver.AppleBluetoothMultitouch.trackpad

Found 24 keys in domain 'com.apple.driver.AppleBluetoothMultitouch.trackpad': {
    Clicking = 1;
    DragLock = 0;
    Dragging = 0;
    TrackpadCornerSecondaryClick = 2;
    TrackpadFiveFingerPinchGesture = 2;
    TrackpadFourFingerHorizSwipeGesture = 2;
    TrackpadFourFingerPinchGesture = 2;
    TrackpadFourFingerVertSwipeGesture = 2;
    TrackpadHandResting = 1;
    TrackpadHorizScroll = 1;
    TrackpadMomentumScroll = 1;
    TrackpadPinch = 1;
    TrackpadRightClick = 1;
    TrackpadRotate = 1;
    TrackpadScroll = 1;
    TrackpadThreeFingerDrag = 1;
    TrackpadThreeFingerHorizSwipeGesture = 1;
    TrackpadThreeFingerTapGesture = 0;
    TrackpadThreeFingerVertSwipeGesture = 0;
    TrackpadTwoFingerDoubleTapGesture = 1;
    TrackpadTwoFingerFromRightEdgeSwipeGesture = 3;
    USBMouseStopsTrackpad = 0;
    UserPreferences = 1;
    version = 5;

Then convert them into items in the format seen here: https://github.com/zph/zph/blob/master/home/.osx to create your own custom configuration to apply during next system setup.

Use defaults domains to find all the key domains.

02 Nov 2022

Dune's Litany on Reliability

I must not fear outages.

Fear is the mind-killer.

Fear is the little-death that brings total obliteration.

I will face my outages.

I will permit it to pass over me and through me.

And when it has gone past I will turn the scorecard to see its path.

Where the outages have gone there will be 5x 9s. Only availability will remain.

02 Nov 2022

One hour on blog post and two hours fixing blog builds


I spent one hour tonight writing a new blog post about storage and then my 9 month idle blog

  1. Refused to build from md to html
  2. Couldn’t compile node-gyp with dependency on EOL’d python2
  3. Debugging alpine build depdencencies for node-gyp
  4. Had webpack v1 issues on M1 apple silicon
  5. I refused to upgrade and stay on webpack
  6. Which led to me switching to esbuild for js
  7. Then I needed a custom plugin for esbuild sass
  8. Finally I learned how to execute node in yarn’s package context

Entropy rules everything around me and this is after setting up CI, pinning versions, and using a static site generator for my blog.

I had to sleep on the issue and debug it with fresh eyes… it took 2.5 hrs of debugging and that speed of tool rot makes me consider reducing blog to even more basic form for improved longevity.