April 24, 2024

Most of Slack runs on a monolithic service merely referred to as “The Webapp”. It’s huge – lots of of builders create lots of of modifications each week.

Deploying at this scale is a singular problem. When individuals discuss steady deployment, they’re usually fascinated about deploying to methods as quickly as modifications are prepared. They discuss microservices and 2-pizza groups (~8 individuals). However what does steady deployment imply if you’re taking a look at 150 modifications on a traditional day? That’s plenty of pizzas…

Graph showing changes opened, merged, and deployed per day, from October 16th to October 20th. Changes deployed is between 150 and 190.
Adjustments per day.


Steady deployments are preferable to massive, one-off deployments.

  1. We would like our clients to see the work of our builders as quick as attainable in order that we will iterate rapidly. This enables us to reply rapidly to buyer suggestions, whether or not that suggestions is a characteristic request or bug studies.
  2. We don’t wish to launch a ton of modifications without delay. There’s a better probability of errors and people errors are tougher to debug inside a sea of modifications.

So we have to transfer quick – and we do transfer quick. We deploy from our Webapp repository 30-40 occasions a day to our manufacturing fleet, with a median deploy measurement of three PRs. We handle an affordable PR-to-deploy ratio regardless of the dimensions of our system’s inputs.

A graph showing deploys per day, from October 16th to October 20th. The number bounces between 32 and 37.


We handle these deployment speeds and sizes utilizing our ReleaseBot. It runs 24/7, regularly deploying new builds. Nevertheless it wasn’t all the time like this. We used to schedule Deploy Commanders (DCs), recruiting them from our Webapp builders. DCs would work a 2 hour shift the place they’d stroll Webapp by its deployment steps, watching dashboards and executing guide checks alongside the best way.

The Launch Engineering staff managed the deployment tooling, dashboards, and the DC schedule. The strongest, most frequent, suggestions Launch Engineering heard from DCs was that they weren’t assured making choices. It’s tough to watch the deployment of a system this massive. DCs have been on a rotation with lots of of different builders. How do you get comfy with a system that you could be solely work together with each few months? What’s regular? What do you do if one thing goes incorrect? We had coaching and documentation, nevertheless it’s not possible to cowl each edge case.

So Launch Engineering began fascinated about how we might give DCs higher indicators. Absolutely automating deployments wasn’t on the radar at this level. We simply wished to provide DCs higher-level, clearer “go/no-go” indicators.

We labored on the ReleaseBot for 1 / 4 and let it run alongside DCs for 1 / 4 earlier than realizing that ReleaseBot could possibly be trusted to deal with deployments by itself. It caught points sooner and extra persistently than people, so why not put it within the driver’s seat?

The center of ReleaseBot is its anomaly detection and monitoring. That is each the scariest and most essential piece in any automated deployment system. Bots transfer sooner than people, that means you’re one bug and a really quick time period away from bringing down manufacturing.

The dangers that include automation are value it for two causes:

  1. It’s safer if you will get the monitoring proper. Computer systems are each sooner and extra vigilant than people.
  2. Human time is our most beneficial, constrained useful resource. What number of hours do your organization’s engineers spend observing dashboards?

Screenshot of Slack Message from Release Bot saying "ReleaseBot started for webapp"

Monitoring by no means feels “carried out”

Any engineer that’s been on-call will know this cycle:

  1. You monitor the whole lot with tight thresholds.
  2. These tight thresholds, mixed with a loud service, result in frequent pages.
  3. Pissed off and drained, you delete a number of alerts and enhance some thresholds
  4. You lastly get some sleep.
  5. An incident happens as a result of that noisy service really broke one thing however you didn’t get paged.
  6. Somebody in an incident evaluation asks why you weren’t monitoring one thing.
  7. Go to step 1.


This cycle stops plenty of groups from implementing automated deployments. I’ve been in conferences like this a number of occasions all through my profession:

  • Particular person 1: “Why don’t we simply automate deployments?”
  • Everybody: *Nods*
  • Particular person 2: “What if one thing breaks?”
  • Everybody: *Seems unhappy*


The dialog doesn’t make it previous this level. Everyone seems to be satisfied it received’t work as a result of it seems like we don’t have a strong maintain on our alarms as-is – and that’s with people within the loop!

Even in case you have strong alerting and an affordable on-call burden, you in all probability end up making small tweaks to alerts each few months. Advanced methods expertise a low hum of background errors and the whole lot from efficiency traits, to dependencies, to the methods themselves change over time. Defining a selected quantity as “unhealthy” for a posh system is open to subjective interpretation. It’s a judgment name. Is 100 errors unhealthy? What a couple of 200 millisecond common latency?  Is one unhealthy information level sufficient to web page somebody or ought to we wait a couple of minutes? Will your solutions be the identical in a month?

Given these constraints, writing a program we belief to deal with deployments can appear insurmountable however, in some methods, it’s simpler than monitoring on the whole.

How deployments are totally different

The variety of errors a system experiences in a steady-state isn’t essentially related to a deployment. If each model 1 and model 2 of an utility emit 100 errors per second, then model 2 didn’t introduce any new, breaking modifications. By evaluating the state of model 1 and model 2 and figuring out that the state of the system didn’t change, we might be assured that model 2 is a “good” deployment.

You’re principally involved with anomalies within the system when deploying. This necessitates a unique method.

That is intuitive if you concentrate on the way you watch a dashboard throughout a deployment. Think about you simply deployed some new code. You’re taking a look at a dashboard. Which of those two graphs catches your consideration?

Two graphs with a line on each denoting a deployment. The left graph is at 1, then spikes to 10 and 15 immediately after the deployment. The right graph is a flat line at 100 before and after the deployment.


Clearly, the graph with a spike is regarding. We don’t even know what this metric represents. Possibly it’s spike! Both manner, you recognize to search for these spikes. They’re a sign one thing is tangibly totally different. And also you’re good at it. You possibly can simply scan the dashboard, ignoring particular numbers, on the lookout for anomalies. It’s simpler and sooner than expecting thresholds on each particular person graph.

So how can we train a pc to do that?

Picture of a robot emoji with a robot cat in a thought bubble. They are in front of a graph in the rough shape of a cat. The text reads "It's easy for humans to spot anomalies in data. For example, this PHP Errors chart resembles my cat".


Fortunately for us, defining “anomalous” is mathematically easy. If a traditional alert threshold is a judgment name involving tradeoffs between below and over alerting, a deployment threshold is a statistical query. We don’t must outline “unhealthy” in absolute phrases. If we will see that the brand new model of the code has an anomalous error price, we will assume that’s unhealthy – even when we don’t know anything in regards to the system.

In brief, you in all probability have all of the metrics you want to begin automating your deployments at present. You simply want to take a look at them a bit in a different way.

Our give attention to “anomalous” is, after all, a bit overfit. Monitoring arduous thresholds throughout a deployment is cheap. That data is accessible, and a easy threshold offers us the sign that we’re on the lookout for more often than not, so why wouldn’t we use it? Nonetheless, you will get indicators on-par with a human scanning a dashboard for those who can implement anomaly detection.

The nitty-gritty

Let’s get into the main points of anomaly detection. We now have 2 methods of detecting anomalous habits: z scores and dynamic thresholds.

Your new greatest pal, the z rating

The best mathematical solution to discover an anomaly is a z rating. A z rating represents the variety of commonplace deviations from the imply for a selected information level (if that each one sounds too math-y, I promise it will get higher). The bigger the quantity, the bigger the outlier.

A picture of a robot emoji with sunglasses on the cover of Kenny Loggins Danger Zone, in front of a graph show a normal distribution with standard deviations. The text reads "A z-score tells us how far a value is from the mean, measured in terms of standard deviation. For example, a z-score of 2.5 or -2.5 means that the value is between 2 to 3 standard deviations from the mean.


Mainly, we’re mathematically detecting a spike in a graph.

This generally is a little intimidating for those who’re not aware of statistics or z scores, however that’s why we’re right here! Learn on to learn the way we do it, the way you would possibly implement it, and some classes we discovered alongside the best way.

First, what’s a z rating? The precise equation for figuring out the z rating for a selected information level is ((information level – imply) / commonplace deviation).

Utilizing the above equation, we will calculate the z scores for each information level in a selected time interval.

Fortunately, calculating a z rating is computationally easy. ReleaseBot is a Python utility. Right here’s our implementation of z scores in Python, utilizing scipy’s stats library:

from scipy import stats

def calculate_zscores(self) -> checklist[float]:
	# Seize our information factors
	values = ChartHelper.all_values_in_automation_metrics(
	# Calculate zscores
	return checklist(stats.zscore(values))

You are able to do the identical factor in Prometheus, Graphite, and in most different monitoring instruments. These instruments often have built-in features for calculating the imply and the usual deviation of datapoints. Right here’s a z rating calculation for the final 5 minutes of knowledge factors in PromQL:

/ stddev_over_time(metric[3h])

Now that ReleaseBot has the z scores, we verify for z rating threshold breaches and ship a sign to our automation. ReleaseBot will routinely cease deployments and notify a Slack channel.

Virtually all of our z rating thresholds are 3 and/or -3 (-3 detects a drop within the graph). A z rating of three typically represents a datapoint above the 99th percentile. I say “typically” as a result of this actually will depend on the form of your information. A z rating of three can simply be the 99.seventh percentile for a dataset.

So a z rating of three is a big outlier, nevertheless it doesn’t must be a big distinction in absolute phrases. Right here’s an instance in Python:

>>> from scipy import stats
# Record representing a metric that alternates between 
# 1 and three for 3 hours (180 minutes)
>>> x = [1 if i % 2 == 0 else 3 for i in range(180)]
# Our most up-to-date datapoint jumps to five.5
>>> x.append(5.5)
# Calculate our zscores and seize the rating for the 5.5 datapoint
>>> rating = stats.zscore(x)[-1]
>>> rating

The identical scenario, in graph kind:

A graph that bounces between 1 and 3 continually, then jumps to 5.5 at the last datapoint. A red arrow points to 5.5 with "z score = 3.37".


So if we’ve a graph that’s been hanging out between 1 and three for 3 hours, a leap to five.5 would have a z rating of three.37. It is a threshold breach. Our metric solely elevated by 2.5 in absolute numerical phrases, however that leap was an enormous statistical outlier. It wasn’t an enormous leap, nevertheless it was undoubtedly an uncommon leap.

That is precisely the kind of sample that’s apparent to a human scanning a dashboard, however could possibly be missed by a static threshold as a result of the precise change in worth is so low.

It’s actually that easy. You should use built-in features within the software of your option to calculate the z rating and now you may detect anomalies as an alternative of wrestling with hard-coded thresholds.

Some additional suggestions:

  1. We’ve discovered a z rating threshold of three is an effective place to begin. We use 3 for almost all of our metrics.
  2. Your commonplace deviation can be 0 if your whole numbers are the identical. The z rating equation requires dividing by the usual deviation. You possibly can’t divide by 0. Be sure that your system handles this.
    1. In our Python utility, scipy.stats.zscore will return “nan” (not a quantity) on this situation. So we simply overwrite “nan” with 0. There was no variation within the metric – the road was flat – so we deal with it like a z rating of 0.
  3. You would possibly wish to ignore both unfavorable or constructive z scores for some metrics. Do you care if errors or latency go down? Possibly! However give it some thought.
  4. You might wish to monitor issues that don’t historically point out points with the system. We, for instance, monitor whole log quantity for anomalies. You in all probability wouldn’t web page an on-call due to elevated informational log messages, however this might point out some surprising change in habits throughout a deployment. (There’s extra on this later.)
  5. Snoozing z rating metrics is a killer characteristic. Generally a change in a metric is an anomaly primarily based on historic information, however you recognize it’s going to be the brand new “regular”. If that’s the case, you’ll wish to snooze your z scores for no matter interval you employ to calculate z scores. ReleaseBot seems on the final 3 hours of knowledge, so the ReleaseBot UI has a “Snooze for 3 Hours” button subsequent to every metric.

How Slack makes use of z scores

We think about z scores “excessive confidence” indicators. We all know one thing has undoubtedly modified and somebody wants to have a look.

At Slack, we’ve an ordinary system of utilizing white, blue, or pink circle emojis inside Slack messages to indicate the urgency of a request, with white being the bottom urgency and pink the best.

A screenshot of a Slack message from Release Bot. The message is a blue circle emoji with text, "Webapp event #2528 opened for char Five Hundred Errors, in tier dogfood and az use1-az2".


A single z rating threshold breach is a blue circle. Think about you noticed one graph spike on the dashboard. That’s not good however you would possibly do some investigation earlier than elevating any alarms.

A number of z rating threshold breaches are a pink circle. You recognize one thing unhealthy simply occurred for those who see a number of graphs leap on the identical time. It’s affordable to take remediation actions earlier than digging right into a root trigger.

We monitor the everyday metrics you’d count on (errors, 500’s, latency, and many others – see Google’s The Four Golden Signals), however listed below are some doubtlessly fascinating ones:

Metric Excessive z rating Low z rating Notes
PHPErrors 1.5 We select to be particularly delicate to error logs.
StatusSlackCom 3 -3 That is the variety of requests to https://status.slack.com – the location customers entry to verify if Slack is having issues. Lots of people all of the sudden curious in regards to the standing of Slack is an effective indication that one thing is damaged.
WebsocketEventsVolume -3 A excessive variety of consumer connections doesn’t essentially imply that we’re overloaded. However an surprising drop in consumer connections might imply we’ve launched one thing particularly unhealthy on the backend.
LogVolume 3 Separate from error logs. Are we creating many extra logs than ordinary? Why? Can our logging system deal with the amount?
EnvoyPanicRouting 3 Envoy routes site visitors to the Webapp hosts. It begins “panic routing” when it may possibly’t find sufficient hosts. Are hosts stopping however not restarting throughout the deployment? Are we deploying too rapidly – taking down too many hosts without delay?


Past the z rating, dynamic thresholds

We nonetheless monitor static thresholds however we think about them “low confidence” alarms (they’re a white circle). We set static thresholds for some key metrics however Releasebot additionally calculates its personal dynamic threshold, utilizing the increased of the 2.

Think about the database staff deploys some element each Wednesday at 3pm. When this deployment occurs, database errors briefly spike above your alert threshold, however your utility handles it gracefully. Because the utility handles it gracefully, customers don’t see the errors and thus we clearly don’t must cease deployments on this scenario.

So how can we monitor a metric utilizing a static threshold whereas filtering out in any other case “regular” habits? We use a median derived from historic information.

“Historic information” deserves some clarification right here. Slack is utilized by enterprises. Our product is usually used throughout the typical workday, 9am to 5pm, Monday by Friday. So we don’t simply seize a bigger, steady window of knowledge after we’re fascinated about historic relevance. We pattern information from comparable time durations.

Let’s say we’re operating this calculation at 6pm on Wednesday. We’ll pull information from:

  • 12pm-6pm Wednesday (at present).
  • 12pm-6pm Tuesday.
  • 12pm-6pm final Wednesday.

We pool all of those home windows collectively and calculate a easy common. Right here’s how you could possibly obtain the identical end result with PromQL:

	+ sum(metric[6h] offset 1d)
	+ sum(metric[6h] offset 1w)
 ) / 3

Once more, this can be a pretty easy algorithm:

  1. Collect historic information and calculate the typical.
  2. Take the bigger of “the typical historic information” and “hard-coded threshold”.
  3. Cease deployments and alarm if the final 5 information factors breach the chosen threshold.

In easy phrases: We watch thresholds however we’re keen to disregard a breach if historic information signifies it’s regular.

Dynamic thresholds are a nice-to-have, however not strictly required, characteristic of ReleaseBot. Static thresholds could also be a bit extra noisy, however don’t carry any further dangers to your manufacturing methods.

Embrace the worry

Concern of breaking manufacturing holds many groups again from automating their deployments, however understanding how deployment monitoring differs from regular monitoring opens the door to easy, efficient instruments.

It’ll nonetheless be scary. We took a cautious, iterative method to ease our fears. We added z rating monitoring to our ReleaseBot platform and in contrast its outcomes to the people operating deployments and watching graphs. The outcomes of ReleaseBot have been much better than we anticipated; to the purpose the place it appeared irresponsible to not put ReleaseBot within the driver’s seat for deployments.

So throw some z scores on a dashboard and see how they work. You would possibly simply unintentionally assist your coworkers keep away from observing dashboards all day.

A screenshot of a message from ReleaseBot with the text "Release Bot has called 'all clear' on that deploy!"

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