Posts By: Will Keleher

Canary Containers at ClassDojo in Too Much Detail

Canary releases are pretty great! ClassDojo uses them as part of our continuous delivery pipeline: having a subset of real users use & validate our app before continuing with deploys allows us to safely & automatically deploy many times a day.

Our canary releases are conceptually simple:

  1. we start canary containers with a new container image
  2. we then route some production traffic to these containers
  3. we monitor them: if a container sees a problem, we stop our pipeline. If they don't see problems, we start a full production deploy

Simple enough, right? There are a few details that go into setting up a system like this, and I'd like to take you through how ClassDojo does it. Our pipeline works well for our company's needs, and I think it's a good example of what this kind of canary-gated deploy can look like.

The key pieces of our system:

  1. We have a logging taxonomy that lets us accurately detect server-errors that we want to fix. ("Errors" that we don't want to fix aren't actually errors!)
  2. HAProxy, Consul, and Nomad let us route a subset of production traffic to a group of canary containers running new code
  3. Our canary containers expose a route with the count of seen errors and the count of total requests that a monitoring script in our jenkins pipeline can hit
  4. The monitoring script will stop our deployment if it sees a single error. If it sees 75,000 successful production requests, it will let the deploy go to production. (75,000 is an arbitrary number that gives us a 99.9% chance of catching errors that happen 1/10^4 requests. )

Starting canary containers

ClassDojo uses Nomad for our container orchestration, so once we've built a docker image and tagged it with our updated_image_id, we can deploy it by running nomad run api-canary.nomad.

1// api-canary.nomad
2job "api-canary" {
3 group "api-canary-group" {
4 count = 8
5 task "api-canary-task" {
6 driver = "docker"
7 config {
8 image = "updated_image_id"
9
10 }
11 service {
12 name = "api-canary"
13 port = "webserver_http"
14 // this registers this port on these containers with consul as eligible for “canary” traffic
15 }
16 resources {
17 cpu = 5000 # MHz
18 memory = 1600
19
20 network {
21 port "webserver_http"{}
22 }
23 }
24 }
25 }
26}

Nomad takes care of running these 8 (count = 8) canary containers on our nomad clients. At this point, we have running containers, but they're not serving any traffic.

Routing traffic to our canary containers

Remember that nomad job file we looked at above? Part of what it was doing was registering a service in consul. We tell consul that the webserver_http port can provide the api-canary service.

1service {
2 name = "api-canary"
3 port = "webserver_http"
4}

We use HAProxy for load-balancing, and we use consul-template to generate updated haproxy configs every 30 seconds based on the service information that consul knows about.

1backend api
2 mode http
3 # I'm omitting a *ton* of detail here!
4 # See https://engineering.classdojo.com/2021/07/13/haproxy-graceful-server-shutdowns talks about how we do graceful deploys with HAProxy
5
6{{ range service "api-canary" }}
7 server canary_{{ .Address }}:{{ .Port }} {{ .Address }}:{{ .Port }}
8{{ end }}
9
10# as far as HAProxy is concerned, the canary containers above should be treated the same as our regularly deployed containers. It will round robin traffic to all of them
11{{ range service "api" }}
12 server api_{{ .Address }}:{{ .Port }} {{ .Address }}:{{ .Port }}
13{{end}}

Monitoring canary

Whenever we see an error, we increment a local counter saying that we saw the error. What counts as an error? For us, an error is something we need to fix (most often 500s or timeouts): if something can't be fixed, it's part of the system, and we need to design around it. If you're curious about our approach to categorizing errors, Creating An Actionable Logging Taxonomy digs into the details. Having an easy way of identifying real problems that should stop a canary deploy is the key piece that makes this system work.

1let errorCount: number = 0;
2export const getErrorCount = () => errorCount;
3export function logServerError(errorDetails: ErrorDetails) {
4 errorCount++;
5 metrics.increment("serverError");
6 winstonLogger.log("error", errorDetails);
7}

Similarly, whenever we finish with a request, we increment another counter saying we saw the request. We can then expose both of these counts on our status route. There are probably better ways of publishing this information to our monitoring script rather than via our main server, but it works well enough for our needs.

1router.get("/api/errorAndRequestCount", () => {
2 return {
3 errorCount: getErrorCount(),
4 requestCount: getRequestsSeenCount(),
5 ...otherInfo,
6 });
7});

Finally, we can use consul-template to re-generate our list of canary hosts & ports, and write a monitoring script to check the /api/errorAndRequestCount route on all of them. If we see an error, we can run nomad job stop api-canary && exit 1, and that will stop our canary containers & our deployment pipeline.

consul-template -template canary.tpl:canary.txt -once

1{{ range service "api-canary" }}
2 {{ .Address }}:{{ .Port }}
3{{end -}}

Our monitoring script watches our canary containers until it sees that they've handled 75,000 requests without an error. (75,000 is a little bit of an arbitrary number: it's large enough that we'll catch relatively rare errors, and small enough that we can serve that traffic on a small number of containers within a few minutes.)

1const fs = require("fs");
2const canaryContainers = fs
3 .readFileSync("./canary.txt")
4 .toString()
5 .split("\n")
6 .map((s) => s.trim())
7 .filter(Boolean);
8const fetch = require("node-fetch");
9const { execSync } = require("child_process");
10const GOAL_REQUEST_COUNT = 75_000;
11
12const delay = (ms) => new Promise((resolve) => setTimeout(resolve, ms));
13
14(async function main() {
15 while (true) {
16 let totalRequestCount = 0;
17 for (const container of canaryContainers) {
18 const { errorCount, requestCount } = await fetch(
19 `${container}/api/errorAndRequestCount`
20 ).then((res) => res.json());
21 totalRequestCount += requestCount;
22 if (errorCount) {
23 // stopping our canary containers is normally handled by the next stage in our pipeline
24 // putting it here for illustration
25 console.error("oh no! canary failed");
26 execSync(`nomad job stop api-canary`);
27 return process.exit(1);
28 }
29 }
30
31 if (totalRequestCount >= GOAL_REQUEST_COUNT) {
32 console.log("yay! canary succeeded");
33 execSync(`nomad job stop api-canary`);
34 return process.exit(0);
35 }
36
37 await delay(1000);
38 }
39})();

Nary an Error with Canary

We've been running this canary setup (with occasional changes) for over eight years now, and it's been a key part of our continuous delivery pipeline, and has let us move quickly and safely. Without it, we would have shipped a lot more errors fully out to production, our overall error rate would likely be higher, and our teams would not be able to move as quickly as they can. Our setup definitely isn't perfect, but it's still hugely valuable, and I hope that sharing our setup will help your team create a better one.

Interested in working in an engineering culture that values automated testing, continuous delivery, and high collaboration? ClassDojo is hiring and we'd love to chat!

I've done a few Advent of Code problems, and one thing that's occasionally tripped me up has been my erroneous assumption that accessing the key of an object is fast. It's a fast constant time operation, but whatever hashing node is doing internally to find a key can sometimes be a performance hot-spot. There's a massive difference in speed between Array[index] and Object[key], and that made me start to wonder whether Object[key] was slow enough that there were situations where it'd make more sense to use Array.includes instead.

When I benchmarked it, I was pretty astounded to see that on a sorted array, Array.includes is faster on arrays of 10,000 strings than Object[key]. Internally, I'm guessing Array.includes is using binary-search when it knows that the array is sorted, which is a cool optimization! Here's the gist if you want to test it out. (I'm running these tests on Node 14 on a Mac Mini)

What about unsorted arrays?

When the array is unsorted, Object[key] look-ups start beating Array.includes pretty quickly: once we have ~20 documents, we'd expect to see faster look-ups from keyed object access. This is much more in line with what I expected when setting up this test.

What about Set.has?

Set.has is consistently slower than Object[key]. It can handle more cases than Object[key] can, but from a speed perspective, it's not in the running. I still often use it over Object[key] because I think it does a better job of expressing the intent to someone who's reading the code, and it's still a fast-enough constant time look-up.

What does this all mean?

Absolutely nothing! I still use Set.has for the majority of situations where I need to check membership in a list. For most of the code I write, expressing clear intent is far more important than optimizing performance, and I have yet to profile non-Advent-of-Code performance and find Object[key] or Set.has being a hotspot.

Benchmark Results

110 object access x 25,715,807 ops/sec ±0.70% (94 runs sampled)
2 10 set x 18,422,013 ops/sec ±0.85% (96 runs sampled)
3 10 array x 28,105,533 ops/sec ±0.88% (89 runs sampled)
4Fastest for ordered 10 is array
5 40 object access x 20,175,473 ops/sec ±0.90% (92 runs sampled)
6 40 set x 19,632,409 ops/sec ±0.73% (92 runs sampled)
7 40 array x 27,999,865 ops/sec ±1.09% (94 runs sampled)
8Fastest for ordered 40 is array
9 100 object access x 21,842,706 ops/sec ±0.70% (95 runs sampled)
10 100 set x 18,283,045 ops/sec ±0.78% (95 runs sampled)
11 100 array x 27,860,691 ops/sec ±1.03% (94 runs sampled)
12Fastest for ordered 100 is array
13 1000 object access x 20,026,184 ops/sec ±0.82% (92 runs sampled)
14 1000 set x 14,656,626 ops/sec ±1.37% (89 runs sampled)
15 1000 array x 23,392,774 ops/sec ±1.31% (90 runs sampled)
16Fastest for ordered 1000 is array
17 10000 object access x 14,367,754 ops/sec ±0.58% (93 runs sampled)
18 10000 set x 12,213,161 ops/sec ±0.51% (95 runs sampled)
19 10000 array x 28,067,504 ops/sec ±0.94% (92 runs sampled)
20Fastest for ordered 10000 is array
21 100000 object access x 9,106,145 ops/sec ±1.55% (93 runs sampled)
22 100000 set x 7,918,164 ops/sec ±0.92% (91 runs sampled)
23 100000 array x 78,374 ops/sec ±58.42% (86 runs sampled)
24Fastest for ordered 100000 is object access
25 1000000 object access x 3,296,346 ops/sec ±1.05% (93 runs sampled)
26 1000000 set x 2,076,943 ops/sec ±1.24% (91 runs sampled)
27 1000000 array x 490 ops/sec ±3.50% (40 runs sampled)
28Fastest for ordered 1000000 is object access
29 10 object access x 24,869,178 ops/sec ±0.80% (91 runs sampled)
30 10 set x 19,181,356 ops/sec ±0.98% (92 runs sampled)
31 10 array x 27,691,570 ops/sec ±0.99% (90 runs sampled)
32Fastest for random 10 is array
33 40 object access x 27,450,529 ops/sec ±1.30% (86 runs sampled)
34 40 set x 12,483,804 ops/sec ±1.30% (91 runs sampled)
35 40 array x 16,590,091 ops/sec ±0.92% (92 runs sampled)
36Fastest for random 40 is object access
37 100 object access x 31,040,999 ops/sec ±1.24% (87 runs sampled)
38 100 set x 12,386,070 ops/sec ±1.14% (91 runs sampled)
39 100 array x 16,389,384 ops/sec ±1.22% (91 runs sampled)
40Fastest for random 100 is object access
41 1000 object access x 25,365,248 ops/sec ±1.52% (88 runs sampled)
42 1000 set x 11,275,049 ops/sec ±1.51% (91 runs sampled)
43 1000 array x 16,294,268 ops/sec ±1.23% (88 runs sampled)
44Fastest for random 1000 is object access
45 10000 object access x 16,907,288 ops/sec ±1.47% (89 runs sampled)
46 10000 set x 8,905,038 ops/sec ±1.47% (91 runs sampled)
47 10000 array x 14,777,156 ops/sec ±1.19% (91 runs sampled)
48Fastest for random 10000 is object access
49...
Engineering

When multiple teams share a resource like a code-base, a database, or an analytics pipeline, that resource can often suffer from the Tragedy of the Commons. Well intentioned engineers can add slow code, not prioritize important improvements because those improvements aren't aligned with team incentives, and just generally not leave the codebase or database better than they found it.

ClassDojo has a monolithic backend web-server API, but it and its backing databases often suffered from hard to diagnose performance problems because engineers would accidentally add code with poorly indexed queries, with database fanouts, or that blocked the event loop, and they wouldn't realize that that code was causing any problems. We intentionally made our Model layer request-agnostic, which helps us create better tests & simpler code, but it meant that we didn't have a way of instrumenting database & performance cost by request until Node 14 started supporting AsyncLocalStorage.

AsyncLocalStorage "creates stores that stay coherent through asynchronous operations." This lets us reference a consistent context over the course of a request to get information about the route, store information about how many database requests that request has triggered, count how many documents we've fetched, and dramatically increase our visibility into what every request is doing. Because every single one of our routes is tied to a product area, and each of our product areas is owned by a team, we were suddenly able to give each team insight into any performance problems they'd inadvertently added to the codebase. None of our engineers and teams ever wanted to cause performance problems, but the lack of pre-AsyncLocalStorage system legibility meant that these issues were difficult to track down, and teams always had higher priority work than tracking down unattributed performance problems.

When we set up our AsyncLocalStorage per-request instrumentation system, we found some crazy things:

  • a route that occasionally made 30,000+ database requests because it was fanning out over a large list of items
  • another route that blocked the event-loop for 15-20 seconds a few times a day, and caused timeouts for any other requests that our server was handling at the same time
  • a third route that was occasionally fetching 500,000+ items to render information to return to clients
  • and plenty of other routes that weren't doing things nearly this egregious, but were still bad enough that we wanted to fix them

Once we had this instrumentation, it was incredibly simple for the right team to find and fix these issues. We'd looked into some of these & similar issues before, but finding & fixing them was incredibly time-consuming. Now, we have a single log that triggers an alert and tells you everything you need to know to track down the problem.

What did we track?

  1. Whenever we query one of our databases, we now include a comment with the query with the route name that triggered the request. When we go through our database's slow query logs, this comment shows us where the query actually came from, and lets us accurately attribute our database load. This makes our database optimization work so much simpler because many of our query patterns could be caused by many different routes!
  2. We also track:
  • the number of database queries made per database and per table-model over the course of a request
  • the number of items returned over the course of a request per database and per table-model
  • the total duration spent querying per-database and per table-model

Whenever we exceed safe thresholds for any of these metrics over the course of a request, we log out the details and send an alert to the appropriate team. We also send all of this data to our data warehouse, Redshift, so we can run more detailed queries about what each route is doing.

  1. We track overall event-loop-lag per container, and whenever event-loop-lag crosses 1 second (which is incredibly high!), we log out the same AsyncLocalStorage based request-cost details for all recent requests. This always points to situations where we've fetched a lot of data, and are then doing something computationally complex with it.

Monoliths need Transparency

It's hard to overestimate how much using AsyncLocalStorage like this has helped easily improve our backend web-server's performance. One of the major drawbacks to a monolith like ours can be that teams aren't able to easily isolate their code's performance from that of the rest of the monolith. Having insight into what is actually happening on a granular level is what's going to allow us to continue having an easy time scaling our monolith for a long time.

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