Posts By: Will Keleher

Using NodeJS's AsyncLocalStorage to Instrument a Webserver

In the dark times before AsyncLocalStorage, it could be hard to tell why a request would occasionally time out. Were there multiple relatively slow queries somewhere in that route's code? Was another request on the same container saturating a database connection pool? Did another request block the event loop? It was possible to use tracing, profiling, and logs to track down problems like these, but it could be tricky; setting up per route metrics using AsyncLocalStorage makes it a ton easier!

When ClassDojo set up our AsyncLocalStorage-based per-route instrumentation we found things like:

  • 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 simple counts to return to clients

I wrote about this a bit more in AsyncLocalStorage Makes the Commons Legible. If you're not familiar with ClassDojo, it's a parent-teacher communication platform. Our monolithic NodeJS web-server backend API normally serves ~10,000 requests per second.

I'd like to go through some of the details of how we set up this per-request instrumentation. For this post, we'll be starting with a relatively standard NodeJS web-server with pre-router middleware, and a router that finds an appropriate route to handle a request. It should look something like this:

1app.use(({ req, res }, next) => {
2 const start =;
3 onFinished(res, () => afterResponse(req, res, start));
4 next();

To add instrumentation to this setup, we'll want to do the following:

  1. Create a per-request async store in our first middleware
  2. Store details about the database request caused by our request in our request's async store
  3. Send the request's database request details to our data lake.
  4. If any of the database request details violate our per-request limits, we log it as a server-error so that a team can see it & take action

Starting our pre-request async store

In a NodeJS web server, each middleware calls the next, so if we start an async local storage context in our very first middleware, every subsequent middleware should have access to the same storage context. (I had a lot of trouble understanding why this worked, so I wrote up a simplified gist that hopefully demonstrates what's going on.)

1import { AsyncLocalStorage } from "async_hooks";
3export const requestTrackingAsyncLocalStore = new AsyncLocalStorage();
5// requestTrackingAsyncLocalStoreMiddleware wraps the downstream koa middlewares inside an async local storage context
6export function requestTrackingAsyncLocalStoreMiddleware({ req, res }, next) {
7 const store = {
8 requestCost,
9 req,
10 res,
11 };
12 // running the next middleware in the chain in the context of this 'run' makes sure that all calls
13 // to getStore() in the scope of this requests are bound to the correct store instance
14 return, next);
17// add this to the router! (this would be in a different file)

Store details about request about behavior in our pre-request async store

Now that we have a pre-request async local store, we can grab it and start using it! We'll want to learn:

  1. How many database requests do we make over the course of an HTTP request? Are we running into the N+1 query problem on any of our routes?
  2. How long do those database requests take in total? Requests that take a long time can indicate spots where we're doing a lot of expensive work.
  3. How many documents are these requests returning? If we're processing 10,000s of documents in NodeJS, that can slow down a server quite a bit, and we may want to move that work to our database instead.
1export function increment(type: "request_count" | "duration" | "document_count", table: string, n: number = 1) {
2 const store = requestTrackingAsyncLocalStore.getStore();
3 // we'll probably want to track this to see if we're losing async context over the course of a request
4 if (!store) return;
5 _.set(store, ["requestCost", type], _.get(store, ["requestCost", type], 0) + n);
6 _.set(store, ["requestCost", "byTable", table, type], _.get(store, ["requestCost", "byTable", table,], 0) + n);

If we add code that wraps our database client's request, it should hopefully be easy to add these increment calls at an appropriate point.

Handle the request report

Once we have this request report, we can do whatever we'd like with it! At ClassDojo, we log a server-error whenever a route is doing anything particularly egregious: that way, we get quick feedback when we've made a mistake. We also use a firehose to send this data to redshift (our data lake) so that we can easily query it. Either way, this is something that we can do after we're done sending our response to the client:

2app.use(({ req, res }, next) => {
3 // this use of new AsyncResource will preserve the async context
4 res.on("finished", new AsyncResource("requestTrackingLogging").bind(() => {
5 const store = requestTrackingAsyncLocalStore.getStore();
6 if (!store) throw new Error(`Something has gone awry with our async tracking!`);
7 if (isEgregiouslyBad(store.requestCost)) logOutBadRequest(store);
8 requestCostFirehose.write(store);
9 }))
10 next();

Tracking down places where we lose async context

While the async store might feel like magic, it's not, and sommon common situations will cause you to lose async context:

  1. using callbacks rather than promises. In those situations, you'll need to create an AsyncResource to bind the current async context
1setTimeout(new AsyncResource("timeout").bind(() => doRequestTrackingThings()), 1);
2redisClient.get("key", new AsyncResource("timeout").bind(() => doRequestTrackingThings()))
  1. Some promise libraries might not support async-hooks. Bluebird does, but requires setting asyncHooks to true: Bluebird.config({ asyncHooks: true });.

It may take a bit of work to track down and fix all of the places where you're losing async context. Setting up your increment calls to log out details about those situations can help!

1export function increment(type: "request_count" | "duration" | "document_count", table: string, n: number = 1) {
2 const store = requestTrackingAsyncLocalStore.getStore();
3 if (!store) {
4 logServerError(`We lack async context for a call to increment ${type} ${table} by ${n}`, new Error().stack);
5 return;
6 }
7 ...

Increased Observability is great!

Putting effort into increasing the observability of a system can make that system much easier to manage. For a NodeJS web-server, we've found a lot of benefits in using AsyncLocalStorage to improve per-request visibility: it has let us improve latency on a few routes, reduced our event-loop blocking, and given us a better view of opportunities to improve performance.

    A single large query in SQL can be hard to understand, test, debug, or change in the same way that an over-large function in code can be. A large query is also much harder to write! Feedback loops while writing large queries are slow and you'll often find yourself needing to guess at where the problem in your query is.

    When I started writing analytics queries, I wrote some pretty rough ones that are now hard to debug and maintain! (Apologies to everyone who has had to deal with any of my old SQL queries.) Over time, I think I've gotten better at them, and I wanted to write down some of the things I'm doing differently now that I hope will be useful to other people in similar situations.

    Finally, I'm a product engineer who rarely writes these sorts of queries! This advice is much less applicable to someone who does this daily & is using better & more focused tools to do analyses.

    Temporary tables, views, and with

    Large queries are hard, but it's pretty simple to break a large query into smaller pieces. For analytics queries, I normally create small temporary tables (often with the temporary keyword) that normalize data, filter out deleted rows and rows I'm not interested in, and organize my data into a format that makes querying easy. Views or with clauses can accomplish similar things, but I like using temporary tables for this because they cache results and make subsequent queries faster.

    I also try to put constants into a temporary table or with clause. When working on a query, it can be easy to forget to update a constant in one spot and then get completely meaningless results. (Shoutout to Ben Haley for showing me this trick!)

    All of this might sound a little abstract: let's take a somewhat contrived query and try to refactor it. We want to bucket and count US-based teachers who were active in 2021 by how many classes they created during that time period. Here's what that might look like as a single query:

    2 case when class_count < 5 then class_count::varchar else 'many' end as bucket,
    3 count(*)
    4 from (
    5 select count(distinct class.classId) as class_count
    6 from teacher
    7 join user_teacher ON teacher.teacherId = user_teacher.teacherid
    8 -- left join class_teacher to make sure we're counting teachers who haven't created classes
    9 left join class_teacher on class_teacher.teacherId = user_teacher.teacherId and class_teacher.creator
    10 left join user USING(userId)
    11 join class using(classId)
    12 join (
    13 select distinct teacherId
    14 from teacher_active
    15 where active_date between '2021-01-01' and '2022-01-01'
    16 ) as ats on teacher.teacherId = ats.teacherId
    17 and class.createdat between '2021-01-01' and '2022-01-01'
    18 and not class.autocreated_demo
    19 and lower( in ('usa', 'us')
    20 group by teacherId
    21 )
    22group by 1

    This query isn't particularly complex, but it's still enough logic that I'd be a little worried about changing it or verifying that it's correct. I'd be tempted to try to pull out constants and then separate out the filtering logic from the calculation logic.

    1drop table if exists _constant;
    2create temporary table _constant as (
    3 select '2021-01-01' as start, '2022-01-01' as end
    6drop table if exists _teacher;
    7create temporary table _teacher as (
    8 -- us_user is probably overkill: this might be better in the `where` clause!
    9 with us_user as (
    10 select userId
    11 from user
    12 where lower(country) in ('usa', 'us')
    13 )
    14 select distinct teacherId
    15 from teacher_active
    16 join user_teacher USING(teacherId)
    17 join us_user using(userid)
    18 where active_date between (select start from _constant)
    19 and (select end from _constant)
    21drop table if exists _class;
    22create temporary table _class (
    23 select classId
    24 from class
    25 where class.created between (select start from _constant)
    26 and (select end from _constant)
    27 and not class.autocreated_demo
    30drop table if exists _classes_created_by_teacher;
    31create temporary table _classes_created_by_teacher (
    32 with class_creator as (
    33 select class_teacher.*
    34 from class_teacher
    35 join _class USING(classId)
    36 where class_teacher.creator
    37 )
    38 select teacherId, count(distinct classId) as classes_created
    39 from _teacher
    40 left join class_creator using(teacherId)
    41 group by teacherId
    45 case when class_count < 5 then class_count::varchar else 'many' end as bucket,
    46 count(*)
    47from _classes_created_by_teacher
    48group by bucket;

    It's arguable whether this is actually better! The initial query is short enough that it's not that much logic to understand: it might be the right size for the team that you're working with. There are also certainly better ways of factoring this same query that could make the logic even more clear. Overall though, I'd much rather work with the updated query:

    • if something is broken in the query, I can easily and quickly examine the tables that I've created to see if my results match my expectations
    • if I have a testing library of some sort, I can set up simple assertions about what the tables I'm using look like
    • the overall query will run faster because results are cached and because query-planners don't always generate optimum plans for large queries. While working on this query, I'll have faster feedback loops
    • I'll be able to tell which parts of this query are slow and optimize if necessary
    • it's easier to focus on adapting & improving a single part
    • The sub-queries that I've turned into tables are things I could take advantage of later if I ever tackle other similar problems by turning them into nice non-temporary cache tables

    I think many data-focused engineers use jupyter notebooks and pandas to break down large queries. I think how you're breaking down a large query into smaller pieces is much less important than doing that breakdown!

    Make feedback loops FAST!

    One of the most frustrating parts of working on a large query is that feedback loops can be slow. Making a change and waiting tens of minutes can completely kill any programming flow or focus that you have.

    • Break up large queries into smaller ones that are quick to run!
    • Use select without a from to quickly test out behavior. You can run queries like select extract('year' from GETDATE()); or select 1 != null, 1 is not null to quickly check your understanding. This can be especially useful for testing out regular expressions and formatting date fields for charts. There's no need for a from or to run a full query1
    • If a full table is slow to query, it might make sense to create a temporary table with representative data that you can use to build up your queries
    • Good indexes or sort-keys can drastically improve query speed! The most common mistake I've made is setting up a compound sort key with a too-precise timestamp followed by other keys I'm interested in. If you use a second or millisecond precision key at the start of a compound sortkey, that key's precision will override any possible benefit from the other key. So, rather than creating a sortkey like (createdAtMs, event), it's probably better to instead sort and query on an index like (day, event). (This might mean that queries need to include both day and createdAtMs.)
    • validate your work as you go! The quicker you can realize that your assumptions about a table or column are incorrect the better. Running a query that checks whether a column is unique, what values a column can hold, or just what data looks like can save a ton of time!

    In general, putting effort into how quickly you get feedback while working makes it much easier to find flow and be effective. A little bit of effort put into setting up nice tables, improving data layout, and optimizing sortkeys can pay large dividends.

      Bob Nystrom's What Color is Your Function does an amazing job of describing why it can be painful when programming languages have different rules for calling synchronous and asynchronous functions. Promises and async/await have simplified things in JavaScript, but it's still a language with "red" (async) and "blue" (sync) functions, and I consistently see a few understandable errors from red vs. blue function confusion. Let's go through some of the most common mistakes – none of these are bad things to get wrong, they're just a symptom of how confusing this can be!

      Omitting await from try/catch blocks

      The most common mistake I see is omitting await from try/catch blocks with async functions. The code looks reasonable, but the catch block will only be able to catch synchronously thrown errors. To make matters worse, error handling logic is often less well tested than the happy path when everything works, which makes this pattern more likely to sneak its way into production code.

      1async function throwsError () {
      2 throw new Error("alas! an error");
      5try {
      6 return throwsError();
      7} catch (err) {
      8 console.error("Oh no! This catch block isn't catching anything", err);

      An async function that throws is the equivalent of a Promise.reject, and when written that way, it's a bit clearer what's going on:

      1try {
      2 return Promise.reject(new Error("alas! an error"));
      3} catch (err) {
      4 console.error("It's clearer that this `catch` can't catch that `Promise.reject`. This is equivalent to the earlier code");

      Personally, I'm starting to wonder whether using try and catch blocks at all is a mistake when dealing with async code. They take up space and don't offer the same pattern matching that a library like Bluebirdjs can add to catch when you only want to catch some specific known errors: await tryThing().catch(NotFoundErrorClass, handleErrPattern) feels substantially cleaner to me than the equivalent try/catch block.

      Array.filter(async () => false)

      In recent years, JavaScript has added lots of useful Array methods like filter, map, forEach, and flatMap, and JavaScript programmers often use libraries like lodash to write functional code rather than writing for loops. Sadly, none of those Array methods or lodash helpers work with red async functions and are a common source of coding errors.

      1const things = [true, false, 1, 0, "", new Date("not a date") - 0];
      2const filteredThings = things.filter(async (thing) => thing);

      How many things do we end up with in filteredThings? Surprisingly, the answer has little to do with JavaScript type coercion: filteredThings will be the same size as things. An async function returns a Promise and even a Promise that resolves to false is still a truthy value: Boolean(Promise.resolve(false)) === true. If we want to do any sort of filtering using an async function, we need to switch out of blue sync mode and into red async mode.

      1(async function () {
      2 // You should use a library like Bluebird rather than filtering like this! this is only for illustration
      3 const things = [true, false, 1, 0, "", new Date("not a date") - 0];
      4 const predicateValues = await Promise.all( (thing) => thing));
      5 const filteredThings = things.filter((_thing, i) => predicateValues[i]);

      When you see Array.filter(async (thing) => thing) written out like that, the mistake is pretty clear. It can be harder to notice when you see code like const goodThings = things.filter(isGoodThing); you need to check whether isGoodThing is red or blue.


      We see a similar problem when people use Array.forEach with an async function:

      1const fruitStatus = {};
      2["apple", "tomato", "potato"].forEach(async (food) => {
      3 fruitStatus[food] = await isFruit(food);
      5return fruitStatus;

      In some ways, this is a more dangerous pattern. Depending on when you check, fruitStatus may have some, none, or all of the correct isFruit values. If isFruit is normally fast, problems and bugs might not manifest until isFruit slows down. A bug that only shows up some of the time is much harder to debug than one that's always there.

      Await off my shoulders

      Despite how easy it is to make mistakes with async/await, I still love it – it feels easier to work with than Promises or callbacks. Dealing with asynchronous code is still one of the harder parts of programming in JavaScript, but tools like bluebird, the TypesScript no-unnecessary-condition rule, and the eslint promise plugin can help surface these easy-to-make red/blue function mistakes early. Hopefully, seeing the mistakes we often make will help you avoid some frustrating minutes debugging.

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