Conditionally trace transactions

[hansott]

Hi Sentry, we have a process that checks for pending work. If there's pending work, we do the work:

while (ok) {
  const transaction = Sentry.startTransaction({
    name: "worker",
  });
  
  Sentry.getCurrentHub().configureScope((scope) => {
    scope.setSpan(transaction);
  });
  
  // Check for pending work and do the actual work
  
  transaction.finish();
}

We don't know upfront whether there's going to be any work. If there's no work, we don't want to sample the transaction for analysis. It's just too much noise (or maybe a lower rate versus transactions with actual work).

We tried to set tags conditionally (based on whether there was work or not):

if (didWork) {
  transaction.setTag("DID_WORK", "true");
}

In tracesSampler, it would be nice to receive the tags in the context object but it seems that this is not the case?

const tracesSampler = (context) => {
  if (context.transactionContext.name.startsWith("worker")) {
    if (context.transactionContext.tags["DID_WORK"] === "true") {
      return higherRate;
    }

    return lowerRate;
  }
}

Next, we tried to use transaction.setName() since the name is passed in context.transactionContext but it seems that the tracesSampler function is called at the start of a transaction?

TL;DR: Be able to conditionally trace transaction based on what happened within the transaction (either by using Tags or changing the name of the transaction)

[lforst]

Hey, you could do transaction.sampled = false at any point before the transaction is finished in order not to send a transaction.

[hansott]

Hey @lforst, I tried this but it doesn't work. I still see transactions with sampled = false appearing in Sentry.

[hansott]

 while (ok) {
    const transaction = Sentry.startTransaction({
      name: "worker",
    });

    Sentry.getCurrentHub().configureScope((scope) => {
      scope.setSpan(transaction);
    });

    console.time("worker");
    const pendingJobs = await jobs.getPending();

    let didWork = false;
    await asyncPool(
      8,
      pendingJobs,
      async (job) => {
        if (await worker.run(job)) {
          didWork = true;
        }
      }
    );

    console.timeEnd("worker");
    transaction.finish();

    if (!didWork) {
      transaction.sampled = false;
    }

    if (ok) {
      await sleep(Duration.fromObject({ milliseconds: 200 }));
    }
  }

[lforst]

As mentioned above, you need to set transaction.sampled = false before calling transaction.finish().

[hansott]

@lforst I tried like this:

if (!ranAJob) {
  transaction.sampled = false;
}

transaction.finish();

but for some reason it's still sampling the transactions (I increased the sampling rate though):

The traces/transactions volume should be way lower.

[hansott]

By the way, I'm using Node.js. The only example of transaction.sampled = false is about frontend/browser (https://docs.sentry.io/platforms/react-native/performance/instrumentation/automatic-instrumentation/). Maybe it's not implemented for the Node.js SDK?

[lforst]

I believe you have some bug or race condition in your ranAJob.

The simplest example I can give is the following:

import * as Sentry from "@sentry/node";

Sentry.init({
  dsn: "__MY_DSN__",
  tracesSampleRate: 1.0,
  debug: true,
});

const transaction = Sentry.startTransaction({ name: "my-txn" });

transaction.sampled = false;

transaction.finish();

Which works perfectly fine as shown by the following log output:

Sentry Logger [log]: Integration installed: InboundFilters
Sentry Logger [log]: Integration installed: FunctionToString
Sentry Logger [log]: Integration installed: Console
Sentry Logger [log]: Integration installed: Http
Sentry Logger [log]: Integration installed: Undici
Sentry Logger [log]: Integration installed: OnUncaughtException
Sentry Logger [log]: Integration installed: OnUnhandledRejection
Sentry Logger [log]: Integration installed: ContextLines
Sentry Logger [log]: Integration installed: LocalVariables
Sentry Logger [log]: Integration installed: Context
Sentry Logger [log]: Integration installed: Modules
Sentry Logger [log]: Integration installed: RequestData
Sentry Logger [log]: Integration installed: LinkedErrors
Sentry Logger [log]: [Tracing] starting undefined transaction - my-txn
Sentry Logger [log]: [Tracing] Discarding transaction because its trace was not chosen to be sampled.

I would need a complete reproduction example to debug this further.