Sampling

Adding Sentry to your app gives you a great deal of very valuable information about errors and performance you wouldn't otherwise get. And lots of information is good -- as long as it's the right information, at a reasonable volume.

Sampling Error Events

To send a representative sample of your errors to Sentry, set the sampleRate option in your SDK configuration to a number between 0 (0% of errors sent) and 1 (100% of errors sent). This is a static rate, which will apply equally to all errors. For example, to sample 25% of your errors:

Sentry.init({ sampleRate: 0.25 });

The error sample rate defaults to 1, meaning all errors are sent to Sentry.

Sampling Transaction Events

We recommend sampling your transactions for two reasons:

1. Capturing a single trace involves minimal overhead, but capturing traces for every page load or every API request may add an undesirable load to your system.
2. Enabling sampling allows you to better manage the number of events sent to Sentry, so you can tailor your volume to your organization's needs.

Choose a sampling rate with the goal of finding a balance between performance and volume concerns with data accuracy. You don't want to collect too much data, but you want to collect sufficient data from which to draw meaningful conclusions. If you’re not sure what rate to choose, start with a low value and gradually increase it as you learn more about your traffic patterns and volume.

Configuring the Transaction Sample Rate

The Sentry SDKs have two configuration options to control the volume of transactions sent to Sentry, allowing you to take a representative sample:

1. Uniform sample rate (tracesSampleRate):

   • Provides an even cross-section of transactions, no matter where in your app or under what circumstances they occur.
   • Uses default inheritance and precedence behavior

2. Sampling function (tracesSampler) which:

   • Samples different transactions at different rates
   • Filters out some transactions entirely
   • Modifies default precedence and inheritance behavior

By default, none of these options are set, meaning no transactions will be sent to Sentry. You must set either one of the options to start sending transactions.

Setting a Uniform Sample Rate

To do this, set the tracesSampleRate option in your Sentry.init() to a number between 0 and 1. With this option set, every transaction created will have that percentage chance of being sent to Sentry. (So, for example, if you set tracesSampleRate to 0.5, approximately 50% of your transactions will get recorded and sent.) That looks like this:

Sentry.init({
  // ...

  tracesSampleRate: 0.5,
});

Setting a Sampling Function

To use the sampling function, set the tracesSampler option in your Sentry.init() to a function that will accept a samplingContext object and return a sample rate between 0 and 1. For example:

// The shape of samplingContext that is passed to the tracesSampler function
interface SamplingContext {
  // Name of the span
  name: string;
  // Initial attributes of the span
  attributes: SpanAttributes | undefined;
  // If the parent span was sampled - undefined if there is no parent span
  parentSampled: boolean | undefined;
}

Sentry.init({
  // ...

  tracesSampler: ({ name, attributes, parentSampled }) => {
    // Do not sample health checks ever
    if (name.includes("healthcheck")) {
      // Drop this completely by setting its sample rate to 0%
      return 0;
    }

    // These are important - take a big sample
    if (name.includes("auth")) {
      return 1;
    }

    // These are less important or happen much more frequently - only take 1%
    if (name.includes("comment")) {
      return 0.01;
    }

    // Continue trace decision, if there is any parentSampled information
    if (typeof parentSampled === "boolean") {
      return parentSampled;
    }

    // Else, use default sample rate
    return 0.5;
  },
});

For convenience, the function can also return a boolean. Returning true is equivalent to returning 1, and will guarantee the transaction will be sent to Sentry. Returning false is equivalent to returning 0 and will guarantee the transaction will not be sent to Sentry.

Sampling Context Data

When a span is started, the tracesSampler function will automatically receive the span name and the initial span attributes to use in its sampling decision. Additionally, it will also receive a parentSampled boolean indicating whether the parent span was sampled. This data can be used to make a more informed decision about whether to sample the span.

Sentry.startSpan({
  name: "GET /search",
  op: "search",
  attributes: {
    "queryParam.animal": "dog",
    "queryParam.type": "very good",
  },
});

// Will result in `tracesSampler` receiving:
function tracesSampler(samplingContext) {
  /*
  samplingContext = {
    name: "GET /search",
    attributes: {
      queryForAnimal: "dog",
      queryForType: "very good"
    },
  }
  */

  // Do not sample this specific span
  return name !== "GET /search";
}

Please note that the name passed to tracesSampler may not be the exact same name that is eventually sent to Sentry. The name may be updated during the lifetime of the span, but the name passed to tracesSampler will always be the initial name. For example, the name for http.server spans will usually not be parametrized yet when passed to tracesSampler, so instead of GET /users/:id you may see a name of GET /users/123.

Inheritance

Whatever a transaction's sampling decision, that decision will be passed to its child spans and from there to any transactions they subsequently cause in other services.

(See Distributed Tracing for more about how that propagation is done.)

If the transaction currently being created is one of those subsequent transactions (in other words, if it has a parent transaction), the upstream (parent) sampling decision will be included in the sampling context data. Your tracesSampler can use this information to choose whether to inherit that decision. In most cases, inheritance is the right choice, to avoid breaking distributed traces. A broken trace will not include all your services.

tracesSampler: samplingContext => {
  // always inherit
  if (samplingContext.parentSampled !== undefined) {
    return samplingContext.parentSampled
  }

  ...
  // rest of sampling logic here
}

If you're using a tracesSampleRate rather than a tracesSampler, the decision will always be inherited.

Precedence

There are multiple ways for a transaction to end up with a sampling decision.

• Random sampling according to a static sample rate set in tracesSampleRate
• Random sampling according to a sample function rate returned by tracesSampler
• Absolute decision (100% chance or 0% chance) returned by tracesSampler
• If the transaction has a parent, inheriting its parent's sampling decision
• Absolute decision passed to startTransaction

When there's the potential for more than one of these to come into play, the following precedence rules apply:

1. If a sampling decision is passed to startTransaction, that decision will be used, overriding everything else.
2. If tracesSampler is defined, its decision will be used. It can choose to keep or ignore any parent sampling decision, use the sampling context data to make its own decision, or choose a sample rate for the transaction. We advise against overriding the parent sampling decision because it will break distributed traces)
3. If tracesSampler is not defined, but there's a parent sampling decision, the parent sampling decision will be used.
4. If tracesSampler is not defined and there's no parent sampling decision, tracesSampleRate will be used.