Sampling Transactions

You can control the volume of transactions sent to Sentry in two ways.

Uniform Sample Rate

Setting a uniform sample rate is a good option if you want an even cross-section of transactions, no matter where in your app or under what circumstances they occur, and are happy with the default inheritance and precedence behavior described below.

Sentry.init({
  // ...

  tracesSampleRate: 0.2,
});

Dynamic Sampling Function

Providing a sampling function is a good option if you:

• want to sample different transactions at different rates
• want to filter out some transactions entirely
• want to modify the default precedence and inheritance behavior described below

Sentry.init({
  // ...

  tracesSampler: samplingContext => {
    // Examine provided context data (including parent decision, if any) along
    // with anything in the global namespace to compute the sample rate or
    // sampling decision for this transaction

    if ("...") {
      // These are important - take a big sample
      return 0.5;
    } else if ("...") {
      // These are less important or happen much more frequently - only take 1%
      return 0.01;
    } else if ("...") {
      // These aren't something worth tracking - drop all transactions like this
      return 0;
    } else {
      // Default sample rate
      return 0.1;
    }
  };
});

Default Sampling Context Data

The information contained in the samplingContext object passed to the tracesSampler when a transaction is created varies by platform and integration.

{
  transactionContext: {
    name: string; // human-readable identifier, like "GET /users"
    op: string; // short description of transaction type, like "pageload"
  }
  parentSampled: boolean; // if this transaction has a parent, its sampling decision
  request: object // in server contexts, information about the incoming request
  ... // custom context as passed to `startTransaction`
}

Custom Sampling Context Data

When manually creating a transaction, you can add data to the samplingContext by passing it as an optional second argument to startTransaction. This is useful if there's data to which you want the sampler to have access but which you don't want to attach to the transaction as tags or data, such as information that's sensitive or that’s too large to send with the transaction. For example:

Sentry.startTransaction(
  {
    // `transactionContext` - will be recorded on transaction
    name: 'GET /search',
    op: 'search',
    data: {
      queryParams: {
        animal: "dog",
        type: "very good"
      }
    },
  },
  // `customSamplingContext` - won't be recorded
  {
    // PII
    userId: '12312012',
    // too big to send
    searchResults: { ... }
  },
);

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 Connecting Backend and Frontend Transactions 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 always be included in the sampling context data, so that your tracesSampler can choose whether and when to inherit that decision. (In most cases, inheritance is the right choice, so that you don't end up with partial traces.)

In some SDKs, for convenience, the tracesSampler function can return a boolean, so that a parent's decision can be returned directly if that's the desired behavior.

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.

Forcing a Sampling Decision

If you know at transaction creation time whether or not you want the transaction sent to Sentry, you also have the option of passing a sampling decision directly to the transaction constructor (note, not in the customSamplingContext object). If you do that, the transaction won't be subject to the tracesSampleRate, nor will tracesSampler be run, so you can count on the decision that's passed not to be overwritten.

Sentry.startTransaction({
  name: "GET /search",
  sampled: true,
});

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 dynamic sample 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 (see Forcing a Sampling Decision above), that decision will be used, regardlesss of anything else
2. If tracesSampler is defined, its decision will be used. It can choose to keep or ignore any parent sampling decision, or use the sampling context data to make its own decision or choose a sample rate for the transaction.
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.