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.

To do this, set the traces_sample_rate option in your sentry_sdk.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 traces_sample_rate to 0.2, approximately 20% of your transactions will get recorded and sent.) That looks like this:

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sentry_sdk.init(
    # ...

    traces_sample_rate=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

To sample dynamically, set the traces_sampler option in your sentry-sdk.init() to a function that will accept a sampling_context dictionary and return a sample rate between 0 and 1. For example:

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def traces_sampler(sampling_context):
    # 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
    elif "...":
        # These are less important or happen much more frequently - only take 1%
        return 0.01
    elif "...":
        # These aren't something worth tracking - drop all transactions like this
        return 0
    else:
        # Default sample rate
        return 0.1

sentry_sdk.init(
    # ...

    traces_sampler=traces_sampler,
)

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.

Default Sampling Context Data

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

For Python-based SDKs, it includes at least the following:

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{
  "transaction_context": {
    "name": <string>  # human-readable identifier, like "GET /users"
    "op": <string>  # short description of transaction type, like "http.request"
  },
  "parent_sampled": <bool>  # if this transaction has a parent, its sampling decision
  ...  # custom context as passed to `start_transaction`
}

Custom Sampling Context Data

When manually creating a transaction, you can add data to the sampling_context by passing it as an optional second argument to start_transaction. 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:

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sentry_sdk.start_transaction(
    # kwargs passed to Transaction constructor - will be recorded on transaction
    name="GET /search",
    op="search",
    data={
      "query_params": {
        "animal": "dog",
        "type": "very good"
      }
    },
    # `custom_sampling_context` - won't be recorded
    custom_sampling_context={
        # PII
        "user_id": "12312012",
        # too big to send
        "search_results": { ... }
    }
);

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 traces_sampler 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.)

For convenience, the traces_sampler function can return a boolean, so that a parent's decision can be returned directly if that's the desired behavior.

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def traces_sampler(sampling_context):
    # always inherit
    if sampling_context["parent_sampled"] is not None:
        return sampling_context["parent_sampled"]

    ...
    # rest of sampling logic here

If you're using a traces_sample_rate rather than a traces_sampler, 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 custom_sampling_context object). If you do that, the transaction won't be subject to the traces_sample_rate, nor will traces_sampler be run, so you can count on the decision that's passed not to be overwritten.

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sentry_sdk.start_transaction(
  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 traces_sample_rate
  • Random sampling according to a dynamic sample rate returned by traces_sampler
  • Absolute decision (100% chance or 0% chance) returned by traces_sampler
  • If the transaction has a parent, inheriting its parent's sampling decision
  • Absolute decision passed to start_transaction

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 start_transaction (see Forcing a Sampling Decision above), that decision will be used, regardlesss of anything else
  2. If traces_sampler 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 traces_sampler is not defined, but there's a parent sampling decision, the parent sampling decision will be used.
  4. If traces_sampler is not defined and there's no parent sampling decision, traces_sample_rate will be used.
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