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 do |config|
  # ...
  config.traces_sample_rate = 0.5
end

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 do |config|
  config.traces_sampler = lambda do |sampling_context|
    # transaction_context is the transaction object in hash form
    # keep in mind that sampling happens right after the transaction is initialized
    # e.g. at the beginning of the request
    transaction = sampling_context[:transaction_context]

    # if the transaction is important, set a higher rate
    if transaction[:name].match?("orders")
      0.5
    # otherwise, give it a lower rate
    else
      0.1
    end
  end
end

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.

{
  transaction_context: {
    name: <string>  # human-readable identifier, like "GET /users"
    op: <string>  # short description of transaction type, like "rack.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:

Sentry.start_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.)

In some SDKs, 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.

config.traces_sampler = lambda do |sampling_context|
  parent_sampled = sampling_context[:parent_sampled]

  if !parent_sampled.nil?
    parent_sampled
  else
    # the rest of sampling logic
  end
end

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.

Sentry.start_transaction(
  name: "Search from navbar",
  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.