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 sample_rate 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:

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Sentry.init do |config|
  # ...
  config.sample_rate = 0.25
end

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:

Setting a Uniform Sample Rate

To do this, set the traces_sample_rate 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 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.init do |config|
  # ...
  config.traces_sample_rate = 0.25
end

Setting a Sampling Function

To use the sampling function, set the traces_sampler option in your Sentry.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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Sentry.init do |config|
  config.traces_sampler = lambda do |sampling_context|
    # if this is the continuation of a trace, just use that decision (rate controlled by the caller)
    unless sampling_context[:parent_sampled].nil?
      next sampling_context[:parent_sampled]
    end

    # transaction_context is the transaction object in hash form
    # keep in mind that sampling happens right after the transaction is initialized
    # for example, at the beginning of the request
    transaction_context = sampling_context[:transaction_context]

    # transaction_context helps you sample transactions with more sophistication
    # for example, you can provide different sample rates based on the operation or name
    op = transaction_context[:op]
    transaction_name = transaction_context[:name]

    case op
    when /request/
      # for Rails applications, transaction_name would be the request's path (env["PATH_INFO"]) instead of "Controller#action"
      case transaction_name
      when /health_check/
        0.0
      when /payment/
        0.5
      when /api/
        0.2
      else
        0.1
      end
    when /sidekiq/
      0.01 # you may want to set a lower rate for background jobs if the number is large
    else
      0.0 # ignore all other transactions
    end
  end
end

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

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 Ruby-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 "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 using custom instrumentation to create 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.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 Services 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, to avoid 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.

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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.

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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 sample function 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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