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:

Copied
import Sentry

SentrySDK.start { options in
    options.sampleRate = 0.25
}

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 when you initialize the SDK 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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import Sentry

SentrySDK.start { options in
    options.tracesSampleRate = 0.2
}

Setting a Sampling Function

To use the sampling function, set the traces-sampler option in your SentrySDK.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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import Sentry

SentrySDK.start { configureOptions in
    options.dsn = "https://examplePublicKey@o0.ingest.sentry.io/0"

    // To set a uniform sample rate
    options.tracesSampleRate = 1.0

    // OR: Determine traces sample rate based on the sampling context
    options.tracesSampler = { context in
        guard let url = context.customSamplingContext["url"] as? String else {
            // The desired information does not exists, do not sample.
            return 0.0    
        }
        
        switch url {
            // These are important - take a big sample
            case "/payment": return 0.5

            // Search is less important and happen much more frequently - only take 1%
            case "/search": return 0.01

            // The health check endpoint is just noise - drop all transactions
            case "/health": return 0.0

            // Default sample rate
            default: return 0.1

            // Or return nil to fallback to options.TracesSampleRate
            // default: return nil
        }
    }
})

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 Apple SDKs, it includes a Transaction Context and a custom sampling context (string to object dictionary).

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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import Sentry

// The following data will take part in the sampling decision
let samplingContext : [String: Any] = {
    "user_id": 12312012,
    "search_results": searchResults
}

var transaction = SentrySDK.startTransaction(
  transactionContext: TransactionContext(name:"GET /search", operation: "http"),
  customSamplingContext: samplingContext
)

// The following is set on transaction, so does not take part in
// the sampling decision
transaction.context.operation = "http"
transaction.context.spanDescription = "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.

Copied
import Sentry

let traceSampler : SentryTracesSamplerCallback = {(context: SamplingContext) -> NSNumber in
  if context.transactionContext.parentSampled != .undecided {
    return context.transactionContext.parentSampled == .yes ? 1.0 : 0.0
  }
  // the rest of sampling logic
}

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.

Copied
import Sentry

let transactionContext = TransactionContext(name:"GET /search", operation:"http", sampled: .yes)
let transaction = SentrySDK.startTransaction(transactionContext: transactionContext)

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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