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:

sentry:
  options:
    sample_rate: 0.25

The error sample rate defaults to 1, meaning all errors are sent to Sentry.

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 (traces_sample_rate):

   • 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 (traces_sampler) which:

   • Samples different transactions at different rates
   • Filters out some transactions entirely
   • Modifies default precedence and inheritance behavior

By default, none of these options are set, meaning no transactions will be sent to Sentry. You must set either one of the options to start sending transactions.

Setting a Uniform Sample Rate

To do this, set the traces_sample_rate option 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:

sentry:
  options:
    traces_sample_rate: 0.2

Setting a Sampling Function

To use the sampling function, set the traces_sampler option to a closure that will accept a \Sentry\Tracing\SamplingContext object and return a sample rate between 0 and 1. For example:

sentry:
  options:
    traces_sampler: 'sentry.callback.traces_sampler'

services:
  sentry.callback.traces_sampler:
    class: 'App\Service\Sentry'
    factory: ['@App\Service\Sentry', 'getTracesSampler']

The service needed for the traces_sampler option can be implemented as follows:

<?php

namespace App\Service;

class Sentry
{
    public function getTracesSampler(): callable
    {
        return function (\Sentry\Tracing\SamplingContext $context): float {
            if ($context->getParentSampled()) {
                // If the parent transaction (for example a JavaScript front-end)
                // is sampled, also sample the current transaction
                return 1.0;
            }

            if (some_condition()) {
                // Drop this transaction, by setting its sample rate to 0
                return 0;
            }

            // Default sample rate for all other transactions (replaces `traces_sample_rate`)
            return 0.25;
        };
    }
}

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 the PHP SDK, it includes at least the following:

/** @var \Sentry\Tracing\SamplingContext $context */

// The context of the transaction (of type \Sentry\Tracing\TransactionContext)
$context->getTransactionContext();

// Sampling decision from the parent transaction, if any
$context->getParentSampled();

// Additional context, depending on where the SDK runs
$context->getAdditionalContext();

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:

\Sentry\startTransaction(
    new \Sentry\Tracing\TransactionContext(
        $name = 'Search from navbar',
        $parentSampled = false
    ),
    $customSamplingContext = [
        // PII
        'userId' => '12312012',
        // too big to send
        'resultsFromLastSearch' => [ ... ]
    ]
);

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.

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 be included in the sampling context data. Your traces_sampler can use this information to choose whether to inherit that decision. In most cases, inheritance is the right choice, to avoid breaking distributed traces. A broken trace will not include all your services.

sentry:
  options:
    traces_sampler: 'sentry.callback.traces_sampler'

services:
  sentry.callback.traces_sampler:
    class: 'App\Service\Sentry'
    factory: ['@App\Service\Sentry', 'getTracesSampler']

The service needed for the traces_sampler option can be implemented as follows:

<?php

namespace App\Service;

class Sentry
{
    public function getTracesSampler(): callable
    {
        return function (\Sentry\Tracing\SamplingContext $context): float {
            // always inherit
            if ($context->getParentSampled()) {
                return 1.0;
            }

            // 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.

$transactionContext = (new \Sentry\Tracing\TransactionContext(
    $name = 'Search from navbar',
    $parentSampled = false
))
    ->setSampled(true);

$transaction = \Sentry\startTransaction($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, that decision will be used, overriding everything else.
2. If traces_sampler is defined, its decision will be used. It can choose to keep or ignore any parent sampling decision, use the sampling context data to make its own decision, or choose a sample rate for the transaction. We advise against overriding the parent sampling decision because it will break distributed traces)
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.