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 SampleRate 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:

#include "SentrySettings.h"

FConfigureSettingsDelegate SettingsDelegate;
SettingsDelegate.BindDynamic(this, &USomeClass::ConfigureSettingsDelegate);

void USomeClass::ConfigureSettingsDelegate(USentrySettings* Settings)
{
    Settings->SampleRate = 0.25f;
}

...

USentrySubsystem* SentrySubsystem = GEngine->GetEngineSubsystem<USentrySubsystem>();

SentrySubsystem->InitializeWithSettings(SettingsDelegate);

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 level loaded 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:

   • 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 one of the options to start sending transactions.

Setting a Uniform Sample Rate

To do this, set the TracesSampleRate 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. For example, if you set TracesSampleRate to 0.2, approximately 20% of your transactions will be recorded and sent. That looks like this:

#include "SentrySettings.h"

FConfigureSettingsDelegate SettingsDelegate;
SettingsDelegate.BindDynamic(this, &USomeClass::ConfigureSettingsDelegate);

void USomeClass::ConfigureSettingsDelegate(USentrySettings* Settings)
{
    Settings->TracesSampleRate = 0.2f;
}

...

USentrySubsystem* SentrySubsystem = GEngine->GetEngineSubsystem<USentrySubsystem>();

SentrySubsystem->InitializeWithSettings(SettingsDelegate);

Setting a Sampling Function

To use the sampling function, set the TracesSampler option in your SDK configuration to a function that will accept a SamplingContext dictionary and return a sample rate between 0 and 1. For example:

UCLASS()
class USomeTraceSampler : public USentryTraceSampler
{
	GENERATED_BODY()

public:
	virtual bool Sample_Implementation(USentrySamplingContext* samplingContext, float& samplingValue) override
    {
        const FString& gameMode =
            *samplingContext->GetCustomSamplingContext().Find("GameMode");

        if (gameMode.Equals(TEXT("ranked")))
        {
            // Ranked matches are important - take a big sample
            samplingValue = 0.5;
        }
        else if (gameMode.Equals(TEXT("quick_match")))
        {
            // Quick matches less important and happen more frequently - only take 20%
            samplingValue = 0.2;
        }
        else if (gameMode.Equals(TEXT("training")))
        {
            // Training matches are just noise - drop all transactions
            samplingValue = 0.0;
        }
        else
        {
            // Default sample rate for other game modes
            samplingValue = 0.1;
        }

	    return true;

        // Or return false to fallback to options.TracesSampleRate (1.0 in this case)
    }
};

...

FConfigureSettingsDelegate SettingsDelegate;
SettingsDelegate.BindDynamic(this, &USomeClass::HandleSettingsDelegate);

void USomeClass::HandleSettingsDelegate(USentrySettings* Settings)
{
    // Set a uniform sample rate
    Settings->TracesSampleRate = 1.0f;

    // OR: Determine traces sample rate based on the sampling context
    Settings->TracesSampler = USomeTraceSampler::StaticCLass();
}

...

USentrySubsystem* SentrySubsystem = GEngine->GetEngineSubsystem<USentrySubsystem>();

SentrySubsystem->InitializeWithSettings(SettingsDelegate);

Additionally, your sampling function may also return false, which indicates that no sampling decision has been made. In such case, the trace will retain the previous decision if it has been made (for example if it was started from an incoming trace header), or fall back to the value configured in TracesSampleRate.

Sampling Context Data

Default Sampling Context Data

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

SamplingContext includes a Transaction Context and a custom sampling context (FString to FString map).

Custom Sampling Context Data

When using custom instrumentation to create a transaction, you can add data to the SamplingContext by passing it as an optional second argument to StartTransaction. This is useful if there's data you want the sampler to have access to, but you don't want to attach it 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:

USentrySubsystem* SentrySubsystem = GEngine->GetEngineSubsystem<USentrySubsystem>();

// The following data will take part in the sampling decision
TMap<FString, FString> options;
options.Add(TEXT("user_id"), TEXT("12312012"));
options.Add(TEXT("search_results"), TEXT("..."));

USentryTransactionContext* transactionContext = NewObject<USentryTransactionContext>();
transactionContext->Initialize(
    TEXT("GET /search"),
    TEXT("http")
);

USentryTransaction* transaction =
    SentrySubsystem->StartTransactionWithContextAndOptions(
        transactionContext,
        options
    );

Inheritance

Whatever a transaction's sampling decision, that decision will be passed to its child spans and from there to any transactions they subsequently trigger in other services.

(See Distributed Tracing 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 be included in the sampling context data. Your TracesSampler 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.

If you're using a TracesSampleRate rather than a TracesSampler, the decision will always be inherited.

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 TracesSampleRate
• Random sampling according to a sample function rate returned by TracesSampler
• Absolute decision (100% chance or 0% chance) returned by TracesSampler
• If the transaction has a parent, inheriting its parent's sampling decision
• Absolute decision passed to StartTransaction

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 StartTransaction, that decision will be used, overriding everything else.
2. If TracesSampler 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 TracesSampler is not defined, but there's a parent sampling decision, the parent sampling decision will be used.
4. If TracesSampler is not defined and there's no parent sampling decision, TracesSampleRate will be used.