---
title: "Sampling"
description: "Learn how to configure the volume of error and transaction events sent to Sentry using the JavaScript SDK."
url: https://docs.sentry.io/platforms/javascript/guides/cordova/sampling/
---

# Sampling for Cordova

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](https://docs.sentry.io/platforms/javascript/guides/cordova/sampling.md#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:

```javascript
Sentry.init({ sampleRate: 0.25 });
```

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

Changing the error sample rate requires re-deployment. In addition, setting an SDK sample rate limits visibility into the source of events. Setting a rate limit for your project (which only drops events when volume is high) may better suit your needs.

## [Sampling Transaction Events](https://docs.sentry.io/platforms/javascript/guides/cordova/sampling.md#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](https://docs.sentry.io/platforms/javascript/guides/cordova/sampling.md#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](https://docs.sentry.io/platforms/javascript/guides/cordova/sampling.md#inheritance) and [precedence](https://docs.sentry.io/platforms/javascript/guides/cordova/sampling.md#precedence) behavior

2. Sampling function (`tracesSampler`) which:

   * Samples different transactions at different rates
   * [Filters](https://docs.sentry.io/platforms/javascript/guides/cordova/configuration/filtering.md) out some transactions entirely
   * Modifies default [precedence](https://docs.sentry.io/platforms/javascript/guides/cordova/sampling.md#precedence) and [inheritance](https://docs.sentry.io/platforms/javascript/guides/cordova/sampling.md#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](https://docs.sentry.io/platforms/javascript/guides/cordova/sampling.md#setting-a-uniform-sample-rate)

To do this, set the `tracesSampleRate` 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 `tracesSampleRate` to `0.5`, approximately 50% of your transactions will get recorded and sent.) That looks like this:

```javascript
Sentry.init({
  // ...

  tracesSampleRate: 0.5,
});
```

### [Setting a Sampling Function](https://docs.sentry.io/platforms/javascript/guides/cordova/sampling.md#setting-a-sampling-function)

To use the sampling function, set the `tracesSampler` option in your `Sentry.init()` to a function that will accept a `samplingContext` object and return a sample rate between 0 and 1. For example:

```javascript
Sentry.init({
  // ...

  tracesSampler: (samplingContext) => {
    const name = samplingContext.transactionContext.name;

    // Do not sample health checks ever
    if (name.includes("healthcheck")) {
      return 0;
    }

    // These are important - take a big sample
    if (name.includes("auth")) {
      return 1;
    }

    // These are less important or happen much more frequently - only take 1%
    if (name.includes("comment")) {
      return 0.01;
    }

    // Otherwise, inherit the sampling decision of the parent, if there is one
    if (samplingContext.parentSampled !== undefined) {
      return samplingContext.parentSampled;
    }

    // Default sample rate
    return 0.5;
  },
});
```

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](https://docs.sentry.io/platforms/javascript/guides/cordova/sampling.md#sampling-context-data)

When a transaction is started, the `tracesSampler` function will automatically receive a `transactionContext` object with the initial `name` and `op` of the transaction to use in its sampling decision. Additionally, it will also receive a `parentSampled` boolean indicating whether the parent transaction was sampled. This data can be used to make a more informed decision about whether to sample the transaction.

```javascript
Sentry.startSpan(
  {
    name: "Search from navbar",
    op: "search",
  },
  () => {
    // ...
  },
);

// Will result in `tracesSampler` receiving:
function tracesSampler(samplingContext) {
  /*
  samplingContext = {
    transactionContext: {
      name: "Search from navbar",
      op: "search",
    },
    parentSampled: undefined,
  }
  */

  // Do not sample this specific transaction
  return samplingContext.transactionContext.name !== "Search from navbar";
}
```

Please note that the `name` passed to `tracesSampler` may not be the exact same name that is eventually sent to Sentry. The name may be updated during the lifetime of the transaction, but the `name` passed to `tracesSampler` will always be the initial name.

## [Inheritance](https://docs.sentry.io/platforms/javascript/guides/cordova/sampling.md#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 `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.

```javascript
tracesSampler: samplingContext => {
  // always inherit
  if (samplingContext.parentSampled !== undefined) {
    return samplingContext.parentSampled
  }

  ...
  // rest of sampling logic here
}
```

If you're using a `tracesSampleRate` rather than a `tracesSampler`, the decision will always be inherited.

## [Precedence](https://docs.sentry.io/platforms/javascript/guides/cordova/sampling.md#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 `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)
2. If `tracesSampler` is not defined, but there's a parent sampling decision, the parent sampling decision will be used.
3. If `tracesSampler` is not defined and there's no parent sampling decision, `tracesSampleRate` will be used.