# Sampling | Sentry for Resque

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/.md#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:

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

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/.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/.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 (`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](https://docs.sentry.io/.md#inheritance) and [precedence](https://docs.sentry.io/.md#precedence) behavior

2. Sampling function (`traces_sampler`) which:

   * Samples different transactions at different rates
   * [Filters](https://docs.sentry.io/platforms/ruby/guides/resque/configuration/filtering.md) out some transactions entirely
   * Modifies default [precedence](https://docs.sentry.io/.md#precedence) and [inheritance](https://docs.sentry.io/.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/.md#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:

```ruby
Sentry.init do |config|
  # ...
  config.traces_sample_rate = 0.2
end
```

### [Setting a Sampling Function](https://docs.sentry.io/.md#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:

```ruby
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

    # the sampling context also has the full rack environment if you want to check the path directly
    rack_env = sampling_context[:env]
    return 0.0 if rack_env && rack_env['PATH_INFO'] =~ /health_check/

    # 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 /http/
      # 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 /queue/
      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](https://docs.sentry.io/.md#sampling-context-data)

### [Default Sampling Context Data](https://docs.sentry.io/.md#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.

### [Custom Sampling Context Data](https://docs.sentry.io/.md#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:

```ruby
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](https://docs.sentry.io/.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.

(See [Distributed Tracing](https://docs.sentry.io/platforms/ruby/guides/resque/tracing/trace-propagation.md) 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 `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.

```ruby
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](https://docs.sentry.io/.md#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.

```ruby
Sentry.start_transaction(
  name: "Search from navbar",
  sampled: true
)
```

## [Precedence](https://docs.sentry.io/.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 `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.
