---
title: "otelcol.processor.tail_sampling | Grafana Alloy documentation"
description: "Learn about otelcol.processor.tail_sampling"
---

# `otelcol.processor.tail_sampling`

`otelcol.processor.tail_sampling` samples traces based on a set of defined policies. All spans for a given trace *must* be received by the same collector instance for effective sampling decisions.

> Note
> 
> `otelcol.processor.tail_sampling` is a wrapper over the upstream OpenTelemetry Collector Contrib [`tail_sampling`](https://github.com/open-telemetry/opentelemetry-collector-contrib/tree/v0.147.0/processor/tailsamplingprocessor) processor. Bug reports or feature requests will be redirected to the upstream repository, if necessary.

You can specify multiple `otelcol.processor.tail_sampling` components by giving them different labels.

## Usage

Alloy ![Copy code to clipboard](/media/images/icons/icon-copy-small-2.svg) Copy

```alloy
otelcol.processor.tail_sampling "<LABEL>" {
  policy {
    ...
  }
  ...

  output {
    traces  = [...]
  }
}
```

## Arguments

You can use the following arguments with `otelcol.processor.tail_sampling`:

Expand table

| Name                                | Type       | Description                                                                                                             | Default | Required |
|-------------------------------------|------------|-------------------------------------------------------------------------------------------------------------------------|---------|----------|
| `decision_wait`                     | `duration` | Wait time since the first span of a trace before making a sampling decision.                                            | `"30s"` | no       |
| `decision_wait_after_root_received` | `duration` | Wait time since receiving the root span before making a sampling decision.                                              | `"0s"`  | no       |
| `num_traces`                        | `int`      | Number of traces kept in memory.                                                                                        | `50000` | no       |
| `block_on_overflow`                 | `boolean`  | If `true`, wait for space when the `num_traces` limit is reached. If `false`, old traces will be evicted to make space. | `false` | no       |
| `expected_new_traces_per_sec`       | `int`      | Expected number of new traces (helps in allocating data structures).                                                    | `0`     | no       |
| `sample_on_first_match`             | `boolean`  | Make a sampling decision as soon as any policy matches.                                                                 | `false` | no       |
| `drop_pending_traces_on_shutdown`   | `boolean`  | Drop pending traces on shutdown instead of deciding with partial data.                                                  | `false` | no       |
| `maximum_trace_size_bytes`          | `int`      | Drop traces early when they exceed this size in bytes. `0` disables early dropping.                                     | `0`     | no       |
| `decision_cache`                    | `object`   | Configures caches for sampling decisions.                                                                               | `{}`    | no       |

`decision_wait` determines the number of batches to maintain on a channel. Its value must convert to a number of seconds greater than zero.

`num_traces` determines the buffer size of the trace delete channel which is composed of trace IDs. Increasing the number will increase the memory usage of the component while decreasing the number will lower the maximum amount of traces kept in memory.

`expected_new_traces_per_sec` determines the initial slice sizing of the current batch. A larger number will use more memory but be more efficient when adding traces to the batch.

If `sample_on_first_match` is `true`, the component makes a decision as soon as one policy matches.

If `drop_pending_traces_on_shutdown` is `true`, the component drops traces that are still waiting for `decision_wait` when shutdown starts.

If `decision_wait_after_root_received` is greater than `0`, the component can decide relative to root-span arrival time.

If `maximum_trace_size_bytes` is greater than `0`, traces over this size are dropped before decision wait to limit memory usage.

`decision_cache` can contain two keys:

- `sampled_cache_size`: Configures the number of trace IDs to be kept in an LRU cache, persisting the “keep” decisions for traces that may have already been released from memory. By default, the size is 0 and the cache is inactive.
- `non_sampled_cache_size`: Configures the number of trace IDs to be kept in an LRU cache, persisting the “drop” decisions for traces that may have already been released from memory. By default, the size is 0 and the cache is inactive.

You may want to vary the size of the `decision_cache` depending on how many “keep” vs “drop” decisions you expect from your policies. For example, you can allocate a larger `non_sampled_cache_size` if you expect most traces to be dropped. Additionally, when you use `decision_cache`, configure it with a much higher value than `num_traces` so decisions for trace IDs are kept longer than the span data for the trace.

## Blocks

You can use the following blocks with `otelcol.processor.tail_sampling`:

No valid configuration blocks found.

### `output`

Required

The `output` block configures a set of components to forward resulting telemetry data to.

The following arguments are supported:

Expand table

| Name      | Type                     | Description                           | Default | Required |
|-----------|--------------------------|---------------------------------------|---------|----------|
| `logs`    | `list(otelcol.Consumer)` | List of consumers to send logs to.    | `[]`    | no       |
| `metrics` | `list(otelcol.Consumer)` | List of consumers to send metrics to. | `[]`    | no       |
| `traces`  | `list(otelcol.Consumer)` | List of consumers to send traces to.  | `[]`    | no       |

You must specify the `output` block, but all its arguments are optional. By default, telemetry data is dropped. Configure the `metrics`, `logs`, and `traces` arguments accordingly to send telemetry data to other components.

### `policy`

Required

The `policy` block configures a sampling policy used by the component. At least one `policy` block is required.

The following arguments are supported:

Expand table

| Name   | Type     | Description                            | Default | Required |
|--------|----------|----------------------------------------|---------|----------|
| `name` | `string` | The custom name given to the policy.   |         | yes      |
| `type` | `string` | The valid policy type for this policy. |         | yes      |

Supported `type` values include: `always_sample`, `latency`, `numeric_attribute`, `probabilistic`, `status_code`, `string_attribute`, `rate_limiting`, `bytes_limiting`, `span_count`, `trace_state`, `boolean_attribute`, `ottl_condition`, `trace_flags`, `and`, `not`, `drop`, and `composite`.

Each policy results in a decision, and the processor evaluates them to make a final decision:

- When there’s a “drop” decision, the trace isn’t sampled.
- When there’s an “inverted not sample” decision, the trace isn’t sampled. ***Deprecated***
- When there’s a “sample” decision, the trace is sampled.
- When there’s an “inverted sample” decision and no “not sample” decisions, the trace is sampled. ***Deprecated***
- In all other cases, the trace isn’t sampled.

An “inverted” decision is the one made based on the `invert_match` attribute, such as the one from the string, numeric or boolean tag policy. There is an exception to this if the policy is within an and or composite policy, the resulting decision will be either sampled or not sampled. The “inverted” decisions have been deprecated, please make use of `drop` policy to explicitly not sample select traces.

### `boolean_attribute`

The `boolean_attribute` block configures a policy of type `boolean_attribute`. The policy samples based on a boolean attribute (resource and record).

The following arguments are supported:

Expand table

| Name           | Type     | Description                                                                          | Default | Required |
|----------------|----------|--------------------------------------------------------------------------------------|---------|----------|
| `key`          | `string` | Attribute key to match against.                                                      |         | yes      |
| `value`        | `bool`   | The boolean value, `true` or `false`, to use when matching against attribute values. |         | yes      |
| `invert_match` | `bool`   | Indicates that values must not match against attribute values.                       | `false` | no       |

### `latency`

The `latency` block configures a policy of type `latency`. The policy samples based on the duration of the trace. The duration is determined by looking at the earliest start time and latest end time, without taking into consideration what happened in between.

The following arguments are supported:

Expand table

| Name                 | Type     | Description                                            | Default | Required |
|----------------------|----------|--------------------------------------------------------|---------|----------|
| `threshold_ms`       | `number` | Lower latency threshold for sampling, in milliseconds. |         | yes      |
| `upper_threshold_ms` | `number` | Upper latency threshold for sampling, in milliseconds. | `0`     | no       |

For a trace to be sampled, its latency should be greater than `threshold_ms` and lower than or equal to `upper_threshold_ms`.

An `upper_threshold_ms` of `0` results in a policy which samples anything greater than `threshold_ms`.

### `numeric_attribute`

The `numeric_attribute` block configures a policy of type `numeric_attribute`. The policy samples based on number attributes (resource and record).

The following arguments are supported:

Expand table

| Name           | Type     | Description                                                    | Default | Required |
|----------------|----------|----------------------------------------------------------------|---------|----------|
| `key`          | `string` | Tag that the filter is matched against.                        |         | yes      |
| `min_value`    | `number` | The minimum value of the attribute to be considered a match.   |         | yes      |
| `max_value`    | `number` | The maximum value of the attribute to be considered a match.   |         | yes      |
| `invert_match` | `bool`   | Indicates that values must not match against attribute values. | `false` | no       |

### `ottl_condition`

The `ottl_condition` block configures a policy of type `ottl_condition`. The policy samples based on a given boolean [OTTL](https://github.com/open-telemetry/opentelemetry-collector-contrib/tree/v0.147.0/pkg/ottl) condition (span and span event).

The following arguments are supported:

Expand table

| Name         | Type           | Description                                         | Default | Required |
|--------------|----------------|-----------------------------------------------------|---------|----------|
| `error_mode` | `string`       | Error handling if OTTL conditions fail to evaluate. |         | yes      |
| `span`       | `list(string)` | OTTL conditions for spans.                          | `[]`    | no       |
| `spanevent`  | `list(string)` | OTTL conditions for span events.                    | `[]`    | no       |

The supported values for `error_mode` are:

- `ignore`: Ignore errors returned by conditions, log them, and continue on to the next condition. This is the recommended mode.
- `silent`: Ignore errors returned by conditions, don’t log them, and continue on to the next condition.
- `propagate`: Return the error up the pipeline. This results in the payload being dropped from Alloy.

At least one of `span` or `spanevent` should be specified. Both `span` and `spanevent` can also be specified.

### `probabilistic`

The `probabilistic` block configures a policy of type `probabilistic`. The policy samples a percentage of traces.

The following arguments are supported:

Expand table

| Name                  | Type     | Description                                      | Default | Required |
|-----------------------|----------|--------------------------------------------------|---------|----------|
| `sampling_percentage` | `number` | The percentage rate at which traces are sampled. |         | yes      |
| `hash_salt`           | `string` | See below.                                       |         | no       |

Use `hash_salt` to configure the hashing salts. This is important in scenarios where multiple layers of collectors have different sampling rates. If multiple collectors use the same salt with different sampling rates, passing one layer may pass the other even if the collectors have different sampling rates. Configuring different salts avoids that.

### `rate_limiting`

The `rate_limiting` block configures a policy of type `rate_limiting`. The policy samples based on rate.

The following arguments are supported:

Expand table

| Name               | Type     | Description                                                         | Default | Required |
|--------------------|----------|---------------------------------------------------------------------|---------|----------|
| `spans_per_second` | `number` | Sets the maximum number of spans that can be processed each second. |         | yes      |

### `bytes_limiting`

The `bytes_limiting` block configures a policy of type `bytes_limiting`. The policy samples based on the rate of bytes per second using a token bucket algorithm.

The following arguments are supported:

Expand table

| Name               | Type     | Description                                                                                                     | Default | Required |
|--------------------|----------|-----------------------------------------------------------------------------------------------------------------|---------|----------|
| `bytes_per_second` | `number` | Sets the sustained byte throughput limit.                                                                       |         | yes      |
| `burst_capacity`   | `number` | Sets the maximum burst size in bytes. If omitted, it defaults to `2 * bytes_per_second` in the upstream policy. | `0`     | no       |

### `span_count`

The `span_count` block configures a policy of type `span_count`. The policy samples based on the minimum number of spans within a batch. If all traces within the batch have fewer spans than the threshold, the batch isn’t sampled.

The following arguments are supported:

Expand table

| Name        | Type     | Description                         | Default | Required |
|-------------|----------|-------------------------------------|---------|----------|
| `min_spans` | `number` | Minimum number of spans in a trace. |         | yes      |
| `max_spans` | `number` | Maximum number of spans in a trace. | `0`     | no       |

Set `max_spans` to `0`, if you don’t want to limit the policy samples based on the maximum number of spans in a trace.

### `status_code`

The `status_code` block configures a policy of type `status_code`. The policy samples based upon the status code.

The following arguments are supported:

Expand table

| Name           | Type           | Description                                                                               | Default | Required |
|----------------|----------------|-------------------------------------------------------------------------------------------|---------|----------|
| `status_codes` | `list(string)` | Holds the configurable settings to create a status code filter sampling policy evaluator. |         | yes      |

`status_codes` values must be `"OK"`, `"ERROR"`, or `"UNSET"`.

### `string_attribute`

The `string_attribute` block configures a policy of type `string_attribute`. The policy samples based on string attributes (resource and record) value matches. Both exact and regular expression value matches are supported.

The following arguments are supported:

Expand table

| Name                     | Type           | Description                                                                                                                                                 | Default | Required |
|--------------------------|----------------|-------------------------------------------------------------------------------------------------------------------------------------------------------------|---------|----------|
| `key`                    | `string`       | Tag that the filter is matched against.                                                                                                                     |         | yes      |
| `values`                 | `list(string)` | Set of values or regular expressions to use when matching against attribute values.                                                                         |         | yes      |
| `enabled_regex_matching` | `bool`         | Determines whether to match attribute values by regular expression string.                                                                                  | `false` | no       |
| `cache_max_size`         | `string`       | The maximum number of attribute entries of Least Recently Used (LRU) Cache that stores the matched result from the regular expressions defined in `values.` |         | no       |
| `invert_match`           | `bool`         | Indicates that values or regular expressions must not match against attribute values.                                                                       | `false` | no       |

### `trace_state`

The `trace_state` block configures a policy of type `trace_state`. The policy samples based on TraceState value matches.

The following arguments are supported:

Expand table

| Name     | Type           | Description                                                      | Default | Required |
|----------|----------------|------------------------------------------------------------------|---------|----------|
| `key`    | `string`       | Tag that the filter is matched against.                          |         | yes      |
| `values` | `list(string)` | Set of values to use when matching against `trace_state` values. |         | yes      |

### `not`

The `not` block configures a policy of type `not`. This policy inverts the decision of a nested `not_sub_policy`.

### `not_sub_policy`

The `not_sub_policy` block configures a sampling policy used by the `not` block. Exactly one `not_sub_policy` block is required inside a `not` block.

The following arguments are supported:

Expand table

| Name   | Type     | Description                            | Default | Required |
|--------|----------|----------------------------------------|---------|----------|
| `name` | `string` | The custom name given to the policy.   |         | yes      |
| `type` | `string` | The valid policy type for this policy. |         | yes      |

### `and`

The `and` block configures a policy of type `and`. The policy samples based on multiple policies by creating an `and` policy.

### `and_sub_policy`

The `and_sub_policy` block configures a sampling policy used by the `and` block. At least one `and_sub_policy` block is required inside an `and` block.

The following arguments are supported:

Expand table

| Name   | Type     | Description                            | Default | Required |
|--------|----------|----------------------------------------|---------|----------|
| `name` | `string` | The custom name given to the policy.   |         | yes      |
| `type` | `string` | The valid policy type for this policy. |         | yes      |

### `drop`

The `drop` block configures a policy of type `drop`. This policy drops traces when all `drop_sub_policy` blocks match.

### `drop_sub_policy`

The `drop_sub_policy` block configures a sampling policy used by the `drop` block. At least one `drop_sub_policy` block is required inside a `drop` block.

The following arguments are supported:

Expand table

| Name   | Type     | Description                            | Default | Required |
|--------|----------|----------------------------------------|---------|----------|
| `name` | `string` | The custom name given to the policy.   |         | yes      |
| `type` | `string` | The valid policy type for this policy. |         | yes      |

### `composite`

The `composite` block configures a policy of type `composite`. This policy samples based on a combination of the above samplers, with ordering and rate allocation per sampler. Rate allocation allocates certain percentages of spans per policy order. For example, if `max_total_spans_per_second` is set to 100, then `rate_allocation` is set as follows:

1. test-composite-policy-1 = 50% of `max_total_spans_per_second` = 50 `spans_per_second`
2. test-composite-policy-2 = 25% of `max_total_spans_per_second` = 25 `spans_per_second`
3. To ensure remaining capacity is filled, use `always_sample` as one of the policies.

### `composite_sub_policy`

The `composite_sub_policy` block configures a sampling policy used by the `composite` block. At least one`composite_sub_policy` block is required inside a `composite` block.

The following arguments are supported:

Expand table

| Name   | Type     | Description                            | Default | Required |
|--------|----------|----------------------------------------|---------|----------|
| `name` | `string` | The custom name given to the policy.   |         | yes      |
| `type` | `string` | The valid policy type for this policy. |         | yes      |

### `debug_metrics`

The `debug_metrics` block configures the metrics that this component generates to monitor its state.

The following arguments are supported:

Expand table

| Name                               | Type      | Description                                          | Default | Required |
|------------------------------------|-----------|------------------------------------------------------|---------|----------|
| `disable_high_cardinality_metrics` | `boolean` | Whether to disable certain high cardinality metrics. | `true`  | no       |

`disable_high_cardinality_metrics` is the Alloy equivalent to the `telemetry.disableHighCardinalityMetrics` feature gate in the OpenTelemetry Collector. It removes attributes that could cause high cardinality metrics. For example, attributes with IP addresses and port numbers in metrics about HTTP and gRPC connections are removed.

> Note
> 
> If configured, `disable_high_cardinality_metrics` only applies to `otelcol.exporter.*` and `otelcol.receiver.*` components.

## Exported fields

The following fields are exported and can be referenced by other components:

Expand table

| Name    | Type               | Description                                                      |
|---------|--------------------|------------------------------------------------------------------|
| `input` | `otelcol.Consumer` | A value that other components can use to send telemetry data to. |

`input` accepts `otelcol.Consumer` data for any telemetry signal (metrics, logs, or traces).

## Component health

`otelcol.processor.tail_sampling` is only reported as unhealthy if given an invalid configuration.

## Debug information

`otelcol.processor.tail_sampling` doesn’t expose any component-specific debug information.

## Example

This example batches trace data from Alloy before sending it to \[otelcol.exporter.otlphttp]\[] for further processing. This example shows an impractical number of policies for the purpose of demonstrating how to set up each type.

Alloy ![Copy code to clipboard](/media/images/icons/icon-copy-small-2.svg) Copy

```alloy
tracing {
  sampling_fraction = 1
  write_to          = [otelcol.processor.tail_sampling.default.input]
}

otelcol.processor.tail_sampling "default" {
  decision_cache = {
    sampled_cache_size     = 100000,
    non_sampled_cache_size = 100000,
    }
  decision_wait               = "10s"
  num_traces                  = 100
  expected_new_traces_per_sec = 10
  sample_on_first_match       = true
  drop_pending_traces_on_shutdown = true

  policy {
    name = "test-policy-1"
    type = "always_sample"
  }

  policy {
    name = "test-policy-2"
    type = "latency"

    latency {
      threshold_ms = 5000
    }
  }

  policy {
    name = "test-policy-3"
    type = "numeric_attribute"

    numeric_attribute {
      key       = "key1"
      min_value = 50
      max_value = 100
    }
  }

  policy {
    name = "test-policy-4"
    type = "probabilistic"

    probabilistic {
      sampling_percentage = 10
    }
  }

  policy {
    name = "test-policy-5"
    type = "status_code"

    status_code {
      status_codes = ["ERROR", "UNSET"]
    }
  }

  policy {
    name = "test-policy-6"
    type = "string_attribute"

    string_attribute {
      key    = "key2"
      values = ["value1", "value2"]
    }
  }

  policy {
    name = "test-policy-7"
    type = "string_attribute"

    string_attribute {
      key                    = "key2"
      values                 = ["value1", "val*"]
      enabled_regex_matching = true
      cache_max_size         = 10
    }
  }

  policy {
    name = "test-policy-8"
    type = "rate_limiting"

    rate_limiting {
      spans_per_second = 35
    }
  }

  policy {
    name = "test-policy-9"
    type = "bytes_limiting"

    bytes_limiting {
      bytes_per_second = 2048
      burst_capacity   = 4096
    }
  }

  policy {
    name = "test-policy-10"
    type = "string_attribute"

    string_attribute {
      key                    = "http.url"
      values                 = ["/health", "/metrics"]
      enabled_regex_matching = true
      invert_match           = true
    }
  }

  policy {
    name = "test-policy-11"
    type = "span_count"

    span_count {
      min_spans = 2
    }
  }

  policy {
    name = "test-policy-12"
    type = "trace_state"

    trace_state {
      key    = "key3"
      values = ["value1", "value2"]
    }
  }

  policy {
    name = "test-policy-13"
    type = "ottl_condition"
    ottl_condition {
      error_mode = "ignore"
      span = [
        "attributes[\"test_attr_key_1\"] == \"test_attr_val_1\"",
        "attributes[\"test_attr_key_2\"] != \"test_attr_val_1\"",
      ]
      spanevent = [
        "name != \"test_span_event_name\"",
        "attributes[\"test_event_attr_key_2\"] != \"test_event_attr_val_1\"",
      ]
    }
  }

  policy {
    name = "drop-policy-1"
    type = "drop"

    drop {
      drop_sub_policy {
        name = "test-drop-policy-1"
        type = "string_attribute"

        string_attribute {
          key                    = "url.path"
          values                 = ["/health", "/metrics"]
          enabled_regex_matching = true
        }
      }
    }
  }

  policy {
    name = "and-policy-1"
    type = "and"

    and {
      and_sub_policy {
        name = "test-and-policy-1"
        type = "numeric_attribute"

        numeric_attribute {
          key       = "key1"
          min_value = 50
          max_value = 100
        }
      }

      and_sub_policy {
        name = "test-and-policy-2"
        type = "string_attribute"

        string_attribute {
          key    = "key1"
          values = ["value1", "value2"]
        }
      }
    }
  }

  policy {
    name = "composite-policy-1"
    type = "composite"

    composite {
      max_total_spans_per_second = 1000
      policy_order               = ["test-composite-policy-1", "test-composite-policy-2", "test-composite-policy-3"]

      composite_sub_policy {
        name = "test-composite-policy-1"
        type = "numeric_attribute"

        numeric_attribute {
          key       = "key1"
          min_value = 50
          max_value = 100
        }
      }

      composite_sub_policy {
        name = "test-composite-policy-2"
        type = "string_attribute"

        string_attribute {
          key    = "key1"
          values = ["value1", "value2"]
        }
      }

      composite_sub_policy {
        name = "test-composite-policy-3"
        type = "always_sample"
      }

      rate_allocation {
        policy  = "test-composite-policy-1"
        percent = 50
      }

      rate_allocation {
        policy  = "test-composite-policy-2"
        percent = 50
      }
    }
  }

  output {
    traces = [otelcol.exporter.otlphttp.production.input]
  }
}

otelcol.exporter.otlphttp "production" {
  client {
    endpoint = sys.env("<OTLP_SERVER_ENDPOINT>")
  }
}
```

## Compatible components

`otelcol.processor.tail_sampling` can accept arguments from the following components:

- Components that export [OpenTelemetry `otelcol.Consumer`](../../../compatibility/#opentelemetry-otelcolconsumer-exporters)

`otelcol.processor.tail_sampling` has exports that can be consumed by the following components:

- Components that consume [OpenTelemetry `otelcol.Consumer`](../../../compatibility/#opentelemetry-otelcolconsumer-consumers)

> Note
> 
> Connecting some components may not be sensible or components may require further configuration to make the connection work correctly. Refer to the linked documentation for more details.