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

# otelcol.processor.tail\_sampling

> **BETA**: This is a [beta](/docs/agent/v0.43/stability/#beta) component. Beta components are subject to breaking changes, and may be replaced with equivalent functionality that cover the same use case.

`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.

The `tail_sampling` component uses both soft and hard limits, where the hard limit is always equal or larger than the soft limit. When memory usage goes above the soft limit, the processor component drops data and returns errors to the preceding components in the pipeline. When usage exceeds the hard limit, the processor forces a garbage collection in order to try and free memory. When usage is below the soft limit, no data is dropped and no forced garbage collection is performed.

> **Note**: `otelcol.processor.tail_sampling` is a wrapper over the upstream OpenTelemetry Collector Contrib `tail_sampling` processor. Bug reports or feature requests will be redirected to the upstream repository, if necessary.

Multiple `otelcol.processor.tail_sampling` components can be specified 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

`otelcol.processor.tail_sampling` supports the following arguments:

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       |
| `num_traces`                  | `int`      | Number of traces kept in memory.                                             | `50000` | no       |
| `expected_new_traces_per_sec` | `int`      | Expected number of new traces (helps in allocating data structures).         | `0`     | 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.

## Blocks

The following blocks are supported inside the definition of `otelcol.processor.tail_sampling`:

Expand table

| Hierarchy                                                                 | Block                                                     | Description                                                                                                      | Required |
|---------------------------------------------------------------------------|-----------------------------------------------------------|------------------------------------------------------------------------------------------------------------------|----------|
| policy                                                                    | [policy](#policy-block) \[]                               | Policies used to make a sampling decision.                                                                       | yes      |
| policy > latency                                                       | [latency](#latency-block)                                 | The policy will sample based on the duration of the trace.                                                       | no       |
| policy > numeric\_attribute                                            | [numeric\_attribute](#numeric_attribute-block)            | The policy will sample based on number attributes (resource and record).                                         | no       |
| policy > probabilistic                                                 | [probabilistic](#probabilistic-block)                     | The policy will sample a percentage of traces.                                                                   | no       |
| policy > status\_code                                                  | [status\_code](#status_code-block)                        | The policy will sample based upon the status code.                                                               | no       |
| policy > string\_attribute                                             | [string\_attribute](#string_attribute-block)              | The policy will sample based on string attributes (resource and record) value matches.                           | no       |
| policy > rate\_limiting                                                | [rate\_limiting](#rate_limiting-block)                    | The policy will sample based on rate.                                                                            | no       |
| policy > span\_count                                                   | [span\_count](#span_count-block)                          | The policy will sample based on the minimum number of spans within a batch.                                      | no       |
| policy > boolean\_attribute                                            | [boolean\_attribute](#boolean_attribute-block)            | The policy will sample based on a boolean attribute (resource and record).                                       | no       |
| policy > ottl\_condition                                               | [ottl\_condition](#ottl_condition-block)                  | The policy will sample based on a given boolean OTTL condition (span and span event).                            | no       |
| policy > trace\_state                                                  | [trace\_state](#trace_state-block)                        | The policy will sample based on TraceState value matches.                                                        | no       |
| policy > and                                                           | [and](#and-block)                                         | The policy will sample based on multiple policies, creates an `and` policy.                                      | no       |
| policy > and > and\_sub\_policy                                     | [and\_sub\_policy](#and_sub_policy-block) \[]             | A set of policies underneath an `and` policy type.                                                               | no       |
| policy > and > and\_sub\_policy > latency                        | [latency](#latency-block)                                 | The policy will sample based on the duration of the trace.                                                       | no       |
| policy > and > and\_sub\_policy > numeric\_attribute             | [numeric\_attribute](#numeric_attribute-block)            | The policy will sample based on number attributes (resource and record).                                         | no       |
| policy > and > and\_sub\_policy > probabilistic                  | [probabilistic](#probabilistic-block)                     | The policy will sample a percentage of traces.                                                                   | no       |
| policy > and > and\_sub\_policy > status\_code                   | [status\_code](#status_code-block)                        | The policy will sample based upon the status code.                                                               | no       |
| policy > and > and\_sub\_policy > string\_attribute              | [string\_attribute](#string_attribute-block)              | The policy will sample based on string attributes (resource and record) value matches.                           | no       |
| policy > and > and\_sub\_policy > rate\_limiting                 | [rate\_limiting](#rate_limiting-block)                    | The policy will sample based on rate.                                                                            | no       |
| policy > and > and\_sub\_policy > span\_count                    | [span\_count](#span_count-block)                          | The policy will sample based on the minimum number of spans within a batch.                                      | no       |
| policy > and > and\_sub\_policy > boolean\_attribute             | [boolean\_attribute](#boolean_attribute-block)            | The policy will sample based on a boolean attribute (resource and record).                                       | no       |
| policy > and > and\_sub\_policy > ottl\_condition                | [ottl\_condition](#ottl_condition-block)                  | The policy will sample based on a given boolean OTTL condition (span and span event).                            | no       |
| policy > and > and\_sub\_policy > trace\_state                   | [trace\_state](#trace_state-block)                        | The policy will sample based on TraceState value matches.                                                        | no       |
| policy > composite                                                     | [composite](#composite-block)                             | This policy will sample based on a combination of above samplers, with ordering and rate allocation per sampler. | no       |
| policy > composite > composite\_sub\_policy                         | [composite\_sub\_policy](#composite_sub_policy-block) \[] | A set of policies underneath a `composite` policy type.                                                          | no       |
| policy > composite > composite\_sub\_policy > latency            | [latency](#latency-block)                                 | The policy will sample based on the duration of the trace.                                                       | no       |
| policy > composite > composite\_sub\_policy > numeric\_attribute | [numeric\_attribute](#numeric_attribute-block)            | The policy will sample based on number attributes (resource and record).                                         | no       |
| policy > composite > composite\_sub\_policy > probabilistic      | [probabilistic](#probabilistic-block)                     | The policy will sample a percentage of traces.                                                                   | no       |
| policy > composite > composite\_sub\_policy > status\_code       | [status\_code](#status_code-block)                        | The policy will sample based upon the status code.                                                               | no       |
| policy > composite > composite\_sub\_policy > string\_attribute  | [string\_attribute](#string_attribute-block)              | The policy will sample based on string attributes (resource and record) value matches.                           | no       |
| policy > composite > composite\_sub\_policy > rate\_limiting     | [rate\_limiting](#rate_limiting-block)                    | The policy will sample based on rate.                                                                            | no       |
| policy > composite > composite\_sub\_policy > span\_count        | [span\_count](#span_count-block)                          | The policy will sample based on the minimum number of spans within a batch.                                      | no       |
| policy > composite > composite\_sub\_policy > boolean\_attribute | [boolean\_attribute](#boolean_attribute-block)            | The policy will sample based on a boolean attribute (resource and record).                                       | no       |
| policy > composite > composite\_sub\_policy > ottl\_condition    | [ottl\_condition](#ottl_condition-block)                  | The policy will sample based on a given boolean OTTL condition (span and span event).                            | no       |
| policy > composite > composite\_sub\_policy > trace\_state       | [trace\_state](#trace_state-block)                        | The policy will sample based on TraceState value matches.                                                        | no       |
| output                                                                    | [output](#output-block) \[]                               | Configures where to send received telemetry data.                                                                | yes      |

### policy block

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      |

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

- When there’s an “inverted not sample” decision, the trace is not sampled.
- 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.
- In all other cases, the trace is *not* sampled.

An “inverted” decision is the one made based on the “invert\_match” attribute, such as the one from the string tag policy.

### latency block

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` will result in a policy which samples anything greater than `threshold_ms`.

### numeric\_attribute block

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       |

### probabilistic block

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.

### status\_code block

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 block

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 regex 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 regexp 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       |

### rate\_limiting block

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      |

### span\_count block

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 is not 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 do not want to limit the policy samples based on the maximum number of spans in a trace.

### boolean\_attribute block

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 bool value (`true` or `false`) to use when matching against attribute values. |         | yes      |

### ottl\_condition block

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/main/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, do not log them, and continue on to the next condition.
- `propagate`: Return the error up the pipeline. This will result in the payload being dropped from Grafana Agent.

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

### trace\_state block

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      |

### and block

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

### and\_sub\_policy block

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      |

### composite block

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 block

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      |

### output block

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.

## 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` does not expose any component-specific debug information.

## Example

This example batches trace data from Grafana Agent Flow before sending it to [otelcol.exporter.otlp](../otelcol.exporter.otlp/) 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_wait               = "10s"
  num_traces                  = 100
  expected_new_traces_per_sec = 10

  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 = "string_attribute"

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

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

    span_count {
      min_spans = 2
    }
  }

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

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

  policy {
    name = "test-policy-12"
    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 = "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.otlp.production.input]
  }
}

otelcol.exporter.otlp "production" {
  client {
    endpoint = 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.