---
name: otel-ottl
description: OpenTelemetry Transformation Language (OTTL) expert. Use when writing or debugging OTTL expressions for any OpenTelemetry Collector component that supports OTTL (processors, connectors, receivers, exporters). Triggers on tasks involving telemetry transformation, filtering, attribute manipulation, data redaction, sampling policies, routing, or Collector configuration. Covers syntax, contexts, functions, error handling, and performance.
metadata:
  author: dash0
  version: '1.0.0'
---

# OpenTelemetry Transformation Language (OTTL)

## Components that use OTTL

OTTL is not limited to the transform and filter processors.
Processors (transform, filter, tailsampling, logdedup) and connectors (routing, count, sum, signaltometrics) accept OTTL expressions.
See [components](./rules/components.md) for the full list with use cases.

## OTTL syntax

### Path expressions

Navigate telemetry data using dot notation:

```
span.name
span.attributes["http.method"]
resource.attributes["service.name"]
```

**Contexts** (first path segment): `resource`, `scope`, `span`, `spanevent`, `metric`, `datapoint`, `log`.

### Enumerations

Use int64 constants for enumeration fields:

```
span.status.code == STATUS_CODE_ERROR
span.kind == SPAN_KIND_SERVER
```

### Operators

Assignment: `=` — Comparison: `==`, `!=`, `>`, `<`, `>=`, `<=` — Logical: `and`, `or`, `not`

### Functions

**Converters** (uppercase, return values):

```
ToUpperCase(span.attributes["http.request.method"])
Substring(log.body.string, 0, 1024)
Concat(["prefix", span.attributes["request.id"]], "-")
IsMatch(metric.name, "^k8s\\..*$")
```

**Editors** (lowercase, modify data in-place):

```
set(span.attributes["region"], "us-east-1")
delete_key(resource.attributes, "internal.key")
limit(log.attributes, 10, [])
```

See [function-reference](./rules/function-reference.md) for the full list of editors and converters.

### Conditional statements

Use `where` to apply transformations conditionally:

<!-- eval:skip -->
```
span.attributes["db.statement"] = "REDACTED" where resource.attributes["service.name"] == "accounting"
```

### Nil checks

Use `nil` for absence checking (not `null`):

```
resource.attributes["service.name"] != nil
```

## Validation workflow

1. **Validate config syntax** — run `otelcol validate --config=config.yaml` to catch compilation errors before starting the Collector.
2. **Test with the debug exporter** — route transformed telemetry to a `debug` exporter and inspect the output:

```yaml
exporters:
  debug:
    verbosity: detailed

service:
  pipelines:
    traces:
      receivers: [otlp]
      processors: [transform]
      exporters: [debug]   # swap in production exporter once validated
```

3. **Set `error_mode: ignore` in production** — see [Error handling](#error-handling).
4. **Promote to production exporters** — replace `debug` with the production exporter.

## Common patterns

<!-- keep-in-sync: each entry must match a ## heading in ./rules/patterns.md -->
- [Set attributes](./rules/patterns.md#set-attributes)
- [Drop telemetry by pattern](./rules/patterns.md#drop-telemetry-by-pattern)
- [Drop stale data](./rules/patterns.md#drop-stale-data)
- [Backfill missing timestamps](./rules/patterns.md#backfill-missing-timestamps)
- [Filter processor example](./rules/patterns.md#filter-processor-example)
- [Transform processor example](./rules/patterns.md#transform-processor-example)
- [Defensive nil checks](./rules/patterns.md#defensive-nil-checks)
- [Redact sensitive data](./rules/redaction.md) — strategies: replace, mask, hash, delete, and drop.
- [Normalize high-cardinality attributes](./rules/cardinality.md) — path segments, IP masking, and attribute count/length limits.
- [Enrich telemetry with static attributes](./rules/enrichment.md)

## Error handling

### Compilation errors

Occur during processor initialization and prevent Collector startup:
- Invalid syntax (missing quotes)
- Unknown functions
- Invalid path expressions
- Type mismatches

### Runtime errors

Occur during telemetry processing:
- Accessing non-existent attributes
- Type conversion failures
- Function execution errors

### Error mode configuration

Always set `error_mode` explicitly; the default differs by component.
The transform and filter processors default to `ignore` since Collector Contrib v0.153.0 and to `propagate` before it.
The routing and signaltometrics connectors default to `propagate`.
The count and sum connectors have no `error_mode` key and always propagate errors.

| Mode | Behavior | When to use |
|------|----------|-------------|
| `ignore` | Logs the error and continues to the next statement | **Production and general use** — the default for the transform and filter processors |
| `propagate` | Returns the error up the pipeline, dropping the payload from the Collector | Development and strict environments where you want to catch every error |
| `silent` | Ignores errors without logging | High-volume pipelines with known-safe transforms where error logs are noise |

```yaml
processors:
  transform:
    error_mode: ignore
    trace_statements:
      - set(span.attributes["parsed"], ParseJSON(span.attributes["json_body"]))
```

Statements are a flat list; the Collector infers the context (`span`, `metric`, `datapoint`, `log`, and so on) from the path prefixes.
Use the object form with an explicit `context:` only when a statement group mixes paths that cannot be inferred to a single context, or when you need a group-level condition or `error_mode`.

## Performance

Use `where` clauses to skip items early.

```
# BAD — runs replace_pattern on every span
replace_pattern(span.attributes["url.path"], "/\\d+", "/{id}")

# GOOD — skips spans that lack the attribute
replace_pattern(span.attributes["url.path"], "/\\d+", "/{id}") where span.attributes["url.path"] != nil
```

## References

- [OTTL Guide](https://www.dash0.com/guides/opentelemetry-transformation-language-ottl)
- [OTTL Specification](https://github.com/open-telemetry/opentelemetry-collector-contrib/tree/main/pkg/ottl)
- [OTTL Functions Reference](https://github.com/open-telemetry/opentelemetry-collector-contrib/tree/main/pkg/ottl/ottlfuncs)
- [OTTL Playground](https://ottl.run)