OpenTelemetry Transformation Language (OTTL)

OTTL transforms or selects telemetry inside Collector components. This skill is pinned to collector-contrib v0.163.0. Function, path, default, and feature-gate availability varies by release; when the user's version differs, verify against the matching upstream tag.

Dated OTTL feature-gate status for v0.162.0 and v0.163.0 is in references/functions.md.

Workflow

1. Choose the component. transform rewrites, filter drops, tail_sampling decides whether to retain traces, and routing sends telemetry to pipelines. A component controls its available contexts and functions.
2. Choose the lowest usable context. Lower contexts can read their parents (for example, a span can read resource.attributes), but parents cannot read children. Use datapoint for point attributes instead of traversing metric.data_points.
3. Verify every emitted function, then write the statement. Confirm each function's exact identifier and signature in references/functions.md. If an identifier is absent, treat it as unsupported instead of deriving or substituting a plausible name. An editor such as set or delete_key mutates data and may have a where condition. Converters such as ParseJSON and IsMatch return values; they do not mutate.
4. Set error behavior deliberately. ignore logs statement errors and continues; silent continues without logging; propagate returns the error and can cause the component to drop the payload. Transform and filter default to ignore; the transform default-error gate was removed in v0.162.0, so naming it in --feature-gates fails startup (history: the otel-collector skill's transform quirks), and the filter gate is stable and cannot be disabled. Routing also defaults to ignore while its beta default-error feature gate is enabled. For routing, ignore sends an errored payload to default_pipelines; configure that fallback or the payload is dropped.
5. Verify end to end. Validate the exact Collector version (since v0.163, validate also parses routing connector conditions), then send known telemetry and inspect file-exporter output. Use the telemetrygen recipe.

set(span.attributes["env"], "prod") where resource.attributes["env"] == nil

Load only what the task needs

• Contexts — exact paths, hierarchy, enums, and request metadata.
• Functions — editor/converter signatures and release availability.
• Quick reference — component YAML, recipes, escaping, troubleshooting, and safe skeletons.

For a single path or function, read only the relevant section instead of loading the full catalogs.

Safety and correctness gates

• Guard optional or polymorphic input before conversion: where x != nil, IsString(x), or the appropriate type check.
• For JSON-object-only work, guard both the type and shape before calling ParseJSON, for example IsString(log.body) and IsMatch(log.body.string, "(?s)^\\s*\\{.*\\}\\s*$"). The RE2 (?s) flag admits pretty-printed objects containing newlines. Checking IsMap after parsing does not prevent arrays or scalar JSON from being parsed.
• On a version-pinned request, confirm every chosen path and function against that release tag; do not assume a function listed for this skill's v0.163 anchor exists in an older release. For v0.156 JSON-object parsing, ParseJSON, IsString, and IsMatch are available without the v0.157 alpha lambda feature gate.
• Request metadata is read-only and may contain credentials. Copy only explicitly allowlisted, non-sensitive keys. OTLP metadata routing requires include_metadata: true on the receiver. HTTP/client header spelling may retain its form (otelcol.client.metadata["X-Tenant"][0]); gRPC metadata keys are lowercase (otelcol.grpc.metadata["x-tenant"][0]).
• The routing request context is deprecated as of v0.156; use otelcol.client.metadata or otelcol.grpc.metadata.
• Log-record-specific rewrites of shared resource or scope data require flatten_data: true and the alpha transform.flatten.logs gate. This copies and regroups data; do not enable it accidentally.
• Since v0.161, set(target, nil) is no longer a no-op: the ottl.set.allowNil gate became beta in v0.161 and stable (cannot be disabled) in v0.162. A nil value empties a map, slice, or pcommon.Value target (an attribute key is created with an empty value) and errors on scalar targets. Use a where source != nil guard when the destination must remain unchanged for missing input.
• Hashing an identifier does not necessarily anonymize it. Apply the organization's data-handling policy before retaining deterministic hashes of personal data.

Frequent syntax traps

• In Collector YAML, write an OTTL replacement backreference ${1} as $${1}. A replacement such as $1REDACTED is literal and silently fails to substitute the capture.
• Go RE2 rejects large counted repetitions such as (.{1024}).*; use Substring with a nil/type guard and Len, or truncate_all for a map.
• Current span-event paths use spanevent.*, not span_event.*. Cache paths are context-qualified, such as span.cache["parsed"].
• Quote any OTTL statement containing a map literal when YAML includes a space after :, for example 'set(log.attributes["a"], {"foo": "bar"})'; otherwise YAML parses : as a mapping.
• Since v0.159, polymorphic pcommon.Value paths compare by their underlying type; maps and slices support only equality and inequality, while primitive values also support ordering.
• Use Decode(value, "base64"); Base64Decode was removed in v0.161.
• Regex escapes inside OTTL strings are doubled (\\d, \\s, \\.).

Upstream sources

• OTTL package
• Transform processor
• Filter processor
• Routing connector