Configuration Data Model

Status: Stable

Overview

The OpenTelemetry configuration data model is part of the declarative configuration interface.

The data model defines data structures which allow users to specify an intended configuration of OpenTelemetry SDK components and instrumentation.

The data model is defined in opentelemetry-configuration using JSON Schema.

The data model itself is an abstraction with multiple built-in representations:

• File-based configuration model
• SDK in-memory configuration model

Versioning policy and stability guarantees

See opentelemetry-configuration versioning policy.

File-based configuration model

A configuration file is a serialized file-based representation of the configuration data model.

Configuration files SHOULD use one the following serialization formats:

• YAML file format

YAML file format

YAML configuration files SHOULD follow YAML spec revision >= 1.2.

YAML configuration files SHOULD be parsed using v1.2 YAML core schema.

YAML configuration files MUST use file extensions .yaml or .yml.

Environment variable substitution

Configuration files support environment variables substitution for references, defined using the Augmented Backus-Naur Form:

SUBSTITUTION-REF = "${" [PREFIX ":"] GENERIC-SUBSTITUTION "}"; generic substitution reference

; when PREFIX is absent or "env", GENERIC-SUBSTITUTION MUST conform to ENV-SUBSTITUTION:
ENV-SUBSTITUTION = ENV-NAME [":-" DEFAULT-VALUE]; env var substitution

PREFIX = ALPHA *(ALPHA / DIGIT / "_"); substitution prefix, e.g. "env" (universal) or language-specific
GENERIC-SUBSTITUTION = 1*(VCHAR-WSP-NO-RBRACE); substitution content, interpretation depends on PREFIX
ENV-NAME = (ALPHA / "_") *(ALPHA / DIGIT / "_"); the name of the environment variable to be substituted
DEFAULT-VALUE = *(VCHAR-WSP-NO-RBRACE); any number of VCHAR-WSP-NO-RBRACE
VCHAR-WSP-NO-RBRACE = %x21-7C / "~" / WSP; printable chars and whitespace, except }

ALPHA = %x41-5A / %x61-7A; A-Z / a-z
DIGIT = %x30-39 ; 0-9

SUBSTITUTION-REF defines a valid substitution reference:

• Must start with ${
• Optionally followed by PREFIX:, where PREFIX is env (universal) or a language-specific identifier
• Must follow with GENERIC-SUBSTITUTION, at least one printable character or whitespace except }
• Must follow with }

When PREFIX is absent or env, GENERIC-SUBSTITUTION MUST conform to ENV-SUBSTITUTION:

• Must start with ENV-NAME, the name of the environment variable to be substituted
  • Must start with alphabetic or _ character
  • Must follow with any number of alphanumeric or _ characters
• Optionally followed by default value
  • Must start with :-
  • Must follow with DEFAULT-VALUE, any number of printable characters and whitespace except }

Language implementations MAY support additional prefixes beyond env: to access language-specific configuration sources. The GENERIC-SUBSTITUTION content is interpreted according to the PREFIX scheme. For example, Java implementations may support sys: to access system properties: ${sys:otel.service.name}. Language implementations SHOULD document any additional prefixes they support.

For convenience, SUBSTITUTION-REF and ENV-SUBSTITUTION are expressed below as PCRE2 regular expressions. Note that these expressions are non-normative.

// SUBSTITUTION-REF
\$\{(?:(?<PREFIX>[a-zA-Z][a-zA-Z0-9_]*):)?(?<GENERIC_SUBSTITUTION>[^}]+)\}

// ENV-SUBSTITUTION (applied to GENERIC_SUBSTITUTION when PREFIX is absent or "env")
(?<ENV_NAME>[a-zA-Z_][a-zA-Z0-9_]*)(:-(?<DEFAULT_VALUE>[^\n]*))?

For example, ${API_KEY} and ${env:API_KEY} are valid env var substitutions, while ${1API_KEY} and ${API_$KEY} are invalid because they do not conform to ENV-SUBSTITUTION.

Environment variable substitution MUST only apply to scalar values. Mapping keys are not candidates for substitution.

The DEFAULT-VALUE component of SUBSTITUTION-REF is an optional fallback value which is substituted if ENV-NAME is null, empty, or undefined. If a referenced environment variable is not defined and does not have a DEFAULT-VALUE, it MUST be replaced with an empty value.

The $ character is an escape sequence, such that $$ in the input is translated to a single $ in the output. The resolved $ from an escape sequence MUST NOT be considered when matching input against the environment variable substitution regular expression. For example, $${API_KEY} resolves to ${API_KEY}, and the value of the API_KEY environment variable is NOT substituted. See table below for more examples. In practice, this implies that parsers consume the input from left to right, iteratively identifying the next escape sequence, and matching the content since the prior escape sequence against SUBSTITUTION-REF.

For example, the pseudocode for processing input $${FOO} ${BAR} $${BAZ} where FOO=a, BAR=b, BAZ=c would resemble:

• Identify escape sequence $$ at index 0. Perform substitution against input.substring(0, 0)="" => "" and append to output. Append $ to output. Current output: "$".
• Identify escape sequence $$ at index 15. Perform substitution against input.substring(0+2, 15)="{FOO} ${BAR} " => "{FOO} b " and append to output. Append $ to output. Current output: "${FOO} b $".
• Reach end of input without escape sequence. Perform substitution against input.substring(15+2, input.length)="{BAZ}" => "{BAZ}" and append to output. Return output: "${FOO} b ${BAZ}".

When parsing a configuration file that contains a SUBSTITUTION-REF where GENERIC-SUBSTITUTION does not conform to the scheme’s validation rules (e.g., does not conform to ENV-SUBSTITUTION when PREFIX is absent or env), the parser must return an empty result (no partial results are allowed) and an error describing the parse failure to the user.

Node types MUST be interpreted after environment variable substitution takes place. This ensures the environment string representation of boolean, integer, or floating point properties can be properly converted to expected types.

It MUST NOT be possible to inject YAML structures by environment variables. For example, see references to INVALID_MAP_VALUE environment variable below.

It MUST NOT be possible to inject environment variable by environment variables. For example, see references to DO_NOT_REPLACE_ME environment variable below.

The table below demonstrates environment variable substitution behavior for a variety of inputs. Examples assume environment variables are set as follows:

export STRING_VALUE="value"
export BOOL_VALUE="true"
export INT_VALUE="1"
export FLOAT_VALUE="1.1"
export HEX_VALUE="0xdeadbeef"                   # A valid integer value (i.e. 3735928559) written in hexadecimal
export INVALID_MAP_VALUE="value\nkey:value"     # An invalid attempt to inject a map key into the YAML
export DO_NOT_REPLACE_ME="Never use this value" # An unused environment variable
export REPLACE_ME='${DO_NOT_REPLACE_ME}'        # A valid replacement text, used verbatim, not replaced with "Never use this value"
export VALUE_WITH_ESCAPE='value$$'              # A valid variable substituted without escaping