Configuration Data Model

Status: Development

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

Stability definition

See opentelemetry-configuration stability definition.

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 which match the following PCRE2 regular expression:

\$\{(?:env:)?(?<ENV_NAME>[a-zA-Z_][a-zA-Z0-9_]*)(:-(?<DEFAULT_VALUE>[^\n]*))?\}

The ENV_NAME MUST start with an alphabetic or _ character, and is followed by 0 or more alphanumeric or _ characters.

For example, ${API_KEY} and ${env:API_KEY} are valid, while ${1API_KEY} and ${API_$KEY} are invalid.

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

The DEFAULT_VALUE is an optional fallback value which is substituted if ENV_NAME is null, empty, or undefined. DEFAULT_VALUE consists of 0 or more non line break characters (i.e. any character except \n). 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 the environment variable substitution regular expression.

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 reference not matching the references regular expression but does match the following PCRE2 regular expression, the parser MUST return an empty result (no partial results are allowed) and an error describing the parse failure to the user.

\$\{(?<INVALID_IDENTIFIER>[^}]+)\}

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 replacement text, used verbatim, not replaced with "Never use this value"