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Documentation Grafana Agent Flow mode Configuration language Expressions Types and values

Types and values

Types

River uses the following types for its values:

• number: Any numeric value, like 3 or 3.14.
• string: A sequence of Unicode characters representing text, like "Hello, world!".
• bool: A boolean value, either true or false.
• array: A sequence of values, like [1, 2, 3]. Elements within the list are indexed by whole numbers, starting with zero.
• object: A group of values which are identified by named labels, like { name = "John" }.
• function: A value representing a routine which can be executed with arguments to compute another value, like env("HOME"). Functions take zero or more arguments as input and always return a single value as output.
• null: A type that has no value.

Naming convention

In addition to the types above, component reference documentation will use the following conventions for referring to types:

• any: A value of any type.
• map(T): an object where the value type is T. For example, map(string) is an object where all the values are strings. The key type of an object is always a string, or an identifier which is converted into a string.
• list(T): an array where the value type is T. For example, list(string) is an array where all the values are strings.
• duration: a string denoting a duration of time, such as "1d", "1h30m", "10s". Valid units are d (for days), h (for hours), m (for minutes), s (for seconds), ms (for milliseconds), ns (for nanoseconds). Values of descending units can be combined to add their values together; "1h30m" is the same as "90m".

Numbers

River handles integers, unsigned integers and floating-point values as a single ’number’ type which simplifies writing and reading River configuration files.

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3    == 3.00     // true
5.0  == (10 / 2) // true
1e+2 == 100      // true
2e-3 == 0.002    // true

Strings

Strings are represented by sequences of Unicode characters surrounded by double quotes "":

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"Hello, world!"

A \ in a string starts an escape sequence to represent a special character. The supported escape sequences are as follows:

Bools

Bools are represented by the symbols true and false.

Arrays

Array values are constructed by a sequence of comma separated values surrounded by square brackets []:

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[0, 1, 2, 3]

Values in array elements may be placed on separate lines for readability. A comma after the final value must be present if the closing bracket ] is on a different line as the final value:

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[
  0,
  1,
  2,
]

Objects

Object values are constructed by a sequence of comma separated key-value pairs surrounded by curly braces {}:

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{
  first_name = "John",
  last_name  = "Doe",
}

A comma after the final key-value pair may be omitted if the closing curly brace } is on the same line as the final pair:

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{ name = "John" }

If the key is not a valid identifier, it must be wrapped in double quotes like a string:

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{
  "app.kubernetes.io/name"     = "mysql",
  "app.kubernetes.io/instance" = "mysql-abcxyz",
  namespace                    = "default",
}

NOTE: Be careful not to confuse objects with blocks.

An object is a value assigned to an Attribute, where commas must be provided between key-value pairs on separate lines.

A Block is a named structural element composed of multiple attributes, where commas must not be provided between attributes.

Functions

Function values cannot be constructed by users, but can be called from the standard library or when exported by a component.

Null

The null value is represented by the symbol null.

Special Types

Secrets

A secret is a special type of string which is never displayed to the user. string values may be assigned to an attribute expecting a secret, but never the inverse; it is not possible to convert a secret to a string or assign a secret to an attribute expecting a string.

Capsules

River has a special type called a capsule, which represents a category of internal types used by Flow. Each capsule type has a unique name and will be represented to the user as capsule("SOME_INTERNAL_NAME"). Capsule values cannot be constructed by the user, but can be used in expressions as any other type. Capsules are not inter-compatible and an attribute expecting a capsule can only be given a capsule of the same internal type. That means, if an attribute expects a capsule("prometheus.Receiver"), it can only be assigned a capsule("prometheus.Receiver") type. The specific type of capsule expected is explicitly documented for any component which uses or exports them.

In the following example, the prometheus.remote_write component exports a receiver, which is a capsule("prometheus.Receiver") type. This can then be used in the forward_to attribute of prometheus.scrape, which expects an array of capsule("prometheus.Receiver")s:

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prometheus.remote_write "default" {
  endpoint {
    url = "http://localhost:9090/api/v1/write"
  }
}

prometheus.scrape "default" {
  targets    = [/* ... */]
  forward_to = [prometheus.remote_write.default.receiver]
}