Open source


The River syntax is designed to be easy to read and write. Essentially, there are just two high-level elements to it: Attributes and Blocks.

River is a declarative language used to build programmable pipelines. As such, the ordering of blocks and attributes within the River configuration file is not important; the language will consider all direct and indirect dependencies between elements to determine their relationships.


River configuration files support single-line // as well as block /* */ comments.


River considers an identifier as valid if it consists of one or more UTF-8 letters (A through Z, both upper- and lower-case), digits or underscores, but doesn’t start with a digit.

Attributes and Blocks


Attributes are used to configure individual settings. They always take the form of ATTRIBUTE_NAME = ATTRIBUTE_VALUE. They can appear either as top-level elements or nested within blocks.

The following example sets the log_level attribute to "debug".

log_level = "debug"

The ATTRIBUTE_NAME must be a valid River identifier.

The ATTRIBUTE_VALUE can be either a constant value of a valid River type (eg. string, boolean, number) or an expression to represent or compute more complex attribute values.


Blocks are used to configure the Agent behavior as well as Flow components by grouping any number of attributes or nested blocks using curly braces. Blocks have a name, an optional label and a body that contains any number of arguments and nested unlabeled blocks.

Pattern for creating an unlabeled block

  // Block body can contain attributes and nested unlabeled blocks

    // Nested block body

Pattern for creating a labeled block

// Pattern for creating a labeled block:
  // Block body can contain attributes and nested unlabeled blocks

    // Nested block body

Block naming rules

The BLOCK_NAME has to be recognized by Flow as either a valid component name or a special block for configuring global settings. If the BLOCK_LABEL has to be set, it must be a valid River identifier wrapped in double quotes. In these cases the label will be used to disambiguate between multiple top-level blocks of the same name.

The following snippet defines a block named local.file with its label set to “token”. The block’s body sets the to the contents of the TOKEN_FILE_PATH environment variable by using an expression and the is_secret attribute is set to the boolean true, marking the file content as sensitive.

local.file "token" {
  filename  = env("TOKEN_FILE_PATH") // Use an expression to read from an env var.
  is_secret = true


All block and attribute definitions are followed by a newline, which River calls a terminator, as it terminates the current statement.

A newline is treated as terminator when it follows any expression, ], ) or }. Other newlines are ignored by River and and a user can enter as many newlines as they want.