Configure components

Components are the defining feature of Alloy. Components are small, reusable pieces of business logic that perform a single task like retrieving secrets or collecting Prometheus metrics, and you can wire them together to form programmable pipelines of telemetry data.

The component controller is responsible for scheduling components, reporting their health and debug status, re-evaluating their arguments, and providing their exports.

Configuring components

You create components by defining a top-level block. All components are identified by their name, describing what the component is responsible for, and a user-specified label.

Arguments and exports

Most user interactions with components center around two basic concepts, arguments and exports.

Arguments are settings that modify the behavior of a component. They can be any number of attributes or nested unlabeled blocks, some required and some optional. Any optional arguments that aren’t set take on their default values.
Exports are zero or more output values that other components can refer to and can be of any Alloy type.

The following block defines a local.file component labeled “targets”. The local.file.targets component exposes the file content as a string in its exports.

The filename attribute is a required argument. You can also define a number of optional arguments, in this case, detector, poll_frequency, and is_secret, that configure how and how often the file should be polled and whether its contents are sensitive.

local.file "targets" {
  // Required argument
  filename = "/etc/alloy/targets"

  // Optional arguments: Components may have some optional arguments that
  // do not need to be defined.
  //
  // The optional arguments for local.file are is_secret, detector, and
  // poll_frequency.

  // Exports: a single field named `content`
  // It can be referred to as `local.file.targets.content`
}

Referencing components

To wire components together, one can use the exports of one as the arguments to another by using references. References can only appear in components.

For example, here’s a component that scrapes Prometheus metrics. The targets field is populated with two scrape targets, a constant target localhost:9001 and an expression that ties the target to the value of local.file.targets.content.

prometheus.scrape "default" {
  targets = [
    { "__address__" = local.file.targets.content }, // tada!
    { "__address__" = "localhost:9001" },
  ]

  forward_to = [prometheus.remote_write.default.receiver]
  scrape_config {
    job_name = "default"
  }
}

Each time the file contents change, the local.file updates its exports. The new value is sent to the prometheus.scrape targets field.

Each argument and exported field has an underlying type. Alloy checks the expression type before assigning a value to an attribute. The documentation of each component provides more information about how to wire components together.

In the previous example, the contents of the local.file.targets.content expression is evaluated to a concrete value. The value is type-checked and substituted into prometheus.scrape.default, where you can configure it.