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Components

Components are the building blocks of Alloy. Each component performs a single task, such as retrieving secrets or collecting Prometheus metrics.

Components consist of the following:

• Arguments: Settings that configure a component.
• Exports: Named values that a component makes available to other components.

Each component has a name that describes its responsibility. For example, the local.file component retrieves the contents of files on disk.

You define components in the configuration file by specifying the component’s name with a user-defined label, followed by arguments to configure the component.

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discovery.kubernetes "pods" {
  role = "pod"
}

discovery.kubernetes "nodes" {
  role = "node"
}

You reference components by combining their name with their label. For example, you can reference a local.file component labeled foo as local.file.foo.

The combination of a component’s name and label must be unique within the configuration file. This naming approach allows you to define multiple instances of a component, as long as each instance has a unique label.

Pipelines

Most arguments for a component in a configuration file are constant values, such as setting a log_level attribute to "debug".

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log_level = "debug"

You use expressions to compute an argument’s value dynamically at runtime. Expressions can retrieve environment variable values (log_level = sys.env("LOG_LEVEL")) or reference an exported field of another component (log_level = local.file.log_level.content).

A dependent relationship is created when a component’s argument references an exported field of another component. The component’s arguments depend on the other component’s exports. The input of the component is re-evaluated whenever the referenced component’s exports are updated.

The flow of data through these references forms a pipeline.

An example pipeline might look like this:

1. A local.file component watches a file containing an API key.
2. A prometheus.remote_write component receives metrics and forwards them to an external database using the API key from the local.file for authentication.
3. A discovery.kubernetes component discovers and exports Kubernetes Pods where metrics can be collected.
4. A prometheus.scrape component references the exports of the previous component and sends collected metrics to the prometheus.remote_write component.

The following configuration file represents the pipeline.

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// Get our API key from disk.
//
// This component has an exported field called "content", holding the content
// of the file.
//
// local.file will watch the file and update its exports any time the
// file changes.
local.file "api_key" {
  filename  = "/var/data/secrets/api-key"

  // Mark this file as sensitive to prevent its value from being shown in the
  // UI.
  is_secret = true
}

// Create a prometheus.remote_write component, which other components can send
// metrics to.
//
// This component exports a "receiver" value, which can be used by other
// components to send metrics.
prometheus.remote_write "prod" {
  endpoint {
    url = "https://prod:9090/api/v1/write"

    basic_auth {
      username = "admin"

      // Use the password file to authenticate with the production database.
      password = local.file.api_key.content
    }
  }
}

// Find Kubernetes pods where we can collect metrics.
//
// This component exports a "targets" value, which contains the list of
// discovered pods.
discovery.kubernetes "pods" {
  role = "pod"
}

// Collect metrics from Kubernetes pods and send them to prod.
prometheus.scrape "default" {
  targets    = discovery.kubernetes.pods.targets
  forward_to = [prometheus.remote_write.prod.receiver]
}