Open source

Component controller

The component controller is the core part of Grafana Agent Flow which manages components at runtime.

It is responsible for:

  • Reading and validating the config file.
  • Managing the lifecycle of defined components.
  • Evaluating the arguments used to configure components.
  • Reporting the health of defined components.

Component graph

As discussed in Components, a relationship between components is created when an expression is used to set the argument of one component to an exported field of another component.

The set of all components and the relationships between them define a directed acyclic graph (DAG), which informs the component controller which references are valid and in what order components must be evaluated.

For a config file to be valid, components must not reference themselves or contain a cyclic reference:

// INVALID: local.file.some_file can not reference itself:
local.file "self_reference" {
  filename = local.file.self_reference.content
// INVALID: cyclic reference between local.file.a and local.file.b:
local.file "a" {
  filename = local.file.b.content
local.file "b" {
  filename = local.file.a.content

Component evaluation

A component is evaluated when its expressions are computed into concrete values. The computed values are then used to configure the component’s runtime behavior. The component controller is finished loading once all components are evaluated, configured, and running.

The component controller only evaluates a given component after evaluating all of that component’s dependencies. Component that do not depend on other components can be evaluated at any time during the evaluation process.

Component reevaluation

As mentioned in Components, a component is dynamic: a component can update its exports any number of times throughout its lifetime.

When a component updates its exports, a controller reevaluation is triggered: the component controller reevaluates any component which references the changed component, any components which reference those components, and so on, until all affected components are reevaluated.

Component health

At any given time, a component can have one of the following health states:

  1. Unknown: default state, the component isn’t running yet.
  2. Healthy: the component is working as expected.
  3. Unhealthy: the component is not working as expected.
  4. Exited: the component has stopped and is no longer running.

By default, the component controller determines the health of a component. The component controller marks a component as healthy as long as that component is running and its most recent evaluation succeeded.

Some components can report their own component-specific health information. For example, the local.file component reports itself as unhealthy if the file it was watching gets deleted.

The overall health of a component is determined by combining the controller-reported health of the component with the component-specific health information.

An individual component’s health is independent from the health of any other components it references: a component can be marked as healthy even if it references an exported field of an unhealthy component.

Handling evaluation failures

When a component fails to evaluate, it is marked as unhealthy with the reason for why the evaluation failed.

When an evaluation fails, the component continue operating as normal: it continues using its previous set of evaluated arguments, and it can continue exporting new values.

This prevents failure propagation: if your local.file component which watches API keys suddenly stops working, other components continues using the last valid API key until the component returns to a healthy state.

Updating the config file

Both the /-/reload HTTP endpoint and the SIGHUP signal can be used to inform the component controller to reload the config file. When this happens, the component controller will synchronize the set of running components with the ones in the config file, removing components which are no longer defined in the config file and creating new components which were added to the config file. All components managed by the controller will be reevaluated after reloading.