How It Works

Mon, 01 Jan 0001 00:00:00 +0000

The Crossplane Function Pipeline

Crossplane composition functions run as steps in a pipeline. Each step receives the current desired state, may modify it, and passes it on to the next step. function-hcl is one such step.

Crossplane
 |
 v
+---------------------+
| function-hcl | <-- reads req.composite, evaluates HCL, writes desired resources
+---------------------+
 |
 v
 (next function or final reconcile)

What function-hcl Does

When Crossplane calls function-hcl, it:

Unpacks the input – reads the HCL scripts from the Composition pipeline step’s input field.
Evaluates the HCL – runs the script with access to the current composite resource, observed resources, and context.
Builds desired resources – each resource block that evaluates successfully contributes a desired composed resource to the output. Each resources block produces additional resources in the desired state.
Defers incomplete blocks – resource blocks that contain incomplete values (e.g. a status field from a resource that hasn’t been created yet) are silently deferred rather than causing an error.
Writes back status / connection details / context – composite status, composite connection, and context blocks write values back to the composite resource or pipeline context.

The Reconcile Loop

Crossplane runs composition functions repeatedly as the state of the world changes. This is important for understanding function-hcl’s behavior:

Deferred Rendering

Mon, 01 Jan 0001 00:00:00 +0000

Deferred rendering is the headline feature of function-hcl. It eliminates the boilerplate conditional logic that is common in Crossplane compositions.

The Problem

In a typical Crossplane composition, if resource B uses a status field from resource A, you need to handle the case where resource A hasn’t been created yet. This often requires conditional logic scattered throughout your composition.

How function-hcl Solves It

function-hcl uses a simple rule: if any expression in a block evaluates to an incomplete value (null, missing attribute, missing index), the entire block is deferred — dropped from the output for that reconcile cycle.