extraction-and-guards

Data Extraction and Guards

This example demonstrates the declared data API: a cell carries a value from one resource's extraction to a later resource's data guard and mutation, within the same component.

What it shows

• Cells: concepts.NewData[string]("db-host") creates a named, typed cell. Cells are created inside the component assembly function, once per reconcile, and passed to the resource factories that need them.
• Declared extraction: The ConfigMap resource calls configmap.ExtractInto(builder, dbHost, fn). After the ConfigMap is reconciled, fn runs against the reconciled object and its return value is written into the cell.
• Data guard: The Secret resource calls builder.WithDataGuard(dbHost). The component blocks the Secret (and anything registered after it) until dbHost has been set, with a generated reason explaining which cell it is waiting on.
• Require in a mutation: The Secret also registers a mutation that calls dbHost.Require() to read the value and copy it into the Secret's data, so the credentials and the endpoint they connect to travel together.
• Registration order matters: The ConfigMap is registered before the Secret. Build() validates that every guarded or read cell has a producer registered strictly earlier, and rejects the component otherwise.
• Topology introspection: Component.DataTopology() returns the declared data flow, one edge per cell, without running any extraction. main.go prints it before reconciling.

Reconciliation steps

1. Normal reconciliation: the ConfigMap is created, its extraction writes dbHost, the Secret's guard unblocks, and the Secret is created with the db-host entry copied in.
2. Steady-state: both resources reconcile normally.

Testing cluster-free previews

A mutation that calls Require() needs the cell to already be set, but a golden-file preview never runs reconciliation, so no extraction ever executes. The resource- and component-level tests seed the cell directly (dbHost.Set("postgres.default.svc")) before asserting the golden file, simulating the value a real reconcile would have extracted. BuildComponent returns the cell for exactly this reason.

Running

go run ./examples/extraction-and-guards/.