ApplicationSets

ApplicationSets fan out a single template into many Application CRs. They are the primary mechanism for multi-cluster, multi-environment, and PR-preview deployments.

In one sentence

An ApplicationSet is a template + a set of generators; the controller produces one Application per element of the generators' product.

Generator types

Argo CD ships eight generators, all under applicationset/generators/. Each implements the small interface in applicationset/generators/interface.go.

Generator	File	Use case
List	list.go	A static, hand-written list of param maps. The simplest one.
Cluster	cluster.go	One Application per registered destination cluster. Filterable by labels.
Git	git.go	One Application per directory or per file in a Git repo (e.g., one app per environment folder).
Matrix	matrix.go	Cartesian product of two child generators. Frequently Cluster × Git.
Merge	merge.go	Field-merges the outputs of two child generators on a key.
SCM Provider	scm_provider.go + services/scm_provider/	One Application per repo in a GitHub/GitLab/Bitbucket/Gitea/Azure/AWS org.
Pull Request	pull_request.go + services/pull_request/	One Application per open PR — used for preview environments.
Plugin	plugin.go + services/plugin/	One Application per item from a user-supplied HTTP service.
DuckType	duck_type.go	One Application per item from any cluster-scoped CRD that lists items.

The generator dispatcher is applicationset/generators/generator_spec_processor.go, which validates the AppSet, resolves overrides, and emits the rendered parameter sets.

Templating

Each generator emits a list of parameter maps. The controller substitutes them into spec.template (a partial Application spec). The template engine is Go text/template + sprig with custom helpers in applicationset/utils/.

Recursive interpolation is implemented in applicationset/generators/value_interpolation.go so values in nested maps can reference earlier-resolved keys.

Reconciliation policies

Set via the controller flag --policy:

Policy	Effect
sync (default)	Create + update + delete child Applications.
create-only	Only create new Applications; never modify or delete.
create-update	Create and update; never delete.
create-delete	Create and delete; never update.

Per-AppSet ignore rules can be added via the applicationset.spec.ignoreApplicationDifferences block.

Progressive rollouts

When ARGOCD_APPLICATIONSET_CONTROLLER_ENABLE_PROGRESSIVE_SYNCS=true, the controller respects applicationset.spec.strategy.rollingSync.steps. Each step has a matchExpressions selector that targets a cohort of child Applications. The controller waits for that cohort to become healthy before proceeding to the next.

Code: applicationset/controllers/applicationset_controller.go and applicationset/status/.

Webhook fast path

applicationset/webhook/webhook.go listens for Git/PR webhooks and triggers immediate re-reconciliation of any ApplicationSet whose Git/SCM/PR generator targets the touched source.

Cluster fan-out

Cluster-aware generators (Cluster + Matrix-with-Cluster + Merge-with-Cluster) react to changes in the cluster Secrets through applicationset/controllers/clustereventhandler.go.

Common patterns

• All clusters get app X: Cluster generator with no filters → one App per cluster.
• Per-environment fanout: Git directory generator over envs/*/values.yaml → one App per env.
• Per-cluster, per-environment: Matrix of Cluster × Git.
• PR previews: Pull Request generator targeting open PRs → one App per PR, deleted on close.
• Bring your own: Plugin generator pointed at an internal HTTP service.

See applicationset/examples/ for runnable examples per generator.

See also

• ApplicationSet controller — the runtime.
• primitives/applicationset — the CRD schema.