cs-image-system-contract

The plugin contract: the one package a plugin is written against. It carries the protocols a model and a builder satisfy, the constants and field helpers a model is declared with, the lifecycle phases a hook keys on, and the data carriers a hook returns. It depends on no other package of the system -- pydantic and packaging only -- and the core (base) depends on it, never the reverse: the contract is upstream, the core conforms to it, and a new hook is a contract change first. The core's base classes are the convenient implementation of the contract, not the contract itself; a class that satisfies the protocols structurally is a first-class plugin, and testing.assert_conforms is the check that says so. Stage 30 made the package (2026-10-29); until then the protocols lived inside base, imported the registry, and a plugin's pyproject.toml depended on the CLI host.

One module per kind of builder, over a shared core:

What moved where

Stage 30 moved these from the core into the package. Every old path but cs_image_system.base.protocols still resolves: the module that held a name re-exports it from here (the shims), kept on purpose while the package is provisional. Removing the shims is the commitment to the contract, and is done together with moving the package to a repository of its own; until then an undo of the stage is a swap of which side holds the bodies, with no import rewritten anywhere.

The protocols carry no implementation. Where a default body existed (global_id, get_classification, get_self_injected_name, the parent reference's registry lookup) it lives in the core's base classes and mixins. The kind modules import core and only the kinds they declare they read (runtime reads image for the provider-specific image's enums); tests/test_v2_contract.py holds that direction, holds the package to importing nothing from the core, and runs assert_conforms over every base class and every built-in plugin's models and builders.

Writing a plugin

A plugin is a Python distribution with an entry point in the group cs_image_system.plugins.<kind> (PLUGIN_TYPES in constants: runtime, state, modification, storage, group, user, image, os, instance, other). The entry point loads to a zero-argument callable that returns one PluginMetadata or a list of them.

1. Depend on the contract. In pyproject.toml, dependencies = ["cs-image-system-contract>=X"], with X the first release that carries the contract (0.1.3 and before do not): a floor, not a pin -- the workspace's own packages pin each other at == because they release together, an external plugin does not. Add cs-image-system-base only if the plugin subclasses the core's base classes (every built-in plugin does; it is the convenient way, not the required one). Never depend on cs-image-system-system, the CLI host.
2. Write the model: a pydantic dataclass, @dataclass(kw_only=True, config=CSIS_MODEL_CONFIG), that satisfies the kind's <Kind>BuilderModelProtocol. The universal fields are name, type (the field is type_, aliased type), description, aliases, executable, is_default, config, gitignore, tags, with the get_* readers and finalize; the kind's module says what more the core reads. The class methods csis_name() (the value a YAML type: names) and csis_classifier() (VCT.<KIND>_BUILDER_MODEL) are how the registry resolves a type: to the class. A foreign key to another builder is fk_field(target=VCT....); a value the resolver renders is templated_field(...); an item that belongs to a builder declares _model_id = parent_field(VCT....).
3. Write the builder: a class that satisfies the kind's <Kind>BuilderProtocol: the universal surface in BuilderProtocol (generate_items_{before,during,after} return an AssetSet, get_commands_to_run_{before,during,after} a CFExecutables, pre_finalize_phase/post_finalize_phase/post_failed_phase fire around a phase's deferred commands, finalize once the configuration is complete) plus the kind's domain hooks, and the same two class methods, csis_classifier() returning VCT.<KIND>_BUILDER. The core's BuilderBase and <Kind>BuilderBase implement every hook with a default; subclass them to override only what the plugin does.
4. Return the metadata: PluginMetadata(services={KEY: [Model, Builder]}, builders_for_models={Model: Builder}), where KEY is the csis_name. Item models a plugin injects under a classification go in injected_models.
5. Prove it. In the plugin's tests, assert_conforms(Model, <Kind>BuilderModelProtocol) and assert_conforms(Builder, <Kind>BuilderProtocol); then a dry cs-image-system run --all over a tree that declares the plugin's type:.

A state backend is the smallest kind: a model that answers to_backend_registration() with a BackendRegistration naming its type's BackendKindProtocol (where a workspace's state lives, the settings the workspace's backend file carries, the settings a consumer's remote-state data source carries), and a builder that emits nothing; the core hands the registration to the terraform collector once the state backends are loaded. The contract example plugin is exactly that, written against this package alone with nothing of the core, and the proof the bar runs; the local state plugin is the same kind written the convenient way on the base classes; the dummy plugin is the template for a group or user builder and for a hook plugin.

Not in the contract yet, so a plugin that needs them depends on base: the run context (GlobalTypeContext, reached today through a base builder's _get_context(); its protocol is a later stage), the cs_image_system.plugins.hooks entry point (HookSet, typed on the context) and the cs_image_system.bootstrap entry point (the bootstrap's sections).

Prerequisites and integration

Nothing outside Python: the package needs Python 3.13 or later, pydantic and packaging, and no account, tool, credential or network. A plugin finds it by import (cs_image_system.contract.<module>); the core finds it the same way and pins it at its own version.

Configuration reference

The contract reads no YAML of its own. What it declares about every builder's entry, read by the core through BuilderModelProtocol:

The kind modules say what more each kind reads (a runtime's defaults and credentials, an OS builder's family and query, a user builder's email templates); the plugin's own README is the reference for the fields it adds. Unknown keys are refused by CSIS_MODEL_CONFIG (extra="forbid"). The sentinels default, self, none, the empty string and null (OOPS_DEFAULTS) mean "not set" wherever a value is read, so nothing may be named after one of them. Variations: none; the contract behaves the same under a dry run and a real one, on every runtime.

What it tests and verifies

• assert_conforms(cls, protocol) (testing): every member the protocol declares exists on the class, every method takes the protocol's parameters by name and kind in order (an implementation may add parameters with defaults, or take *args and **kwargs), a property is a property or a dataclass field, a class method a class method. Annotations are not compared here; pyright does that where a class lists its protocol as a base (its override check, reportIncompatibleMethodOverride, is on as an error since hygiene XIX), and a protocol a class lists as a base is the question, not an answer, so its stubs never count. The core's tests/test_v2_contract.py runs it over every base class and every built-in plugin's models and builders at just test; a plugin's own tests should run it over theirs. The verdict is an AssertionError naming every shortfall.
• The import direction, in the same test: the package's sources import nothing from cs_image_system.base, .system or .hashicorp_utils, and each kind module imports only core and the kinds it declares. tests/test_v2_package_metadata.py holds the same line from the manifest: the package declares exactly the third-party distributions it imports and reaches no sibling.
• At load, the loader checks what an entry point returned with isinstance(..., PluginMetadataProtocol) and logs an error for anything else; the registry refuses a csis_name registered twice under one classification (Type Collision).

When it fails

• AssertionError: <class> does not conform to <protocol>: <member>: ... from assert_conforms: the class lacks a member or a method's parameters differ from the protocol's. Add the member or match the signature; the protocol is the specification. If the shortfall is in a core base class, the contract changed and the core did not follow: that is the test doing its job, and the fix is in base.
• Plugin <name> did not return a valid PluginMetadataProtocol instance in the log at load, and the plugin's types unknown to every type:: the entry point returned something other than a PluginMetadata (or a list of them) -- a class instead of an instance, a dict, nothing.
• Type Collision: <name> at load: two plugins register one csis_name under one classification; rename one.
• BackendWithoutKind (state): a BackendRegistration was built without its kind; the model's to_backend_registration() must name the type's BackendKindProtocol object.
• <Model> ... Extra inputs are not permitted at load: a YAML key the model does not declare (CSIS_MODEL_CONFIG refuses unknown keys); the plugin's README names the fields it reads.
• ModuleNotFoundError: No module named 'cs_image_system.base.protocols' from a plugin written before stage 30 running on this tree: the protocols moved here, with no shim; import them from cs_image_system.contract.core.

No failure of this package has happened in the field yet (2026-10-29).

Related

• docs/PLUGINS.md, the index of every package.
• base, the core that implements the contract.