feat: module contract and registry

1 files changed, 45 insertions, 0 deletions

diff --git a/deckit/module.py b/deckit/module.py
new file mode 100644
index 0000000..b30fbf5
--- /dev/null
+++ b/deckit/module.py
@@ -0,0 +1,45 @@
+"""The module contract and the registry that dispatches to modules.
+
+A module is a data source plus an allow-listed set of actions. The client sends
+a module name and an action id; it never sends a command. The registry is the
+single place that maps ids to code, so adding a module never touches the server.
+"""
+
+from dataclasses import dataclass, field
+from typing import Callable
+
+
+class ModuleError(Exception):
+    """A bad module/action id, or a module that cannot serve the request."""
+
+
+@dataclass
+class Module:
+    name: str
+    title: str
+    state: Callable[[], dict]
+    actions: dict[str, Callable[[dict], dict | None]] = field(default_factory=dict)
+    start: Callable[[], None] | None = None
+    stop: Callable[[], None] | None = None
+
+
+class Registry:
+    def __init__(self, modules):
+        self._modules = {m.name: m for m in modules}
+
+    def names(self) -> list[dict]:
+        return [{"name": m.name, "title": m.title} for m in self._modules.values()]
+
+    def _get(self, name: str) -> Module:
+        if name not in self._modules:
+            raise ModuleError(f"unknown module: {name}")
+        return self._modules[name]
+
+    def state(self, name: str) -> dict:
+        return self._get(name).state()
+
+    def run_action(self, name: str, action: str, body: dict) -> dict | None:
+        module = self._get(name)
+        if action not in module.actions:
+            raise ModuleError(f"unknown action: {name}/{action}")
+        return module.actions[action](body)