Initial commit

yksa_orbital/registry.py

@@ -0,0 +1,52 @@
+"""Which propagator answers, and one instance of it per process.
+
+``get_backend()`` with no argument returns the deployment's configured default;
+with a name it returns that one specifically. Both matter. The default is what
+the request path should use, and the explicit name is what a caller needing
+physics the default cannot do -- space weather, drag fitting, decay -- must pass.
+
+That second case is not hypothetical. The default is ``sgp4``, which raises on
+any of those; a view that forgot to name ``"orekit"`` and swallowed the error
+rendered an empty chart for weeks without anything logging a complaint. If your
+call needs the sidecar, say so.
+"""
+
+from __future__ import annotations
+
+from django.conf import settings
+
+from odm import PropagatorBackend
+
+from .backends.orekit import OrekitBackend
+from .backends.sgp4 import Sgp4Backend
+
+BACKENDS: dict[str, type[PropagatorBackend]] = {
+    "sgp4": Sgp4Backend,
+    "orekit": OrekitBackend,
+}
+
+#: One instance per name. Backends are stateless apart from an HTTP client, so
+#: sharing them saves a connection pool rather than risking shared state.
+_INSTANCES: dict[str, PropagatorBackend] = {}
+
+DEFAULT_BACKEND = "sgp4"
+
+
+def register(name: str, backend: type[PropagatorBackend]) -> None:
+    """Add a backend under ``name``. For a service with its own propagator."""
+    BACKENDS[name] = backend
+    _INSTANCES.pop(name, None)
+
+
+def get_backend(name: str | None = None) -> PropagatorBackend:
+    key = name or getattr(settings, "ORBITAL_PROPAGATOR_BACKEND", DEFAULT_BACKEND)
+    try:
+        cls = BACKENDS[key]
+    except KeyError as exc:
+        raise ValueError(
+            f"unknown propagation backend {key!r}; "
+            f"expected one of {', '.join(sorted(BACKENDS))}"
+        ) from exc
+    if key not in _INSTANCES:
+        _INSTANCES[key] = cls()
+    return _INSTANCES[key]