feat: backend service

gateway/internal/session/backend.go

@@ -0,0 +1,50 @@
+package session
+
+import (
+	"context"
+	"errors"
+	"fmt"
+)
+
+// BackendLookup describes the slice of `backendclient.RESTClient`
+// SessionCache depends on. The narrow interface keeps this package free
+// of any backendclient import.
+type BackendLookup interface {
+	LookupSession(ctx context.Context, deviceSessionID string) (Record, error)
+}
+
+// BackendCache resolves authenticated device sessions by issuing one
+// synchronous REST call to backend per request. The canonical implementation replaces the
+// previous Redis-backed projection with this thin wrapper; gateway no
+// longer keeps a process-local snapshot. See ARCHITECTURE.md §11
+// «backend (sync REST), no Redis projection».
+type BackendCache struct {
+	backend BackendLookup
+}
+
+// NewBackendCache constructs a Cache that delegates every Lookup to
+// backend over REST. backend must not be nil.
+func NewBackendCache(backend BackendLookup) (*BackendCache, error) {
+	if backend == nil {
+		return nil, errors.New("session.NewBackendCache: backend lookup must not be nil")
+	}
+	return &BackendCache{backend: backend}, nil
+}
+
+// Lookup resolves deviceSessionID via backend. ErrNotFound is forwarded
+// unchanged so callers can keep using the existing equality check.
+func (c *BackendCache) Lookup(ctx context.Context, deviceSessionID string) (Record, error) {
+	if c == nil {
+		return Record{}, errors.New("session backend cache: nil cache")
+	}
+	if c.backend == nil {
+		return Record{}, errors.New("session backend cache: nil backend lookup")
+	}
+	rec, err := c.backend.LookupSession(ctx, deviceSessionID)
+	if err != nil {
+		return Record{}, fmt.Errorf("session backend cache: %w", err)
+	}
+	return rec, nil
+}
+
+var _ Cache = (*BackendCache)(nil)