feat: edge gateway service

gateway/internal/session/readthrough.go

@@ -0,0 +1,68 @@
+package session
+
+import (
+	"context"
+	"errors"
+	"fmt"
+)
+
+// ReadThroughCache resolves authenticated sessions from a process-local
+// SnapshotStore first and falls back to another Cache only on a local miss.
+type ReadThroughCache struct {
+	local    SnapshotStore
+	fallback Cache
+}
+
+// NewReadThroughCache constructs a hot-path cache that seeds local snapshots
+// from fallback on demand.
+func NewReadThroughCache(local SnapshotStore, fallback Cache) (*ReadThroughCache, error) {
+	if local == nil {
+		return nil, errors.New("new read-through session cache: nil local cache")
+	}
+	if fallback == nil {
+		return nil, errors.New("new read-through session cache: nil fallback cache")
+	}
+
+	return &ReadThroughCache{
+		local:    local,
+		fallback: fallback,
+	}, nil
+}
+
+// Lookup resolves deviceSessionID from local first, then performs one fallback
+// lookup on a local miss and seeds the local cache with the returned snapshot.
+func (c *ReadThroughCache) Lookup(ctx context.Context, deviceSessionID string) (Record, error) {
+	if c == nil {
+		return Record{}, errors.New("lookup session from read-through cache: nil cache")
+	}
+
+	record, err := c.local.Lookup(ctx, deviceSessionID)
+	switch {
+	case err == nil:
+		return record, nil
+	case !errors.Is(err, ErrNotFound):
+		return Record{}, fmt.Errorf("lookup session from read-through cache: %w", err)
+	}
+
+	record, err = c.fallback.Lookup(ctx, deviceSessionID)
+	if err != nil {
+		return Record{}, err
+	}
+
+	if err := c.local.Upsert(record); err != nil {
+		return Record{}, fmt.Errorf("lookup session from read-through cache: seed local cache: %w", err)
+	}
+
+	return cloneRecord(record), nil
+}
+
+// Local returns the mutable process-local snapshot store used by c.
+func (c *ReadThroughCache) Local() SnapshotStore {
+	if c == nil {
+		return nil
+	}
+
+	return c.local
+}
+
+var _ Cache = (*ReadThroughCache)(nil)