Changeset 0.22.2 (#96)

server/routing/fallback.go

@@ -0,0 +1,59 @@
+package routing
+
+import (
+	"fmt"
+	"log"
+)
+
+// ── Fallback Runner ─────────────────────────
+
+// DispatchFunc is called by the fallback runner to attempt a request
+// against a specific candidate. Returns nil on success, error on failure.
+// The caller is responsible for setting up SSE headers, streaming, etc.
+// This is NOT the actual dispatch — it's the "try this candidate" callback.
+type DispatchFunc func(candidate Candidate) error
+
+// RunWithFallback tries candidates in order until one succeeds.
+// Returns the candidate that succeeded, the decision (updated with fallback
+// depth), and any error if all candidates fail.
+//
+// maxRetries controls how many fallback attempts are made beyond the first.
+// maxRetries=0 means only try the first candidate (no fallback).
+// maxRetries=-1 means try all candidates.
+func RunWithFallback(candidates []Candidate, decision *Decision, maxRetries int, dispatch DispatchFunc) (Candidate, *Decision, error) {
+	if len(candidates) == 0 {
+		return Candidate{}, decision, fmt.Errorf("no routing candidates available")
+	}
+
+	// Determine how many candidates to try
+	limit := 1 // just the first
+	if maxRetries < 0 {
+		limit = len(candidates) // try all
+	} else if maxRetries > 0 {
+		limit = 1 + maxRetries
+		if limit > len(candidates) {
+			limit = len(candidates)
+		}
+	}
+
+	var lastErr error
+	for i := 0; i < limit; i++ {
+		c := candidates[i]
+		err := dispatch(c)
+		if err == nil {
+			// Success
+			if decision != nil {
+				decision.Selected = c.ConfigID
+				decision.FallbackDepth = i
+			}
+			return c, decision, nil
+		}
+
+		lastErr = err
+		if i < limit-1 {
+			log.Printf("[routing] candidate %s (%s) failed: %v — trying next", c.ConfigID, c.ProviderID, err)
+		}
+	}
+
+	return Candidate{}, decision, fmt.Errorf("all %d routing candidates failed; last error: %w", limit, lastErr)
+}