Zero-downtime Fly.io deploys: bluegreen + startup reorder

Three changes to eliminate 502s during proxy deploys:

1. Start nginx after Tailscale connects (not before) so MagicDNS is
   always available when the first request arrives. This is the
   community-recommended pattern for Tailscale sidecars on Fly.io.

2. Switch deploy strategy to bluegreen — the old machine keeps serving
   traffic until the new one passes health checks, then Fly.io cuts
   over. Rolling deploys with a single machine always cause downtime.

3. Replace top-level [checks] with [[http_service.checks]]. Top-level
   checks only monitor; they don't gate traffic routing. Service-level
   checks tell the Fly Proxy to hold traffic until the app is ready.

The sentinel file (/tmp/tailscale-ready) and nginx if-check are removed
since nginx no longer starts before Tailscale.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/changelog.d/fix-zero-downtime-deploy.infra.md

@@ -0,0 +1 @@
+Eliminate 502 errors during Fly.io proxy deploys by starting nginx after Tailscale, switching to bluegreen deploys, and using service-level health checks for traffic gating.

fly/fly.toml

@@ -3,6 +3,9 @@ primary_region = "sjc"
 
 [build]
 
+[deploy]
+strategy = "bluegreen"
+
 [http_service]
 internal_port = 8080
 force_https = true
@@ -10,10 +13,9 @@ auto_stop_machines = "off"
 auto_start_machines = true
 min_machines_running = 1
 
-[checks]
-[checks.health]
-port = 8080
-type = "http"
-interval = "30s"
-timeout = "5s"
+[[http_service.checks]]
+grace_period = "15s"
+interval = "10s"
+method = "GET"
 path = "/healthz"
+timeout = "5s"

fly/nginx.conf

@@ -34,8 +34,8 @@ http {
                      max_size=200m inactive=24h;
 
     # MagicDNS resolver — using a variable in proxy_pass defers upstream DNS
-    # resolution to request time, letting nginx start before Tailscale connects.
-    # Results are cached for 30s per worker to avoid per-request DNS lookups.
+    # resolution to request time (not config time). Results are cached for
+    # 30s per worker to avoid per-request DNS lookups.
     resolver 100.100.100.100 valid=30s;
     resolver_timeout 5s;
 
@@ -76,9 +76,6 @@ http {
         listen 8080 default_server;
 
         location /healthz {
-            if (!-f /tmp/tailscale-ready) {
-                return 503 "starting\n";
-            }
             return 200 "ok\n";
         }
 

fly/start.sh

@@ -1,26 +1,22 @@
 #!/bin/sh
 set -e
 
-# Start nginx immediately so port 8080 is bound (avoids connection refused).
-# Health check returns 503 until /tmp/tailscale-ready exists, so Fly.io
-# keeps the old machine serving traffic until Tailscale connects.
-nginx -g "daemon off;" &
-NGINX_PID=$!
-echo "Nginx started (waiting for Tailscale before proxying)"
-
-# Start tailscale daemon. Fly.io runs Firecracker microVMs which support
-# TUN devices natively — no need for --tun=userspace-networking.
+# Connect to tailnet first — nginx needs MagicDNS for upstream resolution.
+# With bluegreen deploys, the old machine serves traffic until this one is
+# fully ready. Fly.io runs Firecracker microVMs that support TUN devices
+# natively — no need for --tun=userspace-networking.
 tailscaled --statedir=/var/lib/tailscale &
 sleep 2
 
-# Authenticate and join tailnet
 tailscale up --authkey="${TS_AUTHKEY}" --hostname=flyio-proxy
-
-# Wait for tailscale to be ready, then signal nginx health check
 until tailscale status > /dev/null 2>&1; do sleep 1; done
-touch /tmp/tailscale-ready
 echo "Tailscale connected"
 
+# Start nginx — MagicDNS is available, health check passes immediately.
+nginx -g "daemon off;" &
+NGINX_PID=$!
+echo "Nginx started"
+
 # Start Alloy for observability (logs → Loki, metrics → Prometheus)
 alloy run /etc/alloy/config.alloy \
     --server.http.listen-addr=127.0.0.1:12345 \