# 12 — Data Flow

## Summary

This chapter follows a user's request end to end, hop by hop. It's the
consolidated picture of Chapters 3, 6, 7, 8, 9 working together. Use
this chapter to answer "when X doesn't work, which layer failed?"

## Scenario: User creates a task

A user in Austin opens the mobile app and adds a new task for their
property. The client sends `POST https://api.myhoneydue.com/api/tasks/`
with a JSON body and an auth token. We trace every hop.

## Hop 1 — Mobile client → Cloudflare edge

```mermaid
sequenceDiagram
    participant App as iOS client
    participant DNS as Local DNS
    participant CFE as Cloudflare edge (DFW)

    App->>DNS: Resolve api.myhoneydue.com
    DNS->>App: 104.21.13.7 (Cloudflare edge IP)
    App->>CFE: TCP SYN :443
    CFE-->>App: TCP SYN+ACK
    App->>CFE: TLS ClientHello
    CFE->>App: TLS ServerHello + cert
    Note over App,CFE: TLS 1.3 handshake<br/>~1 RTT
    App->>CFE: HTTP/2 stream<br/>POST /api/tasks/<br/>Authorization: Token <xxx>
```

- Client resolves `api.myhoneydue.com` via OS resolver, gets Cloudflare
  edge IP (not our origin IP)
- Client establishes TLS 1.3 to CF's nearest POP (Dallas for Austin)
- Cert presented by CF is `sni.cloudflaressl.com` or a CF-issued
  `*.myhoneydue.com` — our origin cert is never seen by the client
- Latency: ~5–15 ms Austin → DFW

## Hop 2 — Cloudflare edge → Origin (hetzner)

```mermaid
sequenceDiagram
    participant CFE as Cloudflare DFW POP
    participant DNS as CF internal DNS
    participant HN as hetzner node (random of 3)
    participant Traefik as Traefik pod<br/>(host network)

    CFE->>DNS: Which origin for api.myhoneydue.com?
    DNS->>CFE: One of 178.104.247.152, 178.105.32.198, 178.104.249.189
    CFE->>HN: TCP SYN :80
    HN-->>CFE: SYN+ACK
    CFE->>HN: HTTP/1.1 POST /api/tasks/<br/>Host: api.myhoneydue.com<br/>X-Forwarded-For: <user IP><br/>X-Forwarded-Proto: https<br/>CF-Connecting-IP: <user IP>
    Note over HN: UFW: allow 80/tcp from<br/>anywhere (anywhere for now)
    HN->>Traefik: delivered to listener
```

- CF picks one of the 3 node IPs via DNS round-robin. This is per-connection, not per-request.
- Protocol between CF and origin: **HTTP/1.1 plaintext** (SSL=Flexible).
  A future Full-strict upgrade would make this HTTPS.
- Latency: ~90–120 ms DFW → Nuremberg
- CF adds headers: `CF-Connecting-IP`, `X-Forwarded-For`, `X-Forwarded-Proto`

## Hop 3 — Traefik → api Service

```mermaid
sequenceDiagram
    participant Traefik as Traefik pod
    participant CoreDNS as CoreDNS (10.43.0.10)
    participant KP as kube-proxy IPVS<br/>(kernel)
    participant APIPod as api pod<br/>(some node)

    Note over Traefik: Match Host: api.myhoneydue.com<br/>→ honeydue-api Ingress<br/>→ backend: api Service :8000
    Traefik->>CoreDNS: Resolve "api"
    CoreDNS->>Traefik: 10.43.167.83 (Service ClusterIP)
    Traefik->>KP: TCP SYN to 10.43.167.83:8000
    KP->>KP: IPVS: pick endpoint<br/>from Service endpoint set
    KP->>APIPod: Rewrite destination<br/>to 10.42.2.6:8000<br/>(Flannel VXLAN if remote node)
```

- Traefik resolves `api` via CoreDNS → gets the Service ClusterIP
- Traefik sends to `10.43.167.83:8000`
- kube-proxy IPVS (running in-kernel on the node where Traefik lives)
  intercepts, picks a live endpoint, rewrites
- Destination might be local (same node) or remote (VXLAN tunnel to
  another node)
- Latency: <3 ms even cross-node

## Hop 4 — api → Postgres (Neon)

```mermaid
sequenceDiagram
    participant API as api pod (Go)
    participant Resolv as Pod resolv.conf
    participant Neon as Neon pooler<br/>AWS us-east-1

    API->>Resolv: Resolve ep-floral-truth-...-pooler.us-east-1.aws.neon.tech
    Note over Resolv: Goes to CoreDNS<br/>which forwards to upstream<br/>(Hetzner's DNS, then public root)
    Resolv->>API: Neon pooler IP (e.g., 34.206.177.121)
    API->>Neon: TCP :5432
    API->>Neon: TLS 1.3 handshake (DB_SSLMODE=require)
    API->>Neon: Postgres startup (user, database)
    API->>Neon: BEGIN<br/>SELECT ... FROM task_task WHERE residence_id = ?<br/>INSERT INTO task_task (...) VALUES (...)<br/>COMMIT
    Neon-->>API: Query results
```

- Go's database/sql pool may already have an idle connection. If so,
  skip handshake.
- If new connection: ~50 ms TLS handshake + Postgres startup
- Query itself: typically ~5–20 ms (single-row read/write on indexed
  columns)
- Total for this hop: often <10 ms on a warm connection, ~80 ms cold

## Hop 5 — api → Redis (cache miss invalidation)

```mermaid
sequenceDiagram
    participant API as api pod
    participant CoreDNS
    participant KP as kube-proxy
    participant Redis as redis pod

    API->>CoreDNS: Resolve "redis"
    CoreDNS->>API: 10.43.7.10
    API->>KP: TCP :6379
    KP->>Redis: rewritten to 10.42.x.y:6379
    API->>Redis: DEL tasks:user:<user_id>  (invalidate cached list)
    Redis-->>API: OK
```

- Redis connection is usually kept alive in the api's pool
- Latency: <1 ms (Redis is on hetzner2, usually a short hop)

## Hop 6 — api → worker (enqueue side effect)

For some task creation events, api enqueues a background job
(send-notification, update-lookup-table, etc.):

```mermaid
sequenceDiagram
    participant API as api pod
    participant Redis as redis pod (acting as Asynq queue)
    participant Worker as worker pod

    API->>Redis: RPUSH asynq:queue:default <job JSON>
    Redis-->>API: OK
    Note over API,Worker: (Async, no response blocking)
    Worker->>Redis: BLPOP asynq:queue:default
    Redis-->>Worker: <job JSON>
    Worker->>Worker: Process job<br/>(send email, push, etc.)
```

api returns to the caller without waiting for the job.

## Hop 7 — Response back to user

Reverse the path:

1. api returns JSON response to Traefik
2. Traefik returns to Cloudflare
3. Cloudflare re-encrypts TLS to user
4. User receives response

## End-to-end latency budget

For a typical "create task" operation:

| Hop | Latency |
|---|---|
| User → CF (Austin → DFW) | 5–15 ms |
| CF → hetzner (cross-Atlantic) | 90–120 ms |
| UFW + kernel + Traefik accept | <1 ms |
| Traefik → api (same or cross-node) | 1–3 ms |
| api request parsing, auth validation | 1–3 ms |
| api → Postgres (query) | 20–60 ms |
| api → Redis (invalidate) | <1 ms |
| api response generation | 1–5 ms |
| Return path | same as forward, reversed |

**Total**: ~220–310 ms typical. Dominated by the cross-Atlantic CF→origin
hop and the Postgres query round trip.

## Read path (GET /api/tasks/)

Similar but simpler:

```mermaid
sequenceDiagram
    participant App as iOS client
    participant CF as Cloudflare
    participant Traefik
    participant API as api pod
    participant Redis
    participant Neon

    App->>CF: GET /api/tasks/
    CF->>Traefik: (no cache hit)
    Traefik->>API: Route via Service
    API->>Redis: GET tasks:user:<user_id>
    alt Cache hit
        Redis-->>API: cached JSON
    else Cache miss
        API->>Neon: SELECT ...
        Neon-->>API: rows
        API->>Redis: SET tasks:user:<user_id> <json> EX 300
    end
    API-->>Traefik: 200 JSON
    Traefik-->>CF: 200
    CF-->>App: 200 (may cache per response headers)
```

## Admin panel data flow

A different dance because the admin is Next.js:

```mermaid
sequenceDiagram
    participant Browser
    participant CF
    participant Traefik
    participant Admin as admin pod (Next.js)
    participant AdminAPI as api pod<br/>(via public URL)
    participant Neon

    Browser->>CF: GET admin.myhoneydue.com/users
    CF->>Traefik: HTTP :80
    Traefik->>Admin: Service /users
    Note over Admin: Next.js SSR:<br/>fetch from NEXT_PUBLIC_API_URL
    Admin->>CF: GET api.myhoneydue.com/api/admin/users/
    CF->>Traefik: (api ingress)
    Traefik->>AdminAPI: Service
    AdminAPI->>Neon: SELECT ... FROM auth_user
    Neon-->>AdminAPI: rows
    AdminAPI-->>Admin: JSON
    Admin->>Admin: Render HTML
    Admin-->>Traefik: HTML
    Traefik-->>CF: HTML
    CF-->>Browser: HTML
```

Notably, the admin pod's calls to api go **back out to Cloudflare** and
in through the public URL. Not the in-cluster Service IP. This is
because `NEXT_PUBLIC_API_URL=https://api.myhoneydue.com` — Next.js builds
use the same URL for browser-side and server-side fetches.

This is **suboptimal** — server-side (SSR) calls could use the internal
`api.honeydue.svc:8000` URL and skip the CF round-trip. Future
optimization: separate `NEXT_PUBLIC_API_URL` (browser) from `API_URL`
(server-side).

## Static asset flow

For the marketing landing page at `https://myhoneydue.com/`:

1. CF caches HTML per `Cache-Control` (the Go app sets short TTLs)
2. CF caches CSS / JS / images aggressively (via default CF rules)
3. First request hits origin, subsequent requests served from CF edge

The static assets live inside the api container at `/app/static/`.
Served by Echo's static file handler at routes `/css`, `/js`, `/images`.

## Request flow during a rolling update

When a new api image is deployed, some requests will hit old pods and
some will hit new pods for a few minutes:

```mermaid
sequenceDiagram
    participant CF
    participant Traefik
    participant OldPod as api pod v1
    participant NewPod as api pod v2 (starting)

    Note over NewPod: kubelet starts new pod
    Note over NewPod: pod connects to Postgres<br/>MigrateWithLock runs (no-op)<br/>HTTP server starts<br/>readinessProbe passes
    Note over NewPod: kube-proxy updates endpoints<br/>NewPod added to Service pool
    CF->>Traefik: request 1
    Traefik->>OldPod: routed (old pod still in pool)
    CF->>Traefik: request 2
    Traefik->>NewPod: routed (new pod now in pool)
    Note over OldPod: Kubelet terminates old pod<br/>(graceful SIGTERM, then SIGKILL after grace)
    CF->>Traefik: request 3
    Traefik->>NewPod: routed (OldPod gone from pool)
```

Both old and new handle traffic simultaneously until the rolling update
completes. As long as the new code is API-compatible, users don't
notice.

## Failure modes in the data path

See [Chapter 16 — Failure Modes](./16-failure-modes.md) for a full
catalog.

Quick summary:

| Layer fails | User sees | Recovery |
|---|---|---|
| Cloudflare DNS down | Can't resolve api.myhoneydue.com | Manual DNS fallback; extremely rare |
| Cloudflare edge down (single POP) | Slow, CF routes to another POP | Automatic |
| Node NIC fails | Some requests time out (CF routes away) | Cluster reschedules pods |
| UFW misconfig blocks :80 | 521 errors at CF | Re-add rule |
| Traefik pod down on one node | CF routes to other nodes | Automatic |
| kube-proxy broken on one node | Pods on that node can't reach Services | Restart kubelet |
| CoreDNS down | New connections fail DNS | Restart CoreDNS |
| Flannel broken between nodes | Cross-node pod communication fails | Restart flannel or node |
| api pod OOM | 502 to user briefly | kubelet restarts pod |
| Postgres down | 500 errors from api | Neon-side issue; outage |
| Redis down | api serves without cache (degraded) | Restart Redis pod |
| B2 down | Uploads fail, existing content served if cached | Backblaze-side outage |

## References

- [Chapter 3 — Networking](./03-networking.md) for the overlay mechanics
- [Chapter 6 — Traefik](./06-traefik-ingress.md) for routing details
- [Chapter 7 — Services](./07-services.md) for per-service specifics
- [Chapter 16 — Failure Modes](./16-failure-modes.md) for what-if scenarios
