svcforge/ARCHITECTURE.md

# svcforge — how it works

A control plane for internal service provisioning. A team asks for an Elasticsearch; a
worker installs one; a control loop keeps reality matching the database.

This document explains the design and **why each piece is the way it is**. Every "why" here
was paid for by a specific failure mode.

---

## The one-paragraph version

`POST /v1/instances` writes two rows in one transaction: the instance, and a task to build
it. It returns `202` immediately. A worker claims that task with a single `SELECT ... FOR
UPDATE SKIP LOCKED` statement, runs `helm upgrade --install`, and marks the instance ready.
A reconciler sweeps every 60 seconds for the things that go wrong when a process dies at
the wrong moment. The queue is a Postgres table, not Redis, and that single decision
determines most of the rest of the design.

---

## The shape

```mermaid
flowchart LR
    subgraph tenant["Tenant"]
        CLI["svcforge CLI<br/><i>HTTP only, never the DB</i>"]
    end

    subgraph plane["svcforge control plane"]
        API["<b>api</b><br/>FastAPI, N replicas<br/>validates, authenticates,<br/>enqueues"]
        WORKER["<b>worker</b><br/>N replicas<br/>claims tasks, runs helm"]
        RECON["<b>reconciler</b><br/><i>exactly 1</i><br/>4 checks every 60s"]
    end

    subgraph state["State"]
        PG[("<b>Postgres</b><br/>instances + tasks<br/><i>the truth</i>")]
        REDIS[("<b>Redis</b><br/>rate limit, cache<br/><i>derived only</i>")]
    end

    K8S["<b>Kubernetes</b><br/>helm releases<br/><i>the real world</i>"]

    CLI -->|"POST /v1/instances"| API
    API -->|"one transaction:<br/>instance + task"| PG
    API -.->|"best effort"| REDIS
    WORKER -->|"claim<br/>SKIP LOCKED"| PG
    WORKER -->|"helm upgrade --install"| K8S
    RECON -->|"drift, leases,<br/>TTL, versions"| PG
    RECON -->|"helm list"| K8S

    classDef truth fill:#2d4a22,stroke:#5a8f3d,color:#fff
    classDef derived fill:#4a3222,stroke:#8f6a3d,color:#fff
    class PG truth
    class REDIS derived
```

Three services, four layers, two stores. The layer rule is one line:

```
transport/   HTTP handlers, CLI entrypoints. Knows FastAPI. Knows nothing about SQL.
domain/      Pure logic: state machine, catalog, backoff, windows. No I/O. No async.
repo/        SQL. Rows in, domain objects out. Knows psycopg. Knows nothing about HTTP.
adapters/    The outside world: helm, kubectl, webhooks, Redis, the clock.
```

Dependencies point inward: `transport → domain ← repo/adapters`. `domain/` imports nothing
from the other three, which is why its tests need no mocks and run in milliseconds.

---

## The decision everything else follows from

**The queue is a Postgres table.**

A task and the instance state it describes must commit atomically. Split them across two
stores and you own a distributed commit problem with no winning move: the process can die
between the two writes, and whichever you write first is the one that lies.

- Task first, then instance → an orphan task pointing at an instance that never existed.
- Instance first, then task → an instance nobody will ever build.

In one table, in one transaction, neither is possible:

```sql
BEGIN;
  INSERT INTO instances (...);   -- state='requested'
  INSERT INTO tasks (...);       -- kind='provision'
COMMIT;                          -- both, or neither
```

Redis cannot do this, so Redis is not the queue. It holds derived state only — things that
can be recomputed and whose loss is an inconvenience, never a corruption.

| | Postgres | Redis |
|---|---|---|
| Holds | instances, tasks | rate limits, idempotency keys, cache |
| If it is down | the platform is down | the platform is fine |
| If it disagrees with reality | reality wins, reconciler fixes it | discard it |
| Lives on the | control loop **and** request path | request path only |

That last row is a budget constraint. Upstash's free tier is 500K
commands/month = **0.19 commands/second sustained**. One worker polling Redis every 5
seconds is 518,400/month — the entire budget, spent by one pod doing nothing.

---

## Provisioning, end to end

```mermaid
sequenceDiagram
    autonumber
    participant T as Tenant
    participant A as api
    participant P as Postgres
    participant W as worker
    participant K as Kubernetes

    T->>A: POST /v1/instances {elasticsearch, small}
    A->>A: verify JWT, look up catalog
    rect rgb(45, 74, 34)
    A->>P: BEGIN
    A->>P: INSERT instance (state=requested)
    A->>P: INSERT task (kind=provision, traceparent)
    A->>P: COMMIT
    end
    A-->>T: 202 Accepted + Location

    Note over W,P: every 5s, while a semaphore slot is free
    W->>P: UPDATE ... FOR UPDATE SKIP LOCKED
    P-->>W: task (attempts now 1, locked_by=me)
    W->>K: helm upgrade --install --create-namespace --wait
    K-->>W: release ready
    W->>P: CAS provisioning -> ready, set endpoint
    W->>P: complete(task, worker_id)

    T->>A: GET /v1/instances/{id}
    A-->>T: {state: ready, endpoint: ...}
```

**Why 202 and not 201.** A provision is `helm --wait` on a StatefulSet: minutes. Holding
an HTTP connection open for that is a request that dies to any proxy timeout, and a client
that cannot tell "still working" from "lost". The queue absorbs the API's output, which is
the property that lets 50 simultaneous POSTs all return instantly.

---

## The claim query

This is the heart of the system. It is one statement, and it must stay one statement.

```sql
WITH claimed AS (
  UPDATE tasks SET state='running', attempts=attempts+1,
                   locked_by=%(worker)s, locked_at=now()
  WHERE id = (
    SELECT id FROM tasks
    WHERE state='queued' AND run_after <= now()
    ORDER BY run_after
    FOR UPDATE SKIP LOCKED     -- step over rows other workers hold, do not queue behind them
    LIMIT 1
  )
  RETURNING *
)
SELECT claimed.*, instances.team
  FROM claimed LEFT JOIN instances ON instances.id = claimed.instance_id;
```

- **`FOR UPDATE SKIP LOCKED`** is what makes N workers scale. Without `SKIP LOCKED` they
  queue single-file behind whoever holds the oldest row.
- **The subquery exists because Postgres has no `UPDATE ... LIMIT`.**
- **Select-then-update as two statements is the bug this prevents.** Between the `SELECT`
  and the `UPDATE`, a second worker reads the same id and both provision. The window is
  small, which means you will not hit it in testing and you will hit it in production.
- **`LEFT` join, not inner.** The `UPDATE` has already taken effect when the outer select
  runs. An inner join matching nothing would return no row, so `claim()` would report
  "queue empty" for a task it had just marked `running` — stranding it until the lease
  expires, having silently burned an attempt.
- **`attempts` increments at claim time, not on failure.** A worker that dies without
  reporting has still burned an attempt, so a task that reliably kills its worker cannot
  retry forever.

---

## The instance lifecycle

```mermaid
stateDiagram-v2
    [*] --> requested: POST /v1/instances
    requested --> provisioning: worker claims
    provisioning --> ready: helm --wait succeeded
    ready --> deleting: DELETE, or TTL expired
    deleting --> deleted: helm uninstall succeeded

    requested --> failed: attempts exhausted
    provisioning --> failed: attempts exhausted
    ready --> failed: drift — the release vanished
    failed --> provisioning: retry
    failed --> deleting: give up, tear it down
    deleting --> failed: attempts exhausted

    deleted --> [*]: terminal
```

`LEGAL` is a `dict[InstanceState, frozenset[InstanceState]]` in `domain/states.py`, not a
chain of `if`s. `deleted` maps to an **empty frozenset** rather than being absent, so
"terminal" is stated rather than implied by a missing key.

**The state machine is enforced in SQL too.** `TaskRepo.fail` writes `instances.state`
directly, so it derives its guard from the same `LEGAL` table:

```python
_CAN_FAIL = tuple(s.value for s, allowed in LEGAL.items() if InstanceState.FAILED in allowed)
...
UPDATE instances SET state='failed' WHERE id=%s AND state = ANY(%s)
```

Without that, a deprovision exhausting its retries against an already-`deleted` instance
would resurrect it into `failed` — a transition `transition()` explicitly forbids,
performed by raw SQL that never asked it. A state machine only one layer respects is
decoration.

---

## What happens when things die

This is the part that matters. Every guarantee below has a test.

```mermaid
flowchart TD
    START["worker claims task<br/>state=running, locked_by=me"] --> WORK["helm upgrade --install"]
    WORK -->|success| REPORT["complete(task, worker_id)"]
    WORK -->|"raises"| FAIL["fail(task, err, worker_id)"]
    WORK -->|"pod SIGKILLed"| DEAD["nothing reported<br/>row stuck at 'running'"]

    REPORT --> CAS{"still locked_by me?"}
    CAS -->|yes| DONE["state=done"]
    CAS -->|"no — lease was stolen"| DROP["log and drop.<br/>the new owner reports"]

    FAIL --> ATT{"attempts < max?"}
    ATT -->|yes| REQUEUE["state=queued<br/>run_after += backoff+jitter"]
    ATT -->|no| DEADLETTER["state=failed<br/>error copied to instance"]

    DEAD --> LEASE["reconciler: locked_at older<br/>than lease_seconds"]
    LEASE --> REQUEUE

    REQUEUE --> START

    classDef bad fill:#4a2222,stroke:#8f3d3d,color:#fff
    classDef good fill:#2d4a22,stroke:#5a8f3d,color:#fff
    class DEAD,DEADLETTER bad
    class DONE,DROP good
```

**Recovery is by lease.** No distributed lock survives a power cut. A SIGKILLed worker leaves
`state='running'` with `locked_by` set and nobody running it; that row would sit there
forever. The lease is the only thing that recovers it, which is why `locked_at` exists.

**Ownership is checked on report.** A worker that hangs past its lease has its task
requeued and re-claimed by someone else. When it finally returns, `complete()` and `fail()`
both check `AND state='running' AND locked_by=%s`. Without that check, the stale worker
marks the task done while the new owner is still running it — and if the new owner then
fails, a *third* worker provisions the same instance. That is the double-provision the
claim query exists to prevent, arriving through the back door.

**Idempotency is what makes all of this safe.** It is bought in two places:

1. A deterministic release name: `f"{team}-{service_type}-{id[:8]}"`, `UNIQUE` in the schema.
2. Adapters that state desired state — `helm upgrade --install`, `kubectl apply` — instead
   of issuing imperative commands.

Running a handler twice equals running it once, so redelivery is boring.

---

## The reconciler

One replica. Four checks. Every 60 seconds.

```mermaid
flowchart LR
    TICK(("tick<br/>every 60s")) --> D["<b>drift</b><br/>helm list vs DB"]
    TICK --> L["<b>lease expiry</b><br/>running + locked_at old"]
    TICK --> T["<b>TTL</b><br/>ready + expires_at passed"]
    TICK --> V["<b>version drift</b><br/>chart_version ≠ catalog"]

    D --> D1["release missing → re-enqueue provision"]
    D --> D2["release unknown → <b>log only</b>"]
    L --> L1["→ queued, locked_by=null"]
    T --> T1["→ deleting + deprovision task"]
    V --> V1["→ upgrade task, inside<br/>the maintenance window"]
```

**The drift check never auto-deletes.** A bug in a delete path is unrecoverable; a bug in a
report path is a Tuesday. Unknown releases are logged for a human.

**It is a singleton because it sweeps.** Two reconcilers double-enqueue and race on TTL.
`replicas: 1` and `strategy: Recreate` in the chart, plus per-check idempotency guards that
re-verify under `FOR UPDATE`.

---

## Day 2: upgrading the fleet

The team that runs this also patches it. That is the whole module, and it is **two columns
and one query**.

```mermaid
flowchart TD
    EDIT["edit catalog.yaml<br/>21.3.15 → 21.3.16"] --> Q

    Q["<b>the work list</b><br/>state=ready AND chart_version ≠ catalog<br/>AND NOT halted<br/>ORDER BY own_team DESC<br/>LIMIT max_in_flight"]

    Q --> U["upgrade task<br/>run_after = next maintenance window"]
    U --> H["helm upgrade --install"]
    H --> W["write instances.chart_version<br/><i>only after helm succeeds</i>"]
    W --> VER["verify task"]
    VER -->|healthy| NEXT["next instance"]
    VER -->|"failed"| HALT["catalog_versions.rollout_state='halted'<br/><i>work list goes empty</i>"]

    NEXT --> Q
    HALT --> HUMAN["cleared by hand, with SQL"]

    classDef stop fill:#4a2222,stroke:#8f3d3d,color:#fff
    class HALT,HUMAN stop
```

- **`instances.chart_version` is written only after helm succeeds.** Write it optimistically
  and the fleet looks upgraded while it is not.
- **`ORDER BY team = own_team DESC`** puts your own instances first, so you are the tenant
  who discovers the chart is broken. Eating your own dog food is enforced by an `ORDER BY`
  rather than left to policy.
- **`max_in_flight` starts at 1** — a config value, not a scheduler. One at a time is what
  makes the halt meaningful: the fleet stops after the first casualty, not after all of them.
- **The halt is one column, cleared by hand.** An automatic un-halt would just resume
  breaking things.

Deliberately **not** built: `resize`, `backup`/`restore`, `helm rollback` automation,
deprecation timers, a rollouts table with history, a pause/resume CLI. Backup is a whole
subsystem and an untested restore is a rumour.

---

## Observability: one trace across the queue

```mermaid
sequenceDiagram
    participant A as api
    participant P as tasks table
    participant W as worker

    A->>A: span "POST /v1/instances" (trace abc123)
    A->>P: INSERT ... traceparent='00-abc123-...'
    Note over P: minutes pass. different pod.
    W->>P: claim → row carries traceparent
    W->>W: span "task.provision", parent=abc123
    Note over A,W: one trace: POST → queue → helm
```

Trace context does **not** survive a queue on its own. The worker picks the row up in
another process with no ambient context. So the traceparent rides in the table. Skip this
and Tempo shows two unrelated traces for one provision, which is worse than no tracing
because it looks like it works.

Logs carry `instance_id` / `task_id` / `team` on every line via contextvars, bound once at
claim. Metrics are deliberately few, and two of them are named to prevent a specific
mistake: `svcforge_task_attempts_failed_total` counts *attempts* that raised, while
`svcforge_tasks_dead_lettered_total` counts tasks that gave up. Alerting on the first
pages you for ordinary retries that later succeed.

---

## Delivery: CI never touches the cluster

```mermaid
flowchart LR
    PUSH["push to master"] --> GATES

    subgraph GATES["gates — all required, none advisory"]
        direction TB
        G1["ruff"] --> G2["mypy --strict"] --> G3["pytest unit + coverage"]
        G3 --> G4["migrate + pytest integration"]
        G4 --> G5["bandit / gitleaks / pip-audit"]
        G5 --> G6["hadolint + helm lint/template"]
    end

    GATES --> BUILD["build image (--load)"]
    BUILD --> SCAN["trivy HIGH,CRITICAL"]
    SCAN -->|clean| PUSHIMG["push by digest"]
    SCAN -->|"CVE"| STOP["pipeline fails"]
    PUSHIMG --> BUMP["commit digest to values.yaml<br/>[skip ci]"]
    BUMP --> ARGO["ArgoCD notices the commit"]
    ARGO --> CLUSTER["cluster"]

    classDef stop fill:#4a2222,stroke:#8f3d3d,color:#fff
    class STOP stop
```

**CI holds no kubeconfig, and must never hold one.** Its last act is a git commit; ArgoCD
pulls. The maximum blast radius of a compromised pipeline is a bad commit, which is
revertable.

**Build once, promote the artifact.** The image is loaded locally, scanned, and only then
pushed — push-then-scan means a CRITICAL sits in the registry behind a green checkmark.
Deploys are **by digest**, never a mutable tag.

**Migrations never run on app startup.** N replicas would race. They run as a Helm
`pre-upgrade,pre-install` hook Job, once, before any new pod serves traffic. Forward-only,
expand/contract: a rename is three deploys.

### The chart works with or without ArgoCD

The chart is **pure Helm**. It contains no `argocd.argoproj.io/*` annotations, no sync
waves, and no ArgoCD-specific ordering. Both paths are supported and both are verified:

```bash
# Path 1: plain Helm, no ArgoCD anywhere
helm upgrade --install svcforge deploy/chart -n svcforge \
  --set image.api.digest=sha256:... --set image.worker.digest=sha256:... \
  --set image.reconciler.digest=sha256:...

# Path 2: GitOps. ArgoCD watches master and applies the same chart.
kubectl apply -f deploy/argocd/app.yaml
```

Ordering survives both because **ArgoCD translates Helm hooks into its own sync phases**
rather than ignoring them:

| Annotation | Plain Helm | ArgoCD |
|---|---|---|
| `helm.sh/hook: pre-install,pre-upgrade` | runs before the release, aborts it on failure | mapped to the **PreSync** phase |
| `helm.sh/hook-weight: "-5"` | orders hooks within the phase | mapped to hook ordering |
| `helm.sh/hook-delete-policy: before-hook-creation` | deletes the previous Job first | mapped to `BeforeHookCreation` |

Using `argocd.argoproj.io/hook` instead would have been the trap: plain `helm install`
does not understand that annotation, so it would create the migration Job as an ordinary
resource with no ordering guarantee — the migration and the new pods would start together,
and the failure would appear only in whichever path nobody tested.

Verified with `helm install --dry-run=server` against a real cluster, which validates every
manifest through the API server rather than only rendering the templates locally.

---

## User stories, and what each one exercises

| As a… | I want… | So that… | Exercised by |
|---|---|---|---|
| tenant team | to request an Elasticsearch without filing a ticket | I am unblocked in minutes | `POST /v1/instances` → 202 |
| tenant team | to see why my instance failed | I can fix my own request | `instances.error`, `svcforge status` |
| tenant team | a throwaway instance to clean itself up | I do not pay for what I forgot | `ttl_days` → reconciler TTL sweep |
| platform team | a worker pod to be killable at any instant | a rolling deploy is not an outage | SIGTERM drain + lease recovery |
| platform team | to patch a CVE across every tenant | one edit, not N | `catalog.yaml` bump → work list |
| platform team | a bad chart to stop after the first casualty | I do not break 40 tenants | `verify` → `rollout_state='halted'` |
| platform team | to know the queue is stuck before a tenant tells me | I look competent | `SvcforgeQueueDepthRising` → RUNBOOK |
| on-call | a copy-pasteable diagnosis at 3am | I do not have to think | [RUNBOOK.md](RUNBOOK.md) |

---

## Where to read the code

| To understand | Read | Then |
|---|---|---|
| the data model | `domain/models.py`, `migrations/001_init.sql` | `domain/states.py` |
| **the queue** | `repo/tasks.py` — read `_CLAIM_SQL` twice | `services/worker/main.py` |
| crash safety | `services/worker/handlers.py` | `tests/integration/test_worker.py` |
| the API contract | `services/api/routes/instances.py` | `tests/integration/test_api.py` |
| subprocess discipline | `adapters/helm.py::_run` | `tests/integration/test_helm_timeout.py` |
| day 2 | `domain/windows.py`, `InstanceRepo.list_upgradable` | `services/reconciler/main.py` |
| how it ships | `.gitea/workflows/ci.yaml`, `deploy/chart/` | [RUNBOOK.md](RUNBOOK.md) |

Related: [README.md](README.md) for how to read this repo without spoiling the course,
[RUNBOOK.md](RUNBOOK.md) for operating it.