# Roles: one binary, several processes

Status: design, 2026-10-19 (overnight loop 17). H2 (`--role` and the

leader lock, #272) is built; the event bus (H3) is next.

OxygenHub is one process today: pages, API, the `oxy` wire protocol, Flow's

scheduler and every background task share one runtime and one data

directory. That is the single-node edition, and it stays one command. This

document says how the same binary runs as several processes, each taking

some of the work (its **roles**), so the web tier can scale out and the

background work runs exactly once, without a second product to maintain.

## Principles

1. **One binary, many roles.** `oxygenhub serve --role web,worker` (all

   roles when none is given). A cluster runs the roles apart; a small site

   runs them together.

2. **Seams before services.** Each piece of shared state goes behind a

   trait inside the process first, with today's behaviour as one

   implementation. Splitting into processes is then a deployment choice.

3. **Postgres before new infrastructure.** Leader election (advisory

   locks), the job queue (`FOR UPDATE SKIP LOCKED`) and the event bus

   (`LISTEN/NOTIFY`) run on the Postgres the hub already uses (#211). NATS

   or JetStream come only when a measurement asks for them.

4. **Lean on Oxygen's model.** Changes are immutable and content

   addressed; only bookmark and tag moves are mutable, and they move by

   compare-and-swap of the operation head. Two processes writing one

   repository are already safe at the storage layer: that is what makes

   web nodes cheap to add.

## The roles

| Role | Runs | Instances |

|---|---|---|

| **web** | pages, the JSON API, the `oxy` wire protocol (fetch and push), live pages (SSE), the runner API (claims, logs, finish) | many, behind a load balancer |

| **worker** | the background tasks below; one instance of each singleton at a time, chosen by a leader lock | one or more |

| **edge** (later) | the wire protocol and large uploads, routed to the node that holds a repository | many |

| **store** (later) | repositories on its disks, behind a service interface | several, by repository |

Tonight's slices are **web** and **worker**. Edge and store are the

repository service (a later phase): until then every node reaches every

repository either through one shared data directory (one machine) or

through bucket-backed repositories (oxygenhub#96), which any node can open.

## What runs where today, and what each needs

Background tasks started in `lib.rs::open`:

| Task | What it does | Role | Singleton? | Needs for several processes |

|---|---|---|---|---|

| `hooks::worker` | delivers webhooks from the `deliveries` table, nudged by `events.deliveries` | worker | yes today; many once deliveries are claimed with SKIP LOCKED | the nudge over the event bus |

| `tlog::checkpointer` | signs checkpoints, gathers witness cosignatures | web, beside the log's appends, until #274 | **yes** | the log's single writer (below) |

| `flow::reaper` | ends jobs whose runner stopped reporting | worker | yes | leader lock |

| `flow::scheduler` | starts `schedule` runs on their cron lines | worker | **yes** (two would start each run twice) | leader lock |

| `flow::placement` | the placement pass: why each job waits | worker | yes | leader lock |

| `maintenance::nightly` | nightly upkeep (gc, prunes) | worker | yes | leader lock |

| `backups::scheduler` | scheduled backups | worker | **yes** | leader lock |

| `diffusion::backfill`, `diffusion::reconciler` | the pin index and the bumps it drives | worker | yes | leader lock |

| `images::learn_archs` | reads kept images' architectures once | worker | no (idempotent) | none |

Work spawned by requests (runs on whichever web node took the request):

- `autoland::on_check_task` after a check is recorded: serialised today by

  a process-wide `TURN` mutex, so two nodes could land one proposal twice.

  Needs a cluster-wide lock (an advisory lock per proposal).

- `diffusion::propagate` / `try_land` / `catch_up` / `land_ready` after a

  push: idempotent by design, but racing; the same per-proposal lock.

- The transparency log's appends in `after_push` ("logged before the push

  is answered"): **one writer only**. The log's files live in the data

  directory; two processes appending would corrupt them. Needs either an

  advisory lock around each append (one machine, shared directory), or a

  log role that the web nodes call (a cluster).

State kept in the process:

| State | Today | In a cluster |

|---|---|---|

| state cache, summaries | per process, content-addressed | stays per node (safe: keys are hashes) |

| a repository's writer lock (`repos::Locks`, #249) | per process | the operation head's compare-and-swap already arbitrates; the lock only saves retries, so per node is correct |

| Flow placement hints (`CACHES`, `HELD`) | per process | per node is acceptable (a hint); the runner's own claim facts are the truth |

| live pages (`events`, `rooms`) | `tokio::sync::broadcast` | **the event bus** |

| rate limits (`throttle`) | per process | per node (limits per node), or Postgres later |

| sessions, tokens, accounts | the database | unchanged |

| keys (`data/keys/`) | files | shared directory or configuration |

## Leader lock

`leader::run(app, name, task)` (`crates/hub/src/leader.rs`): a process

runs a singleton task only while it holds a lock named for it. On

Postgres, `pg_try_advisory_lock(key)`, the key a stable 64-bit hash

(BLAKE3) of the hub's schema and the task's name, so two hubs in two

schemas of one database never share a lock. Every lock of one process is

held on one connection of its own, outside the pool, for the process's

life. A process that does not hold a lock asks again every 2 seconds. One

that holds it checks the connection every 2 seconds (one `SELECT 1`

answers for every task); if it has gone, the server has freed its locks,

so each task it led is stopped (its future dropped) and the process

contends again on a new connection. A process that dies closes its

connection and frees its locks; another takes over within seconds (2 s in

the test). A holder whose connection drops can overlap its successor for

at most those 2 seconds, which the tasks tolerate. On SQLite (one machine

by definition) the lock is always held. Each task is safe to run again

after a failover: they read their work from the database. A task that

ends by itself (`diffusion.backfill`) gives its lock back; the next

worker to take it runs it again, harmlessly.

The locks: `hooks.worker`, `flow.reaper`, `flow.scheduler`,

`flow.placement`, `maintenance.nightly`, `backups.scheduler`,

`diffusion.backfill`, `diffusion.reconciler` (worker role) and

`tlog.checkpointer` (web role, below). `images.learn_archs` runs in every

worker, without a lock.

`/health` says what the process does and which locks it holds:

`"roles": ["web", "worker"], "leading": ["backups.scheduler", …]`.

## Running the roles apart (H2)

`oxygenhub serve --role web,worker` (or `[server] roles` in `hub.toml`,

`OXYHUB_SERVER_ROLES`); every role when none is given. An unknown role is

an error.

- **web** serves the whole router. It also owns the transparency log: it

  appends to it (`after_push`, kept receipts and approvals) and runs its

  checkpointer, under the `tlog.checkpointer` lock. The checkpointer moved

  from the worker's list above because the log's state is in the process

  that appends: a checkpointer elsewhere would sign a log it never sees

  grow, and the web process would have no checkpoint to serve.

- **worker** starts the background tasks, and serves only `/health` and

  `/metrics` (when `metrics_token` is set) on its bind address, so it can

  be watched; with no bind address it serves nothing. It opens the

  transparency log to read only: it never cuts short an append the web

  process is making.

- **Until the log has a cluster-wide writer (#274), only one process per

  data directory may have the web role.** Any number may be workers.

  Webhook nudges from a push reach only the process that took it, so a

  separate worker delivers on its once-a-minute pass until the event bus

  (H3) carries the nudge.

## Event bus: publish and subscribe

A trait with two operations, `publish(topic, payload)` and

`subscribe(topic) -> Stream`, for the events live pages and workers react

to: a bookmark moved, a proposal changed, a check recorded, a run or job

changed state, a delivery is waiting, a live room's choice.

- **In process:** `tokio::sync::broadcast` per topic, what the hub does

  today. The single-node edition keeps it.

- **Postgres:** `LISTEN`/`NOTIFY` on one channel per topic. A payload is at

  most 8000 bytes, so events carry ids and kinds, and subscribers read the

  rows they need. A node publishes once; every node's subscriber relays

  the event to its local broadcast, so live pages on any node see it.

- **Later, NATS JetStream** (oxygenhub#208, #209): durable streams, replay

  from a position, and subscribers outside the hub (a product feature:

  "subscribe to this repository's changes" for integrations, beyond

  webhooks). The same trait.

Delivery is at most once; anything that must not be lost is a row in a

table (the job queue), and the event only says "look now".

## The job queue

A `jobs` table (kind, payload, run after, attempts, claimed by, claimed

until). Workers claim with `SELECT … FOR UPDATE SKIP LOCKED` on Postgres,

and with the same table polled under the write lock on SQLite. First

tenants: webhook deliveries (they already have a table), mail, the Git

mirror export, the apt rebuild, Diffusion's bumps, `oxy gc`. Runner claims

stay on their own path (oxygenflow#34 may later move them to the queue).

## The single-node edition

`oxygenhub serve` with no `--role` runs every role, with the in-process

bus, the SQLite or Postgres database and the local leader: exactly today's

behaviour. Every slice must keep it working and its tests green.

## Order of work

1. **H2: `--role web,worker`** and the leader lock (Postgres advisory

   locks; always held on SQLite). **Built (#272):** two hubs on one

   Postgres run each singleton on exactly one, and the other takes over

   within seconds when it stops or loses its connection. One web process

   per data directory until step 3.

2. **H3: the event bus trait**, in process and Postgres `LISTEN/NOTIFY`;

   live pages on one node see a push made through the other.

3. The transparency log's appends under a cluster-wide lock (#274), and

   its checkpointer back with the workers.

4. Autoland and Diffusion under per-proposal advisory locks.

5. The job queue, webhooks and mail first.

6. Later: edge and store (the repository service), NATS when measured.