# ADR 0012 — A WebSocket that carries notices, not data

- Status: accepted
- Date: 2026-08-04

## Context

[ADR 0003](0003-sync-protocol.md) built a delta pull that is cheap enough to run on a timer, and
the client does: one pass a minute. That is the difference between a colleague's change appearing
"soon" and appearing *now*, and it shows up in three places that are not equally forgivable.

- **A vault shared with you** arrives on the next pass. `AdmitNewVaultsAsync` says so in its own
  remarks — "the recipient is handed nothing — there is no push channel" — and the README repeats
  it. Sharing works and looks broken.
- **Two people editing one keychain** see each other up to a minute late, which is long enough to
  make the same edit twice and produce a conflict that nobody needed to have.
- **A revoked grant** keeps serving a client that has not noticed yet, for up to a pass.

Shortening the interval is the obvious answer and the wrong one: it costs a request per client per
interval whether or not anything happened, and it does not converge on *immediate* — it converges
on a busier server that is still late.

There is also a second thing coming that this decision has to not preclude. The intended feature is
a **shared terminal session** — one person's shell, watched or driven by another, TeamViewer-shaped.
That is bidirectional, continuous, and latency-sensitive in a way a keychain notice is not.

## Decision

### One WebSocket per signed-in client, at `GET /api/v1/events`

Subprotocol `dodossh.events.v1`. The client opens it after unlock and keeps it open; the server
sends a notice whenever something the client can read has changed.

**Not SSE.** Server-sent events would carry today's notices perfectly well and would be less code.
It is one-directional, so the shared-session feature would need a second mechanism next to it, and
then two transports would need reconnection, authorization and lifetime rules that agree. The cost
of a WebSocket over SSE is small; the cost of two transports is not.

**Not SignalR.** It brings hub protocol negotiation, its own serialisation and transport fallbacks,
none of which are wanted here: `DodoSSH.Contracts` and its source-generated serialiser are "the
actual contract between the two sides", and a second wire format alongside it is exactly the silent
drift `Setup/Json.cs` records having already cost this project once.

### The notice carries no ciphertext

A `vault.changed` frame is `{ kind, vaultId, sequence }` and nothing else. The client's answer to it
is the pull it would have done on the timer anyway.

This is the load-bearing decision, and it is worth being explicit about why the tempting alternative
is refused. Pushing the changed items themselves would save a round trip and would fork the code
path that applies a change into two — one that arrives by pull and one that arrives by socket — with
the cursor, the merge and the tombstone rules duplicated across both. ADR 0003 put every mutation
through one write path for exactly that reason; this keeps every *read* on one path for the same
one. The socket decides *when* to sync. It never decides *what* a vault contains.

It also means a dropped notice is harmless, which is what lets everything below be simple.

### Polling stays, and is the fallback rather than a legacy path

The one-minute pass is unchanged. The socket makes it *early*; it does not make it *necessary*. A
client on a network that eats WebSockets, an older client, a server that has the feature off, a
notice dropped under backpressure, a second API replica that did not see the write — every one of
those degrades to what the product does today, which is correct and up to a minute late.

Nothing may be reachable only by socket. That is a rule about future features, not an observation
about this one.

### Authorization: the bearer token on the upgrade, not a ticket

[ADR 0004](0004-relay-authorization.md) gives the relay a two-step ticket so its WebSocket carries
no API authority. This one goes the other way and takes the ordinary bearer JWT on the upgrade
request, which is an ordinary authenticated HTTP request. The difference is not inconsistency:

- The relay's socket is a **byte pipe to a third party**, and its whole authorization decision —
  which host, which IPs, which port — is made *before* the socket opens and never revisited. It is
  also the extraction seam for a standalone relay process that must not hold ACL code.
- This socket is a **view of the caller's own vault list**, and it has to keep answering "what may
  this account read" for as long as it is open. It needs the full ACL context, in-process, for the
  life of the connection. A ticket would carry that context in a token instead, and it would be
  wrong the moment the account's access changed.

A long-lived connection authorised by a short-lived token is the problem this creates, and it is met
head-on rather than ignored:

1. **The socket does not outlive the token.** The `exp` claim is read at accept, and the connection
   is closed with `4401` when it passes. The client reconnects with a fresh token; that is a
   sub-second gap in a channel whose failure mode is already "poll instead".
2. **The vault set is re-resolved periodically** (`Events:AccessRefreshInterval`, default five
   minutes) as well as on the changes that are known to affect it. A withdrawn grant therefore stops
   producing notices within that window at the latest, and immediately in the ordinary case.

Both are bounds on **metadata** — the fact that a vault changed and roughly when — because that is
all a notice contains. Nobody's ciphertext is behind this socket, and a client that stayed subscribed
one interval too long could still not read a byte of it: reading requires a vault key grant, which
this server has never held.

### The frames

Text frames, JSON, `DodoSshJsonContext`. Server to client:

| kind | meaning |
| --- | --- |
| `hello` | accepted; carries the heartbeat interval and the vault count subscribed |
| `vault.changed` | `vaultId` moved to `sequence`; pull it |
| `vaults.changed` | the set of vaults this account can reach is different; re-read it |
| `ping` | heartbeat; the client answers `pong` |

Client to server: `ping`, answered with `pong`. Nothing else — subscription is decided by the server
from the caller's access, not asked for by the client, because a client that could ask to subscribe
to a vault id is a client that can probe for vault ids.

`kind` is a **string**, not an enum, and that is deliberate. `UseStringEnumConverter` throws on a
value it does not know, so a newer server sending a kind an older client has never heard of would
not add an unknown frame — it would break that client's socket entirely. A string is ignored
instead, which is what makes the table above extensible. `ProblemCodes` is the same shape for the
same reason.

### Where the shared session will attach

The socket is the seam, and one thing about it is chosen now so that it need not be renegotiated
later: **session data will be binary frames on this same connection, not JSON on the table above.**
Terminal output base64'd into a JSON envelope would cost a third of the bandwidth for nothing, on
the one payload here that is continuous rather than occasional. Control — offer, accept, resize,
end — is JSON like everything else.

That is as far as this ADR goes. Two questions are open and are not being answered by implication:
whether a shared session's bytes go through the API at all or peer-to-peer past it, and what
end-to-end encryption means when the second party is watching a stream rather than holding a key.
Both are ADR 0001 questions and deserve their own decision. What this one buys is that they will not
also be transport questions.

## Consequences

- **Fan-out is in-process, and the deployment is therefore single-node for this feature.** Every
  connection is held by the node that accepted it; a write handled by another node produces no
  notice on this one. `IVaultEventPublisher` is the seam a backplane implements — PostgreSQL
  `LISTEN`/`NOTIFY` needs no infrastructure this stack does not already run — and it is deliberately
  **not implemented**, because an untested backplane is worse than a documented gap. Multiple API
  replicas do not break: they degrade to polling, which is the state before this ADR. `/api/v1/meta`
  advertises `events` so a client knows which it is getting.
- **Per-connection queues are bounded and drop the oldest.** A notice is "pull vault X, which is at
  least at sequence N", so the newest is strictly more useful than the one it displaces and the
  client's answer is identical either way. A slow reader costs itself latency, never the publisher's
  progress — the publish path never blocks and never awaits a socket.
- **The publish happens after the transaction commits**, outside the advisory lock ADR 0003 takes.
  A notice sent from inside it would name a sequence a reader cannot yet see, and would hold the
  per-vault write lock across a socket write.
- **A client is notified of its own writes.** It pushed, so it already pulled; the extra pass finds
  nothing. The client coalesces notices over a short window rather than the server suppressing an
  echo, because suppressing it correctly needs a per-*device* identity on the socket and the same
  user's other machines must still be told.
- **Connections are capped** per user and per node (`Events:MaxConnectionsPerUser`,
  `Events:MaxConnectionsTotal`). A socket is cheap but not free, and an unbounded count of them is a
  denial of service that authenticates first.
- The feature can be turned off entirely (`Events:Enabled`). A deployment behind a proxy that will
  not upgrade should say so rather than have every client discover it by failing.

### Rejected

- **Shorter polling.** Cheaper to build, converges on a busier server that is still late.
- **Long polling.** No new transport and genuinely immediate, but it holds a request thread and a
  connection per client for the same money as a WebSocket while offering none of the bidirectionality
  the shared session needs.
- **Pushing the changed items down the socket.** Saves a round trip; forks the apply path in two. See
  above.
- **Client-chosen subscriptions.** A `subscribe(vaultId)` frame is an existence oracle for vault ids,
  which is the disclosure `SyncPullEndpoint` answers 404 rather than 403 to avoid.
