sausage/PROPOSAL.md

# Proposal: plugins, processes, and JSON-RPC

Status: a proposal on the branch `plugins`. When we accept it, its content
goes into `PROTOCOL.md` (the contract) and `DECISIONS.md` (the reasons).
Then this file goes away.

## 1. The idea

Sausage is a world of small WASI programs that an agent grows by talking to
it. The design uses three ideas that models already know:

1. **JSON-RPC calls.** A call has a result, like an LLM tool call. LSP and
   MCP use the same protocol.
2. **Processes.** Each call to a plugin starts a new WASI process. WASI has no
   threads, so processes are the unit of parallelism. Calls are asynchronous,
   so one process can make many calls at the same time.
3. **Files to inspect, calls to act.** The core shows its state as virtual
   files in `/proc`, like Plan 9 and Linux. Each change of state is a call.

## 2. Terms

| Term | Meaning |
| --- | --- |
| world | The folder that a `sausage` host runs. It is `/` for every process. |
| core | The host. It runs processes, routes calls, serves the virtual files, and keeps the journal. |
| plugin | A folder `/plugins/<name>/` with a `plugin.wasm`. A plugin does not run by itself. |
| process | One run of a plugin version. Its ID is `<plugin>-<uuidv7>`, for example `notes-0197f3a2-7c1e-7d2b-9a41-5f0c2e8b1d34`. |
| call | A JSON-RPC request. `notes.add` calls the method `add` of the plugin `notes`. |
| interface | A set of methods. A plugin has one main interface. It can also declare endpoint interfaces. |
| endpoint | An interface that a running process serves on a TCP port. An endpoint file tells others where it is. |
| service | A call in `/config/boot.json`. The core starts it at boot and again after an upgrade of its plugin. |
| import | An interface of another plugin that a plugin uses. The SDK generates it from that plugin's description. |
| provider, consumer | The plugin that has an interface, and a plugin that imports it. |
| journal | The central log of the world. |

## 3. The world folder

```text
<world>/
  plugins/<name>/
    plugin.wasm               the current version
    plugin.prev.wasm          the version before, for `core.revert`
    source/                   the source. `cargo.build` builds it from here.
      imports/<plugin>.json   descriptions of the imported plugins
  state/<plugin>/             persistent state of a plugin, by convention
  state/core/journal.sqlite   the journal
  config/                     global configuration
    boot.json                 the services
    agent.json                API, base URL, model, and token of the agent
```

- Every process can read and write all of it. There are no permissions.
- The world folder must not contain `proc`, `run`, or `self`. These names are
  mount points (section 5). The host does not start a world that has them.

## 4. Calls and processes

### 4.1 The protocol

- JSON-RPC 2.0 on stdin and stdout, one message per line, as in MCP's stdio
  transport.
- A line is a message only if it is a JSON object with `"jsonrpc": "2.0"`.
- The core discards every other line on stdout. For the first discarded line
  of a process, it writes one warning to the journal, without the content of
  the line. Logs go to stderr.
- Each line on stderr goes to the journal.
- `params` is always an object. Batches are not supported.
- The SDK writes each message with one blocking write through std's stdout,
  and it writes a newline before each message. So a `print!` without a
  newline cannot damage a message.

### 4.2 Method names

- A method name has the form `<target>.<method>`. The core splits it at the
  first `.`.
- Plugin names use `a-z`, `0-9`, `-`, and `_`. The name `core` is reserved.
- `notes.add` starts a new process of `notes` and sends it the request `add`.
  `notes.rpc.discover` sends it `rpc.discover`.
- `core.<method>` is a method of the core (section 10).
- Method names that start with `rpc.` are reserved: `rpc.discover`,
  `rpc.retired`, and `rpc.endpoint`.

### 4.3 What a call does

1. The core gives the call a UUIDv7 and starts the process `<plugin>-<uuid>`
   with the current version of the plugin.
2. The core writes the request to the stdin of the process. The request ID is
   the process ID.
3. The process writes the response to stdout. The core sends the response to
   the caller, with the caller's request ID.
4. If the process ends before it answers, the caller gets the error -32002.
   Its `data` has the exit status and the last journal lines of the process.
   Before the core decides this, it reads the stdout of the process to the
   end.

- A process can write calls to stdout at any time, to the core and to
  plugins. The responses come on stdin. A process can have many calls in
  flight.
- The core keys each pending request by the process and the exact JSON ID.
  The core drops a response with an unknown or late ID. The SDK writes a note
  about such a response to stderr.
- A notification to `plugin.method` (without an ID) starts an attached
  process. That process gets the notification without an ID, and nobody gets
  a result.

### 4.4 Detached calls

`core.call {"method": "www.start", "params": {...}, "detach": true}` makes the
call and returns its result to the caller. The new process has no parent, so
it does not end with its caller.

Sent as a notification (without an ID), the same `core.call` starts the
process and forgets it.

### 4.5 Notifications to the caller

A process can tell its caller about progress. It sends the notification
`core.notify {"name": "text", "params": {...}}`. The core gives it to the
caller as `core.notify {"call": <ID of the caller's request>, "name": "text",
"params": {...}}`. After the call has ended, the core drops such
notifications.

So a process never needs to know who called it. Endpoint calls send their
notifications over the endpoint connection (section 6).

### 4.6 Lifetime

A process lives while one of these runs:

- a handler of a call,
- a background task,
- an endpoint that listens.

The SDK's own reader of stdin does not count. When nothing of this runs, the
SDK returns from `main`, and the process ends.

A process has three states:

1. **running**
2. **stopping**: the core has closed stdin. EOF on stdin means "stop now".
   - New calls of the process fail at once, and its pending calls fail.
   - A stop hook can still write files and send notifications. It cannot
     make calls, because no response can arrive.
   - After 5 seconds, the core kills the process.
3. **ended**: the core releases the locks of the process (section 7) and
   removes its folder in `/proc`.

Rules:

- `core.stop {pid}` starts the stopping state. `core.kill {pid}` kills the
  process at once.
- When a process ends, the core stops each of its attached children. All
  children stop in parallel, so a whole tree takes 5 seconds at most.
- A detached process has no parent. Only `core.stop`, `core.kill`, an
  upgrade (section 4.7), and the end of the host stop it.
- At host shutdown, the core stops every process in parallel.

Note: on Unix, children outlive their parent. Here, an attached child ends
with its parent. Only `detach` makes a process independent.

### 4.7 Upgrades

A process is an instance of one plugin version. When a new version becomes
active (section 11.3):

1. New calls get the new version at once.
2. Each process of an older version gets a grace period to finish its work.
   The default is 60 seconds. A plugin can declare its own value in
   `rpc.discover` (`x-sausage-grace`). Then the core stops the process.
3. When the old process of a service has ended, the core starts the service
   again with the new version.
4. When the last process of an older version has ended, the core starts a
   process of the new version and calls `rpc.retired {"previous":
   "<sha256 of the older file>"}`. A plugin can use this hook to migrate its
   state, or to write a marker that its other new processes wait for. The
   result goes to the journal.

During the grace period, an old process can call a method that a new
provider no longer has. The time is limited, so this risk is acceptable.

### 4.8 Environment and network

- `SAUSAGE_PID`: the process ID.
- `SAUSAGE_PLUGIN`: the plugin name.
- `SAUSAGE_PARENT`: the parent: `console`, `core`, or a process ID.

Every process gets the network: outgoing HTTP and HTTPS through `wasi:http`,
and TCP, UDP, and DNS through `wasi:sockets`.

### 4.9 Scheduling

- Each process is a task on the core's multi-thread tokio runtime, with work
  stealing.
- A scheduler thread advances Wasmtime's epoch every 10 milliseconds. At
  each tick, a process that computes yields to the runtime. So a busy process
  cannot block the core or the other processes.
- `core.kill` aborts the task. It takes effect at the next tick at the latest.
- Most processes wait for IO most of the time. A better scheduler can come
  later.

### 4.10 Errors

The standard JSON-RPC codes apply. The core adds these codes:

| Code | Meaning |
| --- | --- |
| -32001 | There is no such plugin. |
| -32002 | The process ended without an answer. `data` has the exit status and the last journal lines. |
| -32003 | The activation was rejected. `data` has the reasons. |

## 5. The filesystem

Each process gets one preopen, `/`. The VFS resolves each path through a
mount table:

| Path | Layer | Access |
| --- | --- | --- |
| `/` | host: the world folder | read-write |
| `/proc` | generated by the core | read-only |
| `/self` | memory: the folder `/proc/<own pid>/` | read-write |
| `/run` | memory, shared, empty at each host start | read-write |

A listing of `/` shows `proc`, `run`, and `self`. A path from any folder
descriptor goes through the same mount table.

```text
/proc/
  core/
    PROTOCOL.md          the contract
    graph.json           plugins, imports, and the result of each check
    graph.dot            the same graph for Graphviz
    locks.json           held locks and waiting processes
    plugins/<name>/
      discover.json      the rpc.discover document of the current plugin.wasm
      client.rs          the Rust code that import!("<name>") generates
  <pid>/
    info.json            plugin, version, method, params, parent, detached, start time, state
    log                  the journal lines of this process
    <files>              the files that the process writes to /self
```

Rules:

- A process writes its own folder only through `/self`. Writes through
  `/proc` always fail.
- In a process folder, the names `info.json`, `log`, and `fs` belong to the
  core. A write to `/self/info.json` fails.
- A read of a generated file returns a snapshot that ends. For example,
  `/proc/<pid>/log` returns the lines up to the time of the open.
- The folder of a process goes away when the process ends. Its files are in
  memory. Large or persistent data belongs in `/state`.
- Do not send a `/self` path to another process. There it means a different
  folder, like `localhost` on another machine.

Principle: **inspect through files, act through calls.** A file read is
cheaper than a call, and every language and tool can read a file.

### 5.1 The VFS crate

The VFS is a crate of its own, `sausage-vfs`, so that it can become public
later. It knows nothing about Sausage.

- One trait with the operations of a filesystem: stat, list, open, read and
  write at an offset, set size, create a folder, remove, and rename.
- The layers:
  - **host:** a real folder, sandboxed with `cap-std`. Each file operation
    runs in a blocking task. That task holds a guard of its process until
    the system call has returned, even if the process was killed.
  - **memory:** files and folders in memory.
  - **generated:** files that a function makes at each open.
  - **mounts:** sends each path to the right layer. Each process has its own
    mount table, which shares most layers with the other processes.
- An adapter implements `wasi:filesystem` for Wasmtime on top of the trait.
  It replaces the filesystem of `wasmtime-wasi` completely.
- Not now: overlays (a union of two layers). Deletes and renames across
  layers need whiteouts and copy-up. The trait leaves room for overlays.
- Tests:
  1. Unit tests for each layer.
  2. Differential tests: `proptest` generates sequences of operations. They
     run on the host layer and on `std::fs` in a temporary folder, and the
     results must be equal.
  3. A model test for the memory layer.
  4. WASI tests: Rust guests use `std::fs` through the adapter. `rusqlite`
     and `tempfile` must work.

## 6. Endpoints

- `listen(path, interface)` binds a free port on 127.0.0.1. When the listener
  is ready, it writes an endpoint file at `path`, in one atomic step. The
  file has one line: the address, the owner, and the interface. Example:
  `tcp:127.0.0.1:41234 www-0197… control`.
- Usually the path is in `/self`. A service that others must find uses a
  fixed path in `/run`, for example `/run/secrets`.
- `connect(path)` reads the file and connects. Its first request is the
  built-in `rpc.endpoint`, which returns the owner, the plugin, and the
  interface. If they do not match the file, `connect` fails. So a stale file
  or a reused port cannot connect a client to the wrong process.
- If the file does not exist yet, `connect` waits a short time, for example
  while the services start.
- JSON-RPC runs on the connection, one message per line. In an endpoint
  handler, `ctx.notify` sends the notification over the connection.
- `listen` accepts only interfaces that the plugin declares (section 11.1).

## 7. Locks

- `core.lock {"path": "/state/notes/notes.json"}` returns when the caller
  holds the lock.
- Each acquisition gets its own place in a FIFO queue. The same process
  cannot take one lock twice: its second acquisition waits behind its first.
- With `"wait": false`, the call returns at once with `{"held": true}` or
  `{"held": false}`.
- `core.unlock {path}` releases the lock. The SDK sends it as a notification
  when a guard drops. If the future of an acquisition was dropped, the SDK
  releases the lock when the grant arrives.
- When a process ends, the core releases all of its locks, also after a
  crash or a kill. It waits until the last file operation of the process has
  returned (section 5.1). So a write of a killed process cannot land after
  the next process got the lock.
- Locks are advisory. The core does not block writes to the file: the path
  is only a name.
- SQLite has no file locks in WASI. Hold the lock while the database is open.

## 8. The journal

- An SQLite file at `/state/core/journal.sqlite`. Only the core writes it.
- Each entry has a UUIDv7 (it sorts by time), the process ID, the plugin, the
  stream (`stderr` or `core`), and the line.
- The core writes its own events too: start (with method and params), exit
  status, stop, kill, upgrades, and the warning about discarded stdout.
- `core.log {plugin?, pid?, since?, until?, grep?, limit?}` queries the
  journal. `/proc/<pid>/log` shows the lines of one running process.
- Later: structured logging, like `tracing`.

## 9. Boot and services

At start, the core starts each service in `/config/boot.json`, in order:

```json
[
  {"method": "secrets.serve", "params": {}},
  {"method": "www.start", "params": {"port": 8080}}
]
```

- A service is a detached call. The core waits for its result before it
  starts the next one. The result shows that the service is ready.
- If a service fails, the core writes the error to the journal and continues.
- After an upgrade of the plugin of a service, the core starts the service
  again with the new version (section 4.7).
- The core does not restart other detached processes.
- A singleton is a service with a fixed endpoint in `/run`. Only the core
  starts it, so two instances cannot start at the same time.

## 10. The core API

| Method | Params | Result |
| --- | --- | --- |
| `core.call` | `method`, `params`, `detach`? | the result of the method |
| `core.notify` | `name`, `params` | none (a notification) |
| `core.stop` | `pid` | `{}` when the process has ended |
| `core.kill` | `pid` | `{}` |
| `core.lock` | `path`, `wait`? | `{"held": bool}` |
| `core.unlock` | `path` | none (a notification) |
| `core.activate` | `plugin` and `wasm`, or `plugins: [{plugin, wasm}]`. `force`? | the new `rpc.discover` documents |
| `core.revert` | `plugin`, `force`? | like `core.activate` |
| `core.log` | filters (section 8) | journal entries |
| `core.exit` | none | `{}`. Then the host stops with exit code 75. |

Everything else is a file.

## 11. Interfaces

### 11.1 `rpc.discover`

Each plugin answers `rpc.discover` with an OpenRPC document. The SDK
generates it from the code, with all doc comments. The schemas are JSON
Schema Draft 7, as OpenRPC requires. The document has these extension
fields:

- `x-sausage-params` for each method: a `$ref` to the schema of the params
  struct. So the name and the docs of the struct are not lost.
- `x-sausage-endpoints`: the endpoint interfaces by name, each with its
  methods.
- `x-sausage-imports`: the methods that the plugin uses from each provider,
  with their schemas.
- `x-sausage-grace`: the grace period in seconds, if the plugin declares one.

The schema of params describes what the provider can deserialize. The schema
of a result describes what the provider serializes. (Schemars generates both
kinds. They differ, for example for fields with `#[serde(default)]`.)

```json
{
  "openrpc": "1.2.6",
  "info": {"title": "www", "version": "0.1.0"},
  "methods": [{
    "name": "start",
    "summary": "Start the web server and return the ID of its process.",
    "paramStructure": "by-name",
    "params": [{"name": "port", "required": true,
                "description": "The TCP port to listen on, on all interfaces.",
                "schema": {"type": "integer", "minimum": 0, "maximum": 65535}}],
    "result": {"name": "pid", "schema": {"type": "string"}},
    "x-sausage-params": {"$ref": "#/components/schemas/StartParams"}
  }],
  "components": {"schemas": {"StartParams": {"…": "…"}}},
  "x-sausage-endpoints": {"control": {"methods": [{"name": "stats", "…": "…"}]}},
  "x-sausage-imports": {"notes": {"methods": [{"name": "add", "…": "…"}]}}
}
```

### 11.2 Compatibility

A provider can add methods, optional params, and result fields. For each
imported method:

1. The provider has the method.
2. Each params value that the consumer can send is valid for the provider.
3. Each result that the provider can return is valid for the consumer.

- The same rules apply to endpoint interfaces.
- There are no version numbers. This check replaces them.
- The check is our own. It covers a fixed subset of JSON Schema: objects with
  `properties` and `required`, the primitive types, arrays, optional and
  nullable values, string enums, and `$ref`. `#[schema]` gives a compile
  error for a type outside this subset. So the check can decide every case.
- The check covers calls, not state. Old and new processes share the files in
  `/state`, and `core.revert` restores only code. A plugin must keep its
  state format compatible, or migrate it in `rpc.retired`.

### 11.3 Activation

`core.activate {"plugin": "notes", "wasm": "/state/cargo/builds/7/notes.wasm"}`:

1. The core copies the file. All further steps use this copy, even if the
   file changes.
2. It starts the copy and calls `rpc.discover`.
3. It waits until it has the documents of all plugins (section 11.4). A
   missing document never counts as "no consumers".
4. It checks the imports of the new version against the current providers.
5. It checks each current consumer of the plugin against the new version.
6. If all checks pass, `plugin.wasm` becomes `plugin.prev.wasm`, and the copy
   becomes `plugin.wasm`. Then the upgrade starts (section 4.7).

If a check fails, nothing changes. The error lists each problem, with a
counterexample where possible:

```text
notes was not activated: 1 consumer breaks.
- agent imports notes.add(text: string) -> Added.
  The new notes.add has no param "text". It requires "body": string.
  Counterexample: {"text": "milk"} is not valid for the new params.
Update agent first, or activate both together.
```

- The core runs one activation at a time.
- `plugins: [...]` activates several plugins together. The checks then use
  the new set. The core writes all files under temporary names first, and
  then renames them.
- `force: true` skips steps 4 and 5.
- `core.revert {plugin}` activates `plugin.prev.wasm` in the same way, and
  the current version becomes `plugin.prev.wasm`.
- A person can also copy a file to `plugin.wasm` by hand. That skips the
  checks (section 11.4).

### 11.4 The graph

- The core keeps the `rpc.discover` documents of all plugins in memory, by
  file hash. At start, it gets them in the background.
- A file watcher on `/plugins/*/plugin.wasm` updates them, for example after
  a copy by hand. Such a copy skips the checks. `graph.json` then shows each
  broken import, and the journal records it.
- An import of a plugin that does not exist is "missing".
- Untyped calls (`call("x.y", ...)`) are not in the graph.

## 12. Building and tests

`cargo.build {"plugin": "notes"}` is a native method of the host. A call
still gets a process ID, a folder in `/proc`, and journal entries, like other
calls. The method does these steps:

1. It copies `/plugins/notes/source/` once. The tests and the build both use
   this copy.
2. It runs the tests under WASI.
3. It builds the release `.wasm` for `wasm32-wasip2`.
4. It returns `{"wasm": "/state/cargo/builds/<n>/notes.wasm", "tests":
   {"passed", "failed", "output"}, "warnings"}`. If a step fails, it returns
   an error with the compiler or test output.

- The dependency `sausage-plugin = "0.1"` points to the SDK copy at
  `/plugins/cargo/sdk/`.
- Build scripts can compile C: `CC_wasm32_wasip2` and `AR_wasm32_wasip2`
  point to a WASI clang.
- The warnings include unused imported methods. Remove them from
  `imports/<plugin>.json`, so that fewer changes of the provider break the
  plugin.
- A kill of the call stops the whole process group of cargo, with rustc, the
  build scripts, and the tests.

Tests:

- `sausage` is the test runner (`CARGO_TARGET_WASM32_WASIP2_RUNNER`). It runs
  each test in its own instance, in parallel. A panic ends only its own test.
  Without this, the first panic would end the whole test run, because WASI
  has `panic=abort`.
- The normal Rust test setup works: `#[test]`, `tests/*.rs`, name filters,
  and `--ignored`. Async tests use `#[sausage_plugin::test]`, like
  `#[tokio::test]`. `#[should_panic]` cannot work, because WASI cannot catch
  a panic.
- Each test gets its own mount table: an empty temporary world as `/`, the
  package folder (so that fixture files work), the variables that cargo sets,
  and its own `/proc`, `/self`, and `/run`.
- The runner answers `core.lock`, `core.unlock`, `core.notify`, and
  `core.log`. A call to a plugin fails with a clear error: a test has no
  other plugins.

## 13. The SDK

Two crates: `sausage-plugin`, and `sausage-plugin-macros`, which
`sausage-plugin` re-exports. A plugin depends only on `sausage-plugin`. The
SDK also re-exports `serde` and `schemars` for the macros.

| Item | Purpose |
| --- | --- |
| `#[rpc]` | Makes an `impl` an interface. On the main interface: `#[rpc(endpoints(name = Type), grace_seconds = N)]`. |
| `#[on_retired]` | Marks the method that answers `rpc.retired`. |
| `#[schema]` | For each type that a method takes or returns: adds the serde and schemars derives, and checks the subset of section 11.2. |
| `run(T)` | In `main`: serves the main interface on stdin and stdout. |
| `call(method, params)` | An untyped call. Returns a JSON value. |
| `.detach()`, `.on_notify(f)` | Options of a call. |
| `Ctx` | An optional second argument of a method: `ctx.notify(name, params)`, `ctx.exit_after_response()`. |
| `lock(path)` | A lock guard. `.try_now()` does not wait. `.keep()` holds the lock until the process ends. |
| `listen(path, value)`, `connect(path)` | Endpoints. |
| `background(future)` | Runs a task in this process. The process lives while a task runs. |
| `import!("name")` | Generates `mod name` from `imports/name.json`. |
| `state_dir()`, `write_atomic(path, bytes)`, `pid()` | Helpers. |
| `#[sausage_plugin::test]` | An async test. A plain `#[test]` also works. |

Rules that the compiler enforces:

- A method takes `&self`, an optional `ctx: &Ctx`, and at most one params
  argument.
- The params argument is a `#[schema]` struct with named fields. Its fields
  are the params of the method.
- Each method, each `#[schema]` type, and each field has a doc comment.
  Other agents read these docs to use the interface. Write them like good
  Rust documentation.

Other rules:

- Method docs become `summary` (the first paragraph) and `description` in
  OpenRPC. Type and field docs become `description` in the JSON Schema. The
  generated imports contain them again as doc comments.
- Each `import!` registers its document in a static list with the
  `inventory` crate. `run` reads the list for `rpc.discover`. `import!` uses
  `include_str!`, so cargo rebuilds the plugin when the file changes.
- The value that `run` receives must be cheap to create, because the core
  also starts a plugin only for `rpc.discover`. Open files and databases in
  the methods.
- Log with `eprintln!`. stdout belongs to the protocol.

### 13.1 A plugin

```toml
[package]
name = "notes"
version = "0.1.0"
edition = "2024"

[dependencies]
sausage-plugin = "0.1"
serde_json = "1"
```

```rust
//! Notes: a list of short texts, stored in `/state/notes/notes.json`.
//!
//! Each call runs in its own process, so two calls can run at the same time.
//! `add` takes a lock on the file, so that no note gets lost.

use std::path::PathBuf;

use sausage_plugin::{Result, lock, rpc, schema, state_dir, write_atomic};

/// The notes plugin. It keeps no state in memory: each call reads the file.
struct Notes;

/// The params of `add`.
#[schema]
struct AddParams {
    /// The text of the new note. It can contain newlines.
    text: String,
}

/// The result of `add`.
#[schema]
struct Added {
    /// The number of notes after the call, with the new note.
    count: usize,
}

#[rpc]
impl Notes {
    /// Add a note at the end of the list.
    ///
    /// Returns the number of notes after the call.
    async fn add(&self, AddParams { text }: AddParams) -> Result<Added> {
        let _lock = lock(file()).await?; // released on drop, or when the process ends
        let mut notes = load()?;
        notes.push(text);
        write_atomic(file(), serde_json::to_vec(&notes)?)?;
        Ok(Added { count: notes.len() })
    }

    /// List all notes, the oldest first.
    fn list(&self) -> Result<Vec<String>> {
        load() // no lock: write_atomic replaces the file in one step
    }
}

/// The file with the notes: a JSON list of strings.
fn file() -> PathBuf {
    state_dir().join("notes.json")
}

/// Read all notes. A missing file means no notes.
fn load() -> Result<Vec<String>> {
    match std::fs::read(file()) {
        Ok(bytes) => Ok(serde_json::from_slice(&bytes)?),
        Err(e) if e.kind() == std::io::ErrorKind::NotFound => Ok(vec![]),
        Err(e) => Err(e.into()),
    }
}

fn main() {
    sausage_plugin::run(Notes);
}

#[cfg(test)]
mod tests {
    use super::*;

    #[sausage_plugin::test]
    async fn add_appends_and_counts() {
        assert_eq!(Notes.add(AddParams { text: "milk".into() }).await.unwrap().count, 1);
        assert_eq!(Notes.add(AddParams { text: "eggs".into() }).await.unwrap().count, 2);
        assert_eq!(Notes.list().unwrap(), ["milk", "eggs"]);
    }

    #[test]
    fn list_is_empty_without_a_file() {
        assert!(Notes.list().unwrap().is_empty());
    }
}
```

### 13.2 A server with an endpoint

```rust
//! A web server for the files in `/state/www/`.
//!
//! `start` starts the server in its own process and returns at once. Run it
//! as a service, or call it detached, or the server ends with its caller.
//! The running server serves the endpoint `control` at
//! `/proc/<pid>/control`.

use std::{cell::Cell, rc::Rc};

use sausage_plugin::{Ctx, Result, background, listen, pid, rpc, schema};
use wstd::net::TcpListener;

/// The www plugin.
struct Www;

/// The params of `start`.
#[schema]
struct StartParams {
    /// The TCP port to listen on, on all interfaces.
    port: u16,
}

#[rpc(endpoints(control = Control), grace_seconds = 5)]
impl Www {
    /// Start the web server and return the ID of its process.
    ///
    /// The server runs until it gets `stop` on its endpoint, until
    /// `core.stop`, or until its caller ends. Fails if the port is in use.
    async fn start(&self, StartParams { port }: StartParams) -> Result<String> {
        let tcp = TcpListener::bind(&format!("0.0.0.0:{port}")).await?;
        let control = Rc::new(Control { hits: Cell::new(0) });
        listen("/self/control", control.clone()).await?;
        background(serve(tcp, control)); // the process lives while this task runs
        Ok(pid())
    }
}

/// The control endpoint of a running server.
struct Control {
    /// The number of HTTP requests that the server answered.
    hits: Cell<u64>,
}

#[rpc]
impl Control {
    /// The number of HTTP requests that the server answered since its start.
    fn stats(&self) -> Result<u64> {
        Ok(self.hits.get())
    }

    /// Stop the server. The process ends after this response.
    fn stop(&self, ctx: &Ctx) -> Result<()> {
        ctx.exit_after_response();
        Ok(())
    }
}

/// Accept HTTP connections, and answer each request from `/state/www/`.
async fn serve(tcp: TcpListener, control: Rc<Control>) {
    // …
}

fn main() {
    sausage_plugin::run(Www);
}
```

The same type can serve stdin and an endpoint. The example uses two types
for a reason: `www.stats` through the core would start a fresh process, and
that process would answer 0.

### 13.3 A service with a fixed endpoint

```rust
//! Secrets: keeps secrets in memory, for other plugins.
//!
//! Run it as a service: `{"method": "secrets.serve", "params": {}}` in
//! `/config/boot.json`. Clients connect to the endpoint `vault` at
//! `/run/secrets`.

use std::rc::Rc;

use sausage_plugin::{Result, listen, rpc};

/// The secrets plugin.
struct Secrets;

#[rpc(endpoints(vault = Vault))]
impl Secrets {
    /// Open the vault and serve it at `/run/secrets`.
    ///
    /// Returns when the endpoint is ready. The process then lives while the
    /// endpoint listens.
    async fn serve(&self) -> Result<()> {
        listen("/run/secrets", Rc::new(Vault::open()?)).await
    }
}
```

A client imports `secrets` and calls
`secrets::vault::connect("/run/secrets").await?`.

### 13.4 Imports

To use another plugin, an agent does these steps:

1. Copy `/proc/core/plugins/notes/discover.json` to
   `source/imports/notes.json`.
2. Read `/proc/core/plugins/notes/client.rs`: the exact code that the import
   generates.
3. Write `import!("notes")` and the code that uses it.
4. Call `cargo.build`, and remove the imported methods that the warnings call
   unused.
5. Call `core.activate`.

```rust
sausage_plugin::import!("notes"); // imports/notes.json
sausage_plugin::import!("www"); // imports/www.json, with its endpoint "control"

/// Add a note, then start a web server and return its number of answered requests.
async fn example() -> Result<u64> {
    let added = notes::add(notes::AddParams { text: "milk".into() }).await?;
    eprintln!("{} notes", added.count); // stderr goes to the journal
    let pid = www::start(www::StartParams { port: 8080 }).detach().await?;
    let control = www::control::connect(&format!("/proc/{pid}/control")).await?;
    control.stats().await
}
```

- The params of each method are one struct. The fields have the same names as
  the JSON params.
- Types take their names from the schemas. When the provider uses
  `#[schema]`, those are the names of its Rust types: `AddParams` stays
  `AddParams`.
- The generated items are private to the crate, so rustc warns about each
  unused method.

## 14. The agent

- `agent.chat {"text": "..."}` answers a message from a person. The result is
  the final text. The interim texts come as `core.notify` with the name
  `text`.
- The agent takes a lock on `/state/agent/history.jsonl`. So a second message
  waits until the first turn has ended.
- The agent declares a long grace period (for example 10 minutes), so that a
  turn can finish after an upgrade. A new version applies at the next
  `chat`.
- The tools: `call` (method, params, detach), `read_file`, `write_file`,
  `list_dir`, and `copy_dir`.
- The configuration is in `/config/agent.json`.
- The prompt stays short. It says:
  - where the contract is (`/proc/core/PROTOCOL.md`),
  - where the SDK is (`/plugins/cargo/sdk/`),
  - how to build and activate a plugin,
  - where the agent's own source is (`/plugins/agent/source/`),
  - that logs go to stderr,
  - that the interface check does not cover state.

## 15. The console

- `rustyline` gives line editing and history. Notifications appear while you
  type.
- A short prompt. Results are indented. Agent notifications are dimmed.

| Input | Effect |
| --- | --- |
| text | `agent.chat` |
| `<plugin>.<method> {json}` | a call. The console prints the result. |
| `ls [path]`, `cat <path>` | the filesystem as a process sees it, with `/proc` and `/run` |
| `methods <plugin>` | the methods from `rpc.discover`, with params and docs |
| `ps` | the running processes |
| `log [plugin or pid] [-n N]` | the journal |
| `activate <plugin> <wasm>`, `revert <plugin>` | `core.activate`, `core.revert` |
| `stop <pid>`, `kill <pid>` | `core.stop`, `core.kill` |
| `help`, `quit` | |

`--headless` stays: no console input, and the host runs until SIGTERM or
`core.exit`.

## 16. Caches

- Wasmtime's own compile cache stores the compiled code of each `.wasm` file.
  It supports components.
- The folder is `$XDG_CACHE_HOME/sausage/`, or `~/.cache/sausage/` without
  that variable. `--cache-dir` changes the folder.

## 17. Decisions

- **JSON-RPC, not component interfaces.** Every language can read and write
  lines of JSON, and models know JSON-RPC from LSP and MCP.
- **A process per call.** We get parallelism without threads in WASI. A new
  version applies at the next call, and old processes retire after a grace
  period.
- **Attached by default.** A process does not outlive its caller by
  accident.
- **Services in one file.** Only the core starts and restarts them, so
  upgrades and singletons need no logic in plugins.
- **Files to inspect, calls to act.** This keeps the core API small, and the
  agent's file tools are enough to explore the world.
- **Generated imports and interface checks, without version numbers.** A model
  that translates a description by hand makes errors. A generator does not.
  The check catches the most common error in a world that changes itself: a
  change to one plugin breaks another one.
- **A small schema subset.** With Draft 7 and a fixed subset, our own check
  can decide every case.
- **Docs are part of the interface.** Other agents work from them, so the
  compiler requires them.
- **No permissions.** The checks protect against mistakes, not against
  plugins. Locks are advisory.
- **Our own VFS, in its own crate.** It works on Linux and macOS, it makes
  `/proc` live, and plugin-served files need it later. Mounts now, overlays
  later.
- **TCP endpoints for now.** They need no host code. VFS pipes can replace
  them later behind the same SDK API.
- **Simple scheduling for now.** An epoch tick every 10 milliseconds keeps
  the core responsive and makes kills reliable.

## 18. Later

- Plugin-served files at `/proc/<pid>/fs/`, for example for an FTP plugin. A
  request gets a timeout of 10 seconds, because a process that reads its own
  served files would wait for itself forever.
- Overlays in the VFS.
- VFS pipes for endpoints, instead of TCP ports.
- Subscriptions to file changes.
- Structured logging.
- A secrets service.
- Cancellation of calls.
- A better scheduler.

## 19. Plan

The work happens on the branch `plugins`. Each step includes its tests.

Already confirmed:

- `inventory` works on `wasm32-wasip2`.
- Wasmtime's compile cache supports components.
- C code builds for `wasm32-wasip2`.

Checks of risks first:

1. An end-to-end pair: a provider and a consumer with a generated import,
   through the real transport. This includes `typify` with Draft 7 from
   schemars, and our compatibility check on real documents.
2. Fixture paths in tests under the runner.

Then the steps:

1. `PROTOCOL.md` and `DECISIONS.md` from this proposal.
2. `sausage-vfs`: the trait, the layers, the Wasmtime adapter, and its tests.
3. The core: a process per call, JSON-RPC routing, the process tree, upgrades,
   services, locks, the journal, scheduling, and the compile cache.
4. The SDK and its macros.
5. `cargo.build`, the test runner, activation, and the graph.
6. The agent and the console.