folk-api

Plugin contract for the Folk PHP application server.

Status: in active development. See folk-spec for the roadmap.

Requirements

• Rust 1.85+
• Tokio async runtime

Installation

# Cargo.toml
folk-api = "0.1"

Quick start

A minimal plugin that logs on boot and shutdown (~25 lines):

use anyhow::Result;
use async_trait::async_trait;
use folk_api::{
    Plugin, PluginContext, PluginFactory, ServerPlugin, ServerPluginWrapper,
    RpcMethodDef,
};
use serde::Deserialize;
use serde_json::Value;

#[derive(Debug, Deserialize, Default)]
struct GreetConfig {
    message: Option<String>,
}

struct GreetPlugin {
    message: String,
}

#[async_trait]
impl ServerPlugin for GreetPlugin {
    fn name(&self) -> &'static str {
        "greet"
    }

    async fn run(&self, ctx: PluginContext) -> Result<()> {
        tracing::info!(msg = %self.message, "greet plugin running");
        let mut shutdown = ctx.shutdown;
        let _ = shutdown.changed().await;
        tracing::info!("greet plugin stopping");
        Ok(())
    }
}

struct GreetFactory;

impl PluginFactory for GreetFactory {
    fn create(&self, config: Value) -> Result<Box<dyn Plugin>> {
        let cfg: GreetConfig = serde_json::from_value(config).unwrap_or_default();
        let message = cfg.message.unwrap_or_else(|| "Hello, Folk!".into());
        Ok(Box::new(ServerPluginWrapper::new(GreetPlugin { message })))
    }
}

/// Required entry point — the builder calls this by name.
pub fn folk_plugin_factory() -> Box<dyn PluginFactory> {
    Box::new(GreetFactory)
}

Register it in folk.build.toml:

[[plugin]]
crate_name = "my_greet_plugin"
path = "../my-greet-plugin"
config_key = "greet"

And configure it in folk.toml:

[greet]
message = "Howdy"

Configuration

Plugins receive their config section as an opaque serde_json::Value. Each plugin deserializes it into its own struct. There is no global schema — the plugin owns its config shape.

How it works

Plugin lifecycle

1. Factory — The builder calls folk_plugin_factory() once per plugin crate. The returned PluginFactory receives the plugin's config as JSON and constructs a Box<dyn Plugin>.
2. Boot — plugin.boot(ctx) is called in registration order at server startup. Returning Err is fatal: already-booted plugins are shut down in reverse order and the server exits.
3. Run — For ServerPlugin implementations, run() executes as a long-lived task. Watch ctx.shutdown to know when to stop.
4. Shutdown — plugin.shutdown() is called in reverse registration order. Errors are logged but do not block the shutdown sequence.

PluginContext

Every plugin receives a PluginContext at boot:

Field	Type	Description
executor	Arc<dyn Executor>	Send work to the PHP worker pool
shutdown	watch::Receiver<bool>	Fires when the server is shutting down
rpc_registrar	Option<Arc<dyn RpcRegistrar>>	Register admin RPC methods
health_registry	Option<Arc<dyn HealthRegistry>>	Register health checks
metrics_registry	Option<Arc<dyn MetricsRegistry>>	Register Prometheus metrics

Optional registries are None when the corresponding plugin (metrics, etc.) is not loaded. Check before use.

Key traits

• Plugin — 3 required methods: name(), boot(), shutdown(). Optional: rpc_methods().
• ServerPlugin — Convenience trait for the common "spawn a task, wait for shutdown" pattern. Wrap with ServerPluginWrapper to get a Plugin.
• PluginFactory — Single method create(config: Value) -> Result<Box<dyn Plugin>>.
• Executor — async fn execute(&self, payload: Bytes) -> Result<Bytes>. Sends MessagePack-encoded payloads to the PHP worker pool and returns the response.

RPC methods

Advertise methods from rpc_methods() and register handlers via rpc_registrar.register_raw(name, handler). Handlers receive and return Bytes (typically MessagePack-encoded).

Health checks

Register via health_registry.register(name, check_fn). Return HealthStatus::ok() or HealthStatus::degraded(msg).

Metrics

Create metric families via metrics_registry.counter_vec(name, help, &labels), then .with_labels(&values) to get a handle. Supports counters, gauges, and histograms. All metrics render in Prometheus text format.

See ADR 0006 — Plugin API shape for the full design rationale.

License

MIT