feat(WIP): multithreaded runtime

Basic framework for a multithreaded runtime has been put in place. Needs
plenty of fixes to keep the logic straightforward and performant.

examples/hello.rs

@@ -1,4 +1,7 @@
-use swactor::{ActorAddress, ActorInterface, Message, Runtime, RuntimeFlavor};
+use swactor::{
+    actor::{ActorAddress, ActorInterface},
+    runtime::{Context, Runtime, RuntimeFlavor},
+};
 
 #[derive(Debug, Default)]
 struct Greeter {
@@ -13,13 +16,15 @@ struct GreetMessage {
     /// who do we send out greeting back to?
     return_addr: ActorAddress,
 }
-impl Message for GreetMessage {}
+
+#[derive(Debug, Default, Clone)]
+struct GreetResponse(String);
 
 impl ActorInterface for Greeter {
     type Incoming = GreetMessage;
     type Response = GreetResponse;
 
-    fn handle(&mut self, ctx: &Runtime, msg: GreetMessage) {
+    fn handle(&mut self, ctx: &Context, msg: GreetMessage) {
         let res = GreetResponse(format!("Hello, {}!", msg.who));
         self.num_greeted += 1;
         if let Err(_) = ctx.send_to(msg.return_addr, res) {
@@ -29,12 +34,8 @@ impl ActorInterface for Greeter {
     }
 }
 
-#[derive(Debug, Default, Clone)]
-struct GreetResponse(String);
-impl Message for GreetResponse {}
-
 fn main() {
-    let mut rt = Runtime::new(100, Some(RuntimeFlavor::SingleThreaded));
+    let rt = Runtime::new(100, RuntimeFlavor::SingleThreaded);
     let addr = rt
         .spawn(Greeter::default())
         .expect("failed to spawn greeter");

examples/ring.rs

@@ -0,0 +1,68 @@
+use swactor::{
+    actor::{ActorAddress, ActorInterface},
+    runtime::{Context, Inbox, Runtime, RuntimeFlavor},
+};
+
+#[derive(Debug, Default, Clone)]
+pub struct RingMessage {
+    count: usize,
+}
+
+impl RingMessage {
+    pub fn next(self) -> Self {
+        Self {
+            count: self.count + 1,
+        }
+    }
+}
+
+#[derive(Debug, Default)]
+struct RingActor {
+    next: ActorAddress,
+}
+
+impl RingActor {
+    pub fn new(next: ActorAddress) -> Self {
+        Self { next }
+    }
+}
+
+impl ActorInterface for RingActor {
+    type Incoming = RingMessage;
+    type Response = ();
+    fn handle(&mut self, ctx: &Context, msg: Self::Incoming) {
+        if let Err(_) = ctx.send_to(self.next, msg.next()) {
+            // do nothing
+        }
+    }
+}
+
+fn main() {
+    let rt = Runtime::new(10_000, RuntimeFlavor::SingleThreaded);
+    let inbox: Inbox<RingMessage> = rt.new_inbox();
+
+    let mut next = rt
+        .spawn(RingActor::new(*inbox.addr()))
+        .expect("failed to spawn");
+    for _ in 0..500 {
+        let new = rt.spawn(RingActor::new(next)).expect("failed to spawn");
+        next = new;
+    }
+    rt.send_to(next, RingMessage { count: 0 })
+        .expect("failed to start message ring");
+
+    let msg: RingMessage;
+    loop {
+        match inbox.try_recv() {
+            Some(m) => {
+                msg = m;
+                break;
+            }
+            None => {
+                rt.tick();
+            }
+        }
+    }
+
+    println!("{msg:?}")
+}

src/actor.rs

@@ -0,0 +1,62 @@
+use crate::{ring_buffer::Receiver, runtime::Context, WATERLEVEL};
+
+pub trait Message: 'static + Sized + Clone + Send {}
+impl<T: 'static + Sized + Clone + Send> Message for T {}
+
+pub trait ActorInterface: 'static + Send {
+    type Incoming: Message;
+    type Response: Message;
+    fn handle(&mut self, ctx: &Context, msg: Self::Incoming);
+}
+
+pub type ActorAddress = u64;
+
+/// FIXME: If we never have the actor struct reference its own address, should
+/// we even include it as a variable here? We could instead grab this information
+/// from the runtime or router.
+pub struct Actor<A>
+where
+    A: ActorInterface,
+{
+    _addr: ActorAddress,
+    inbox: Receiver<A::Incoming>,
+    inner: A,
+}
+
+impl<A: ActorInterface> Actor<A> {
+    pub(crate) fn new(addr: ActorAddress, inbox: Receiver<A::Incoming>, inner: A) -> Self {
+        Self {
+            _addr: addr,
+            inbox,
+            inner,
+        }
+    }
+}
+
+/// Trait for type-erased actors
+pub(crate) trait AnyActor: Send {
+    fn tick(&mut self, ctx: &Context);
+}
+
+impl<A> AnyActor for Actor<A>
+where
+    A: ActorInterface,
+{
+    fn tick(&mut self, ctx: &Context) {
+        let total_messages = self.inbox.len();
+        let messages_to_process = if total_messages < WATERLEVEL {
+            total_messages
+        } else {
+            total_messages >> 1
+        };
+
+        for _ in 0..messages_to_process {
+            match self.inbox.try_recv() {
+                Some(msg) => self.inner.handle(ctx, msg),
+                None => unreachable!(
+                    "We checked number of unprocessed messages in the queue ahead of processing"
+                ),
+            }
+        }
+    }
+}

src/error.rs

@@ -1,6 +1,5 @@
 #[derive(Debug)]
 pub struct Error(Box<dyn std::error::Error + Send + Sync + 'static>);
-pub type Result<T> = std::result::Result<T, Error>;
 pub(crate) fn convert_err<E: std::fmt::Debug>(e: E) -> Error {
     Error(format!("{e:?}").into())
 }

src/lib.rs

@@ -1,251 +1,28 @@
+pub mod actor;
+
+pub(crate) mod error;
+pub use error::Error;
+
 mod ring_buffer;
+mod router;
+pub mod runtime;
 
-use std::collections::HashMap;
+// Re-export commonly used types
-
+pub use actor::{ActorAddress, ActorInterface, Message};
-use crossbeam_queue::ArrayQueue;
+pub use runtime::{Context, Inbox, Runtime, RuntimeFlavor};
-use ring_buffer::{Receiver, Sender};
-
-pub mod error;
-use error::Error;
 
 #[cfg(feature = "getrandom")]
-pub fn get_random(buf: &mut [u8]) {
+pub(crate) fn get_random(buf: &mut [u8]) {
     getrandom::getrandom(buf).unwrap()
 }
 
 /// The strategy for message processing is such:
+/// 
+/// ```ignore
 /// if total_messages < WATERLEVEL:
 ///     process all
 /// else
-///     process total_messages // 2
+///     process total_messages >> 1
+/// ```
 const WATERLEVEL: usize = 10;
-
+const DEFAULT_INBOX_CAPACITY: usize = 1_000;
-const DEFAULT_INBOX_CAPACITY: usize = 100;
-
-pub trait Message: 'static + Sized + Clone + Send {}
-pub type Envelope = Box<dyn std::any::Any + Send>;
-
-pub trait ActorInterface: 'static + Send {
-    type Incoming: Message;
-    type Response: Message;
-    fn handle(&mut self, ctx: &Runtime, msg: Self::Incoming);
-}
-
-pub type ActorAddress = u64;
-
-pub struct Actor<A>
-where
-    A: ActorInterface,
-{
-    _addr: ActorAddress,
-    inbox: Receiver<A::Incoming>,
-    inner: A,
-}
-
-/// Trait for type-erased actors
-trait AnyActor: Send {
-    fn tick(&mut self, ctx: &Runtime);
-}
-
-impl<A> AnyActor for Actor<A>
-where
-    A: ActorInterface,
-{
-    fn tick(&mut self, ctx: &Runtime) {
-        let total_messages = self.inbox.len();
-        let messages_to_process = if total_messages < WATERLEVEL {
-            total_messages
-        } else {
-            total_messages >> 1
-        };
-
-        for _ in 0..messages_to_process {
-            match self.inbox.try_recv() {
-                Some(msg) => self.inner.handle(ctx, msg),
-                None => unreachable!(
-                    "We checked number of unprocessed messages in the queue ahead of processing"
-                ),
-            }
-        }
-    }
-}
-
-pub struct Inbox<M: Message> {
-    addr: ActorAddress,
-    inner: Receiver<M>,
-}
-
-impl<M: Message> Inbox<M> {
-    pub fn addr(&self) -> &ActorAddress {
-        &self.addr
-    }
-
-    pub fn try_recv(&self) -> Option<M> {
-        self.inner.try_recv()
-    }
-}
-
-#[derive(Debug, Default)]
-pub enum RuntimeFlavor {
-    #[default]
-    SingleThreaded,
-    Multithreaded(usize),
-}
-
-pub struct Runtime {
-    flavor: RuntimeFlavor,
-    router: Router,
-    router_inbox: Sender<RouterMessage>,
-    actor_queue: ArrayQueue<Box<dyn AnyActor>>,
-}
-
-impl Runtime {
-    pub fn new(capacity: usize, flavor: Option<RuntimeFlavor>) -> Self {
-        let router = Router::new(DEFAULT_INBOX_CAPACITY);
-        let router_inbox = router.new_sender();
-        Self {
-            flavor: flavor.unwrap_or_default(),
-            router,
-            router_inbox,
-            actor_queue: ArrayQueue::new(capacity),
-        }
-    }
-
-    pub fn spawn<A: ActorInterface>(&self, actor: A) -> Result<ActorAddress, Error> {
-        let addr = {
-            let mut bytes = u64::to_le_bytes(0);
-            get_random(&mut bytes);
-            u64::from_le_bytes(bytes)
-        };
-        let inbox = Receiver::<A::Incoming>::new(DEFAULT_INBOX_CAPACITY);
-        let sender = inbox.new_sender();
-
-        // Register the sender with the router
-        let _ = self
-            .router_inbox
-            .try_send(RouterMessage::AddAddr(addr, Box::new(sender)));
-
-        self.actor_queue
-            .push(Box::new(Actor {
-                _addr: addr,
-                inbox,
-                inner: actor,
-            }))
-            .map_err(|_| Error::from("Runtime error: Failed to spawn actor."))?;
-
-        Ok(addr)
-    }
-
-    pub fn send_to<M: Message>(&self, addr: ActorAddress, msg: M) -> Result<(), ()> {
-        let envelope: Envelope = Box::new(msg);
-        self.router_inbox
-            .try_send(RouterMessage::SendToAddr {
-                addr,
-                msg: envelope,
-            })
-            .map_err(|_| ())
-    }
-
-    pub fn tick(&mut self) {
-        // Pop actor, tick it, push it back
-        if let Some(mut actor) = self.actor_queue.pop() {
-            actor.tick(self);
-            let _ = self.actor_queue.push(actor);
-        }
-
-        match self.flavor {
-            RuntimeFlavor::Multithreaded(_) => (), // router has its own thread
-            RuntimeFlavor::SingleThreaded => self.router.tick(),
-        }
-    }
-
-    pub fn new_inbox<M: Message>(&self) -> Inbox<M> {
-        let addr = {
-            let mut bytes = u64::to_le_bytes(0);
-            get_random(&mut bytes);
-            u64::from_le_bytes(bytes)
-        };
-        let receiver = Receiver::<M>::new(DEFAULT_INBOX_CAPACITY);
-        let sender = receiver.new_sender();
-        // Register the sender with the router
-        let _ = self
-            .router_inbox
-            .try_send(RouterMessage::AddAddr(addr, Box::new(sender)));
-        Inbox {
-            addr,
-            inner: receiver,
-        }
-    }
-}
-
-pub trait SenderT: Send {
-    fn try_send(&self, envelope: Envelope);
-}
-
-impl<M: Message> SenderT for Sender<M> {
-    fn try_send(&self, envelope: Envelope) {
-        if let Ok(msg) = envelope.downcast::<M>() {
-            let _ = Sender::try_send(self, *msg);
-        }
-    }
-}
-
-/// Internal messages for the Router's own inbox
-pub enum RouterMessage {
-    /// register addrs <addr> with sender <sender>
-    AddAddr(ActorAddress, Box<dyn SenderT>),
-    /// remove an actor from the address book
-    RemoveAddr(ActorAddress),
-    /// send <msg> to <addr>
-    SendToAddr { addr: ActorAddress, msg: Envelope },
-}
-
-struct Router {
-    directory: HashMap<ActorAddress, Box<dyn SenderT>>,
-    inbox: Receiver<RouterMessage>,
-}
-
-impl Router {
-    pub fn new(cap: usize) -> Self {
-        Self {
-            directory: HashMap::new(),
-            inbox: Receiver::new(cap),
-        }
-    }
-
-    pub fn tick(&mut self) {
-        let total_messages = self.inbox.len();
-        let messages_to_process = if total_messages < WATERLEVEL {
-            total_messages
-        } else {
-            total_messages >> 1
-        };
-
-        for _ in 0..messages_to_process {
-            match self.inbox.try_recv() {
-                Some(msg) => self.handle(msg),
-                None => unreachable!("We ran checks on total messages before processing."),
-            }
-        }
-    }
-
-    pub fn new_sender(&self) -> Sender<RouterMessage> {
-        self.inbox.new_sender()
-    }
-
-    fn handle(&mut self, msg: RouterMessage) {
-        match msg {
-            RouterMessage::AddAddr(addr, sender) => {
-                self.directory.insert(addr, sender);
-            }
-            RouterMessage::RemoveAddr(addr) => {
-                self.directory.remove(&addr);
-            }
-            RouterMessage::SendToAddr { addr, msg } => {
-                if let Some(sender) = self.directory.get(&addr) {
-                    sender.try_send(msg);
-                }
-            }
-        }
-    }
-}

src/ring_buffer.rs

@@ -48,6 +48,17 @@ pub(crate) struct Sender<T> {
     queue: Arc<ArrayQueue<T>>,
 }
 
+/// FIXME: I don't like this. Why do we need to clone the Sender
+/// Because there are no guarentees on the existence of the Receiver
+/// we need to be very careful about passing around access to the buffer.
+impl<T> Clone for Sender<T> {
+    fn clone(&self) -> Self {
+        Self {
+            queue: Arc::clone(&self.queue),
+        }
+    }
+}
+
 impl<T> Sender<T> {
     /// Attempt to push a value to the queue. Returns Err(value) if the queue is full.
     pub fn try_send(&self, value: T) -> Result<(), T> {

src/router.rs

@@ -0,0 +1,86 @@
+use std::collections::HashMap;
+
+use crate::{
+    actor::{ActorAddress, Message},
+    ring_buffer::{Receiver, Sender},
+    WATERLEVEL,
+};
+
+pub(crate) type Envelope = Box<dyn std::any::Any + Send>;
+
+pub(crate) trait SenderT: Send {
+    fn try_send(&self, envelope: Envelope);
+}
+
+impl<M: Message> SenderT for Sender<M> {
+    fn try_send(&self, envelope: Envelope) {
+        if let Ok(msg) = envelope.downcast::<M>() {
+            let _ = Sender::try_send(self, *msg);
+        }
+    }
+}
+
+/// Internal messages for the Router's own inbox
+pub(crate) enum RouterMessage {
+    /// register addrs <addr> with sender <sender>
+    AddAddr(ActorAddress, Box<dyn SenderT>),
+    
+    /// FIXME: this will be active when we allow actors to shut themselves
+    /// down. For now, disable the warning.
+    #[allow(dead_code)] 
+    /// remove an actor from the address book
+    RemoveAddr(ActorAddress),
+    
+    /// send <msg> to <addr>
+    SendToAddr { addr: ActorAddress, msg: Envelope },
+}
+
+pub(crate) struct Router {
+    directory: HashMap<ActorAddress, Box<dyn SenderT>>,
+    inbox: Receiver<RouterMessage>,
+}
+
+impl Router {
+    pub fn new(cap: usize) -> Self {
+        Self {
+            directory: HashMap::new(),
+            inbox: Receiver::new(cap),
+        }
+    }
+
+    pub fn tick(&mut self) {
+        let total_messages = self.inbox.len();
+        let messages_to_process = if total_messages < WATERLEVEL {
+            total_messages
+        } else {
+            total_messages >> 1
+        };
+
+        for _ in 0..messages_to_process {
+            match self.inbox.try_recv() {
+                Some(msg) => self.handle(msg),
+                None => unreachable!("We ran checks on total messages before processing."),
+            }
+        }
+    }
+
+    pub fn new_sender(&self) -> Sender<RouterMessage> {
+        self.inbox.new_sender()
+    }
+
+    fn handle(&mut self, msg: RouterMessage) {
+        match msg {
+            RouterMessage::AddAddr(addr, sender) => {
+                self.directory.insert(addr, sender);
+            }
+            RouterMessage::RemoveAddr(addr) => {
+                self.directory.remove(&addr);
+            }
+            RouterMessage::SendToAddr { addr, msg } => {
+                if let Some(sender) = self.directory.get(&addr) {
+                    sender.try_send(msg);
+                }
+            }
+        }
+    }
+}

src/runtime.rs

@@ -0,0 +1,316 @@
+use std::sync::atomic::{AtomicBool, Ordering};
+use std::sync::{Arc, Mutex};
+use std::thread::{self, JoinHandle};
+
+use crossbeam_queue::ArrayQueue;
+
+use crate::{
+    actor::{Actor, ActorAddress, ActorInterface, AnyActor, Message},
+    get_random,
+    ring_buffer::{Receiver, Sender},
+    router::{Envelope, Router, RouterMessage},
+    Error, DEFAULT_INBOX_CAPACITY,
+};
+
+#[derive(Debug, Clone, Default)]
+pub enum RuntimeFlavor {
+    #[default]
+    SingleThreaded,
+    Multithreaded {
+        workers: usize,
+    },
+}
+
+pub struct Inbox<M: Message> {
+    addr: ActorAddress,
+    inner: Receiver<M>,
+}
+
+impl<M: Message> Inbox<M> {
+    pub fn addr(&self) -> &ActorAddress {
+        &self.addr
+    }
+
+    pub fn try_recv(&self) -> Option<M> {
+        self.inner.try_recv()
+    }
+}
+
+/// A lightweight handle for sending messages to actors
+/// This is what actors receive in their handle() method
+#[derive(Clone)]
+pub struct Context {
+    router_inbox: Sender<RouterMessage>,
+}
+
+impl Context {
+    /// Send a message to an actor address
+    pub fn send_to<M: Message>(&self, addr: ActorAddress, msg: M) -> Result<(), Error> {
+        let envelope: Envelope = Box::new(msg);
+        self.router_inbox
+            .try_send(RouterMessage::SendToAddr {
+                addr,
+                msg: envelope,
+            })
+            .map_err(|_| Error::from("Failed to send message: router inbox full"))
+    }
+}
+
+/// Shared runtime state, wrapped in Arc for thread sharing
+/// FIXME: We check the `running` variable too often. Should
+/// push it somewhere where it is infreqently checked, and instead
+/// have shutdown logic for everything else.
+struct RuntimeInner {
+    running: AtomicBool,
+    router_inbox: Sender<RouterMessage>,
+    actor_queue: ArrayQueue<Box<dyn AnyActor>>,
+}
+
+/// Thread handles for the multithreaded runtime
+struct RuntimeHandles {
+    workers: Vec<JoinHandle<()>>,
+    router_thread: Option<JoinHandle<()>>,
+}
+
+/// The main runtime for executing actors
+pub struct Runtime {
+    inner: Arc<RuntimeInner>,
+    flavor: RuntimeFlavor,
+    /// Router is only accessed from a single thread (either main or dedicated router thread)
+    /// 
+    /// FIXME: If single threaded, why do we have a mutex
+    router: Mutex<Router>,
+    /// Thread handles, created lazily when run() is called
+    handles: Mutex<Option<RuntimeHandles>>,
+}
+
+impl Runtime {
+    /// Create a new runtime with given actor queue capacity and flavor
+    /// 
+    /// FIXME: I don't like this interface. Maybe a builder or config pattern. I shouldnt
+    /// have to read code to understand what these variable names are.
+    pub fn new(capacity: usize, flavor: RuntimeFlavor) -> Self {
+        // FIXME: Avoid hard coded defaults, or at least put them all in one place
+        let router = Router::new(DEFAULT_INBOX_CAPACITY);
+        let router_inbox = router.new_sender();
+        Self {
+            inner: Arc::new(RuntimeInner {
+                running: AtomicBool::new(false),
+                router_inbox,
+                actor_queue: ArrayQueue::new(capacity),
+            }),
+            flavor,
+            router: Mutex::new(router),
+            handles: Mutex::new(None),
+        }
+    }
+
+    /// Get a context handle for sending messages
+    /// 
+    /// FIXME: Do we need all this indirection?
+    pub fn context(&self) -> Context {
+        Context {
+            router_inbox: self.inner.router_inbox.clone(),
+        }
+    }
+
+    /// Spawn an actor, returns its address
+    pub fn spawn<A: ActorInterface>(&self, actor: A) -> Result<ActorAddress, Error> {
+        // FIXME: Figure out what to do with this.
+        // Its distracting to include here, but not used elsewhere for now. 
+        let addr = {
+            let mut bytes = u64::to_le_bytes(0);
+            get_random(&mut bytes);
+            u64::from_le_bytes(bytes)
+        };
+        let inbox = Receiver::<A::Incoming>::new(DEFAULT_INBOX_CAPACITY);
+        let sender = inbox.new_sender();
+
+        // Register the sender with the router
+        let _ = self
+            .inner
+            .router_inbox
+            .try_send(RouterMessage::AddAddr(addr, Box::new(sender)));
+
+        self.inner
+            .actor_queue
+            .push(Box::new(Actor::new(addr, inbox, actor)))
+            .map_err(|_| Error::from("Runtime error: Failed to spawn actor."))?;
+
+        Ok(addr)
+    }
+
+    /// Send a message to an actor address
+    pub fn send_to<M: Message>(&self, addr: ActorAddress, msg: M) -> Result<(), Error> {
+        self.context().send_to(addr, msg)
+    }
+
+    /// Create an external inbox for receiving messages outside actors
+    pub fn new_inbox<M: Message>(&self) -> Inbox<M> {
+        let addr = {
+            let mut bytes = u64::to_le_bytes(0);
+            get_random(&mut bytes);
+            u64::from_le_bytes(bytes)
+        };
+        let receiver = Receiver::<M>::new(DEFAULT_INBOX_CAPACITY);
+        let sender = receiver.new_sender();
+        // Register the sender with the router
+        let _ = self
+            .inner
+            .router_inbox
+            .try_send(RouterMessage::AddAddr(addr, Box::new(sender)));
+        Inbox {
+            addr,
+            inner: receiver,
+        }
+    }
+
+    /// Process one actor tick + router messages
+    /// Works in both single/multi mode (useful for testing and fine-grained control)
+    /// 
+    /// FIXME: `tick` does not make sense in multithreaded context. Add a check to ensure single threaded
+    pub fn tick(&self) {
+        let ctx = self.context();
+
+        if let Some(mut actor) = self.inner.actor_queue.pop() {
+            actor.tick(&ctx);
+            let _ = self.inner.actor_queue.push(actor);
+        }
+
+        // In single-threaded mode, also tick the router
+        if matches!(self.flavor, RuntimeFlavor::SingleThreaded)
+            && let Ok(mut router) = self.router.lock()
+        {
+            router.tick();
+        }
+    }
+
+    /// Run the runtime (blocking)
+    /// - SingleThreaded: runs in current thread until shutdown
+    /// - Multithreaded: spawns workers + router thread, blocks until shutdown
+    pub fn run(&self) {
+        self.inner.running.store(true, Ordering::Release);
+
+        match &self.flavor {
+            RuntimeFlavor::SingleThreaded => {
+                self.run_single_threaded();
+            }
+            RuntimeFlavor::Multithreaded { workers } => {
+                self.run_multi_threaded(*workers);
+            }
+        }
+    }
+
+    /// FIXME: I don't like this loop. We can send shutdown signals from the process
+    /// that calls the actor runtime instead. It also does not make sense to have this
+    /// around for single threaded runtimes (people can instead loop over `runtime.tick()`).
+    fn run_single_threaded(&self) {
+        let ctx = self.context();
+
+        while self.inner.running.load(Ordering::Relaxed) {
+            // Pop actor, tick it, push it back
+            if let Some(mut actor) = self.inner.actor_queue.pop() {
+                actor.tick(&ctx);
+                let _ = self.inner.actor_queue.push(actor);
+            }
+
+            // Tick the router
+            if let Ok(mut router) = self.router.lock() {
+                router.tick();
+            }
+
+            thread::yield_now();
+        }
+    }
+
+    fn run_multi_threaded(&self, num_workers: usize) {
+        // Take ownership of router for the dedicated router thread
+        // FIXME: this is weird and concerning. Introduces a class of logic errors
+        // wherein we try and call a dummy router.
+        let router = {
+            let mut guard = self.router.lock().unwrap();
+            std::mem::replace(&mut *guard, Router::new(1)) // placeholder
+        };
+
+        // Spawn dedicated router thread
+        // FIXME: what is this, these variables are named terribly
+        let router_running = Arc::clone(&self.inner);
+        let router_handle = {
+            let running = router_running;
+            thread::spawn(move || {
+                router_loop(router, running);
+            })
+        };
+
+        // Spawn worker threads
+        let worker_handles: Vec<_> = (0..num_workers)
+            .map(|_| {
+                let inner = Arc::clone(&self.inner);
+                thread::spawn(move || {
+                    worker_loop(inner);
+                })
+            })
+            .collect();
+
+        // Store handles
+        *self.handles.lock().unwrap() = Some(RuntimeHandles {
+            workers: worker_handles,
+            router_thread: Some(router_handle),
+        });
+
+        // Block until shutdown - wait for all threads to complete
+        self.wait_for_shutdown();
+    }
+
+    fn wait_for_shutdown(&self) {
+        // Wait for the running flag to be set to false, then join threads
+        while self.inner.running.load(Ordering::Relaxed) {
+            thread::yield_now();
+        }
+
+        // Join all threads
+        let handles = self.handles.lock().unwrap().take();
+        if let Some(h) = handles {
+            for worker in h.workers {
+                let _ = worker.join();
+            }
+            if let Some(rt) = h.router_thread {
+                let _ = rt.join();
+            }
+        }
+    }
+
+    /// Signal all workers to stop
+    pub fn shutdown(&self) {
+        self.inner.running.store(false, Ordering::Release);
+    }
+
+    /// Check if runtime is still active
+    pub fn is_running(&self) -> bool {
+        self.inner.running.load(Ordering::Acquire)
+    }
+}
+
+/// Worker thread loop - processes actors from the shared queue
+fn worker_loop(inner: Arc<RuntimeInner>) {
+    let ctx = Context {
+        router_inbox: inner.router_inbox.clone(),
+    };
+
+    while inner.running.load(Ordering::Relaxed) {
+        if let Some(mut actor) = inner.actor_queue.pop() {
+            actor.tick(&ctx);
+            let _ = inner.actor_queue.push(actor);
+        } else {
+            thread::yield_now();
+        }
+    }
+}
+
+/// Router thread loop - processes router messages
+fn router_loop(mut router: Router, inner: Arc<RuntimeInner>) {
+    while inner.running.load(Ordering::Relaxed) {
+        router.tick();
+        thread::yield_now();
+    }
+}

tests/runtime_tests.rs

@@ -0,0 +1,155 @@
+use std::sync::Arc;
+use std::thread;
+use std::time::Duration;
+
+use swactor::{
+    actor::{ActorAddress, ActorInterface},
+    runtime::{Context, Inbox, Runtime, RuntimeFlavor},
+};
+
+// ============================================================================
+// Test Helpers
+// ============================================================================
+
+#[derive(Clone)]
+struct PingMessage {
+    reply_to: ActorAddress,
+}
+
+#[derive(Clone)]
+struct PongMessage;
+
+struct PongActor;
+
+impl ActorInterface for PongActor {
+    type Incoming = PingMessage;
+    type Response = PongMessage;
+
+    fn handle(&mut self, ctx: &Context, msg: PingMessage) {
+        let _ = ctx.send_to(msg.reply_to, PongMessage);
+    }
+}
+
+/// An actor that forwards messages to another address
+struct ForwarderActor {
+    target: ActorAddress,
+}
+
+#[derive(Clone)]
+struct ForwardMessage(usize);
+
+impl ActorInterface for ForwarderActor {
+    type Incoming = ForwardMessage;
+    type Response = ();
+
+    fn handle(&mut self, ctx: &Context, msg: ForwardMessage) {
+        let _ = ctx.send_to(self.target, msg);
+    }
+}
+
+#[test]
+fn test_single_threaded_ping_pong() {
+    let rt = Runtime::new(100, RuntimeFlavor::SingleThreaded);
+    let inbox: Inbox<PongMessage> = rt.new_inbox();
+
+    let pong_addr = rt.spawn(PongActor).expect("spawn pong");
+
+    // Send ping
+    rt.send_to(
+        pong_addr,
+        PingMessage {
+            reply_to: *inbox.addr(),
+        },
+    )
+    .unwrap();
+
+    // Tick until we get a response
+    for _ in 0..10 {
+        rt.tick();
+        if inbox.try_recv().is_some() {
+            return; // Success!
+        }
+    }
+
+    panic!("Did not receive pong response");
+}
+
+#[test]
+fn test_single_threaded_message_chain() {
+    let rt = Runtime::new(100, RuntimeFlavor::SingleThreaded);
+    let inbox: Inbox<ForwardMessage> = rt.new_inbox();
+
+    // Create a chain: A -> B -> C -> inbox
+    let c_addr = rt
+        .spawn(ForwarderActor {
+            target: *inbox.addr(),
+        })
+        .unwrap();
+    let b_addr = rt.spawn(ForwarderActor { target: c_addr }).unwrap();
+    let a_addr = rt.spawn(ForwarderActor { target: b_addr }).unwrap();
+
+    // Send message to start of chain
+    rt.send_to(a_addr, ForwardMessage(42)).unwrap();
+
+    // Tick until message arrives
+    for _ in 0..20 {
+        rt.tick();
+        if let Some(ForwardMessage(val)) = inbox.try_recv() {
+            assert_eq!(val, 42);
+            return;
+        }
+    }
+
+    panic!("Message did not traverse the chain");
+}
+
+#[test]
+fn test_multithreaded_message_passing() {
+    let rt = Arc::new(Runtime::new(
+        1000,
+        RuntimeFlavor::Multithreaded { workers: 4 },
+    ));
+    let inbox: Inbox<ForwardMessage> = rt.new_inbox();
+
+    // Create a longer chain to exercise multi-threading
+    let mut target = *inbox.addr();
+    for _ in 0..20 {
+        target = rt.spawn(ForwarderActor { target }).unwrap();
+    }
+
+    let start_addr = target;
+
+    // Send message
+    rt.send_to(start_addr, ForwardMessage(999)).unwrap();
+
+    // Spawn thread to check for result and shutdown
+    let rt_clone = Arc::clone(&rt);
+    let inbox_check = thread::spawn(move || {
+        for _ in 0..100 {
+            thread::sleep(Duration::from_millis(10));
+            if let Some(ForwardMessage(val)) = inbox.try_recv() {
+                rt_clone.shutdown();
+                return Some(val);
+            }
+        }
+        rt_clone.shutdown();
+        None
+    });
+
+    rt.run();
+
+    let result = inbox_check.join().unwrap();
+    assert_eq!(result, Some(999));
+}
+
+#[test]
+fn test_is_running_flag() {
+    let rt = Runtime::new(100, RuntimeFlavor::SingleThreaded);
+
+    // Before run(), is_running should be false
+    assert!(!rt.is_running());
+
+    // After shutdown before run, still false
+    rt.shutdown();
+    assert!(!rt.is_running());
+}