Sim Scaffold

Scaffold Simulation-Testable Module

Generate boilerplate for new modules that are deterministic simulation testing (DST) ready from the start. Prevents the costly retrofitting that FoundationDB avoided by building simulation infrastructure before writing database code.

Evidence Base

When to Use

• Creating a new module in gossip-coordination/
• Adding a new gossip protocol component
• Building a new pipeline stage that touches coordination
• Any new code that manages distributed state, leases, or shard lifecycle

When NOT to Use

• Adding helper functions to existing modules (use /sim-review instead)
• Detection engine rules or regex patterns
• Pure data types with no state transitions

Procedure

Step 1: Determine Module Type

Ask the user which type of module they are building:

Step 2: Gather Invariants

Ask the user:

1. What are the key state transitions? (e.g., Open -> Acquired -> Checkpointing -> Done)
2. What invariants must hold? (e.g., "only one worker holds a lease at a time")
3. What are the error conditions? (e.g., "lease expired", "fence rejected")
4. What external dependencies exist? (e.g., persistence, network, timers)

Step 3: Generate Scaffold

Type A: Coordination Module

Generate this file structure under the appropriate crate:

src/<module>/
  mod.rs            — Public API, module docs, re-exports
  state.rs          — State machine types
  logic.rs          — Pure state transition functions
  sim.rs            — Simulation harness (behind test-support feature)
  tests/
    proptest_sm.rs  — proptest state machine tests
    invariants.rs   — Invariant assertion functions

Prerequisite: LogicalTime must be defined in gossip_contracts::coordination before the scaffold compiles. If the coordination contracts module is still a doc stub, define LogicalTime there first (or use a temporary type alias).

state.rs template:

//! State machine types for <module>.
//!
//! All types are plain data — no I/O, no time reads, no randomness.

use gossip_contracts::coordination::LogicalTime;

/// The states this module can be in.
#[derive(Debug, Clone, PartialEq, Eq)]
pub enum State {
    // TODO: Fill in states from Step 2
}

/// Inputs that drive state transitions.
#[derive(Debug, Clone)]
pub enum Input {
    // TODO: Fill in inputs from Step 2
}

/// Side effects produced by state transitions.
///
/// Effects are returned as data — the caller is responsible for executing them.
/// This keeps the state machine pure and deterministically testable.
#[derive(Debug, Clone, PartialEq, Eq)]
pub enum Effect {
    // TODO: Fill in effects from Step 2
}

logic.rs template:

//! Pure state transition logic for <module>.
//!
//! Every function in this module is a pure function:
//! `fn(current_state, input, now) -> (new_state, Vec<Effect>)`
//!
//! No I/O, no clock reads, no randomness without explicit seed.

use super::state::{Effect, Input, State};
use gossip_contracts::coordination::LogicalTime;

/// Apply an input to the current state, producing a new state and effects.
///
/// # Invariants
///
/// TODO: Document invariants from Step 2
pub fn transition(
    state: &State,
    input: &Input,
    now: LogicalTime,
) -> (State, Vec<Effect>) {
    match (state, input) {
        // TODO: Implement transitions
        _ => (state.clone(), vec![]),
    }
}

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

    // TODO: Unit tests for each transition
}

sim.rs template:

//! Simulation harness for <module>.
//!
//! Connects the pure state machine to a deterministic execution environment
//! with seeded PRNG, simulated clock, and fault injection.

#![cfg(feature = "test-support")]

use std::collections::BinaryHeap;

use rand::rngs::StdRng;
use rand::SeedableRng;

use gossip_contracts::coordination::LogicalTime;

use super::logic;
use super::state::{Effect, Input, State};

/// Deterministic execution context for simulation.
pub struct SimContext {
    /// Seeded PRNG for reproducible randomness.
    pub rng: StdRng,
    /// Simulated logical clock.
    pub clock: LogicalTime,
    /// Seed used to create this context (for reproduction).
    pub seed: u64,
}

impl SimContext {
    /// Create a new simulation context with the given seed.
    pub fn new(seed: u64) -> Self {
        Self {
            rng: StdRng::seed_from_u64(seed),
            clock: LogicalTime::default(),
            seed,
        }
    }

    /// Advance the simulated clock by `ticks`.
    pub fn advance(&mut self, ticks: u64) {
        self.clock = self.clock.advance(ticks);
    }
}

/// Fault injection configuration.
#[derive(Debug, Clone, Default)]
pub struct FaultConfig {
    /// Drop probability for effects (0.0 = none, 1.0 = all).
    pub effect_drop_rate: f64,
    /// Whether to inject lease expiry after N operations.
    pub expire_lease_after: Option<usize>,
    /// Whether to inject process pause (freeze clock).
    pub inject_pause: bool,
}

/// Simulation harness orchestrating multiple state machine instances.
pub struct SimHarness {
    pub ctx: SimContext,
    pub states: Vec<State>,
    pub effects_log: Vec<(usize, Effect)>,
    pub fault_config: FaultConfig,
    op_count: usize,
}

impl SimHarness {
    pub fn new(seed: u64, initial_states: Vec<State>) -> Self {
        Self {
            ctx: SimContext::new(seed),
            states: initial_states,
            effects_log: Vec::new(),
            fault_config: FaultConfig::default(),
            op_count: 0,
        }
    }

    /// Apply an input to a specific state machine instance.
    pub fn apply(&mut self, instance: usize, input: &Input) -> Vec<Effect> {
        let (new_state, effects) =
            logic::transition(&self.states[instance], input, self.ctx.clock);
        self.states[instance] = new_state;
        self.op_count += 1;

        // Log effects for invariant checking.
        for effect in &effects {
            self.effects_log
                .push((instance, effect.clone()));
        }

        effects
    }

    /// Check all invariants against current state.
    ///
    /// Returns a list of violated invariant descriptions.
    pub fn check_invariants(&self) -> Vec<String> {
        let mut violations = Vec::new();

        // TODO: Add invariant checks from Step 2
        // Example:
        // if self.states.iter().filter(|s| matches!(s, State::Active)).count() > 1 {
        //     violations.push("MUTUAL_EXCLUSION: More than one instance active".into());
        // }

        violations
    }
}

tests/proptest_sm.rs template:

//! proptest state machine tests for <module>.
//!
//! Generates random sequences of valid inputs and verifies that
//! invariants hold after every transition.

#![cfg(all(test, feature = "test-support"))]

use proptest::prelude::*;

use super::sim::{FaultConfig, SimHarness};
use super::state::Input;

/// Strategy for generating valid inputs.
fn arb_input() -> impl Strategy<Value = Input> {
    // TODO: Generate valid inputs based on module types
    prop_oneof![
        // Just(Input::VariantA),
        // Just(Input::VariantB { ... }),
    ]
}

/// Strategy for generating input sequences.
fn arb_input_sequence(max_len: usize) -> impl Strategy<Value = Vec<Input>> {
    prop::collection::vec(arb_input(), 1..=max_len)
}

proptest! {
    /// Invariants hold after every transition under normal conditions.
    #[test]
    fn invariants_hold_sunny_day(
        seed in any::<u64>(),
        inputs in arb_input_sequence(50),
    ) {
        let mut harness = SimHarness::new(seed, vec![/* initial state */]);

        for input in &inputs {
            harness.apply(0, input);

            let violations = harness.check_invariants();
            prop_assert!(
                violations.is_empty(),
                "Invariant violations after {:?}: {:?}",
                input,
                violations,
            );
        }
    }

    /// Invariants hold under fault injection.
    #[test]
    fn invariants_hold_with_faults(
        seed in any::<u64>(),
        inputs in arb_input_sequence(50),
        drop_rate in 0.0..0.5f64,
    ) {
        let mut harness = SimHarness::new(seed, vec![/* initial state */]);
        harness.fault_config = FaultConfig {
            effect_drop_rate: drop_rate,
            ..Default::default()
        };

        for input in &inputs {
            harness.apply(0, input);

            let violations = harness.check_invariants();
            prop_assert!(
                violations.is_empty(),
                "Invariant violations under faults (drop_rate={}) after {:?}: {:?}",
                drop_rate,
                input,
                violations,
            );
        }
    }
}

tests/invariants.rs template:

//! Invariant assertion functions for <module>.
//!
//! Each function checks one invariant and returns Ok(()) or
//! Err(description) if violated.

#![cfg(all(test, feature = "test-support"))]

use super::state::State;

/// Check: TODO describe invariant
pub fn check_invariant_name(states: &[State]) -> Result<(), String> {
    // TODO: Implement invariant check
    Ok(())
}

Type B: Gossip Protocol Module

Generate this file structure:

src/<module>/
  mod.rs              — Public API, module docs
  protocol.rs         — Sans-IO state machine
  messages.rs         — Protocol message types
  sim.rs              — SimNetwork connecting N protocol instances
  tests/
    proptest_convergence.rs  — Convergence property tests
    proptest_partitions.rs   — Partition tolerance tests

protocol.rs template (sans-IO pattern):

//! Sans-IO gossip protocol state machine.
//!
//! This module contains NO I/O. The protocol is driven by four methods:
//!
//! - `handle_input(msg, now)` — process an incoming message
//! - `poll_transmit()` — dequeue the next outbound message
//! - `poll_timeout()` — query when the next timer fires
//! - `handle_timeout(now)` — process a timer expiry
//!
//! The caller (runtime or simulation harness) is responsible for actually
//! sending messages and managing real/simulated time.
//!
//! # Evidence
//!
//! This pattern is used by:
//! - sled: `receive(msg, at) -> [(msg, destination)]`
//! - Firezone: sans-IO connlib architecture
//! - Stateright: `Actor::on_msg(state, msg) -> Vec<Action>`

use std::collections::VecDeque;

use gossip_contracts::coordination::LogicalTime;

use super::messages::{GossipMessage, NodeId, Transmit};

/// Configuration for the gossip protocol.
#[derive(Debug, Clone)]
pub struct ProtocolConfig {
    /// This node's identity.
    pub node_id: NodeId,
    /// Gossip interval (in logical time ticks).
    pub gossip_interval: u64,
    /// Failure detection timeout (in logical time ticks).
    pub failure_timeout: u64,
    /// Fan-out: number of peers to gossip to per round.
    pub fanout: usize,
}

/// Sans-IO gossip protocol state machine.
pub struct GossipProtocol {
    config: ProtocolConfig,
    outbox: VecDeque<Transmit>,
    next_gossip: Option<LogicalTime>,
    // TODO: Protocol-specific state (membership table, suspicion map, etc.)
}

impl GossipProtocol {
    /// Create a new protocol instance with the given configuration.
    pub fn new(config: ProtocolConfig, now: LogicalTime) -> Self {
        Self {
            next_gossip: Some(now.advance(config.gossip_interval)),
            outbox: VecDeque::new(),
            config,
        }
    }

    /// Process an incoming message from another node.
    ///
    /// This may enqueue outbound messages (retrievable via `poll_transmit`)
    /// and update internal state, but performs NO I/O.
    pub fn handle_input(&mut self, msg: &GossipMessage, now: LogicalTime) {
        match msg {
            // TODO: Handle each message type
            _ => {}
        }
    }

    /// Dequeue the next outbound message, if any.
    ///
    /// The caller is responsible for actually sending this over the network
    /// (or routing it in-process during simulation).
    pub fn poll_transmit(&mut self) -> Option<Transmit> {
        self.outbox.pop_front()
    }

    /// Query when the next timeout should fire.
    ///
    /// Returns `None` if no timers are pending.
    pub fn poll_timeout(&self) -> Option<LogicalTime> {
        self.next_gossip
    }

    /// Process a timer expiry.
    ///
    /// Call this when the logical clock reaches or exceeds the value
    /// returned by `poll_timeout()`.
    pub fn handle_timeout(&mut self, now: LogicalTime) {
        if self.next_gossip.is_some_and(|t| now >= t) {
            self.do_gossip_round(now);
            self.next_gossip = Some(now.advance(self.config.gossip_interval));
        }
    }

    fn do_gossip_round(&mut self, _now: LogicalTime) {
        // TODO: Select peers, build gossip messages, enqueue via self.outbox
    }
}

sim.rs template (SimNetwork):

//! Simulation network connecting N gossip protocol instances.
//!
//! Messages are delivered via an in-process priority queue ordered by
//! logical time, following sled's discrete-event simulation pattern.

#![cfg(feature = "test-support")]

use std::cmp::Reverse;
use std::collections::BinaryHeap;

use rand::rngs::StdRng;
use rand::SeedableRng;

use gossip_contracts::coordination::LogicalTime;

use super::messages::{NodeId, Transmit};
use super::protocol::{GossipProtocol, ProtocolConfig};

/// A scheduled event in the simulation.
#[derive(Debug)]
struct ScheduledEvent {
    time: LogicalTime,
    kind: EventKind,
}

#[derive(Debug)]
enum EventKind {
    Deliver(Transmit),
    Timeout(NodeId),
}

impl PartialEq for ScheduledEvent {
    fn eq(&self, other: &Self) -> bool {
        self.time == other.time
    }
}

impl Eq for ScheduledEvent {}

impl PartialOrd for ScheduledEvent {
    fn partial_cmp(&self, other: &Self) -> Option<std::cmp::Ordering> {
        Some(self.cmp(other))
    }
}

impl Ord for ScheduledEvent {
    fn cmp(&self, other: &Self) -> std::cmp::Ordering {
        // Min-heap: earliest time first.
        Reverse(self.time).cmp(&Reverse(other.time))
    }
}

/// Network fault injection configuration.
#[derive(Debug, Clone, Default)]
pub struct NetworkFaults {
    /// Message drop probability (0.0 = reliable, 1.0 = total partition).
    pub drop_rate: f64,
    /// Maximum message delay in ticks (0 = instant delivery).
    pub max_delay: u64,
    /// Set of (from, to) pairs that are partitioned.
    pub partitions: Vec<(NodeId, NodeId)>,
}

/// Simulated network of gossip protocol instances.
pub struct SimNetwork {
    pub nodes: Vec<GossipProtocol>,
    pub rng: StdRng,
    pub clock: LogicalTime,
    event_queue: BinaryHeap<ScheduledEvent>,
    pub faults: NetworkFaults,
    pub seed: u64,
}

impl SimNetwork {
    /// Create a network of `n` nodes with the given seed.
    pub fn new(seed: u64, n: usize, config_fn: impl Fn(usize) -> ProtocolConfig) -> Self {
        let clock = LogicalTime::default();
        let nodes = (0..n)
            .map(|i| GossipProtocol::new(config_fn(i), clock))
            .collect::<Vec<_>>();

        let mut net = Self {
            nodes,
            rng: StdRng::seed_from_u64(seed),
            clock,
            event_queue: BinaryHeap::new(),
            faults: NetworkFaults::default(),
            seed,
        };

        // Schedule initial timeouts.
        for (i, node) in net.nodes.iter().enumerate() {
            if let Some(t) = node.poll_timeout() {
                net.event_queue.push(ScheduledEvent {
                    time: t,
                    kind: EventKind::Timeout(NodeId(i as u64)),
                });
            }
        }

        net
    }

    /// Run the simulation for up to `max_ticks` logical time ticks.
    ///
    /// Returns the number of events processed.
    pub fn run(&mut self, max_ticks: u64) -> usize {
        let deadline = self.clock.advance(max_ticks);
        let mut events_processed = 0;

        while let Some(event) = self.event_queue.pop() {
            if event.time > deadline {
                self.event_queue.push(event);
                break;
            }

            self.clock = event.time;
            events_processed += 1;

            match event.kind {
                EventKind::Deliver(transmit) => {
                    let dst = transmit.destination.0 as usize;
                    if dst < self.nodes.len() {
                        self.nodes[dst].handle_input(&transmit.message, self.clock);
                        self.drain_outbox(dst);
                    }
                }
                EventKind::Timeout(node_id) => {
                    let idx = node_id.0 as usize;
                    if idx < self.nodes.len() {
                        self.nodes[idx].handle_timeout(self.clock);
                        self.drain_outbox(idx);

                        // Re-schedule next timeout.
                        if let Some(t) = self.nodes[idx].poll_timeout() {
                            self.event_queue.push(ScheduledEvent {
                                time: t,
                                kind: EventKind::Timeout(node_id),
                            });
                        }
                    }
                }
            }
        }

        events_processed
    }

    fn drain_outbox(&mut self, node_idx: usize) {
        while let Some(transmit) = self.nodes[node_idx].poll_transmit() {
            // TODO: Apply fault injection (drops, delays, partitions)
            // using self.faults and self.rng
            self.event_queue.push(ScheduledEvent {
                time: self.clock.advance(1), // Minimum 1-tick delivery delay
                kind: EventKind::Deliver(transmit),
            });
        }
    }

    /// Check convergence: do all nodes agree on the same state?
    pub fn check_convergence(&self) -> bool {
        // TODO: Define convergence check based on protocol semantics
        true
    }
}

Type C: Pipeline Component

Generate this file structure:

src/<component>/
  mod.rs          — Public API
  state.rs        — Processing state types
  logic.rs        — Pure processing logic
  traits.rs       — I/O trait boundaries
  sim.rs          — Simulation with mock I/O
  tests/
    proptest_processing.rs

Use the same SimContext pattern as Type A, but the trait boundaries focus on I/O operations (read source data, write checkpoints, emit findings) rather than distributed coordination.

Step 4: Verify Scaffold Compiles

After generating the scaffold:

cargo check --all-features

If compilation fails, fix the generated code. Common issues:

• Missing imports (add use statements)
• Type mismatches with existing crate types
• Feature gate misalignment

Step 5: Generate Initial Tests

Run the generated proptest to verify the scaffold works:

cargo test --features test-support -- <module>::tests

The initial tests should pass (they test the empty/default state machine).

Step 6: Report

SIM-SCAFFOLD REPORT
════════════════════
Module:          <name>
Type:            {Coordination | Gossip Protocol | Pipeline Component}
Files created:   {count}
Pattern:         {sans-IO | state machine + effects | trait boundary}

Invariants to implement:
  1. {invariant from Step 2}
  2. {invariant from Step 2}

Next steps:
  1. Implement state transitions in logic.rs
  2. Add proptest strategies for Input variants
  3. Implement invariant checks in sim.rs
  4. Run /sim-review on completed implementation

Quick Reference: Module Types

Related Skills

• /sim-review — Review existing code for DST compatibility
• /sim-run — Execute simulation tests
• /test-strategy — Choose between unit/property/fuzz/simulation testing
• /dist-sys-auditor — Validate distributed systems design decisions