494 lines
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<main>
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<h1 id="adr-026-arc-based-shared-state-management"><a class="header" href="#adr-026-arc-based-shared-state-management">ADR-026: Arc-Based Shared State Management</a></h1>
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<p><strong>Status</strong>: Accepted | Implemented
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<strong>Date</strong>: 2024-11-01
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<strong>Deciders</strong>: Backend Architecture Team
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<strong>Technical Story</strong>: Managing thread-safe shared state across async Tokio handlers</p>
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<hr />
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<h2 id="decision"><a class="header" href="#decision">Decision</a></h2>
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<p>Implementar <strong>Arc-wrapped shared state</strong> con <code>RwLock</code> (read-heavy) y <code>Mutex</code> (write-heavy) para coordinación inter-handler.</p>
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<hr />
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<h2 id="rationale"><a class="header" href="#rationale">Rationale</a></h2>
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<ol>
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<li><strong>Cheap Clones</strong>: <code>Arc</code> enables sharing without duplication</li>
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<li><strong>Thread-Safe</strong>: <code>RwLock</code>/<code>Mutex</code> provide safe concurrent access</li>
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<li><strong>Async-Native</strong>: Works with Tokio async/await</li>
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<li><strong>Handler Distribution</strong>: Each handler gets Arc clone (scales across threads)</li>
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</ol>
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<hr />
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<h2 id="alternatives-considered"><a class="header" href="#alternatives-considered">Alternatives Considered</a></h2>
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<h3 id="-direct-shared-references"><a class="header" href="#-direct-shared-references">❌ Direct Shared References</a></h3>
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<ul>
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<li><strong>Pros</strong>: Simple</li>
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<li><strong>Cons</strong>: Borrow checker issues in async, unsafe</li>
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</ul>
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<h3 id="-message-passing-only-channels"><a class="header" href="#-message-passing-only-channels">❌ Message Passing Only (Channels)</a></h3>
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<ul>
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<li><strong>Pros</strong>: Avoids shared state</li>
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<li><strong>Cons</strong>: Overkill for read-heavy state, latency</li>
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</ul>
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<h3 id="-arcrwlock--arcmutex-chosen"><a class="header" href="#-arcrwlock--arcmutex-chosen">✅ Arc<RwLock<>> / Arc<Mutex<>> (CHOSEN)</a></h3>
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<ul>
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<li>Right balance of simplicity and safety</li>
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</ul>
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<hr />
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<h2 id="trade-offs"><a class="header" href="#trade-offs">Trade-offs</a></h2>
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<p><strong>Pros</strong>:</p>
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<ul>
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<li>✅ Cheap clones via Arc</li>
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<li>✅ Type-safe via Rust borrow checker</li>
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<li>✅ Works seamlessly with async/await</li>
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<li>✅ RwLock for read-heavy workloads (multiple readers)</li>
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<li>✅ Mutex for write-heavy/simple cases</li>
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</ul>
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<p><strong>Cons</strong>:</p>
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<ul>
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<li>⚠️ Lock contention possible under high concurrency</li>
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<li>⚠️ Deadlock risk if not careful (nested locks)</li>
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<li>⚠️ Poisoned lock handling needed</li>
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</ul>
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<hr />
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<h2 id="implementation"><a class="header" href="#implementation">Implementation</a></h2>
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<p><strong>Shared State Definition</strong>:</p>
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<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span>// crates/vapora-backend/src/api/state.rs
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pub struct AppState {
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pub project_service: Arc<ProjectService>,
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pub task_service: Arc<TaskService>,
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pub agent_service: Arc<AgentService>,
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// Shared mutable state
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pub task_queue: Arc<Mutex<Vec<Task>>>,
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pub agent_registry: Arc<RwLock<HashMap<String, AgentState>>>,
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pub metrics: Arc<RwLock<Metrics>>,
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}
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impl AppState {
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pub fn new(
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project_service: ProjectService,
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task_service: TaskService,
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agent_service: AgentService,
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) -> Self {
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Self {
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project_service: Arc::new(project_service),
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task_service: Arc::new(task_service),
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agent_service: Arc::new(agent_service),
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task_queue: Arc::new(Mutex::new(Vec::new())),
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agent_registry: Arc::new(RwLock::new(HashMap::new())),
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metrics: Arc::new(RwLock::new(Metrics::default())),
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}
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}
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}
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<span class="boring">}</span></code></pre></pre>
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<p><strong>Using Arc in Handlers</strong>:</p>
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<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span>// Handlers receive State which is Arc already
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pub async fn create_task(
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State(app_state): State<AppState>, // AppState is Arc<AppState>
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Json(req): Json<CreateTaskRequest>,
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) -> Result<Json<Task>, ApiError> {
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let task = app_state
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.task_service
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.create_task(&req)
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.await?;
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// Push to shared queue
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let mut queue = app_state.task_queue.lock().await;
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queue.push(task.clone());
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Ok(Json(task))
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}
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<span class="boring">}</span></code></pre></pre>
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<p><strong>RwLock Pattern (Read-Heavy)</strong>:</p>
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<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span>// crates/vapora-backend/src/swarm/registry.rs
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pub async fn get_agent_status(
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app_state: &AppState,
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agent_id: &str,
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) -> Result<AgentStatus> {
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// Multiple concurrent readers can hold read lock
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let registry = app_state.agent_registry.read().await;
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let agent = registry
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.get(agent_id)
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.ok_or(VaporaError::NotFound)?;
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Ok(agent.status)
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}
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pub async fn update_agent_status(
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app_state: &AppState,
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agent_id: &str,
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new_status: AgentStatus,
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) -> Result<()> {
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// Exclusive write lock
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let mut registry = app_state.agent_registry.write().await;
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if let Some(agent) = registry.get_mut(agent_id) {
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agent.status = new_status;
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Ok(())
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} else {
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Err(VaporaError::NotFound)
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}
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}
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<span class="boring">}</span></code></pre></pre>
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<p><strong>Mutex Pattern (Write-Heavy)</strong>:</p>
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<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
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||
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</span><span class="boring">fn main() {
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</span>// crates/vapora-backend/src/api/task_queue.rs
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pub async fn dequeue_task(
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app_state: &AppState,
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|
) -> Option<Task> {
|
||
|
|
let mut queue = app_state.task_queue.lock().await;
|
||
|
|
queue.pop()
|
||
|
|
}
|
||
|
|
|
||
|
|
pub async fn enqueue_task(
|
||
|
|
app_state: &AppState,
|
||
|
|
task: Task,
|
||
|
|
) {
|
||
|
|
let mut queue = app_state.task_queue.lock().await;
|
||
|
|
queue.push(task);
|
||
|
|
}
|
||
|
|
<span class="boring">}</span></code></pre></pre>
|
||
|
|
<p><strong>Avoiding Deadlocks</strong>:</p>
|
||
|
|
<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
|
||
|
|
</span><span class="boring">fn main() {
|
||
|
|
</span>// ✅ GOOD: Single lock acquisition
|
||
|
|
pub async fn safe_operation(app_state: &AppState) {
|
||
|
|
let mut registry = app_state.agent_registry.write().await;
|
||
|
|
// Do work
|
||
|
|
// Lock automatically released when dropped
|
||
|
|
}
|
||
|
|
|
||
|
|
// ❌ BAD: Nested locks (can deadlock)
|
||
|
|
pub async fn unsafe_operation(app_state: &AppState) {
|
||
|
|
let mut registry = app_state.agent_registry.write().await;
|
||
|
|
let mut queue = app_state.task_queue.lock().await; // Risk: lock order inversion
|
||
|
|
// If another task acquires locks in opposite order, deadlock!
|
||
|
|
}
|
||
|
|
|
||
|
|
// ✅ GOOD: Consistent lock order prevents deadlocks
|
||
|
|
// Always acquire: agent_registry → task_queue
|
||
|
|
pub async fn safe_nested(app_state: &AppState) {
|
||
|
|
let mut registry = app_state.agent_registry.write().await;
|
||
|
|
let mut queue = app_state.task_queue.lock().await; // Same order everywhere
|
||
|
|
// Safe from deadlock
|
||
|
|
}
|
||
|
|
<span class="boring">}</span></code></pre></pre>
|
||
|
|
<p><strong>Poisoned Lock Handling</strong>:</p>
|
||
|
|
<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
|
||
|
|
</span><span class="boring">fn main() {
|
||
|
|
</span>pub async fn handle_poisoned_lock(
|
||
|
|
app_state: &AppState,
|
||
|
|
) -> Result<Vec<Task>> {
|
||
|
|
match app_state.task_queue.lock().await {
|
||
|
|
Ok(queue) => Ok(queue.clone()),
|
||
|
|
Err(poisoned) => {
|
||
|
|
// Lock was poisoned (panic inside lock)
|
||
|
|
// Recover by using inner value
|
||
|
|
let queue = poisoned.into_inner();
|
||
|
|
Ok(queue.clone())
|
||
|
|
}
|
||
|
|
}
|
||
|
|
}
|
||
|
|
<span class="boring">}</span></code></pre></pre>
|
||
|
|
<p><strong>Key Files</strong>:</p>
|
||
|
|
<ul>
|
||
|
|
<li><code>/crates/vapora-backend/src/api/state.rs</code> (state definition)</li>
|
||
|
|
<li><code>/crates/vapora-backend/src/main.rs</code> (state creation)</li>
|
||
|
|
<li><code>/crates/vapora-backend/src/api/</code> (handlers using Arc)</li>
|
||
|
|
</ul>
|
||
|
|
<hr />
|
||
|
|
<h2 id="verification"><a class="header" href="#verification">Verification</a></h2>
|
||
|
|
<pre><code class="language-bash"># Test concurrent access to shared state
|
||
|
|
cargo test -p vapora-backend test_concurrent_state_access
|
||
|
|
|
||
|
|
# Test RwLock read-heavy performance
|
||
|
|
cargo test -p vapora-backend test_rwlock_concurrent_reads
|
||
|
|
|
||
|
|
# Test Mutex write-heavy correctness
|
||
|
|
cargo test -p vapora-backend test_mutex_exclusive_writes
|
||
|
|
|
||
|
|
# Integration: multiple handlers accessing shared state
|
||
|
|
cargo test -p vapora-backend test_shared_state_integration
|
||
|
|
|
||
|
|
# Stress test: high concurrency
|
||
|
|
cargo test -p vapora-backend test_shared_state_stress
|
||
|
|
</code></pre>
|
||
|
|
<p><strong>Expected Output</strong>:</p>
|
||
|
|
<ul>
|
||
|
|
<li>Concurrent reads successful (RwLock)</li>
|
||
|
|
<li>Exclusive writes correct (Mutex)</li>
|
||
|
|
<li>No data races (Rust guarantees)</li>
|
||
|
|
<li>Deadlock-free (consistent lock ordering)</li>
|
||
|
|
<li>High throughput under load</li>
|
||
|
|
</ul>
|
||
|
|
<hr />
|
||
|
|
<h2 id="consequences"><a class="header" href="#consequences">Consequences</a></h2>
|
||
|
|
<h3 id="performance"><a class="header" href="#performance">Performance</a></h3>
|
||
|
|
<ul>
|
||
|
|
<li>Read locks: low contention (multiple readers)</li>
|
||
|
|
<li>Write locks: exclusive (single writer)</li>
|
||
|
|
<li>Mutex: simple but may serialize</li>
|
||
|
|
</ul>
|
||
|
|
<h3 id="concurrency-model"><a class="header" href="#concurrency-model">Concurrency Model</a></h3>
|
||
|
|
<ul>
|
||
|
|
<li>Handlers clone Arc (cheap, ~8 bytes)</li>
|
||
|
|
<li>Multiple threads access same data</li>
|
||
|
|
<li>Lock guards released when dropped</li>
|
||
|
|
</ul>
|
||
|
|
<h3 id="debugging"><a class="header" href="#debugging">Debugging</a></h3>
|
||
|
|
<ul>
|
||
|
|
<li>Data races impossible (Rust compiler)</li>
|
||
|
|
<li>Deadlocks prevented by discipline</li>
|
||
|
|
<li>Poisoned locks rare (panic handling)</li>
|
||
|
|
</ul>
|
||
|
|
<h3 id="scaling"><a class="header" href="#scaling">Scaling</a></h3>
|
||
|
|
<ul>
|
||
|
|
<li>Per-core scalability excellent (read-heavy)</li>
|
||
|
|
<li>Write contention bottleneck (if heavy)</li>
|
||
|
|
<li>Sharding option for write-heavy</li>
|
||
|
|
</ul>
|
||
|
|
<hr />
|
||
|
|
<h2 id="references"><a class="header" href="#references">References</a></h2>
|
||
|
|
<ul>
|
||
|
|
<li><a href="https://doc.rust-lang.org/std/sync/struct.Arc.html">Arc Documentation</a></li>
|
||
|
|
<li><a href="https://docs.rs/tokio/latest/tokio/sync/struct.RwLock.html">RwLock Documentation</a></li>
|
||
|
|
<li><a href="https://docs.rs/tokio/latest/tokio/sync/struct.Mutex.html">Mutex Documentation</a></li>
|
||
|
|
<li><code>/crates/vapora-backend/src/api/state.rs</code> (implementation)</li>
|
||
|
|
</ul>
|
||
|
|
<hr />
|
||
|
|
<p><strong>Related ADRs</strong>: ADR-008 (Tokio Runtime), ADR-024 (Service Architecture)</p>
|
||
|
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|
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|
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