RwLock (Reader-Writer Lock)¶

An RwLock protects data and allows either: - Multiple readers at the same time, OR - One writer with exclusive access

This is great when you have data that's read often but rarely written.

Basic Usage¶

import sync

# Create an RwLock protecting a value
let rw = sync.RwLock(42)

# Read access (multiple readers allowed)
using reader = rw.read():
    print(reader.value)  # 42

# Write access (exclusive)
using writer = rw.write():
    print(writer.value)  # Exclusive access

When to Use RwLock¶

Use RwLock when: - Reads are frequent, writes are rare - Readers don't block each other - Data integrity matters - Writers get exclusive access - Performance critical - Better than Mutex for read-heavy workloads

Methods¶

Method	Returns	What it does
`read()`	`ReadGuard<T>`	Get shared read access (blocks if writer active)
`write()`	`WriteGuard<T>`	Get exclusive write access (blocks if any access active)
`try_read()`	`Option<ReadGuard<T>>`	Try to read without blocking
`try_write()`	`Option<WriteGuard<T>>`	Try to write without blocking

The using Guard Pattern¶

Always use using with guards for automatic unlock:

# ✅ Correct - lock released automatically
using reader = rw.read():
    print(reader.value)
# Lock released here

# ⚠️ Warning - manual unlock needed
let reader = rw.read()  # DSY0004 warning
print(reader.value)
# Lock NOT released!

Non-Blocking with try Methods¶

import sync

let rw = sync.RwLock(42)

match rw.try_read():
    Option.Some(guard): print(guard.value)
    Option.Nothing: print("Couldn't get read lock")

Works with Any Type¶

import sync

# Primitives
let rw_int = sync.RwLock(42)
let rw_str = sync.RwLock("hello")
let rw_float = sync.RwLock(3.14)
let rw_bool = sync.RwLock(true)

# Structs
struct Point:
    x: int
    y: int

let rw_point = sync.RwLock(Point(x=10, y=20))

using reader = rw_point.read():
    print(reader.value.x)  # 10

RwLock vs Mutex¶

Feature	RwLock	Mutex
Readers	Multiple concurrent	One at a time
Writers	Exclusive	Exclusive
Best for	Read-heavy data	General protection
Guards	ReadGuard/WriteGuard	MutexGuard

Diagnostics¶

Code	Severity	Message
`DSY0004`	Warning	ReadGuard without `using`
`DSY0005`	Warning	WriteGuard without `using`

Common Patterns¶

Configuration Data¶

# Config is read often, rarely updated
let config = sync.RwLock(load_config())

# Many threads can read simultaneously
using reader = config.read():
    print(reader.value.setting)

Cache with Occasional Updates¶

let cache = sync.RwLock(initial_cache())

# Reading (frequent)
using reader = cache.read():
    if reader.value.has_key("data"):
        return reader.value.get("data")

# Writing (rare)
using writer = cache.write():
    # Exclusive access for update
    pass