Read-Write Lock

If you only lock during write operations, it can partially ensure synchronization, but in some cases, this might not be enough to guarantee thread safety, especially when read and write operations are interdependent.

Read-Write Race Conditions

If you only lock during write operations but allow multiple threads to perform read operations simultaneously without locking, the following issues can arise:

Dirty Reads: One thread might read data while another thread is writing to it, resulting in reading incomplete or outdated data.
Non-Atomic Operations: Even read operations, if dependent on consistent data (e.g., reading a value and then performing some operation based on it), could be problematic if the data is modified by another thread during the read.

Example

Consider a scenario where multiple threads share a global variable counter. Some threads read it, while others modify it. If you lock only during writes and not during reads, you might get inconsistent behavior:

import threading

counter = 0
lock = threading.Lock()

def increment():
    global counter
    with lock:
        counter += 1  # Lock only during write operations

def read_counter():
    print(counter)  # No locking for reads

threads = []

# Create multiple read threads
for _ in range(5):
    t = threading.Thread(target=read_counter)
    threads.append(t)
    t.start()

# Create multiple write threads
for _ in range(5):
    t = threading.Thread(target=increment)
    threads.append(t)
    t.start()

for t in threads:
    t.join()

In this example, since no lock is used for reading, the following issues may occur:

Reading stale data: A read thread might access counter before a write thread updates it.
Inconsistent reads: Multiple read threads could access counter at different times, possibly reading the value in between a write operation, leading to inconsistency.

Locking for Writes Only VS Read-Write Lock

If you’re certain that read operations don’t depend on the result of write operations and won’t be affected by ongoing writes, locking only during writes can be sufficient. However, if read operations and write operations are interdependent (e.g., when logic relies on the current value), locking only for writes won’t ensure consistency.

A more robust solution is using a read-write lock, which allows multiple threads to read simultaneously, but blocks all reads and writes while a thread is writing. Python’s threading library doesn’t provide a built-in read-write lock, but you can implement it yourself or use a third-party library.

Sample Read-Write Lock Implementation

A simple read-write lock allows:

Multiple threads to read at the same time.
Only one thread to write, and no other threads (readers or writers) can operate during a write.

Here’s a basic implementation:

import threading

class ReadWriteLock:
    def __init__(self):
        self.readers = 0
        self.lock = threading.Lock()
        self.read_lock = threading.Lock()

    def acquire_read(self):
        with self.lock:
            self.readers += 1
            if self.readers == 1:
                self.read_lock.acquire()  # First reader locks, blocking writes

    def release_read(self):
        with self.lock:
            self.readers -= 1
            if self.readers == 0:
                self.read_lock.release()  # Last reader releases lock

    def acquire_write(self):
        self.read_lock.acquire()  # Blocks both readers and writers

    def release_write(self):
        self.read_lock.release()

# Usage example
rw_lock = ReadWriteLock()

# Read operation
def read_data():
    rw_lock.acquire_read()
    try:
        # Perform read operation
        print("Reading data")
    finally:
        rw_lock.release_read()

# Write operation
def write_data():
    rw_lock.acquire_write()
    try:
        # Perform write operation
        print("Writing data")
    finally:
        rw_lock.release_write()

This mechanism allows multiple read threads to run concurrently while ensuring that when a write operation is happening, no other reads or writes can occur.

Summary

Locking only during write operations is suitable for simple cases where read and write operations do not interfere with each other.
If there’s interdependency between read and write operations, locking only for writes is insufficient, and you’ll need to lock reads as well.
A read-write lock is a more advanced synchronization mechanism that allows concurrent reads but locks everything during a write.