How do you use volatile, atomic, and synchronized keywords to control memory visibility in Kotlin?

1 What is memory visibility?

Memory visibility is the property that determines when a thread can see the changes made by another thread to a shared variable. In Kotlin, each thread has its own local memory, where it caches the values of shared variables for faster access. However, this means that the local memory might not reflect the actual state of the shared variable in the main memory, where all threads can access it. To guarantee memory visibility, you need to use mechanisms that force the threads to synchronize their local memory with the main memory.

2 What is volatile?

Volatile is a keyword that you can use to mark a variable as volatile. This means that the variable will always be read from and written to the main memory, bypassing the local memory of the threads. This ensures that all threads see the same and latest value of the volatile variable. However, volatile does not guarantee atomicity, which means that the variable might still be modified by multiple threads at the same time, leading to race conditions. For example, if two threads increment a volatile counter, they might both read the same value, add one to it, and write it back, resulting in a lost update.

3 What is atomic?

Atomic is a concept that refers to operations that are indivisible and cannot be interrupted by other threads. Atomic operations guarantee both memory visibility and atomicity, which means that the threads will see the latest and correct value of the shared variable, and that only one thread can modify it at a time. In Kotlin, you can use atomic classes from the kotlinx.atomicfu library to create and manipulate atomic variables. For example, you can use an atomic integer to increment a counter atomically, without using volatile or synchronized.

4 What is synchronized?

Synchronized is a keyword that you can use to create a synchronized block or a synchronized function. A synchronized block or function can only be executed by one thread at a time, and it locks the object that it is synchronized on. This means that other threads that want to access the same object have to wait until the lock is released. This ensures that the shared state of the object is consistent and visible across the threads. However, synchronized also introduces some overhead and complexity, as you have to manage the locks and avoid deadlocks.

5 How to choose the right mechanism?

When deciding which mechanism to use for controlling memory visibility, you should consider your use case and performance requirements. Generally, volatile should be used when you only need to read and write a single variable, and don't require atomicity. Atomic should be used when you need to perform simple operations on a single variable, and want to avoid locks and synchronization. Synchronized is best when multiple variables or complex objects need to be accessed or modified, and consistency and integrity must be ensured.

6 How to test memory visibility?

Testing memory visibility can be done with a variety of tools and techniques, such as logging and debugging, unit testing, and stress testing. Logging statements or breakpoints can be used to inspect the values of shared variables and track the execution flow of different threads. Unit testing frameworks like JUnit or TestNG can simulate multithreaded scenarios and assert the expected outcomes. Additionally, stress testing tools like JMeter or Gatling can generate high load and concurrency to measure the performance and reliability of your application.

7 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?