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Compare-and-set (CAS)

Compare-and-set (CAS) is an atomic instruction used in concurrency control to synchronize access to shared data, such as reference counts in memory management.^[600-developer__big-data__netty__netty-rotate.md]

The operation checks if a variable currently holds an expected value, and if so, updates it to a new value; otherwise, it leaves the variable unchanged.^[600-developer__big-data__netty__netty-rotate.md] This process is typically implemented within a retry loop (Spinlock), ensuring that the update only occurs if the data has not been modified by another thread since it was read.^[600-developer__big-data__netty__netty-rotate.md]

Implementation Example

In frameworks like Netty, CAS is often used to manage the lifecycle of resources (like buffers) by atomically updating an integer reference count (refCnt).^[600-developer__big-data__netty__netty-rotate.md] For instance, AtomicIntegerFieldUpdater.compareAndSet is used to ensure that increments or decrements to the counter are applied safely without race conditions.^[600-developer__big-data__netty__netty-rotate.md]

  • [[Spinlocks]]
  • [[Atomic operations]]
  • [[Concurrency control]]

Sources

  • 600-developer__big-data__netty__netty-rotate.md