Concurrency Control
What is Concurrency Control?
Concurrency control is a major part of database management systems, operating systems, and other computational systems that manage multiple processes, threads, or transactions simultaneously. Its primary goal is to manage shared resources and data to prevent conflicts and ensure the system remains correct and consistent.
Locking is a fundamental mechanism in concurrency control. A lock is a variable associated with a resource (like a database record or a file) that indicates its current state, such as whether it’s in use or available. When a process needs to use a locked resource, it must first acquire the lock. If another process already holds the lock, the new process must wait. Once the process is done with the resource, it releases the lock, making it available for others.
Types of Locks
Common types of locks include:
- Binary Locks: A simple lock that can be in one of two states: locked or unlocked.
- Shared/Exclusive (S/X) Locks:
- Shared (S) locks, also known as read locks, allow multiple processes to read the same resource concurrently.
- Exclusive (X) locks, also known as write locks, grant a process exclusive access, preventing other processes from reading or writing to the resource.
Challenges and Problems
While locking is essential, it can lead to several problems if not managed correctly:
- Deadlock: A situation where two or more processes are blocked indefinitely, each waiting for the other to release a resource. For example, process A has a lock on resource X and needs a lock on resource Y, while process B has a lock on resource Y and needs a lock on resource X. Neither can proceed.
- Starvation: A situation where a process is repeatedly denied access to a shared resource, even though the resource becomes available, because other processes are always given priority.
- Livelock: A situation where two or more processes repeatedly change their state in response to the state of the other processes without making any progress.
What is Locking in Computer Science?
In computer science, a lock 🔒 (also known as a mutex, short for mutual exclusion) is a synchronization primitive used to enforce a concurrency control policy. Its primary purpose is to prevent race conditions by ensuring that only one thread of execution can access a shared resource at a time. Locks are essential in multi-threaded and multi-process environments where multiple entities might try to modify the same data simultaneously, which could lead to data corruption or inconsistencies.
How Locks Work
A lock generally has two states: locked and unlocked. The basic process for using a lock is as follows:
- A thread that needs to access a shared resource (like a variable, file, or database record) first attempts to acquire the lock.
- If the lock is unlocked, the thread successfully acquires it, and the lock’s state becomes locked. The thread then enters a critical section, which is the code segment where the shared resource is accessed and modified.
- If the lock is already locked by another thread, the requesting thread must wait until the lock is released. This waiting can involve either blocking (the thread is suspended by the operating system) or spinning (the thread repeatedly checks if the lock is available).
- Once the thread is done with the critical section, it must release the lock, changing its state back to unlocked. This allows other waiting threads to acquire the lock and proceed.
Types of Locks
There are various types of locks, each suited for different use cases and programming paradigms. Here are some of the most common ones:
- Binary Semaphore (Mutex): A simple lock that provides exclusive access. It can be acquired by only one thread at a time. The thread holding the mutex has exclusive access to the shared resource.
- Spinlock: A lock where the waiting thread repeatedly checks for the lock’s availability in a tight loop instead of being put to sleep. This is efficient for very short critical sections, as it avoids the overhead of context switching. However, it wastes CPU cycles if the waiting period is long.
- Semaphore: A more general synchronization primitive that can be used as a lock. A semaphore maintains a counter and can allow a specified number of threads to access a resource simultaneously. A binary semaphore is just a semaphore with a counter of 1.
- Reader-Writer Lock: This type of lock allows for greater concurrency. It distinguishes between read and write operations. Multiple threads can hold a “read lock” simultaneously to read the shared resource, but only one thread can hold a “write lock” at a time. A write lock prevents any other thread from acquiring either a read or a write lock. This is useful for resources that are read far more often than they are written to.
- Deadlock: A common problem that can arise when using locks. A deadlock occurs when two or more threads are waiting for each other to release a lock, creating a circular dependency where none of them can proceed. For example, Thread A holds a lock on Resource 1 and needs a lock on Resource 2, but Thread B holds the lock on Resource 2 and needs the lock on Resource 1. Both threads are stuck waiting indefinitely.