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Let’s say you work as a company secretary and your job is to take the phone calls, manage meetings etc. Every time the phone rings, you have to stop whatever you are doing. A property of a program that allows tasks to run in overlapping time periods.
- Parallelism, allows 2 or more tasks to run at the same time (Only if the machine has the multiprocessing capability). You asked your boss and he hired a clerk for taking the call. It looks very simple but concurrency introduces a few problems like the Race condition,
- Race Condition, Suppose you have $500 in your bank account, at the same time when you withdraw $200 someone transferred $300 or maybe in simple words same time 2 transaction happened for withdrawing $500.
- Mutual Exclusion (Mutex), Solution for the Race condition. Now, whenever some transaction starts it locks the account. if some transaction is going on with your account you cannot withdraw. But next problem is nobody wants to wait every time when there’s is something going on. Semaphore is the modified solution for Mutex.
- Semaphore (Improper use of Semaphore will give improper results)
- Binary Semaphore (Range is from 0 to 1), Idea is to give specific priority to different types of transaction. Withdraw request has a higher priority than bank transfer. So when you withdraw, another transaction for transfer will stop and it will resume once withdraw is completed. (As simple as that, 1 = ongoing transaction, 0 = waiting) Also known as integer semaphore.
- Counting Semaphore (Range is from -∞ to +∞), Allow more than one process running at the same time. Let’s suppose you’re a key room manager where you have 30 keys if lockers are full users have to wait in the queue. when someone has done they will hand over the key to the first person in the queue.
- Deadlock, Another common issue in concurrency modal. Let’s assume user a transferred amount to b and at the same time user b transfer the amount to the user a. both transactions are waiting to complete another one as they can’t access the locked account.