FirstInFirstOut

First-in, first-out (FIFO) is a method in which items are processed in the order they arrive: the oldest inputs are removed or served before newer ones.

In computer science, a queue implements FIFO: elements are enqueued at the rear and dequeued from the front, ensuring that the order of arrival is preserved. This simple discipline is widely used for managing streams of tasks, IO requests, messages, and events.

In inventory management and accounting, FIFO is a cost-flow assumption that the oldest inventory is sold first. This affects cost of goods sold, ending inventory values, and taxes, and it can influence reported profitability as prices fluctuate over time.

Other uses include memory management (FIFO page replacement), network buffers and printers, and certain task scheduling scenarios in operating systems and applications. The approach is valued for its simplicity and predictable behavior, which can make system performance easier to reason about.

Advantages of FIFO include straightforward implementation, fairness in serving items in arrival order, and clear traceability of processing history. Limitations include potentially suboptimal average waiting times, lack of priority handling, and possible distortion of profitability measurements in accounting when prices move significantly.

Related concepts include FCFS (first-come, first-served) in operating systems, which is closely aligned with FIFO ordering, and various data structure variants such as circular queues that maintain FIFO behavior under wraparound conditions. FIFO can be contrasted with LIFO (last-in, first-out) and with priority-based scheduling approaches.