Firstcomefirstserved

FirstComeFirstServed, often abbreviated FCFS, is a queueing principle by which service is provided to customers in the order of their arrival. It assumes that arriving items or people join a single line and are attended to sequentially, with no reordering unless a tie occurs. The principle is used in a wide range of contexts, from retail and hospitality to computing and networking.

In service settings, FCFS aims to be fair by treating customers equally based on arrival time; it is common in bank tellers, ticket counters, call centers, and online support queues. In computing, FCFS appears as first-come, first-served scheduling, a simple non-preemptive CPU scheduling algorithm where processes are executed in the order they arrive. It is easy to implement and predictable, but can lead to long waiting times if early arrivals require lengthy service and can suffer from the convoy effect, where a long job delays many shorter ones.

In networking and data processing, FCFS queue disciplines are used in some routers and messaging systems; head-of-line blocking and sensitivity to traffic burstiness can degrade performance. Many implementations augment FCFS with priorities or additional scheduling rules to improve fairness and throughput. Variants include priority FCFS, where higher-priority tasks are served first within their class, and hybrid schemes that combine FCFS with other strategies.

FCFS is often discussed alongside other queueing disciplines such as last-come-first-served, round-robin, and fair queuing in theoretical models and practical deployments.