NonFIFO

NonFIFO is a term used to describe systems, policies, or data handling that do not guarantee First-In-First-Out ordering. It is often contrasted with FIFO, where items are processed or delivered in the exact order they arrive.

In computing and data structures, nonFIFO queues may allow processing to be influenced by priorities, parallelism, or multiple producers and consumers. Service order can be non-deterministic, with some items advanced ahead of others or reordered after arrival. Some systems offer configurable semantics, allowing a mix of ordered and unordered delivery depending on workload or quality-of-service requirements.

In networking and messaging, nonFIFO semantics mean messages may be delivered out of the original arrival order. This can improve throughput or reduce latency in high-concurrency environments. To recover the correct sequence, protocols or applications may attach sequence numbers and perform reordering at the receiver. Many message brokers support both FIFO and non-FIFO delivery modes, enabling choices based on use case.

In inventory management and accounting, nonFIFO typically refers to valuation methods that do not assume items sold first are the oldest. The common alternatives are LIFO (last-in, first-out) and weighted-average cost. The choice of a non-FIFO method affects cost of goods sold, reported gross margins, and ending inventory value, and it is subject to regulatory rules that vary by jurisdiction and standard-setting bodies.

Overall, nonFIFO describes a broad class of mechanisms where strict arrival-order processing is not guaranteed, often trading strict ordering for performance, flexibility, or policy reasons. See also FIFO, LIFO, and priority-based scheduling.