Fanout

Fanout is a term used in electronics and computing to denote the number of destinations driven by a single source.

In digital logic, the fan-out of a gate is the number of inputs that its output can reliably drive. It is roughly the ratio of the gate's output current capability to the input current required by each driven gate. For TTL gates, typical fan-outs range from 5 to 10; CMOS devices usually support higher static fan-out because inputs draw negligible DC current, but dynamic loading—measured as total input capacitance—limits practical fan-out by increasing propagation delay and reducing noise margins. Exceeding the specified fan-out degrades rise and fall times, can shift switching thresholds, and increases the risk of errors under adverse conditions.

In software and data systems, fan-out describes how many downstream recipients receive a single event or message. This appears in pub/sub brokers, event streams, or fan-out writes that replicate data to multiple stores. While fan-out can improve parallelism and throughput, it adds latency, raises the chance of message duplication or inconsistency, and complicates error handling. Designers address this with patterns such as idempotent handlers, back-pressure, and careful ordering guarantees.

Overall, fan-out is a general measure of distribution from one source to many sinks; its appropriate value depends on technology, timing requirements, and reliability needs.