ReplaySubject

ReplaySubject is a type of subject in reactive programming (notably in Rx/RxJS) that stores a buffer of previously emitted values and replays them to new subscribers. Unlike a standard Subject, which only delivers new values to listeners from the moment they subscribe, ReplaySubject can provide a history of past events to anyone who subscribes later.

Buffering behavior is configurable. The subject can be created with a bufferSize, which limits how many past values are kept, and with a windowTime, which limits values to those emitted within a certain time frame. When a new subscriber joins, ReplaySubject immediately emits the buffered values in their original order, and then continues to emit any new values as they arrive. If both bufferSize and windowTime are specified, the buffer contains up to bufferSize values within the time window. Some implementations also allow an optional scheduler to control the timing of emissions.

Terminal events are handled in a predictable way. If the ReplaySubject has already completed or errored, new subscribers will receive the buffered values first, followed by the terminal notification (completion or error). This makes ReplaySubject suitable for scenarios where late subscribers need a view of recent history and the overall lifecycle needs to be preserved.

Common use cases include caching the most recent events for late subscribers, sharing a historical log in real-time streams, and implementing features like “last value” or “recent activity” in user interfaces. However, unbounded buffering can consume memory, so practical use often involves setting a finite bufferSize and/or a time window.