forwardonly

Forwardonly, often written as forward-only, is a term used in computing to describe a mode or constraint where data flow or processing proceeds strictly in a single forward direction. In a forward-only system, consumers or processors advance their position through a data stream in arrival order and are not allowed to move backward to reprocess or verify earlier items unless a separate mechanism is provided. This limitation is common in log-structured storage, streaming pipelines, and certain data replication schemes.

Applications and contexts

- Event streams and log-structured storage: a forward-only consumer advances a pointer or offset and cannot rewind to reread past entries. Reprocessing typically requires restarting from a saved offset or restoring from a snapshot.

- Change data capture and replication: a forward-only replication stream applies changes sequentially, preserving order without reverse application. Recovery and resynchronization rely on logs, snapshots, or retained history.

- Database cursors and file readers: forward-only cursors read records sequentially and avoid in-memory buffering for random access, reducing resource usage.

- Streaming analytics: some engines operate in forward-only mode to maximize throughput and minimize latency.

Design considerations

- Checkpointing and offsets record progress to enable later resumption, but data loss is possible if a failure occurs before the next checkpoint.

- Advantages include simplicity, lower memory footprint, and higher throughput in append-only workloads.

- Disadvantages include limited reprocessing capabilities and greater complexity for error handling and data corrections, since replay or backfill typically requires separate mechanisms (e.g., logs, backups, or snapshots).

See also: cursors, offsets, change data capture, streaming, replication.