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At its core, Hudi maintains a timeline of all def~instant-action performed on the def~table at different instants of time that helps provide instantaneous views of the def~table, while also efficiently supporting retrieval of data in the order of arrival. A Hudi `timeline instant` consists of the following components

• def~instant-action: Type of action performed on the def~table
• def~instant-time: typically a timestamp (e.g: 20190117010349), which monotonically increases in the order of action’s begin time.
• instant state: current state of the 135861679

in which it was written. The timeline is akin to a redo/transaction log, found in databases, and consists of a set of def~timeline-instants. Hudi guarantees that the actions performed on the timeline are atomic & timeline consistent based on the instant time.

Timeline is implemented as a set of files under the `.hoodie` def~metadata-folder directly under the def~table-basepath. Specifically, while the most recent instants are maintained as individual files, the older instants are archived to the def~timeline-archival folder, to bound the number of files, listed by writers and queries.