arangodb/arangod/MMFiles

MMFilesEngine

How operations are stored - Overview

All operations like insert or remove are written as markers to a write ahead log (WAL). This WAL consists of files of a certain size and if such a file is full (or is manually flushed), all relevant markers are transferred (transferMarkers()) to the journals of the respective collections. During the transfer any obsolete markers will be thrown away: a sequence of insert, remove, insert on the same document will result in the last insert discarding the previous operations. When a journal file of size (journalSize()) is full, it will be sealed and renamed. By applying these operations it will become a datafile that is read-only. Datafiles will eventually be merged by a compactor thread that does about the same work as the transferMarkers function, reducing the size of the stored data.

Ditches

Ditches are used to pin objects in WAL or journal as long as they are used by other operations.