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MMFilesEngine
How operations are Stored - Overview
All operations like insert or delete are written as markers to a write ahead log (WAL). This WAL consists of slices of a certain size and if such a slice is full for is manually flushed all relevant markers are transferred (transferMarkers()) to the journals of the respective collections. During the transfer 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 this operations it will become a datafile, that is read only. Datafiles will eventually be merged by a compactor task/thread, that does the 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.