It’s also part of storage : the journal is a log that helps the database recover in the event of a hard shutdown. There are several configurable options that allows the journal to strike a balance between performance and reliability that works for your particular use case.
In simple Words:
With journaling, MongoDB’s storage layer has two internal views of the data set:
Private view, used to write to the journal files, and
Shared view, used to write to the data files: MongoDB first applies write operations to the private view.
In this process, a write operation occurs in mongod, which then creates changes in private view.
The first block is memory and the second block is ‘my disc’. After a specified interval, which is called a ‘journal commit interval’, the private view writes those operations in journal directory (residing in the disc).
Once the journal commit happens, mongod pushes data into shared view. As part of the process, it gets written to actual data directory from the shared view (as this process happens in background). The basic advantage is,we have a reduced cycle from 60 seconds to 200 milliseconds.
If you want to change the value of Interval commng and flush shared view follow below the link
For More details about Journaling: Click Here
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