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What are the requirements for synchronization in MongoDB?
The sync source must have newer oplog entries than the member (i.e. the sync source is ahead of the member). The sync source must be visible. The sync source must be within 30 seconds of the newest oplog entry on the primary. If the member builds indexes, the sync source must build indexes.
How does the return array work in MongoDB?
New in version 3.4.4. Converts a document to an array. The return array contains an element for each field/value pair in the original document. Each element in the return array is a document that contains two fields k and v: The k field contains the field name in the original document.
How often does a secondary MongoDB replicate data?
The secondary mongod can restart the initial sync source selection process up to 10 times before exiting with an error. Secondary members replicate data continuously after the initial sync. Secondary members copy the oplog from their sync from source and apply these operations in an asynchronous process. [ 1]
What’s the change in MongoDB version 3.4?
Changed in version 3.4: Initial sync builds all collection indexes as the documents are copied for each collection. In earlier versions of MongoDB, only the _id indexes are built during this stage. Changed in version 3.4: Initial sync pulls newly added oplog records during the data copy.
How to automatically sync a member of a replica set in MongoDB?
MongoDB provides two options for performing an initial sync: Restart the mongod with an empty data directory and let MongoDB’s normal initial syncing feature restore the data. This is the more simple option but may take longer to replace the data. See Automatically Sync a Member.
Why is MongoDB unable to find a member?
The member is in RECOVERING since its performing startup self-checks & it is not able to transient into the SECONDARY state since it contains stale data which is due to the oplog size issue. Also you can specify the oplog size in MB in mongod.conf file,say for 50 GB its 429496 MB