How does replication work?

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How MongoDB replication works

Setting up Replication

• Change operations are written to the oplog

• A single update/delete affecting multiple documents can result in many oplog entries
• The oplog is a capped collection (fixed size)
• Secondaries query the primary's oplog and apply what they find
• All replica set members contain an oplog

• Must have enough space to allow new secondaries to catch up after syncing from a primary, and cope with any applicable slaveDelay