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Type:
Question
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Resolution: Community Answered
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Priority:
Major - P3
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None
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Affects Version/s: 4.4.6
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Component/s: None
The transaction with snapshot read concern provides ACID properties for users.
As I known, it is based on the timestamp transaction in wiredtiger.
A question makes me puzzled that what is the timestamp used in snapshot distributed transaction?
When I refered to the document, it shows
Snapshot read concern will choose a snapshot from which the transaction will read. If it is specified with an `atClusterTime` argument, then that will be used as the transaction's read timestamp. If `atClusterTime` is not specified, then the read timestamp of the transaction will be the [`all_durable`]timestamp when the transaction is started, which ensures a snapshot with no oplog holes.
If the mongodb is deployed in relicat set, the snapshot transaction reads data before all_durable timestamp which ensures a snapshot with no oplog holes.
However, in shard deployment, each shard has its own all_durable timestamp, but the meaning of "snapshot" requires that each shard reads from the same timestamp, I think it is the atClusterTime from mongos and it is the clusterTime in mongos, right?
However, this will not lead to holes in the oplog?