Allow configuration of `ReadConcern` for queries and aggregations

[sad3rasd]

Hi,
I would like to use find and aggregate query with readConcern level "snapshot" (and "atClusterTime") in order to get data at a specified point in time.

I can't find any way to implement this with spring data mongodb, am I miss something?
If not already available, how can be implemented using MongoTemplate? Also could this feature be added?

more info: https://www.mongodb.com/docs/manual/reference/read-concern-snapshot/

[mp911de]

ReadPreferences can be set via:

1. MongoTemplate for commands that operate on a collection
2. Query meta flags via CursorOption.SECONDARY_READS

It would be neat if we could provide the ReadPreference through Query and AggregationOptions.

[mp911de]

Please note that atClusterTime is a driver-internal attribute that is used with client sessions to track the latest cluster time. It is not intended to go back in time. See also the specification on snapshot reads:

https://github.com/mongodb/specifications/blob/master/source/sessions/snapshot-sessions.rst

For snapshot reads the driver MUST first obtain atClusterTime from the server response of a find/aggregate/distinct command, by specifying readConcern with snapshot level field, and store it as snapshotTime in the ClientSession object.

{
    find : <string>, // or other read command
    ... // the rest of the command parameters
    readConcern :
    {
        level : "snapshot"
    }
}

For subsequent reads in the same session, the driver MUST send the snapshotTime saved in the ClientSession as the value of the atClusterTime field of the readConcern with a snapshot level:

{
    find : <string>, // or other read command
    ... // the rest of the command parameters
    readConcern :
    {
        level : "snapshot",
        atClusterTime : <BsonTimestamp>
    }
}

The current documentation creates a slightly different impression of a wayback-machine.