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Note: The read preference does not affect the visibility of data; i.e, clients can see the results of writes before they are made durable: 

• Regardless of write concern (page 130), other clients can see the result of the write operations before the write operation is acknowledged to the issuing client. 

• Clients can read data which may be subsequently rolled back (page 584). 

## Read Preference Modes 

## primary 

All read operations use only the current replica set primary . 5 This is the default read mode. If the primary is unavailable, read operations produce an error or throw an exception. 

The primary (page 657) read preference mode is not compatible with read preference modes that use tag sets (page 590). If you specify a tag set with primary (page 657), the driver will produce an error. 

## primaryPreferred 

