STARTUP2 (page 655) until all data is copied and all indexes are built. Afterwards, the member transitions to RECOVERING (page 656). 

## RECOVERING 

A member of a replica set enters RECOVERING (page 656) state when it is not ready to accept reads. The RECOVERING (page 656) state can occur during normal operation, and doesn’t necessarily reflect an error condition. Members in the RECOVERING (page 656) state are eligible to vote in elections, but are not eligible to enter the PRIMARY (page 655) state. 

A member transitions from RECOVERING (page 656) to SECONDARY (page 655) after replicating enough data to guarantee a consistent view of the data for client reads. The only difference between RECOVERING (page 656) and SECONDARY (page 655) states is that RECOVERING (page 656) prohibits client reads and SECONDARY (page 655) permits them. SECONDARY (page 655) state does not guarantee anything about the staleness of the data with respect to the primary. 

Due to overload, a secondary may fall far enough behind the other members of the replica set such that it may need to resync (page 633) with the rest of the set. When this happens, the member enters the RECOVERING (page 656) state and requires manual intervention. 

Error States Members in any error state can’t vote. 

## UNKNOWN 

Members that have never communicated status information to the replica set are in the UNKNOWN (page 656) state. 

## DOWN 

Members that lose their connection to the replica set are seen as DOWN (page 656) by the remaining members of the set. 

## REMOVED 

Members that are removed from the replica set enter the REMOVED (page 656) state. When members enter the REMOVED (page 656) state, the logs will mark this event with a replSet REMOVED message entry. 

## ROLLBACK 

Whenever the replica set replaces a primary in an election, the old primary may contain documents that did not replicate to the secondary members. In this case, the old primary member reverts those writes. During rollback (page 584), the member will have ROLLBACK (page 656) state. 

## FATAL 

A member in FATAL (page 656) encountered an unrecoverable error. The member must be shut down and restarted; a resync may be required as well. 

## Read Preference Reference 

Read preference describes how MongoDB clients route read operations to the members of a replica set . 

By default, an application directs its read operations to the primary member in a replica set . Because write operations are issued to the single primary, reading from the primary returns the latest version of a document $^{25}$. 

For an application that does not require fully up-to-date data, you can improve read throughput or reduce latency by distributing some or all reads to secondary members of the replica set. 

