An important consideration in debugging issues in the field is which members are syncing from which. It is currently difficult to obtain the history of this and correlate it with other events because this information is currently only available as a string field, which is not preserved as a metric in FTDC.

It would be helpful if the members array had a numeric field which is the _id of the current sync source for that member, or -1 if it has no sync source.

(It is currently possible, but difficult, to obtain this information fro FTDC from the full reference documents stored once per chunk: due to SERVER-27230, when there is a change of sync source typically we also start a new chunk. Once SERVER-27230 is fixed, it will then be impossible to obtain accurate information about changes of sync source from FTDC.)