Here is a summary of the directives available in the members sub-document. As a minimum, you need to specify the _id and host for each replica set member:
| Directive | Data Type | Notes |
| _id | Integer | Represents an arbitrary unique identifier for this member within the replica set. Values can range from 0 to 255 (mandatory). |
| host | String | Either the hostname or host name:port for this replica set member. This hostname must be resolvable (for example, reachable using ping) (mandatory). |
| arbiterOnly | Boolean | Set this value to true if you wish this replica set member to serve as an arbiter (does not have a copy of the database, but participates in elections in order to help prevent a deadlock). The default is false. |
| buildIndexes | Boolean | Set this value to false if you do not wish to have indexes built on this replica set member. Bear in mind that this is only in cases where this member only serves a backup/high-availability purpose and... |
