Read preference for querying
In the previous section, we saw what a write concern is and how it affects the write operations (insert, update, and delete). In this section, we will see what a read preference is and how it affects query operations. We'll discuss how to use a read preference in separate recipes, to use specific programming language drivers.
When connected to an individual node, query operations will be allowed by default when connected to a primary, and in case if it is connected to a secondary node, we need to explicitly state that it is ok to query from secondary instances by executing rs.slaveOk() from the shell.
However, consider connecting to a Mongo replica set from an application. It will connect to the replica set and not a single instance from the application. Depending on the nature of the application, it might always want to connect to a primary; always to a secondary; prefer connecting to a primary node but would be ok to connect to a secondary node in some...
