[DRILL-4882] Drill does not work when it has only access to "secondary nodes"

• Type: Bug
• Status: Open
• Priority: Minor
• Resolution: Unresolved
• Affects Version/s: 1.6.0, 1.7.0, 1.8.0
• Component/s: Storage - MongoDB

Environment

Drill Stand Alone
Drill in MapR

• Assignee: Unassigned
• Reporter: Tugdual Grall
• Created: 07/Sep/16 12:35
• Updated: 04/Jan/17 23:23

Description

In some deployments, especially when doing analytics on MongoDB, the application, Drill in this case, only have access to secondary nodes.

This is a common choice to avoid overloading the Mongo cluster.

When we have such configuration Drill fail with various exception related to timeout/primary read preference.

This is due to the fact that some of the "Schema Oriented" methods of MongoDriver are forcing the read preference to primary. The methods are used in the following place in Drill:

1-
https://github.com/apache/drill/blob/1.8.0/contrib/storage-mongo/src/main/java/org/apache/drill/exec/store/mongo/schema/MongoSchemaFactory.java#L89

2-
https://github.com/apache/drill/blob/1.8.0/contrib/storage-mongo/src/main/java/org/apache/drill/exec/store/mongo/schema/MongoSchemaFactory.java#L109

and also the Run command to get information about the type of topology (sharded or not):

3-
https://github.com/apache/drill/blob/1.8.0/contrib/storage-mongo/src/main/java/org/apache/drill/exec/store/mongo/MongoGroupScan.java#L167

3- could be fixed by simply adding a ReadPreference.primaryPreferred() but not sufficient since we need to have 1 & 2 fixed

Issue Links

• links to: GitHub Pull Request #609