DynamoDB consistent reads for Global Secondary Index

You probably already went through this: http://docs.aws.amazon.com/amazondynamodb/latest/developerguide/GSI.html

The short answer is that you cannot do what you want to do with Global Secondary Indexes (ie it's always eventual consistency).

A solution here would be to have a separate table w/ the attribute you're interested in as a key and do consistent reads there. You would need to ensure you are updating that whenever you are inserting new entities, and you would also have to worry about the edge case in which inserts there succeed, but not in the main table (ie you need to ensure they are in sync)

Another solution would be to scan the whole table, but that would probably be overkill if the table is large.

Why do you care if somebody creates 2 accounts with the same email? You could just use the username as the primary hash key and just not enforce the email uniqueness.

When you try to use putItem, you have a ConditionExpression to use to check if the condition is satisfied to put the item, which means you can check if the email or username exists.

ConditionExpression — (String)
A condition that must be satisfied in order for a conditional PutItem operation to succeed.

An expression can contain any of the following:

Functions: attribute_exists | attribute_not_exists | attribute_type | contains | begins_with | size
These function names are case-sensitive.

Comparison operators: = | <> | < | > | <= | >= | BETWEEN | IN
Logical operators: AND | OR | NOT
For more information on condition expressions, see Condition Expressions in the Amazon DynamoDB Developer Guide.

https://docs.aws.amazon.com/AWSJavaScriptSDK/latest/AWS/DynamoDB.html#putItem-property