DynamoDB consistent reads for Global Secondary Index

Question

Why cant I get consistent reads for global-secondary-indexes?

I have the following setup:

The table: tblUsers (id as hash)

Global Secondary Index: tblUsersEmailIndex (email as hash, id as attribute)

Global Secondary Index: tblUsersUsernameIndex (username as hash, id as attribute)

I query the indexes to check if a given email or username is present, so I dont create a duplicate user.

Now, the problem is I cant do consistent reads for queries on the indexes. But why not? This is one of the few occasions I actually need up-to-date data.

According to AWS documentation:

Queries on global secondary indexes support eventual consistency only.

Changes to the table data are propagated to the global secondary indexes within a fraction of a second, under normal conditions. However, in some unlikely failure scenarios, longer propagation delays might occur. Because of this, your applications need to anticipate and handle situations where a query on a global secondary index returns results that are not up-to-date.

But how do i handle this situation? How can I make sure that a given email or username is not already present in the db?

Solution 1

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.

Solution 2

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