DynamoDB vs MongoDB NoSQL [closed]

Answer 1

Short answer: Start with SQL and add NoSQL only when/if needed. (unless you don't need anything beyond very simple queries)

My personal experience: I haven't used MongoDB for queries but as of April 2015 DynamoDB is still very crippled when it comes to anything beyond the most basic key/value queries. I love it for the basic stuff but if you want query language then look to a real SQL database solution.

In DynamoDB you can query on a hash or on a hash and range key, and you can have multiple secondary global indexes. I'm doing queries on a single table with 4 possible filter parameters and sorting the results, this is supported (barely) through the use of the global secondary indexes with filter expressions. The problem comes in when you try to get the total results matching the filter, you can't just search for the first 10 items matching the filter, but rather it checks 10 items and you may get 0 valid results forcing you to keep re-scanning from the continue key - pain in the neck and consumes too much of your table read quota for a simple scenario.

To be specific about the limit problem with filters in the query, this is from the docs (http://docs.aws.amazon.com/amazondynamodb/latest/developerguide/QueryAndScan.html#ScanQueryLimit):

In a response, DynamoDB returns all the matching results within
the scope of the Limit value. For example, if you issue a Query 
or a Scan request with a Limit value of 6 and without a filter
expression, the operation returns the first six items in the 
table that match the request parameters. If you also supply a
FilterExpression, the operation returns the items within the 
first six items in the table that match the filter requirements.

My conclusion is that queries involving FilterExpressions are only usable on very rare occasions and are not scalable because each query can easily read most or all of your of your table which consumes far too many DynamoDB read units. Once you use too many read units you'll get throttled and see poor performance.

Expert opinion: In the AWS summit on Apr 9, 2015 Brett Hollman, Manager, Solutions Architecture, AWS in his talk on scalling to your first 10 million users advocates starting with a SQL database and then using NoSQL only when and if it makes sense. Because sooner or later you'll probably need a SQL server somewhere in your stack. His slides are here: http://www.slideshare.net/AmazonWebServices/deep-dive-scaling-up-to-your-first-10-million-users See slide 28.