Is it possible to use the Free Tier of AWS DynamoDB On Demand

It really depends on how much your app is cached at CloudFlare. I ran some basic tests a while back (for something completly different) and a micro instance in the US, running Varnish, with a WordPress server in the backend (in Germany) was able to handle 50 users at a time, and was not breaking a sweat. the German Server only had to respond to the Varnish box once. I used LoadImpact to do the testing...

If you can get your site to return fairly static pages, with a fairly long expiration date, it should stay in CloudFlare for a fair amount of time. your mileage may vary though.

We're using DynamoDB and ElastiCache Redis for different reasons.

DynamoDB:

• Has a query language which is able to do more complex things (greater than, between etc.)
• Is reachable via an external internet-facing API (different regions are reachable without any changes or own infrastructure)
• Permissions based on tables or even rows are possible
• Scales in terms of data size to infinity
• You pay per request -> low request numbers means smaller bill, high request numbers means higher bill
• Reads and Writes are different in costs
• Data is saved redundant by AWS in multiple facilities
• DynamoDB is highly available out-of-the-box
• Autoscaling is available in the service itself

ElastiCache Redis:

• Simple query language - no complex features
• Is (out-of-the-box) not reachable from other regions.
• You're always limited to the amount of memory (or the sum of all primary instances in a cluster)
• Sharding over multiple instances is only possible within your application - Redis doesn't do anything here (Redis cluster helps here but the sharding logic is still inside of the driver/sdk you're using in your application) - scale-in and scale-out is not possible without downtime at the moment
• You pay per instance no matter how the load or the number of requests are.
• If you want redundancy of the data you need to setup replication (not possible between different regions)
• You need to use replicas for high availability
• No autoscaling available (see the part about no scaling at all above)

So our setup most of the time is: Simple caches with high volume of requests in Redis backed by DynamoDB as the permanent and long-durable storage. With this we limit the costs as we get an implicit discount for our reads by the pay-per-instance model of Redis but also get the benefit of the redundancy of DynamoDB and are even able to use the DynamoDB query language for more complex stuff (if we need it).

Hope that helps!

Update: With the announcement of Amazon DynamoDB Accelerator (https://aws.amazon.com/de/dynamodb/dax/) we're switching over to use DAX as it is (in the end) exactly what we were doing with the combination of DynamoDB and Redis. As DAX ist fully-managed by AWS and gives us the chance to always use the DynamoDB language in our application but also get the benefits from a write-through cache like Redis.