Amazon DynamoDB vs Riak

• DynamoDB is fully-managed. In the early days, it lacked features like backups, and developers had to either implement some of their own backup functionality or live dangerously. Today, DynamoDB's claim to be fully-managed is more credible. Backups can be configured through the console. Table capacity does not even need to be planned anymore; you can scale (and pay) on-demand.
• DynamoDB is inexpensive for some use-cases. In particular, DynamoDB is very inexpensive when you create a datastore for a low-volume micro-service, or a stateful background job. DynamoDB is much less expensive than RDS or Elasticache for these use-cases, and it allows developers to design systems without worrying about cost.
• DynamoDB is fast when used for the use-cases it was designed for.
• DynamoDB supports the development experience and testing reasonably well. AWS provides an official DynamoDB image that can be used in tests locally or in CI.

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• Consistency model is highly distributed, so uptime of our documents is always available.
• Simple to use RESTful interface, which was easy to access from our Java application.
• Data model using catalogs and indexes was simple, making it easy to store and retrieve our statements.

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• Better querying capability, we had a requirement where we wanted to index and query nested arrays. For example: our sample data structure: {a: [{b: ['array of interest' ]}]}. DynamoDB does not provide neat way to query the 'array of interest'. Hence we had to shift few of our Databases from DynamoDB to Postgres.
• DynamoDB does not offer any sort of database functions or triggers that would help manipulate the data before performing a transaction. This is key for part of our NoSQL datastore. For example: we wanted to manually maintain consistency of order variable in a JSON array i.e. if we add an element to middle of the array, the order of rest of the elements should shuffle within a single transaction. This functionality leaves something to be desired from DynamoDB.
• Transaction management is something that DynamoDB does not offer. For example: if we want to perform a read and write within a same transaction, DynamoDB does not offer this capability. Hence, DynamoDB is not a great fit for highly concurrent environment.

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• At the time I was using Riak, data was stored as blobs so you couldn't query the data directly on the server. This has since been remedied with the addition of full-text search support.

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• Cost savings over Cassandra.
• Less of a maintenance hassle and less downtime for the services running on it.

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• Riak has had a positive impact in the fact that it is still a very fast data store and can scale very well over time.
• Riak has had a negative impact as it is one of the most expensive pieces of our architecture.
• Riak's SOLR engine failed us at a critical moment which seriously hurt the company. Our use case is very different than the norm, so I don't expect this to affect most people.

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