S3 is not a database, but that's not the point. As explained by Capital One, the attacker gained access through a misconfigured web app. This could have happened on any platform (on-premise or cloud), and the underlying AWS services weren't compromised in any way.
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When it comes to scalability, Apache MXNet (http://mxnet.io/) is actually the best choice. Multi-GPU support and distributed training on multiple hosts are extremely easy to set up. It's also supported by Keras (still in beta, though).