DynamoDB: Conditional writes vs. the CAP theorem

Question

Using DynamoDB, two independent clients trying to write to the same item at the same time, using conditional writes, and trying to change the value that the condition is referencing. Obviously, one of these writes is doomed to fail with the condition check; that's ok.

Suppose during the write operation, something bad happens, and some of the various DynamoDB nodes fail or lose connectivity to each other. What happens to my write operations?

Will they both block or fail (sacrifice of "A" in the CAP theorem)? Will they both appear to succeed and only later it turns out that one of them actually was ignored (sacrifice of "C")? Or will they somehow both work correctly due to some magic (consistent hashing?) going on in the DynamoDB system?

It just seems like a really hard problem, but I can't find anything discussing the possibility of availability issues with conditional writes (unlike with, for instance, consistent reads, where the possibility of availability reduction is explicit).

Answer 1

There is a lack of clear information in this area but we can make some pretty strong inferences. Many people assume that DynamoDB implements all of the ideas from its predecessor "Dynamo", but that doesn't seem to be the case and it is important to keep the two separated in your mind. The original Dynamo system was carefully described by Amazon in the Dynamo Paper. In thinking about these, it is also helpful if you are familiar with the distributed databases based on the Dynamo ideas, like Riak and Cassandra. In particular, Apache Cassandra which provides a full range of trade-offs with respect to CAP.

By comparing DynamoDB which is clearly distributed to the options available in Cassandra I think we can see where it is placed in the CAP space. According to Amazon "DynamoDB maintains multiple copies of each item to ensure durability. When you receive an 'operation successful' response to your write request, DynamoDB ensures that the write is durable on multiple servers. However, it takes time for the update to propagate to all copies." (Data Read and Consistency Considerations). Also, DynamoDB does not require the application to do conflict resolution the way Dynamo does. Assuming they want to provide as much availability as possible, since they say they are writing to multiple servers, writes in DyanmoDB are equivalent to Cassandra QUORUM level. Also, it would seem DynamoDB does not support hinted handoff, because that can lead to situations requiring conflict resolution. For maximum availability, an inconsistent read would only have to be at the equivalent of Cassandras's ONE level. However, to get a consistent read given the quorum writes would require a QUORUM level read (following the R + W > N for consistency). For more information on levels in Cassandra see About Data Consistency in Cassandra.

In summary, I conclude that:

• Writes are "Quorum", so a majority of the nodes the row is replicated to must be available for the write to succeed
• Inconsistent Reads are "One", so only a single node with the row need be available, but the data returned may be out of date
• Consistent Reads are "Quorum", so a majority of the nodes the row is replicated to must be available for the read to succeed

So writes have the same availability as a consistent read.

To specifically address your question about two simultaneous conditional writes, one or both will fail depending on how many nodes are down. However, there will never be an inconsistency. The availability of the writes really has nothing to do with whether they are conditional or not I think.