处理Cassandra的最终一致性

Question

I have a 3 node cassandra cluster with RF=2. The read consistency level, call it CL, is set to 1.

I understand that whenever CL=1,a read repair would happen when a read is performed against Cassandra, if it returns inconsistent data. I like the idea of having CL=1 instead of setting it to 2, because then even if a node goes down, my system would run fine. Thinking by the way of the CAP theorem, I like my system to be AP instead of CP. The read requests are seldom(more like 2-3 per second), but are very important to the business. They are performed against log-like data(which is immutable, and hence never updated). My temporary fix for this is to run the query more than once, say 3 times, instead of running it once. This way, I can be sure that that even if I don't get my data in the first read request, the system would trigger read repairs, and I would eventually get my data during the 2nd or 3rd read request. Ofcourse, these 3 queries happen one after the other, without any blocking.

Is there any way that I can direct Cassandra to perform read repairs in the background without having the need to actually perform a read request in order to trigger a repair? Basically, I am looking for ways to tune my system in a way as to circumvent the 'eventual consistency' model, by which my reads would have a high probability of succeeding.

Help would be greatly appreciated.

Answer 1

reads would have a high probability of succeeding

Look at DowngradingConsistencyRetryPolicy this policy allows retry queries with lower CL than the initial one. With this policy your queries will have strong consistency when all nodes are available and you will not lose availability if some node is fail.