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I believe information about empty chunks should not be global, it should probably be per partitioning information. But that is a separate story that we should handle separately.
The New Design
Per Database Meeting 2015-07-15 we will get rid of zookeeper and use mysql instead. We will implement mysql-based KVInterface in C++ and expose it to python layer (thus we will keep just one implementation instead of managing two). We will extend the KVInterface to support updates. And, finally, we will synchronize CSS updates with the new Query Metadata through locks to make sure CSS seen by Qserv is always consistent.
Appendix A: Qserv Interactions with CSS
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