Hello cassandra-dev, I would like to continue the momentum on improving Cassandra's tracing, following Mick's excellent work on pluggable tracing and Zipkin support.
There are a couple of areas we can improve that would make tracing an even more useful tool for cluster operators to diagnose ongoing issues. The control we currently have over tracing is coarse and somewhat cumbersome. Enabling tracing from the client for a specific query is fine for application developers, particularly in an environment where Zipkin is being used to trace all parts of the system and show an aggregated view. For an operator investigating an issue however, this does not always give us the control that we need in order to obtain relevant data. We often need to diagnose an issue without the possibility of making any changes in the client, and often without the prior knowledge of which queries at the application level are experiencing poor performance. Our only other instigator of tracing is nodetool settraceprobability which only affects a single node and gives us no control over precisely which queries get traced. In practise, it is very difficult to find the relevant queries that we want to investigate, so we have often resorted to bulk loading the traces into an external tool for analysis, and this seems sub-optimal when cassandra could reduce much of the friction. I have a few proposals to improve tracing that I'd like to throw out to the mailing list to get feedback before I start implementing. 1. Include trace_probability as a CF level property, so sampled tracing can be enabled on a per-CF basis, cluster-wide, by changing the CF property. https://issues.apache.org/jira/browse/CASSANDRA-13154 2. Allow tracing at the CFS level. If we have a misbehaving host, then it would be useful to enable sampled tracing at the CFS layer on just that host so that we can investigate queries landing on that replica, rather than just queries passing through as a coordinator as is currently possible. https://issues.apache.org/jira/browse/CASSANDRA-13155 3. Add an interface allowing for custom filters which can decide whether tracing should be enabled for a given query. This is a similar idea to CASSANDRA-9193 [1] but following the same pattern that we have for IAuthenticator, IEndpointSnitch, ConfigurationLoader et al. where the intention is that useful default implementations are provided, but abstracted in such a way that custom implementations can be written for deployments where a specific type of functionality is required. This would then allow solutions such as CASSANDRA-11012 [2] without any specific support needing to be written in Cassandra. https://issues.apache.org/jira/browse/CASSANDRA-13156 Thanks for reading! Regards, Sam [1] https://issues.apache.org/jira/browse/CASSANDRA-9193 Facility to write dynamic code to selectively trigger trace or log for queries [2] https://issues.apache.org/jira/browse/CASSANDRA-11012 Allow tracing CQL of a specific client only, based on IP (range)
