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Query tracing¶
The driver has utilities for monitoring the execution of queries.
There are two separate ways to get information about what happened with a query: Tracing and Query Execution History.
Tracing¶
Tracing is a feature provided by Scylla. When sending a query we can set a flag that signifies that we would like it to be traced.
After completing the query Scylla provides a tracing_id which can be used to fetch information about it - which nodes it was sent to, what operations were performed etc.
Queries that support tracing:
After obtaining the tracing id you can use Session::get_tracing_info() to query tracing information.
TracingInfo contains values that are the same in Scylla and Cassandra®, skipping any database-specific ones.
If TracingInfo does not contain some needed value it’s possible to query it manually from the tables
system_traces.sessions and system_traces.events
Query Execution History¶
Tracing provides information about how the query execution went on database nodes, but it doesn’t say anything about what was going on inside the driver.
This is what query execution history was made for.
It allows to follow what the driver was thinking - all query attempts, retry decisions, speculative executions. More information is available in the Query Execution History chapter.