ClickHouse/docs/en/operations/opentelemetry.md

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62	OpenTelemetry Support

[experimental] OpenTelemetry Support

OpenTelemetry is an open standard for collecting traces and metrics from distributed application. ClickHouse has some support for OpenTelemetry.

!!! warning "Warning" This is an experimental feature that will change in backwards-incompatible ways in the future releases.

Supplying Trace Context to ClickHouse

ClickHouse accepts trace context HTTP headers, as described by the W3C recommendation. It also accepts trace context over native protocol that is used for communication between ClickHouse servers or between the client and server. For manual testing, trace context headers conforming to the Trace Context recommendation can be supplied to clickhouse-client using --opentelemetry-traceparent and --opentelemetry-tracestate flags.

If no parent trace context is supplied, ClickHouse can start a new trace, with probability controlled by the opentelemetry_start_trace_probability setting.

Propagating the Trace Context

The trace context is propagated to downstream services in the following cases:

• Queries to remote ClickHouse servers, such as when using Distributed table engine.

• URL table function. Trace context information is sent in HTTP headers.

Tracing the ClickHouse Itself

ClickHouse creates trace spans for each query and some of the query execution stages, such as query planning or distributed queries.

To be useful, the tracing information has to be exported to a monitoring system that supports OpenTelemetry, such as Jaeger or Prometheus. ClickHouse avoids a dependency on a particular monitoring system, instead only providing the tracing data conforming to the standard. A natural way to do so in an SQL RDBMS is a system table. OpenTelemetry trace span information required by the standard is stored in the system table called system.opentelemetry_span_log.

The table must be enabled in the server configuration, see the opentelemetry_span_log element in the default config file config.xml. It is enabled by default.

The table has the following columns:

• trace_id
• span_id
• parent_span_id
• operation_name
• start_time
• finish_time
• finish_date
• attribute.name
• attribute.values

The tags or attributes are saved as two parallel arrays, containing the keys and values. Use ARRAY JOIN to work with them.