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97 lines
4.8 KiB
ReStructuredText
97 lines
4.8 KiB
ReStructuredText
.. _request-tracing:
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###############
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Request Tracing
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###############
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The request tracing framework adds the ability to monitor transactions as they
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move through FoundationDB. Tracing provides a detailed view into where
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transactions spend time with data exported in near real-time, enabling fast
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performance debugging. The FoundationDB tracing framework is based off the
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`OpenTracing <https://opentracing.io/>`_ specification.
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*Disambiguation:* :ref:`Trace files <administration-managing-trace-files>` are
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local log files containing debug and error output from a local ``fdbserver``
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binary. Request tracing produces similarly named *traces* which record the
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amount of time a transaction spent in a part of the system. This document uses
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the term tracing (or trace) to refer to these request traces, not local debug
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information, unless otherwise specified.
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*Note*: Full request tracing capability requires at least ``TLogVersion::V6``.
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==============
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Recording data
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==============
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The request tracing framework produces no data by default. To enable collection
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of traces, specify the collection type using the ``--tracer`` command line
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option for ``fdbserver`` and the ``DISTRIBUTED_CLIENT_TRACER`` :ref:`network
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option <network-options-using-environment-variables>` for clients. Both client
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and server must have the same trace value set to perform correctly.
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========================= ===============
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**Option** **Description**
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------------------------- ---------------
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none No tracing data is collected.
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file, logfile, log_file Write tracing data to FDB trace files, specified with ``--logdir``.
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network_lossy Send tracing data as UDP packets. Data is sent to ``localhost:8889``, but the default port can be changed by setting the ``TRACING_UDP_LISTENER_PORT`` knob. This option is useful if you have a log aggregation program to collect trace data.
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========================= ===============
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-----------
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Data format
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-----------
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Spans are the building blocks of traces. A span represents an operation in the
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life of a transaction, including the start and end timestamp and an operation.
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A collection of spans make up a trace, representing a single transaction. The
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tracing framework outputs individual spans, which can be reconstructed into
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traces through their parent relationships.
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Trace data sent as UDP packets when using the ``network_lossy`` option is
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serialized using `MessagePack <https://msgpack.org>`_. To save on the amount of
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data sent, spans are serialized as an array of length 8 (if the span has one or
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more parents), or length 7 (if the span has no parents).
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The fields of a span are specified below. The index at which the field appears
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in the serialized msgpack array is also specified, for those using the UDP
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collection format.
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================== ========= ======== ===============
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**Field** **Index** **Type** **Description**
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------------------ --------- -------- ---------------
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Source IP:port 0 string The IP and port of the machine where the span originated.
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Trace ID 1 uint64 The 64-bit identifier of the trace. All spans in a trace share the same trace ID.
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Span ID 2 uint64 The 64-bit identifier of the span. All spans have a unique identifier.
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Start timestamp 3 double The timestamp when the operation represented by the span began.
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Duration 4 double The duration in seconds of the operation represented by the span.
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Operation name 5 string The name of the operation the span represents.
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Tags 6 map User defined tags, added manually to specify additional information.
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Parent span IDs 7 vector (Optional) A list of span IDs representing parents of this span.
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================== ========= ======== ===============
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^^^^^^^^^^^^^^^^^^^^^
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Multiple parent spans
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^^^^^^^^^^^^^^^^^^^^^
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Unlike traditional distributed tracing frameworks, FoundationDB spans can have
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multiple parents. Because many FDB transactions are batched into a single
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transaction, to continue tracing the request, the batched transaction must
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treat all its component transactions as parents.
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---------------
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Control options
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---------------
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In addition to the command line parameter described above, tracing can be set
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at a database and transaction level.
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Tracing can be controlled on a global level by setting the
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``TRACING_SAMPLE_RATE`` knob. Set the knob to 0.0 to record no traces, to 1.0
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to record all traces, or somewhere in the middle. Traces are sampled as a unit.
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All individual spans in the trace will be included in the sample.
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Tracing can be enabled or disabled for individual transactions. The special key
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space exposes an API to set a custom trace ID for a transaction, or to disable
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tracing for the transaction. See the special key space :ref:`tracing module
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documentation <special-key-space-tracing-module>` to learn more.
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