OpenTelemetry SDK is a reference implementation of the OpenTelemetry API. It
implements the Logging API, Metrics API, Tracing API, Resource API, and the
Context API. Once a valid SDK is installed and configured all the OpenTelemetry
API methods, which were no-ops without an SDK, will start emitting telemetry.
This SDK also ships with
ILogger
integration to automatically capture and enrich logs emitted using
Microsoft.Extensions.Logging.
The SDK deals with concerns such as sampling, processing pipelines (exporting
telemetry to a particular backend, etc.), metrics aggregation, and other
concerns outlined in the OpenTelemetry
Specification.
In most cases, users indirectly install and enable the SDK when they install an
exporter.
To learn how to set up and configure the OpenTelemetry SDK see: . For additional details about
initialization patterns see: .
All the components shipped from this repo uses
EventSource
for its internal logging. The name of the EventSource used by OpenTelemetry
SDK is "OpenTelemetry-Sdk". To know the EventSource names used by other
components, refer to the individual readme files.
While it is possible to view these logs using tools such as
PerfView,
dotnet-trace
etc., this SDK also ships a self-diagnostics feature, which
helps with troubleshooting.
Self-diagnostics
OpenTelemetry SDK ships with built-in self-diagnostics feature. This feature,
when enabled, will listen to internal logs generated by all OpenTelemetry
components (i.e EventSources whose name starts with "OpenTelemetry-") and writes
them to a log file.
The self-diagnostics feature can be enabled/changed/disabled while the process
is running (without restarting the process). The SDK will attempt to read the
configuration file every 10 seconds in non-exclusive read-only mode. The SDK
will create or overwrite a file with new logs according to the configuration.
This file will not exceed the configured max size and will be overwritten in a
circular way.
To enable self-diagnostics, go to the
current working directory of
your process and create a configuration file named OTEL_DIAGNOSTICS.json with
the following content:
To disable self-diagnostics, delete the above file.
Tip: In most cases, you could just drop the file along your application.
On Windows, you can use Process Explorer,
double click on the process to pop up Properties dialog and find "Current
directory" in "Image" tab.
Internally, it looks for the configuration file located in
GetCurrentDirectory,
and then AppContext.BaseDirectory.
You can also find the exact directory by calling these methods from your code.
Configuration Parameters
LogDirectory is the directory where the output log file will be stored. It
can be an absolute path or a relative path to the current directory.
FileSize is a positive integer, which specifies the log file size in
KiB. This value must be within
range [1024, 131072] (1 MiB /<= size /<= 128 MiB), or it will be rounded to
the closest upper or lower limit. The log file will never exceed this
configured size, and will be overwritten in a circular way.
LogLevel is the lowest level of the events to be captured. It has to be one
of the
values
of the EventLevel
enum.
The level signifies the severity of an event. Lower severity levels encompass
higher severity levels. For example, Warning includes the Error and
Critical levels.
FormatMessage is a boolean value that controls whether log messages should
be formatted by replacing placeholders ({0}, {1}, etc.) with their actual
parameter values. When set to false (default), messages are logged with
unformatted placeholders followed by raw parameter values. When set to
true, placeholders are replaced with formatted parameter values for
improved readability.
Example with FormatMessage: false (default):
2025-07-24T01:45:04.1020880Z:Measurements from Instrument '{0}', Meter '{1}' will be ignored. Reason: '{2}'. Suggested action: '{3}'{dotnet.gc.collections}{System.Runtime}{Instrument belongs to a Meter not subscribed by the provider.}{Use AddMeter to add the Meter to the provider.}
Example with FormatMessage: true:
2025-07-24T01:44:44.7059260Z:Measurements from Instrument 'dotnet.gc.collections', Meter 'System.Runtime' will be ignored. Reason: 'Instrument belongs to a Meter not subscribed by the provider.'. Suggested action: 'Use AddMeter to add the Meter to the provider.'
Remarks
A FileSize-KiB log file named as ExecutableName.ProcessId.log (e.g.
foobar.exe.12345.log) will be generated at the specified directory
LogDirectory, into which logs are written to.
If the SDK fails to parse the LogDirectory, FileSize or LogLevel fields as
the specified format, the configuration file will be treated as invalid and no
log file would be generated.
When the LogDirectory or FileSize is found to be changed, the SDK will create
or overwrite a file with new logs according to the new configuration. The
configuration file has to be no more than 4 KiB. In case the file is larger than
4 KiB, only the first 4 KiB of content will be read.
The log file might not be a proper text file format to achieve the goal of having
minimal overhead and bounded resource usage: it may have trailing NULs if log
text is less than configured size; once write operation reaches the end, it will
start from beginning and overwrite existing text.
