How to add an attribute to all spans in a trace?

[kroehre]

Hi! I'm trying to understand how to add an attribute to all spans in a trace. As I understand it this is sometimes called "correlation context". I've tried using OpenTelemetry::Baggage following the example here but that doesn't seem to add attributes to spans. Thank you

[fbogsany]

Baggage does not automatically add attributes to spans in a trace. It can propagate "correlation context" but that's not its only use.

Typically, the easiest ways to add attributes to all spans are using a SpanProcessor, which receives on_start and on_finish callbacks:

opentelemetry-ruby/sdk/lib/opentelemetry/sdk/trace/span_processor.rb

Lines 15 to 33 in 113e513

	# Called when a {Span} is started, if the {Span#recording?}
	# returns true.
	#
	# This method is called synchronously on the execution thread, should
	# not throw or block the execution thread.
	#
	# @param [Span] span the {Span} that just started.
	# @param [Context] parent_context the parent {Context} of the newly
	# started span.
	def on_start(span, parent_context); end
	# Called when a {Span} is ended, if the {Span#recording?}
	# returns true.
	#
	# This method is called synchronously on the execution thread, should
	# not throw or block the execution thread.
	#
	# @param [Span] span the {Span} that just ended.
	def on_finish(span); end

or using a Sampler, which can return attributes to be added to a span alongside the sampling decision:

opentelemetry-ruby/sdk/lib/opentelemetry/sdk/trace/samplers.rb

Lines 20 to 39 in 113e513

	# Custom samplers can be provided by SDK users. The required interface is:
	#
	# should_sample?(trace_id:, parent_context:, links:, name:, kind:, attributes:) -> Result
	# description -> String
	#
	# Where:
	#
	# @param [String] trace_id The trace_id of the {Span} to be created.
	# @param [OpenTelemetry::Context] parent_context The
	# {OpenTelemetry::Context} with a parent {Span}. The {Span}'s
	# {OpenTelemetry::Trace::SpanContext} may be invalid to indicate a
	# root span.
	# @param [Enumerable<Link>] links A collection of links to be associated
	# with the {Span} to be created. Can be nil.
	# @param [String] name Name of the {Span} to be created.
	# @param [Symbol] kind The {OpenTelemetry::Trace::SpanKind} of the {Span}
	# to be created. Can be nil.
	# @param [Hash<String, Object>] attributes Attributes to be attached
	# to the {Span} to be created. Can be nil.
	# @return [Result] The sampling result.

[kroehre]

This works! Thank you.

Is OpenTelemetry::Context#with_value an appropriate way to assign correlation context attributes to be later assigned to child spans by the span processor?

[robbkidd]

I've used the Baggage#set_value API to set baggage attributes and use the returned Context in a call to Context#with_current. It's been a while since I experimented with the Context and Baggage interfaces, so I'm not positive this is the most ergonomic means to set baggage. Also in that commit is the definition of a custom BaggageSpanProcessor to duplicate baggage data onto spans and the wiring of it into the configuration.

📓 Using Baggage and a span processor for this can only add attributes to spans as they are started. In other words, adding to baggage will not mutate the current span; if you want something on current and children, you'll need to add it to current and to baggage.

📔 As far as I am aware, there is nothing in OpenTelemetry today that allows for pushing attributes up from child spans to parent spans. (There is no mechanism to mutate a parent context to have an effect on a parent span and, even if there were, span processors #on_finish by spec does not have access to any context and spans are immutable when passed in.) Using Baggage and a span processor will not add attributes to all spans within a trace, only the child spans from the current context.

⚠️ Baggage propagation is on by default. When using auto-instrumentation with the defaults, the data you add to baggage will appear in a header added to all outgoing HTTP requests. Do not put sensitive data in Baggage.

[robbkidd]

Following up with an example of using Baggage for this and some shenanigans with a custom, off-spec CarryOn and associated span processor.