Clarification Needed: Globally Unique ID Requirements and W3C TraceContext parent-id Mapping in Distributed Tracing Documentation

[AdamCoulterOz]

I'm seeking clarification on the apparent discrepancies regarding the ID size and uniqueness requirements for distributed trace-related telemetry items, as well as their mapping to the W3C TraceContext standards. This is highlighted in the following document sections:

Data model for telemetry correlation

...

A distributed logical operation typically consists of a set of smaller operations that are requests processed by one of the components. Request telemetry defines these operations. Every request telemetry item has its own id that identifies it uniquely and globally. And all telemetry items (such as traces and exceptions) that are associated with the request should set the operation_parentId to the value of the request id.

Dependency telemetry represents every outgoing operation, such as an HTTP call to another component. It also defines its own id that's globally unique. Request telemetry, initiated by this dependency call, uses this id as its operation_parentId.

You can build a view of the distributed logical operation by using operation_Id, operation_parentId, and request.id with dependency.id. These fields also define the causality order of telemetry calls.

Noting the specifically called out globally unique requirement for all distributed trace related telemetry items (i.e. Request and Dependency, but technically should also include PageView too). This is achieved using GUIDs with these ID fields being 16 bytes.

Example

...

itemType	name	ID	operation_ParentId	operation_Id
pageView	Stock page	STYz		STYz
dependency	GET /Home/Stock	qJSXU	STYz	STYz
request	GET Home/Stock	KqKwlrSt9PA=	qJSXU	STYz
dependency	GET /api/stock/value	bBrf2L7mm2g=	KqKwlrSt9PA=	STYz

The example table is nice and clear in how then mapping occurs, but uses very short ID fields (which may just be for ease of reading) which I doubt those 4 and 12 byte IDs are globally unique.

Correlation headers using W3C TraceContext

...

Application Insights	W3C TraceContext >
Id of Request and Dependency	parent-id
Operation_Id	trace-id
Operation_ParentId	parent-id of this span's parent span. This field must be empty if it's a root span.

Finally, the mapping of Request and Dependency Id fields to the parent-id field of the W3C TraceContext. The parent-id field is only 8 bytes. How can I fit a 16 byte guid into a 8 byte parent-id ?

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Document Details

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• ID: 83691d46-7bf3-8081-1a60-55afcbb984c7
• Version Independent ID: 4b4bb2bc-395b-8adf-f670-cf9894f0610f
• Content: Distributed tracing and telemetry correlation in Azure Application Insights - Azure Monitor
• Content Source: articles/azure-monitor/app/distributed-trace-data.md
• Service: azure-monitor
• Sub-service: application-insights
• GitHub Login: @AaronMaxwell
• Microsoft Alias: aaronmax

[Naveenommi-MSFT]

@AdamCoulterOz Thanks for your feedback! We will investigate and update as appropriate.

[AaronMaxwell]

Thank you for reaching out to us! We appreciate your interest in improving our documentation.

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The best way to get answers to your questions is to create a support ticket. Our support team is available to assist you with any questions or issues you may have. You can create a support ticket by visiting the following link: https://azure.microsoft.com/support/create-ticket/

Once you have received assistance from our support team, we would be happy to document the answer to your questions in the appropriate articles for future reference. #please-close