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rfc(decision): Mobile - Tracing Without Performance V2 #136

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rfc(decision): Mobile - Tracing Without Performance V2 #136

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rfc(decision): Mobile - Tracing Without Performance V2
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rfc(decision): Mobile - Tracing Without Performance V2
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Expand Up @@ -64,3 +64,4 @@ This repository contains RFCs and DACIs. Lost?
- [0126-sdk-spans-aggregator](text/0126-sdk-spans-aggregator.md): SDK Spans Aggregator
- [0129-video-replay-envelope](text/0129-video-replay-envelope.md): Video-based replay envelope format
- [0131-pass-native-sdk-spans-to-hybrid](text/0131-pass-native-sdk-spans-to-hybrid.md): rfc(feature): Pass Native SDKs Spans to Hybrid
- [0136-mobile-tracing-without-performance-v-2](text/0136-mobile-tracing-without-performance-v-2.md): Mobile - Tracing Without Performance V2
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- Start Date: 2024-06-04
- RFC Type: decision
- RFC PR: https://github.com/getsentry/rfcs/pull/136
- RFC Status: draft

# Summary

One paragraph explanation of the feature or document purpose.

# Motivation

Why are we doing this? What use cases does it support? What is the expected outcome?

# Background

The reason this decision or document is required. This section might not always exist.

# Supporting Data

[Metrics to help support your decision (if applicable).]

# Options Considered

If an RFC does not know yet what the options are, it can propose multiple options. The
preferred model is to propose one option and to provide alternatives.

# Drawbacks
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@Lms24 Lms24 Jun 10, 2024 •
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We were recently made aware that another implication of long-running traces is that it potentially increases transaction/span quota usage. This is because in JS we inherit the sampling decision for the trace in subsequent transactions.

For example:

  1. Pageload transaction is sampled by rolling the dice
  2. PropagationContext stores positive sampling decision
  3. Interaction transaction is started
  4. Interaction transaction is sampled because the propagation context already holds a positive sampling decision
  5. Repeat for every started transaction until next pageload or navigation

So either we accept this and move on for now by continuing with this behaviour or we break trace consistency by again rolling the dice for new root spans/transactions.

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@Lms24, please clarify why this wasn't a problem before. I don't understand how this proposed change here will cause this.

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@Lms24 Lms24 Jun 10, 2024 •
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I might be missing a bit of context around how Tracing without Performance and the PropagationContext is implemented in mobile SDKs.

If the proposed change is purely intended for TwP scenarios and does not affect the overall trace lifetime in case a root span/transaction is started ("tracing with performance") I think we're good. That is because for TwP, we defer the sampling decision to the downstream service (i.e. send sentry-trace headers without a sampled flag).

In JS however, we changed the trace lifetime not just for TwP but in general, leading to scenarios like the one above. To illustrate further, why this is problematic, I'm gonna adjust the example a bit from above

  1. Initial Pageload transaction is sampled by rolling the dice
  2. PropagationContext stores positive sampling decision
  3. application, still on the same page but after the pageload span ended, makes an http request to a downstream service and propagates the sentry-trace header with the positive sampling decision, forcing the downstream service to positively sample their transaction.
  4. repeat 3 a lot of times (e.g. an application auto-refreshing some state every 5s) and you have a lot of sampled transactions because one initial transaction was sampled positively in the FE.

So even without an active transaction, we'd still propagate a forced sampled flag to downstream services.

Does this make sense?

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Thanks for the detailed explanation. Yes, that makes sense, but I guess in the long run, you should have a roughly equal amount of transactions. It shouldn't matter if you roll the dice once for 10 transactions or every time for each transaction. If you roll the dice often enough, an equal amount of transactions should be captured.

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Not necessarily unfortunately. This would only hold up if the sample rates on client and server were the same. If users have lower sample rates on the server, they would send significantly less server-side transactions with the previous implementation.

I tried verifying this with a small script: https://gist.github.com/Lms24/9a631295aef58cf22fb8f5307953335c

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When a span starts, SDKs should use the traceID on the PropagationContext

We should say explicitly to use only the traceId, but not the sampling decision of the PropagationContext.
Regardless of client/server side sampling, i think we would break the sampling in general, as it doesn't apply to the PropagationContext. The sampler function is particularly problematic imho


Why should we not do this? What are the drawbacks of this RFC or a particular option if
multiple options are presented.

# Unresolved questions

- What parts of the design do you expect to resolve through this RFC?
- What issues are out of scope for this RFC but are known?