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feat(elixir): Add propagator, LiveView tracing, and Plug.Telemetry note #17336

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sl0thentr0py merged 1 commit into from
Apr 14, 2026
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feat(elixir): Add propagator, LiveView tracing, and Plug.Telemetry note #17336

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40 changes: 39 additions & 1 deletion docs/platforms/elixir/tracing/index.mdx
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Original file line number Diff line number Diff line change
Expand Up @@ -60,18 +60,42 @@ Set up OpenTelemetry to send traces to Sentry:
config :opentelemetry, span_processor: {Sentry.OpenTelemetry.SpanProcessor, []}

config :opentelemetry, sampler: {Sentry.OpenTelemetry.Sampler, []}

# Enable distributed tracing across services via sentry-trace and baggage headers
config :opentelemetry,
text_map_propagators: [
:trace_context,
:baggage,
Sentry.OpenTelemetry.Propagator
]
Comment on lines +65 to +70
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@sentry sentry bot Apr 14, 2026

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Bug: The documentation for text_map_propagators suggests a mixed list of atoms and module references, a pattern not confirmed to be supported by the OpenTelemetry Elixir SDK.
Severity: MEDIUM

Suggested Fix

Verify with the OpenTelemetry Elixir SDK source code or official documentation whether mixed-type lists are supported for text_map_propagators. If not, update the documentation to show a supported configuration pattern, which might involve using only atoms or providing a different mechanism for custom propagators.

Prompt for AI Agent 
Review the code at the location below. A potential bug has been identified by an AI
agent.
Verify if this is a real issue. If it is, propose a fix; if not, explain why it's not
valid.

Location: docs/platforms/elixir/tracing/index.mdx#L65-L70

Potential issue: The documentation suggests configuring `text_map_propagators` with a
mixed list containing both atoms (e.g., `:trace_context`) and module references (e.g.,
`Sentry.OpenTelemetry.Propagator`). There is no authoritative documentation or source
code evidence from the OpenTelemetry Elixir SDK to confirm that it can parse and handle
such a mixed-type list. If a user implements this configuration, the application could
either crash during initialization or, more subtly, fail to register the custom
propagator, leading to incomplete tracing data in production without any obvious error.

Did we get this right? 👍 / 👎 to inform future reviews.

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@sl0thentr0py sl0thentr0py Apr 14, 2026

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@solnic is this a valid problem?

```

## Set Up Phoenix Instrumentation

In your `application.ex`, set up the OpenTelemetry instrumentation:
In your `application.ex`, set up the OpenTelemetry instrumentation.


<Alert title="Note">

`OpentelemetryPhoenix` requires your Phoenix endpoint to include `Plug.Telemetry` in order to correctly trace endpoint calls. Make sure your endpoint contains:

```elixir
plug Plug.Telemetry, event_prefix: [:phoenix, :endpoint]
```

This is included by default in the Phoenix endpoint template.

</Alert>

```elixir
# lib/my_app/application.ex
defmodule MyApp.Application do
use Application

def start(_type, _args) do
# Set up LiveView trace propagation (must be BEFORE OpentelemetryPhoenix)
Sentry.OpenTelemetry.LiveViewPropagator.setup()

# Set up OpenTelemetry instrumentation
OpentelemetryBandit.setup() # for Bandit (Phoenix 1.7+)
# OR OpentelemetryPhoenix.setup(adapter: :cowboy2) # for Cowboy
Expand All @@ -92,6 +116,20 @@ defmodule MyApp.Application do
end
```

Then add `Sentry.Plug.LiveViewContext` to your router's browser pipeline, after `:fetch_session`. This stores the trace context in the session so that LiveView processes can restore it during `mount`, `handle_params`, and `handle_event` callbacks.

```elixir {filename:lib/my_app_web/router.ex}
pipeline :browser do
plug :accepts, ["html"]
plug :fetch_session
plug :fetch_live_flash
plug :put_root_layout, html: {MyAppWeb.Layouts, :root}
plug :protect_from_forgery
plug :put_secure_browser_headers
plug Sentry.Plug.LiveViewContext
end
```

## Advanced Sampling

For more control over sampling, you can use a sampling function:
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