#tracing-layer #trace #logging-tracing #modality #auxon #subscriber #events


This crate provides a tracing Layer (and Subscriber) for emitting trace data to Auxon Modality

5 unstable releases

0.3.0 Apr 1, 2024
0.2.1 Sep 29, 2022
0.2.0 Aug 4, 2022
0.1.1 Jun 28, 2022
0.1.0 Jun 27, 2022

#160 in Debugging

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Used in 2 crates (via modality-plugin-utils)




tracing-modality provides a tracing.rs Layer for tracing systems written in Rust to Auxon Modality.

The quickest way to get started is to let TracingModality register itself as the global default tracer, done most simply using [TracingModality::init()]:

use tracing_modality::TracingModality;
use tracing::info;

async fn main() {
    // first thing in main
    let mut modality = TracingModality::init().await.expect("init");

    info!("my application has started");

    // last thing in main

Some basic configuration options are also available to be set at init with TracingModality::init_with_options.

As this example shows, you must TracingModality::finish at the end of your main thread to ensure the ingest thread handing all trace events has a chance to finish flushing all queued events before your program exits.


After the initial setup, shown above, this library is not used directly, but is used via macros defined in tracing.rs. These macros function much like the log crate's logging macros error, warn, info, debug and trace, except they also allow you to pass in structured values, in the form of fields, that are recorded in the trace data as standalone values rather than being stringified into the log message.

Additionally, tracing provides a similar set of macros for emitting spans: error_span, warn_span, info_span, debug_span, and trace_span. Spans are used to label regions of execution.

There are also many more ways to use tracing, such as the #[instrument] attribute and the Value trait. See tracing's documentation for the full API.

Mappings to Modality Concepts

While tracing and modality both deal with tracing data, there's some difference in the exact data model each uses. This section describes how tracing's concepts are mapped into Modality's concepts.

Modality Timelines

In Modality, a timeline is a linear sequence of events. This library represents each OS thread as a separate timeline.

To record interactions between threads the timeline ID of the remote thread must be known. Each thread can access its own timeline ID with the [timeline_id()] function and should send that ID along with the interaction for the remote thread to record the interaction on its own timeline.

tracing Metadata

tracing implicitly generates some metadata for every event and span and tracing-modality will add some more based on its view of each event or span.

This is how each piece of metadata is mapped into modality:

  • name -> event.name[^1]
  • level -> event.severity
  • module_path -> event.source.module
  • file -> event.source.file
  • line -> event.source.line
  • the kind of event -> event.internal.rs.kind ["span:defined", "span:enter", "span:exit" ]
  • id -> event.internal.rs.span_id (spans only)

[^1]: The event.name field is almost always overridden, see next section for details.

tracing Fields

Fields are the structured data you define when using an event or span macro. They consist of a key that takes the form of a dot separated string and a value of one of tracing's supported types.

All fields are mapped directly as is to event.*, except fields prefixed with modality. which are mapped to the datasource specific namespace event.internal.rs.*. Fields manually set will overwrite any any default values set by metadata, if present.

There is a special case for event.name which will be overridden by the field name, but it that does not exist, will instead be overridden by the field message. If the field message is promoted to name it is recorded only as name and will not also be recorded as message.


Copyright 2022 Auxon Corporation

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at


Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.


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