Expand description


Support for tracing in actix-web apps that target MozLog.

This crate provides a Tracing subscriber as well as an Actix web middleware. Both can be used independently, but using them together implements the full recommended format for MozLog.


To use the subscriber, register it and a JsonStorageLayer with a tracing_subscriber::Registry:

use tracing_actix_web_mozlog::{JsonStorageLayer, MozLogFormatLayer};
use tracing_subscriber::layer::SubscriberExt;

let subscriber = tracing_subscriber::registry()
    .with(MozLogFormatLayer::new("service-name", std::io::stdout));

This subscriber can then be registered with tracing using tracing::subscriber::set_global_default, or any other registration method. It will manage formatting any events logged in MozLog JSON format.

Fields defined on the enclosing spans of an event will be included when logging an event. The event overrides the spans, and inner spans override outer spans.


To make sure that the correct Tracing context is captured, it is important to create the MozLog middleware outside of the HttpServer::new closure:

use tracing_actix_web_mozlog::MozLog;
use actix_web::{HttpServer, App};

let moz_log = MozLog::default();

let server = HttpServer::new(move || {

This middleware will emit request.summary events for each request as it is completed, including timing information.

Message Types

MozLog expects all messages to have a type that defines the schema of their fields. This can be specified with the type field while logging events. Since type is a Rust reserved keyword, this can also be specified using a raw-string-inspired format: r#type = value.

let format_error = "...";
    r#type = "auth.login.invalid-email",
    "A user attempted to register using an email in an invalid format"

Messages that don’t include a type field will be assigned a type of <unknown>. If a message contains both a type field and a r#type field, the type field will take precedence.

Notably, use of the standard log facade’s macros will have an unknown type, as well as most other logging that originates from libraries.

MozLog extensions

In addition to all standard MozLog fields, this crate always adds a spans field to messages. This contains a comma-separated list of the names of the spans enclosing the event, with the outermost span coming first. Top-level events will have an empty string for this value.


A layer to collect information about Tracing spans and provide it to other layers.

Middleware factory that implements the request/response cycle logging required by MozLog.

This layer is exclusively concerned with formatting information using the MozLog format. It relies on the upstream crate::JsonStorageLayer to get access to the fields attached to each span.

A logging message in MozLog format, adapted to Tracing.