Adding Slog Logger to Actix Web

#rust #webdev #actix #logging

We will learn how to use slog logger for logging in Actix web.
Actix web is a powerful, pragmatic, and extremely fast web framework for Rust and Slog is an ecosystem of reusable components for structured, extensible, composable logging for Rust. We will be using two crates of slog : slog-async and slog-term with the core Slog Core Package.

Why Slog over default log crate?

extensible
composable
flexible
structured and both human and machine-readable
contextual

Crates used

Actix Web : powerful web framework.
Slog Core Crate : core package to the gateway of logging modules.
Slog Term : Unix terminal drain and formatter for slog-rs.
Slog Term : Asynchronous drain for slog-rs.

Crated Version and Code

Simply paste the code in the cargo.toml file

slog = "2.7.0"
slog-term = "2.9.0"
slog-async = "2.7.0"

Default template for Actix web

It is a default hello world program of Actix web

use actix_web::{ web, App, HttpServer};

async fn index() -> &'static str {
    "Hello world!"
}

#[actix_web::main]
async fn main() -> std::io::Result<()> {
    println!("Starting the server at 127.0.0.1:8080");

    HttpServer::new(|| {
        App::new()
            .service(web::resource("/index.html").to(|| async { "Hello world!" }))
            .service(web::resource("/").to(index))
    })
    .bind(("127.0.0.1", 8080))?
    .run()
    .await
}

Configure Logger

use slog;
use slog::{Logger,o,Drain,info};
use slog_term;
use slog_async;

fn configure_log()->Logger{
    let decorator = slog_term::TermDecorator::new().build();
    let console_drain = slog_term::FullFormat::new(decorator).build().fuse();

    // It is used for Synchronization
    let console_drain = slog_async::Async::new(console_drain).build().fuse();

    // Root logger
    slog::Logger::root(console_drain,o!("v"=>env!("CARGO_PKG_VERSION")))
}

Let's break the configuration function and understand what is happening behind the scene.

TermDecorator Decorator : IT is used for formatting terminal output implemented using term crate. This decorator will add nice formatting to the logs it’s outputting. Note It does not deal with serialization so is !Sync. Run in a separate thread with slog_async::Async. We will be using the slog async with it. We can other decorator like CompactFormat, PlainRecordDecorator etc according to need.
FullFormat : It is a Drain that will take Decorator as an argument, used for terminal output. Decorator is for formatting and Drain is for outputting.
Synchronization via Async Slog : They are three ways slog to do synchronization out of which PlainSyncDecorator and slog_async are the efficient one depending on the need. Other than the two, the last Synchronization via Mutex is not efficient. You can read more here. We are using the synchronization with slog_async.
Logger::root :Logger is used to execute logging statements. It takes two arguments
1. drain - destination where to forward logging Records for processing.
2. context - list of key-value pairs associated with it.
o! : Macro for building group of key-value pairs used as a content in Logger.

fuse() : It is used for panicking if something went wrong. It is necessary to call fuse as the root logger must take a Drain which is error free.

Passing log instance to the handlers

Add the following line of code in main function

let log = configure_log();

It will configure the logger and ready to use now.

Passing a log instance

HttpServer::new(move || {
        App::new()
            .app_data(web::Data::new(log.clone()))
            .service(web::resource("/index.html").to(|| async { "Hello world!" }))
            .service(web::resource("/").to(index))
    })
    .bind(("127.0.0.1", 8080))?
    .run()
    .await

web::Data::new(log.clone()) : It is an application data stored with App::app_data() method available through the HttpRequest::app_data method at runtime.

Accessing the Log instance in function handlers

async fn index(log: web::Data<Logger>) -> &'static str {
    info!(log,
        "Inside Hello World"
    );
    "Hello world!"
}

info! : It is a macro used for the building Info Level Record Or Context(key-value pair) used by Logger to output. They are a bunch of macros you can be used for different level records

log: web::Data : -
Essentials helper functions and types for application registration.

Request Extractors

Data: Application data item
ReqData: Request-local data item
Path: URL path parameters / dynamic segments
Query: URL query parameters
Header: Typed header
Json: JSON payload
Form: URL-encoded payload
Bytes: Raw payload

We are using the Data method to access the application data initialised in server instance in main function.

Complete Code

use actix_web::{web, App, HttpServer};
// IT is used as a logging middleware. We can even use the default logger with actix.
use slog;
use slog::{Logger,o,Drain,info};
use slog_term;
use slog_async;

fn configure_log()->Logger{
    // Formatting the output https://docs.rs/slog-term/2.9.0/slog_term/index.html#
    let decorator = slog_term::TermDecorator::new().build();

    // Drain for outputting https://docs.rs/slog-term/2.9.0/slog_term/index.html#structs
    // fuse is used for panicking if something went wrong. It is necessary to call fuse as the root logger must take a Drain which is error free.
    let console_drain = slog_term::FullFormat::new(decorator).build().fuse();

    // It is used for Synchronization https://docs.rs/slog-term/2.9.0/slog_term/index.html#structs
    let console_drain = slog_async::Async::new(console_drain).build().fuse();
    slog::Logger::root(console_drain,o!("v"=>env!("CARGO_PKG_VERSION")))
}
async fn index(log: web::Data<Logger>) -> &'static str {
    info!(log,
        "Inside Hello World"
    );
    "Hello world!"
}

#[actix_web::main]
async fn main() -> std::io::Result<()> {
    let log = configure_log();

    info!(log,
        "Starting the server at http://127.0.0.1:8080/"
    );

    HttpServer::new(move || {
        App::new()
            .app_data(web::Data::new(log.clone()))
            .service(web::resource("/index.html").to(|| async { "Hello world!" }))
            .service(web::resource("/").to(index))
    })
    .bind(("127.0.0.1", 8080))?
    .run()
    .await
}