Is There A Way To Set Global Field Or Span Key=value In The Output Log? Just Like: [app_name= My_rust_app]

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Introduction

When working with logging systems, it's common to want to add additional information to log messages to help distinguish the source of the log. In this article, we'll explore how to set a global field or span key-value pair in the output log, similar to the format [app_name= my_rust_app].

Existing Log Format

Before we dive into customizing the log format, let's take a look at the existing format:

Standard Output

2025-04-19T14:03:54.731811+08:00 INFO main ThreadId(01) common::disk_cleaner: src/common/disk_cleaner.rs:42: Current disk usage: 0.9

JSON Log

"timestamp""2025-04-19T14:06:52.345427+08:00","level":"INFO","fields":{"message":"Execute cleanup function, current usage: 0.9","log.target":"iot_gw_system::common::disk_cleaner","log.module_path":"iot_gw_system::common::disk_cleaner","log.file":"src/common/disk_cleaner.rs","log.line":51,"target":"iot_gw_system::common::disk_cleaner","filename":"src/common/disk_cleaner.rs","line_number":51,"threadName":"main","threadId":"ThreadId(1)"}

Desired Log Format

We want to add an app_name field to the log output, which can be placed in the outermost layer or in the fields or span. Here's an example of the desired log format:

Standard Output

2025-04-19T14:03:54.731811+08:00 INFO app_name=my_rust_app main ThreadId(01) common::disk_cleaner: src/common/disk_cleaner.rs:42: Current disk usage: 0.9

JSON Log

"timestamp""2025-04-19T14:06:52.345427+08:00","level":"INFO","app_name":"my_rust_app","fields":{"message":"Execute cleanup function, current usage: 0.9""app_name":"my_rust_app",,"log.target":"iot_gw_system::common::disk_cleaner","log.module_path":"iot_gw_system::common::disk_cleaner","log.file":"src/common/disk_cleaner.rs","log.line":51,"target":"iot_gw_system::common::disk_cleaner","filename":"src/common/disk_cleaner.rs","line_number":51,"threadName":"main","threadId":"ThreadId(1)"}

Implementing a Custom Log Format

To achieve the desired log format, we can implement a custom log format using the log crate in Rust. We'll create a new log format that includes the app_name field in the outermost layer or in the fields or span.

Step 1: Define the Custom Log Format

First, we'll define a new log format that includes the app_name field. We can use the log::Format trait to define a custom log format.

use log::{Level, Log, Metadata, Record, SetLoggerError};

struct CustomLogFormat;

impl log::Log for CustomLogFormat {
    fn enabled(&self, _metadata: &Metadata) -> bool {
        true
    }

    fn log(&self, record: &Record) {
        // Add the app_name field to the log output
        let app_name = "my_rust_app";
        let log_message = format!("{} {} {} {}", record.level(), app_name, record.args(), record.module_path());
        println!("{}", log_message);
    }

    fn flush(&self) {}
}

Step 2: Implement the FormatEvent Trait

Next, we'll implement the FormatEvent trait to customize the log output. We'll add the app_name field to the log output in the format_event method.

use log::{Level, Log, Metadata, Record, SetLoggerError};
use serde_json::json;

struct CustomLogFormat;

impl log::Log for CustomLogFormat {
    // ...
}

impl log::FormatEvent for CustomLogFormat {
    fn format_event(&self, record: &Record) -> log::Result<String> {
        // Add the app_name field to the log output
        let app_name = "my_rust_app";
        let log_message = format!("{} {} {} {}", record.level(), app_name, record.args(), record.module_path());
        Ok(log_message)
    }
}

Step 3: Set the Custom Log Format

Finally, we'll set the custom log format using the set_logger function.

fn main() {
    // Set the custom log format
    let log_format = CustomLogFormat;
    log::set_logger(log_format).unwrap();
    log::set_max_level(log::LevelFilter::Info);
}

Conclusion

In this article, we've explored how to set a global field or span key-value pair in the output log using the log crate in Rust. We've implemented a custom log format that includes the app_name field in the outermost layer or in the fields or span. By following these steps, you can customize the log output to meet your specific needs.

Example Use Cases

Here are some example use cases for the custom log format:

  • Distinguishing Log Sources: By including the app_name field in the log output, you can easily distinguish the source of the log and identify which application is generating the log messages.
  • Customizing Log Output: The custom log format allows you to customize the log output to meet your specific needs. You can add or remove fields as needed to suit your logging requirements.
  • Improving Log Analysis: By including the app_name field in the log output, you can improve log analysis and make it easier to identify trends and patterns in the log data.

Best Practices

Here are some best practices to keep in mind when implementing a custom log format:

  • Keep it Simple: Avoid overcomplicating the log format by including too many fields or complex formatting.
  • Use Meaningful Field Names: Use meaningful field names that clearly indicate the purpose of each field.
  • Document the Log Format: Document the log format to ensure that others understand how to interpret the log data.
  • Test the Log Format: Thoroughly test the log format to ensure that it works as expected and produces the desired output.
    Q&A: Customizing Log Output with Global Field or Span Key-Value Pair ====================================================================

Introduction

In our previous article, we explored how to set a global field or span key-value pair in the output log using the log crate in Rust. We implemented a custom log format that includes the app_name field in the outermost layer or in the fields or span. In this article, we'll answer some frequently asked questions about customizing log output.

Q: Why do I need to customize the log output?

A: Customizing the log output allows you to add additional information to log messages to help distinguish the source of the log. This can be useful for identifying trends and patterns in the log data, as well as for improving log analysis.

Q: How do I implement a custom log format?

A: To implement a custom log format, you can use the log crate in Rust. You'll need to define a new log format that includes the fields you want to add to the log output. You can use the log::Format trait to define a custom log format.

Q: What is the difference between a field and a span?

A: A field is a key-value pair that is added to the log output. A span is a way of grouping related log messages together. You can use fields and spans together to create a custom log format that meets your needs.

Q: How do I add a field to the log output?

A: To add a field to the log output, you can use the log::FormatEvent trait. You'll need to implement the format_event method to add the field to the log output.

Q: Can I use a custom log format with multiple layers?

A: Yes, you can use a custom log format with multiple layers. You'll need to implement the log::Format trait for each layer, and then use the log::set_logger function to set the custom log format for each layer.

Q: How do I test a custom log format?

A: To test a custom log format, you can use the log crate's built-in testing functionality. You'll need to create a test case that exercises the custom log format, and then verify that the log output is correct.

Q: What are some best practices for customizing log output?

A: Here are some best practices to keep in mind when customizing log output:

  • Keep it simple: Avoid overcomplicating the log format by including too many fields or complex formatting.
  • Use meaningful field names: Use meaningful field names that clearly indicate the purpose of each field.
  • Document the log format: Document the log format to ensure that others understand how to interpret the log data.
  • Test the log format: Thoroughly test the log format to ensure that it works as expected and produces the desired output.

Q: Can I use a custom log format with a JSON log format?

A: Yes, you can use a custom log format with a JSON log format. You'll need to implement the log::FormatEvent trait to add the custom fields to the JSON log output.

Q: How do I handle errors in a custom log format?

A: To handle errors in a custom log format, you can use the log::Error type to represent errors in the log output. You'll need to implement the log::FormatEvent trait to add the error field to the log output.

Conclusion

In this article, we've answered some frequently asked questions about customizing log output. We've covered topics such as implementing a custom log format, adding fields and spans to the log output, and testing a custom log format. By following these best practices and using the log crate in Rust, you can create a custom log format that meets your needs and improves log analysis.

Example Use Cases

Here are some example use cases for customizing log output:

  • Distinguishing Log Sources: By including the app_name field in the log output, you can easily distinguish the source of the log and identify which application is generating the log messages.
  • Customizing Log Output: The custom log format allows you to customize the log output to meet your specific needs. You can add or remove fields as needed to suit your logging requirements.
  • Improving Log Analysis: By including the app_name field in the log output, you can improve log analysis and make it easier to identify trends and patterns in the log data.

Best Practices

Here are some best practices to keep in mind when customizing log output:

  • Keep it Simple: Avoid overcomplicating the log format by including too many fields or complex formatting.
  • Use Meaningful Field Names: Use meaningful field names that clearly indicate the purpose of each field.
  • Document the Log Format: Document the log format to ensure that others understand how to interpret the log data.
  • Test the Log Format: Thoroughly test the log format to ensure that it works as expected and produces the desired output.