a contour feather, a penna; a quill, a feather used for writing; a pen; a pencil.

Penna is an opinionated backend for slf4j that focuses on doing one thing right: Logging structured logs in json format to the console.

Penna is currently in alpha and, while usable, has not been tested in production yet.

Please use with caution. Feedback, however, is very welcome.

Penna presents itself as an alternative to logback. It is designed for a specific use case: When you want to have structured logging, straight to the console. This might be a common use-case for jvm apps running in kubernetes. If that is your use case, you might prefer Penna over logback because:

• Penna is specialized for this use-case, working out of the box with sane defaults;
• It does not require any json library (or any dependency other than slf4j);
• It is very optimized, with impressive performance when compared to logback;
• It is also designed not consume almost any runtime memory, so it won't cause GC pressure;
• If you want to configure, the extension config library penna-yaml-config allows you to configure Penna in yaml, which might be a more native configuration format for its runtime environment (i.e. kubernetes);

However, Penna doesn't try to replace logback for all its use cases. If you have to log in multiple formats, to a file or any other target, logback might still be your tool of choice.

Penna is a backend for slf4j, so you don't need to interact with it directly.

In order to use it, add it to the build manager of your preference, for example:

// gradle
runtimeOnly 'com.hkupty.penna:penna-core:0.8.0'

// Penna doesn't have any strict dependencies aside from slf4j.
implementation 'org.slf4j:slf4j-api:2.0.12'

⚠️ Note that Penna is built targeting JVM 21+.

By default, you will get log level INFO enabled as well as the following fields:

• timestamp
• level
• message
• logger
• thread
• mdc
• markers
• data (slf4j's 2.0 .addKeyValue())
• throwable

If you want to configure it, Penna provides a separate convenience library for configuring your log levels in yaml files:

# resources/penna.yaml
---
# Since version 0.8, penna-yaml-config supports setting up a file watcher
# so any updates to this file will be reflected immediately
watch: true
loggers:
    # All the loggers under `com.yourapp` will be configured to debug level.
    com.yourapp: { level: debug }
    org.noisylibrary: { level: warn }

If you want to use penna-yaml-config, you have to add it as a dependency:

runtimeOnly 'com.hkupty.penna:penna-yaml-config:0.8.0'

// penna-yaml-config is a thin layer and uses a yaml parsing libray under the hood.
// You can chose among jackson, snakeyaml (yaml 1.1) or snakeyaml engine (yaml 1.2)

// Jackson
runtimeOnly 'com.fasterxml.jackson.core:jackson-core:2.17.0'
runtimeOnly 'com.fasterxml.jackson.core:jackson-databind:2.17.0'
runtimeOnly 'com.fasterxml.jackson.dataformat:jackson-dataformat-yaml:2.17.0'

// Snakeyaml
runtimeOnly 'org.yaml:snakeyaml:2.2'

// Snakeyaml engine
runtimeOnly 'org.snakeyaml:snakeyaml-engine:2.7'

Logging is supposed to provide meaningful information and, with the evolution of log processing platforms, it is oftentimes difficult to convey the right information as written, natural text, in a way that it both makes sense for humans and is easy for machines to process.

Instead, we should embrace the notion that logs are effectively data and should be treated as such.

Penna comes packed with a sane defaults configuration that allows one to plug it and start using immediately. Although configuration is possible, by rolling with the shipped defaults one can already reap the benefits of structured logging without having to set up any configuration.

The logging framework should not draw out much attention. It should just work. With that in mind, Penna tries to be a simple yet effective component in your architecture. It should not require you to add in more dependencies. Instead, it should work with whatever you have available. Also, it should make its best effort to consume the fewer resources as possible, being efficient and sparing your app of GC pauses when under heavy load. Read more on our performance tests.