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From Debug to Panic - Comparing Log Levels Across Top Languages

· 6 min read
Jeff Dwyer

Log Levels DropDown

Log levels exist to help developers filter through application logs and focus on relevant information. But what levels are available in each language? I recently looked into this because one of the features of Prefab is that it lets you change your log levels on the fly. To build this, we needed to offer users a dropdown of log levels to choose from. But which levels to show?

You might think that providing a list of levels to choose from should be straightforward. We're all trying to achieve the same thing with logging, so I'm sure there's a nice standard solution, right? Well, no. In this blog post, we'll take a lighthearted look at the log levels available in popular languages and frameworks, comparing their similarities and differences, and pointing out the quirks that make them unique.

Log Levels by Language

A Tour of Log Levels:

  1. Java Ever-popular, Java has a clear-cut set of log levels: Trace, Debug, Info, Warn, Error, and Fatal. Straight to the point, Java keeps things simple and functional... as long as you are you using a Simple Logging Facade for Java (SLF4J). Java likes a Facade Pattern.
  2. Java Util Logging If you make the newbie mistake of trying to use the built-in java.util.logging you're in for a surprise. This package brings us one of our most esoteric set of levels: Finest, Finer, Fine, Config, Info, Warn, and Severe. A fine dining experience of log levels, you can smell the overthinking from a few decades away. Presumably this came about from some deep thinking that Fineness is the right way to describe the levels, whereas Debug describes the action you might take upon them. 20 years on, we're still not sure what to do with Finest logs.
  3. Go A language known for its simplicity and efficiency, offers a slightly extended set of log levels: Trace, Debug, Info, Warn, Error, Fatal and Panic. This is the same as the Java set, but we've added Panic. This is because Go wants to make it clear that it's born of real world concerns. Sometimes your can't simply die and have that be the end of it. When you're running the Internet like Google is, sometimes you need to Panic. If you are using Kubernetes, logging at Panic will also send a Google Wave notification to Larry Page.
  4. Elixir Speaking of real-world reliability, Elixir spices things up with a unique set of log levels: Debug, Info, Notice, Warn, Error, Critical, Alert, and Emergency. This is Elixir’s way of reminding you that it runs on top of Erlang and was achieving 6 nines of uptime when you were still writing “Hello World”. It has thought more about error handling that you ever will, which is why logging at Emergency actually increases America’s DEFCON by 1.
  5. Node.js The popular JavaScript runtime sticks to it's philosophy of: the first step in doing X is choosing between 6 different basically equivalent NPM packages. LogLevel keeps it real with Trace, Debug, Info, Warn, Error but there's no reason not to get Silly and go with NPMLog which offers Silly, Verbose, Info, HTTP, Warn & Error. I didn't add Silly to the matrix below because that's just plain silly.
  6. Ruby Known for its elegance, Ruby has a slightly different set of log levels: Debug, Info, Warn, Error, and Fatal. They're the same as Java, but with no Trace. This is because Ruby wants you to enjoy yourself and if it's come to Trace things have already gone too far and you should take a break and meditate upon the problem.
  7. Python Python, the versatile language, keeps it simple with just five log levels: Debug, Info, Warn, Error, and Critical. You’ll note that this is the same as Ruby, but with Critical instead of Fatal. Python is a battlefield medic and can't be calling things dead if it's possible there's a chance they could be revived.
  8. Php/Laravel Laravel, the popular PHP framework, can’t be contained to the big 5. It offers Debug, Info, Notice, Warn, Error, Critical, Alert, and Emergency. Sometimes Php gets a bad rap, so it wants you to know it’s prepared for any situation that might arise. What's the difference between Info and Notice? Why is Alert more serious than Critical? Php knows, but it's not telling.
  9. C# A powerful language from Microsoft, C# opts for a set of log levels similar to its peers: Trace, Debug, Info, Warn, Error, and (like Python) Critical instead of Fatal. It's a solid, no-nonsense approach. Microsoft's version of Java needed to differentiate itself somehow, but didn't have any ideas of its own so stole one from Python.
  10. Rust A systems programming language, Rust has a minimalistic set of log levels: Trace, Debug, Info, Warn, and Error. Rust knows that keeping things simple is key. It doesn't require anything more serious than Error because nothing can go wrong in properly written Rust code.
  11. Kotlin This modern and expressive language JVM language, keeps its log levels consistent with its Java roots: Trace, Debug, Info, Warn, and Error. It's clear that Kotlin values simplicity and familiarity. It removed Fatal because it's not a fan of drama.
  12. Swift Swift, the language for iOS development, has a concise set of log levels: Trace, Debug, Info, Warn, Error, and Critical. Swift ensures that developers can easily manage logs while building apps for Apple devices. Swift wants to make sure that your Fart Sound iPhone can properly log at Critical when notifications are disabled.
  13. Scala No stranger to thinking differently, Scala takes a hardline approach to logging, only allowing us: Debug, Info and Warn. You might've expected the superset of all possible Log Levels for Scala, since Scala generally has the superset of all possible programming idea, but at some point enough is enough and they landed on the simple union of all log levels. And frankly, maybe that's all you need.


After examining the log levels across various languages and frameworks, we were able to pick a core set of log levels that should work well with Prefab. The common log levels we identified are Trace, Debug, Info, Warn, and Error. These levels cover the essential needs of developers, allowing them to effectively debug their applications and handle critical notices.

The good news is that no language decided to place Error before Warn or Info before Debug. This means that a general ordering of log levels can be established and client specific mappings can be created to paper over the differences. These core log levels are universal enough to provide a solid foundation for Prefab's on-the-fly log level changing feature.

Like what you read? You might want to check out what we're building at Prefab. Feature flags, dynamic config, and dynamic log levels. Free trials and great pricing for all of it.
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