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Log levels
I'm launching a new live course: **Idiomatic Testing in Go**. The course begins May 5. Early-bird pricing is in effect until April 28. Why Go avoids assert libraries Better alternatives to mocks Make writing tests fun! See pricing & reserve → log/slog provides some rather sophisticated capabilities around log levels. We’ll get into it, but the good news is, you don’t need to care about how sophisticated it can get, if you don’t care.
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Logging common fields
It’s common that you’ll want to include certain attributes in all logs in an application or component. log/slog makes this pretty easy. Overview … Some attributes are common to many log calls. For example, you may wish to include the URL or trace identifier of a server request with all log events arising from the request. Rather than repeat the attribute with every log call, you can use Logger.With to construct a new Logger containing the attributes:
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Handler configuration
The default slog handlers are quite configurable. Overview … Both TextHandler and JSONHandler can be configured with HandlerOptions. There are options for setting the minimum level (see Levels, below), displaying the source file and line of the log call, and modifying attributes before they are logged. While HandlerOptions only exposes three fields: AddSource bool Level Leveler ReplaceAttr func(groups []string, a Attr) Attr The last one provides an immense amount of flexibility, letting you filter, replace, or augment log key/value pairs as they are processed.
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Chosing an slog handler
log/slog ships with two default handlers: the TextHandler and the JSONHandler. Overview … For more control over the output format, create a logger with a different handler. This statement uses New to create a new logger with a TextHandler that writes structured records in text form to standard error: logger := slog.New(slog.NewTextHandler(os.Stderr, nil)) TextHandler output is a sequence of key=value pairs, easily and unambiguously parsed by machine. This statement: logger.Info("hello", "count", 3) produces this output:
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Overriding the default handler
Today I’m going to jump around a bit in the GoDoc, to talk about a topic I mentioned last time: how to override the default logger. Overview … Setting a logger as the default with slog.SetDefault(logger) will cause the top-level functions like Info to use it. SetDefault also updates the default logger used by the log package, so that existing applications that use log.Printf and related functions will send log records to the logger’s handler without needing to be rewritten.
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The default handler
Much like the older log package, log/slog ships with a “default logger”. You can use this logger without doing any configuration, just by calling any of the package-level logging functions: slog.Error("oh noes!") While you’d probably never want to use the default logger in a serious server application, it can be a convenience for small or throw-away utilities. But how does it work? That’s today’s topic! Overview … The default handler formats the log record’s message, time, level, and attributes as a string and passes it to the log package.
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Intro to slog levels
By default, the log/slog package supports four log levels: Debug, Info, Warn, and Error, each of which has matching logger methods: Overview … The Info top-level function calls the Logger.Info method on the default Logger. In addition to Logger.Info, there are methods for Debug, Warn and Error levels. Besides these convenience methods for common levels, there is also a Logger.Log method which takes the level as an argument. Each of these methods has a corresponding top-level function that uses the default logger.
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1 min read
slog Records and levels
Yesterday we saw that slog uses handlers to process/record logs. But what exactly does it process? Records! Overview … A log record consists of a time, a level, a message, and a set of key-value pairs, where the keys are strings and the values may be of any type. As an example, slog.Info("hello", "count", 3) creates a record containing the time of the call, a level of Info, the message “hello”, and a single pair with key “count” and value 3.
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Anatomy of a log/slog logger
Unlike the older log package, which provides a single *log.Logger type as its primary interface, log/slog has a two-tiered architecture. This is roughly the same architecture used by the database/sql package: One interface implements a handler (or “driver” for database/sql), and another interface is consumed. The package itself provides the intermediate translation. This is essentially a localized example of ports-and-adaptors or hexagonal architecture. Here’s how the GoDoc for the package explains it:
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Let's talk about logging
I’ve been absent far too long. Most days I think about writing something again, then… I don’t. 🤷♂️ I’m going to try to get back into the habit… and this time, I thought I’d talk about one of my favorite packages in the standard library: log/slog. To kick off, I’ll give a general description of the package, then starting tomorrow (I promise! I won’t forget again!) we’ll start into the particulars.
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Happy late new year
I kinda fell off the planet for a while with family, holidays, and… writers’s block. But I’m back now, and prepared to take the time to come up with (hopefully) interesting things to write about again. Today I’m going to talk about a new feature coming in Go 1.26, which relates to my series earlier in 2025 about contexts. The new feature is mentioned very briefly in the release notes: