Blog / Standard Library

Concurrent logging

June 18, 2026

One last note in the performance section…

How does logging work in a concurrent system?

Performance considerations

…

The built-in handlers acquire a lock before calling io.Writer.Write to ensure that exactly one Record is written at a time in its entirety. Although each log record has a timestamp, the built-in handlers do not use that time to sort the written records. User-defined handlers are responsible for their own locking and sorting.

So three points fall out from this:

  1. The built-in logging handlers ensure it’s safe to log from multiple goroutines concurrently. You’ll never get a partial log from one interwoven with a log from another. This won’t happen:
2022/11/08 15:28:26 INFO hello 2022/11/08 15:28:27 INFO hello count=3count=3
  1. Timestamps are not used for sorting. So in theory, this could happen:
2022/11/08 15:28:28 INFO hello count=3
2022/11/08 15:28:27 INFO hello count=3
  1. If you’re implementing your own logger, these guarantees don’t apply, unless you implement them yourself.
Share this

Direct to your inbox, daily. I respect your privacy .

Unsure? Browse the archive .

Related Content

slog.Group

The one Attr constructor not yet covered: func Group func Group(key string, args ...any) Attr Group returns an Attr for a Group Value. The first argument is the key; the remaining arguments are converted to Attrs as in Logger.Log. Use Group to collect several key-value pairs under a single key on a log line, or as the result of LogValue in order to log a single value as multiple Attrs.

The Attr type

Other than logger methods, the slog.Attr type is probably the thing you’ll use the most in your use of the slog package: Types type Attr type Attr struct { Key string Value Value } An Attr is a key-value pair. This type has several type-specific constructors, too, but we’re going to do an abbrevaited look at them, because they’re highly repetitive. func Any func Any(key string, value any) Attr Any returns an Attr for the supplied value.

Backward compatibility

We’ve already discussed when the default log/slog Logger also serves as the default log Logger. But what if you want to send log logs to a log/slog Logger, without using the global defaults? Introducing NewLogLogger! func NewLogLogger func NewLogLogger(h Handler, level Level) *log.Logger NewLogLogger returns a new log.Logger such that each call to its Output method dispatches a Record to the specified handler. The logger acts as a bridge from the older log API to newer structured logging handlers.

Get daily content like this in your inbox!

Subscribe