kv

Package kv provides support for working with collections of key/value pairs.

• Lists
• Errors
• Context
• Parse
• Logging

Lists

The List type represents a sequence of alternating key/value pairs. Lists can render themselves as text in logfmt format, so if you like logging messages with key value pairs but are not ready to move away from the standard library log package you can use something like:

log.Println("this is a log message", kv.With(
    "key1", "value 1",
    "key2", 2,
))

// Output:
// this is a log message key1="value 1" key2=2

The output from the previous example can be easily read by humans, and easily parsed by machines. This makes it an excellent format for structured logging.

Errors

The Error type implements the builtin error interface and renders its error message as a free text message followed by key/value pairs:

cannot open file: permission denied file="/etc/passwd" user=alice

Errors are constructed with message text and key/value pairs.

// create an error
err := kv.NewError("permission denied").With("user", user)
log.Println(err)

// Output:
// permission denied user=alice

Errors can wrap other errors:

err = kv.Wrap(err, "cannot open file").With("file", filename)
log.Println(err)

// Output:
// cannot open file: permission denied file="/etc/passwd" user=alice

Context

Key/value pairs can be stored in the context:

ctx := context.Background()

// associate some key/value pairs with the context
ctx = kv.From(ctx).With("url", "/api/widgets", "method", "get")

// ... later on ...

log.Println("access denied", kv.From(ctx).With("file", filename))

// Output:
// access denied file="/etc/passwd" url="/api/widgets" method=get

Parse

One of the key points of structured logging is that logs are machine readable. The Parse function provides an easy way to read messages.

// read one line
line := []byte(`cannot open file: access denied file="/etc/passwd" user=alice`)

 text, list := kv.Parse(line)
 log.Println("text:", string(text))
 log.Println("list:", list)

// Output:
// text: cannot open file: access denied
// list: file="/etc/passwd" user=alice

Logging

The kvlog subdirectory contains a package that works well with the log package in the Go standard library. Its usage is as simple as:

func main() {
    kvlog.Attach() // attach to the standard logger
    log.Println("program started", kv.With("args", os.Args))
}

See the GoDoc for more details.