Using the logger presets in zap can be a huge time saver, but if you really need to tweak the logger, you need to explore ways to create custom loggers. zap provides an easy way to create custom loggers using a configuration struct. You can either create the logger configuration using a JSON object (possibly kept in a file next to your other app config files), or you can statically configure it using the native zap.Config struct, which we will explore here.

Contents:

• Using the zap config struct to create a logger
• Customizing the encoder
• Metadata field encoder alternatives
• Changing logger behavior on the fly

Using the zap config struct to create a logger

Loggers can be created using a configuration struct zap.Config. You are expected to fill in the struct with required values, and then call the .Build() method on the struct to get your logger.

// general pattern

cfg := zap.Config{...}
logger, err := cfg.Build()

There are no sane defaults for the struct. You have to, at the minimum, provide values for the three classes of settings that zap needs.

• encoder: Just adding a Encoding: "xxx" field is a minimum. Using json here as the value will create a default JSON encoder. The other alternative is using console.
  
  You can customize the encoder (which you almost certainly have to, because the defaults aren’t very useful), by adding a zapcore.EncoderConfig struct to the EncoderConfig field.

• level enabler: This is an interface type which allows zap to determine whether a message at a particular level should be displayed. In the zap config struct, you provide such a type using the AtomicLevel wrapper in the Level field.

• sink: This is the destination of the log messages. You can specify multiple output paths using the OutputPaths field which accepts a list of path names. Output will be sent to all of these files. Magic values like "stderr" and "stdout" can be used for the usual purposes.

Customizing the encoder

Just mentioning an encoder type in the struct is not enough. By default the JSON encoder only outputs fields specifically provided in the log messages.

Here are the least number of struct fields which will not throw an error when you call .Build():

logger, _ = zap.Config{
    Encoding:    "json",
    Level:       zap.NewAtomicLevelAt(zapcore.DebugLevel),
    OutputPaths: []string{"stdout"},
}.Build()

logger.Info("This is an INFO message with fields", zap.String("region", "us-west"), zap.Int("id", 2))

Output:

{"region":"us-west","id":2}

Even the message is not printed!

To add the message in the JSON encoder, you need to specify the JSON key which will have this value in the output.

logger, _ = zap.Config{
    Encoding:    "json",
    Level:       zap.NewAtomicLevelAt(zapcore.DebugLevel),
    OutputPaths: []string{"stdout"},
    EncoderConfig: zapcore.EncoderConfig{  
        MessageKey: "message",  // <--
    },
}.Build()

logger.Info("This is an INFO message with fields", zap.String("region", "us-west"), zap.Int("id", 2))

Output:

{"message":"This is an INFO message with fields","region":"us-west","id":2}

zap can add more metadata to the message like level name, timestamp, caller, stacktrace, etc. Unless you specifically mention the JSON key in the output corresponding to a metadata, it is not displayed.

Note that these metadata field names have to be paired with an encoder else zap just burns and dies (!!).

For example:

cfg := zap.Config{
    Encoding:         "json",
    Level:            zap.NewAtomicLevelAt(zapcore.DebugLevel),
    OutputPaths:      []string{"stderr"},
    ErrorOutputPaths: []string{"stderr"},
    EncoderConfig: zapcore.EncoderConfig{
        MessageKey: "message",

        LevelKey:    "level",
        EncodeLevel: zapcore.CapitalLevelEncoder,

        TimeKey:    "time",
        EncodeTime: zapcore.ISO8601TimeEncoder,

        CallerKey:    "caller",
        EncodeCaller: zapcore.ShortCallerEncoder,
    },
}
logger, _ = cfg.Build()
logger.Info("This is an INFO message with fields", zap.String("region", "us-west"), zap.Int("id", 2))

Will output:

{"level":"INFO","time":"2018-05-02T16:37:54.998-0700","caller":"customlogger/main.go:91","message":"This is an INFO message with fields","region":"us-west","id":2}

Metadata field encoder alternatives

Each of the encoder can be customized to fit your requirements, and some have different implementations provided by zap.

• timestamp can be output in either ISO 8601 format, or as an epoch timestamp in seconds, milliseconds and even nanoseconds.
  
• level can be capital or lowercase. Each of them even have colored options. Note that the colored options don’t make sense in the JSON output encoder because the terminal escape codes are not stripped in the current implementation.
  
• The caller can be shown in short and full formats.

Changing logger behavior on the fly

Loggers can be cloned from an existing logger with certain modification to their behavior. This can often be useful for example, when you want to reduce code duplication by fixing a standard set of fields the logger will always output.

• logger.AddCaller() adds caller annotation
• logger.AddStacktrace() adds stacktraces for messages at and above a given level
• logger.Fields() adds specified fields to all messages output by the new logger. Creating loggers this way, and not specifying additional fields during the actual log call can make your logging faster with less memory allocations.
• logger.WrapCore() allows you to modify or even completely replace the underlying core in the logger which combines the encoder, level and sink. Here is an example:

fmt.Printf("\n*** Using a JSON encoder, at debug level, sending output to stdout, all possible keys specified\n\n")

cfg := zap.Config{
    Encoding:         "json",
    Level:            zap.NewAtomicLevelAt(zapcore.DebugLevel),
    OutputPaths:      []string{"stderr"},
    ErrorOutputPaths: []string{"stderr"},
    EncoderConfig: zapcore.EncoderConfig{
        MessageKey: "message",

        LevelKey:    "level",
        EncodeLevel: zapcore.CapitalLevelEncoder,

        TimeKey:    "time",
        EncodeTime: zapcore.ISO8601TimeEncoder,

        CallerKey:    "caller",
        EncodeCaller: zapcore.ShortCallerEncoder,
    },
}
logger, _ = cfg.Build()

logger.Info("This is an INFO message")

fmt.Printf("\n*** Same logger with console logging enabled instead\n\n")

logger.WithOptions(
    zap.WrapCore(
        func(zapcore.Core) zapcore.Core {
            return zapcore.NewCore(zapcore.NewConsoleEncoder(cfg.EncoderConfig), zapcore.AddSync(os.Stderr), zapcore.DebugLevel)
        })).Info("This is an INFO message")

Output:

*** Using a JSON encoder, at debug level, sending output to stdout, all possible keys specified

{"level":"INFO","time":"2018-05-02T16:37:54.998-0700","caller":"customlogger/main.go:90","message":"This is an INFO message"}

*** Same logger with console logging enabled instead

2018-05-02T16:37:54.998-0700    INFO    customlogger/main.go:99 This is an INFO message