Base fields

You can customize the Base Fields of the Logger instance using the following methods:

// fields.gofunc Fields(fields LogFields) Logger { ... }func RawFields(fields LogFields) Logger { ... }

note

The returned Logger instance will be a copy of the previous one, sharing the same Configuration struct. The only difference will be the Base Fields.

More info at the Loggers clonage page.

There's a third method, used to query Base Fields of the Logger instance:

// fields.gofunc Field(key string) interface{} { ... }

Fields method

This method will return a new copy of the Logger instance, with the given LogFields applied. Note that if the previous Logger had a Base Field with a key that clashes with one of the new ones, it will be overriden:

firstLogger := logger.New(logger.DefaultConfig()).  Fields(logger.LogFields{    "field-A": "value-A",    "field-B": "value-B",    "field-C": "value-C",  }).  Outputs(logger.OutputJsonToWriter(os.Stdout, nil))firstLogger.Info("first log")/*  {    "msg": "first log",    "lvl": 4,    "field-A": "value-A",    "field-B": "value-B",    "field-C": "value-C"  }*/
secondLogger := firstLogger.  Fields(logger.LogFields{    "field-B": "new value",  })secondLogger.Info("second log")/*  {    "msg": "second log",    "lvl": 4,    "field-A": "value-A",    "field-B": "new value",    "field-C": "value-C"  }*/

RawFields method

This method is almost equal to the previous one (returns a new copy of the Logger instance too), with one difference: the given LogFields will be set right away, ignoring any previous Base Fields.

firstLogger := logger.New(logger.DefaultConfig()).  Fields(logger.LogFields{    "field-A": "value-A",    "field-B": "value-B",    "field-C": "value-C",  }).  Outputs(logger.OutputJsonToWriter(os.Stdout, nil))firstLogger.Info("first log")/*  {    "msg": "first log",    "lvl": 4,    "field-A": "value-A",    "field-B": "value-B",    "field-C": "value-C"  }*/
secondLogger := firstLogger.  RawFields(logger.LogFields{    "field-B": "new value",  })secondLogger.Info("second log")/*  {    "msg": "second log",    "lvl": 4,    "field-B": "new value",  }*/

Field method

This helper method can be used to retrieve the values of the Base fields (and just Base fields):

someLogger := logger.NewDefault().  Fields(logger.LogFields{    "field": "value",  })v := someLogger.Field("field")fmt.Println(v)// "value"

info

It's impossible to query PreHooks, AdHoc fields or PostHooks directly from the Logger, because they're not ready yet.

tip

You can use this method to create more complex rules when cloning Loggers. See the implementation of LoggerCLI for a concrete example.

Dynamic Fields

You may have noticed that if you need to calculate the value of some field every time a new log is created (think about calculating the timestamp of your logs, for example), the Base Fields are useless, since the Fields/RawFields methods can only set fields with constant values.

To set dynamic fields, you will need to use Hooks. They will be discussed in the following pages.