Format LogEvents as JSON

A format for formatting LogEvents as jsonlines log files. This provides a nice balance between human- an machine-readability.

See also

read_json_lines(), https://jsonlines.org/

Other Layouts: Layout, LayoutFormat, LayoutGlue

Super class

lgr::Layout -> LayoutJson

Active bindings

toJSON_args

a list

timestamp_fmt

a character scalar or a function that accepts a POSIXct as its single argument

transform_event

a function that accepts a LogEvent as its single argument

transform_event_names

a named character vector or a function that accepts a character vector of field names as its single argument.

Methods

Public methods

• LayoutJson$new()

• LayoutJson$format_event()

• LayoutJson$set_toJSON_args()

• LayoutJson$set_timestamp_fmt()

• LayoutJson$set_transform_event()

• LayoutJson$set_transform_event_names()

• LayoutJson$toString()

• LayoutJson$parse()

• LayoutJson$read()

• LayoutJson$clone()

Inherited methods

• lgr::Layout$set_excluded_fields()

---

Method new()

Creates a new instance of this R6 class.

Usage

LayoutJson$new(
  toJSON_args = list(auto_unbox = TRUE),
  timestamp_fmt = NULL,
  transform_event = function(event) event[["values"]],
  transform_event_names = NULL,
  excluded_fields = "rawMsg"
)

Arguments

toJSON_args

a list of arguments passed to jsonlite::toJSON(),

timestamp_fmt

Format to be applied to the timestamp. This is applied after transform_event but before transform_event_names

• NULL: formatting of the timestamp is left to jsonlite::toJSON(),

• a character scalar as for format.POSIXct(), or

• a function that returns a vector of the same length as its (POSIXct) input. The returned vector can be of any type supported by jsonlite::toJSON().

transform_event

a function with a single argument that takes a LogEvent object and returns a list of values.

transform_event_names

• NULL: don't process names

• a named character vector of the format new_name = old_name

• or a function with a single mandatory argument that accepts a character vector of field names. Applied after transform_event.

excluded_fields

A character vector of field names to exclude from the final output. Applied after transform_event_names.

---

Method format_event()

Usage

LayoutJson$format_event(event)

---

Method set_toJSON_args()

Usage

LayoutJson$set_toJSON_args(x)

Arguments

x

a list

---

Method set_timestamp_fmt()

Usage

LayoutJson$set_timestamp_fmt(x)

Arguments

x

a character scalar or a function that accepts a POSIXct as its single argument

---

Method set_transform_event()

Usage

LayoutJson$set_transform_event(x)

Arguments

x

a function that accepts a LogEvent as its single argument

---

Method set_transform_event_names()

Usage

LayoutJson$set_transform_event_names(x)

Arguments

x

a named character vector or a function that accepts a character vector of field names as its single argument.

---

Method toString()

Represent the LayoutJson class as a string

Usage

LayoutJson$toString()

---

Method parse()

Read and parse file written using this Layout

This can be used by the $data active binding of an Appender

Usage

LayoutJson$parse(file)

Arguments

file

character scalar: path to a file

---

Method read()

Read a file written using this Layout (without parsing)

This can be used by the $show() method of an Appender

Usage

LayoutJson$read(file, threshold = NA_integer_, n = 20L)

Arguments

file

character scalar: path to a file

threshold

character Minimum log level to show. Requires parsing of the log file (but will still display unparsed output)

n

integer number of lines to show

---

Method clone()

The objects of this class are cloneable with this method.

Usage

LayoutJson$clone(deep = FALSE)

Arguments

deep

Whether to make a deep clone.

Examples

# setup a dummy LogEvent
event <- LogEvent$new(
  logger = Logger$new("dummy logger"),
  level = 200,
  timestamp = Sys.time(),
  caller = NA_character_,
  msg = "a test message",
  custom_field = "LayoutJson can handle arbitrary fields"
)

lo <- LayoutJson$new()
lo$format_event(event)
#> {"level":200,"timestamp":"2025-07-23 06:40:29","logger":"dummy logger","caller":null,"msg":"a test message","custom_field":"LayoutJson can handle arbitrary fields"} 

lo <- LayoutJson$new(
  transform_event_names = toupper,
  excluded_fields = c("RAWMSG", "CALLER"))

lo$format_event(event)
#> {"LEVEL":200,"TIMESTAMP":"2025-07-23 06:40:29","LOGGER":"dummy logger","MSG":"a test message","CUSTOM_FIELD":"LayoutJson can handle arbitrary fields"} 

lo <- LayoutJson$new(
  transform_event = function(e) {
    values <- e$values
    values$msg <- toupper(values$msg)
    values
  },
  timestamp_fmt = "%a %b %d %H:%M:%S %Y",
  excluded_fields = c("RAWMSG", "CALLER"))

lo$format_event(event)
#> {"level":200,"timestamp":"Wed Jul 23 06:40:29 2025","logger":"dummy logger","caller":null,"msg":"A TEST MESSAGE","rawMsg":"a test message","custom_field":"LayoutJson can handle arbitrary fields"}