Module log4rs::encode::pattern

A simple pattern-based encoder.

Requires the pattern_encoder feature.

The pattern syntax is similar to Rust's string formatting syntax. It consists of raw text interspersed with format arguments. The grammar is:

format_string := <text> [ format <text> ] *
format := '{' formatter [ ':' format_spec ] '}'
formatter := [ name ] [ '(' argument ')' ] *
name := identifier
argument := format_string

format_spec := [ [ fill ] align ] [ min_width ] [ '.' max_width ]
fill := character
align := '<' | '>'
min_width := number
max_width := number

Special characters

The {, }, (, ), and characters are part of the pattern syntax; they must be escaped to appear in output. Like with Rust's string formatting syntax, type the character twice to escape it. That is, {{ will be rendered as { in output and )) will be rendered as ).

In addition, these characters may also be escaped by prefixing them with a character. That is, { will be rendered as {.

Formatters

A formatter inserts a dynamic portion of text into the pattern. It may be text derived from a log event or from some other context like the current time. Formatters may be passed arguments consisting of parenthesized format strings.

The following formatters are currently supported. Unless otherwise stated, a formatter does not accept any argument.

• d, date - The current time. By default, the ISO 8601 format is used. A custom format may be provided in the syntax accepted by chrono. The timezone defaults to local, but can be specified explicitly by passing a second argument of utc for UTC or local for local time.
  • {d} - 2016-03-20T14:22:20.644420340-08:00
  • {d(%Y-%m-%d %H:%M:%S)} - 2016-03-20 14:22:20
  • {d(%Y-%m-%d %H:%M:%S %Z)(utc)} - 2016-03-20 22:22:20 UTC
• f, file - The source file that the log message came from, or ??? if not provided.
• h, highlight - Styles its argument according to the log level. The style is intense red for errors, red for warnings, blue for info, and the default style for all other levels.
  • {h(the level is {l})} - the level is ERROR
• l``, level - The log level.
• L, line - The line that the log message came from, or ??? if not provided.
• m, message - The log message.
• M, module - The module that the log message came from, or ??? if not provided.
• n - A platform-specific newline.
• t, target - The target of the log message.
• T, thread - The name of the current thread.
• I, thread_id - The ID of the current thread.
• X, mdc - A value from the MDC. The first argument specifies the key, and the second argument specifies the default value if the key is not present in the MDC. The second argument is optional, and defaults to the empty string.
  • {X(user_id)} - 123e4567-e89b-12d3-a456-426655440000
  • {X(nonexistent_key)(no mapping)} - no mapping
• An "unnamed" formatter simply formats its argument, applying the format specification.
  • {({l} {m})} - INFO hello

Format Specification

The format specification determines how the output of a formatter is adjusted before being returned.

Fill/Alignment

The fill and alignment values are used in conjunction with a minimum width value (see below) to control the behavior when a formatter's output is less than the minimum. While the default behavior is to pad the output to the right with space characters (i.e. left align it), the fill value specifies the character used, and the alignment value is one of:

• < - Left align by appending the fill character to the formatter output
• > - Right align by prepending the fill character to the formatter output.

Width

By default, the full contents of a formatter's output will be inserted into the pattern output, but both the minimum and maximum lengths can be configured. Any output over the maximum length will be truncated, and output under the minimum length will be padded (see above).

Examples

The default pattern is {d} {l} {t} - {m}{n} which produces output like 2016-03-20T22:22:20.644420340+00:00 INFO module::path - this is a log message.

The pattern {m:>10.15} will right-align the log message to a minimum of 10 bytes, filling in with space characters, and truncate output after 15 bytes. The message hello will therefore be displayed as hello, while the message hello there, world! will be displayed as hello there, wo.

The pattern {({l} {m}):15.15} will output the log level and message limited to exactly 15 bytes, padding with space characters on the right if necessary. The message hello and log level INFO will be displayed as INFO hello , while the message hello, world! and log level DEBUG will be truncated to DEBUG hello, wo.

Structs

PatternEncoder	An Encoder configured via a format string.
PatternEncoderConfig	The pattern encoder's configuration.
PatternEncoderDeserializer	A deserializer for the PatternEncoder.