Log::Log4perl::Layout::JSON - Layout a log message as a JSON hash, including MDC data


version 0.60


Example configuration:

    log4perl.appender.Example.layout = Log::Log4perl::Layout::JSON
    log4perl.appender.Example.layout.field.message = %m{chomp}
    log4perl.appender.Example.layout.field.category = %c
    log4perl.appender.Example.layout.field.class = %C
    log4perl.appender.Example.layout.field.file = %F{1}
    log4perl.appender.Example.layout.field.sub = %M{1}
    log4perl.appender.Example.layout.include_mdc = 1

    # Optional truncation of specific fields
    log4perl.appender.Example.layout.maxkb.message = 2

    # Note: Appender option!
    # log4perl.appender.Example.warp_message = 0

See below for more configuration options.


This class implements a Log::Log4perl layout format, similar to Log::Log4perl::Layout::PatternLayout except that the output is a JSON hash.

The JSON hash is ASCII encoded, with no newlines or other whitespace, and is suitable for output, via Log::Log4perl appenders, to files and syslog etc.

Contextual data in the Log::Log4perl::MDC hash will be included if "include_mdc" is true.



Specify one or more fields to include in the JSON hash. The value is a string containing one of more Log::Log4perl::Layout::PatternLayout placeholders. For example:

    log4perl.appender.Example.layout.field.message = %m{chomp}
    log4perl.appender.Example.layout.field.category = %c
    log4perl.appender.Example.layout.field.where = %F{1}:%L

If no fields are specified, the default is message = %m{chomp}. It is recommended that message be the first field.


Specify a prefix string for the JSON. For example:

    log4perl.appender.Example.layout.prefix = @cee:



If this is turned on, the prefix is treated as a Log::Log4perl::Layout::PatternLayout string, and will be rendered as a pattern layout.

For example:

    log4perl.appender.Example.layout.prefix = %m{chomp} @cee:
    log4perl.appender.Example.layout.format_prefix = 1

Would log Hello World as:

    Hello World @cee:{ .. MDC as JSON ... }

See also "prefix"


Exclude the message from the JSON (default: 0). If you are logging the message in the prefix for example, you may want to omit the message from the JSON layout.


Include the data in the Log::Log4perl::MDC hash.

    log4perl.appender.Example.layout.include_mdc = 1

See also "name_for_mdc".


Use this name with the field name to specify a maximum length for a specific field. For example:

    log4perl.appender.Example.maxkb.message = 2

Will truncate message if it is more than 2048 bytes in length. Truncated message will have a marker at the end like ...[truncated, was $len chars total]...


Use this name as the key in the JSON hash for the contents of MDC data

    log4perl.appender.Example.layout.name_for_mdc = mdc

If not set then MDC data is placed at top level of the hash.

Where MDC field names match the names of fields defined by the Log4perl configuration then the MDC values take precedence. This is currently construde as a feature.


If true then use canonical order for hash keys when encoding the JSON.

    log4perl.appender.Example.layout.canonical = 1

This is mainly intended for testing.


Set the maximum JSON length in kilobytes. The default is 20KB.

    log4perl.appender.Example.layout.max_json_length_kb = 3.8

This is useful where some downstream system has a limit on the maximum size of a message.

For example, rsyslog has a maxMessageSize configuration parameter with a default of 4KB. Longer messages are simply truncated (which would corrupt the JSON). We use rsyslog with maxMessageSize set to 128KB.

If the JSON is larger than the specified size (not including "prefix") then some action is performed to reduce the size of the JSON.

Currently fields are simply removed until the JSON is within the size. The MDC field/fields are removed first and then the fields specified in the Log4perl config, in reverse order. A message is printed on STDERR for each field removed.

In future this rather dumb logic will be replaced by something smarter.


Switch JSON encoding from ASCII to UTF-8.

warp_message = 0

The warp_message appender option is used to specify the desired behavior for handling log calls with multiple arguments. The default behaviour (warp_message not set>) is to concatenate all arguments using join( $Log::Log4perl::JOIN_MSG_ARRAY_CHAR, @log_args ) and setting a JSON field message to this simple string.

If, on the other hand, warp_message = 0 is applied, then for log calls with multiple arguments these are considered name/value pairs and rendered to a hash-like JSON structure. For log calls with an odd number of arguments (3 or more), the first argument is considered the message and the others are again considered name/value pairs.

See "Appenders Expecting Message Chunks" in Log::Log4perl::Appender for more info on the configuration option.


    local Log::Log4perl::MDC->get_context->{request} = {
        request_uri => $req->request_uri,
        query_parameters => $req->query_parameters

    # ...

    for my $id (@list_of_ids) {

        local Log::Log4perl::MDC->get_context->{id} = $id;



Using code like that shown above, any log messages produced by do_something_useful() will automatically include the 'contextual data', showing the request URI, the hash of decoded query parameters, and the current value of $id.

If there's a $SIG{__WARN__} handler setup to log warnings via Log::Log4perl then any warnings from perl, such as uninitialized values, will also be logged with this context data included.

The use of local ensures that contextual data doesn't stay in the MDC beyond the relevant scope. (For more complex cases you could use something like Scope::Guard or simply take care to delete old data.)


Originally created and maintained through v0.002003 by Tim Bunce. Versions 0.50 and later maintained by Michael Schout <>


The development version is on github at https:// and may be cloned from git://


Please report any bugs or feature requests on the bugtracker website

When submitting a bug or request, please include a test-file or a patch to an existing test-file that illustrates the bug or desired feature.


Michael Schout <>


This software is copyright (c) 2014 by Tim Bunce.

This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.