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LogP6 cpan:ATROXAPER last updated on 2019-04-25


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LogP6 - full customisable and fast logging library inspired by idea of separate a logging and its configuration. You can use it not only in apps but even in your own libraries.



Logger system have to be as much transparent as possible. At the same time it have to be fully customisable. It have to provide possibility to change logging logic without changing any line of code. It is amazing if you can use logger system during developing a library and its user do not feel discomfort of it. LogP6 logger library is all about that.



  1. Possibility to change logger configuration and its behaviour without touching the code;
  2. Configuring from the code and/or configuration files;
  3. Possibility to use any kind of IO::Handler, even for async or database work;
  4. Possibility to use multiple loggers in one app;
  5. Possibility to use multiply IO::Handler's in one logger;
  6. Flexible filter system;
  7. Stack and Map entities associated with each thread and possibility to use values from it in the logger;
  8. Pretty fast work. Using pre-calculation as much as it can be done - parse logger layout pattern only once, reuse current DateTime objects and so on;
  9. Possibility to use logger while development (dynamically change logger settings in runtime), and while production work (maximum fast, without any lock excepts possible IO::Handle implementation's).



Using logger:

use LogP6;                                  # use library in general mode

my \log = get-logger('audit');              # create or get logger with 'audit' trait'property ' ~ 'foo' ~ ' setted as ' ~ 5);  # log string with concatenation'property %s setted as %d', 'foo', 5);     # log sprintf like style

Configure the logger in code:

use LogP6 :configure;   # use library in configure mode
cliche(                 # create a cliche
  :name<cl>,            # obligatory unique cliche name
  :matcher<audit>,      # obligatory matcher
  grooves => (                                          # optional list of writer-filter pairs (or their names)
    writer(:pattern('%level| %msg'), :handle($*ERR)),   # create anonymous (w/o name) writer configuration
    filter(:level($debug))));                           # create anonymous (w/o name) filter configuration

Configure in the configuration file style (same as above):

  "writers": [{
    "type": "std",
    "name": "w",
    "pattern": "%level | %msg",
    "handle": { "type": "std", "path": "err" }
  "filters": [{
    "type": "std",
    "name": "f",
    "level": "debug"
  "cliches": [{
    "name": "cl",
    "matcher": "audit",
    "grooves": [ "w", "f" ] 


LogP6 library adds an object associated with each Thread - Logger Context or just context. You can work with the context directly in filter subsystem or in custom writer implementations. Also logger has special methods for working with NDC and MDC (see below in Logger). For more information please look at methods' declarators in LogP6::Context.


Writer is responsible to write all corresponding data to corresponding output in some format. It has only one method:


Filter is responsible to decide allow the corresponding writer to write log or not. It has three methods:

Nested Diagnostic Context (NDC) and Mapped Diagnostic Context (MDC)

There are cases when we want to trace some information through group of log messages, but not only in one message. For example, user id, http session number or so. In such cases we have to store the information somewhere, pass it through logic subs and methods and pass to log methods over and over again. Since log system have to be separated from the main program logic then we need a special place to store the information. That place is a Nested Diagnostic Context (NDC) - a stack structure and a Mapped Diagnostic Context (MDC) - a map structure. You can push/pop values in NDC and put/remove values in MDC. Standard writer has special placeholders for message pattern (see below in Pattern) for put all values from NDC or some kay associated value from MDC to final log message string.


The logger is an immutable object that contains from zero to several pairs of writer and filter (grooves). For each time the you want to log some message (with or without arguments) the logger compiles message+arguments in one message string, updates the context and goes through grooves - call filter's methods and if it pass then ask a writer to write message. The writer takes all necessary information such as message, log level, NDC/MDC values, current date-time and so from the context.

Logger has the following methods:

Logger Wrapper

System to wrap (or decorate) logger object into another and add additional logic. You can describe logger wrapper factory which will wrap any created logger.

Synchronisation of configuration and Logger instance

An example of logger wrapper usage is synchronisation a logger configuration and logger instance. It may be useful in case of development or debug session to change logger configuration dynamically.

Since a logger object are immutable and cannot know about changes in configuration it produced, we need a logic which checks if user updated corresponding configuration and updates the logger instance.

You can specify any kind of wrapper for logger synchronization. There are helper class LogP6::Wrapper::SyncAbstract to create your own synchronization wrapper.

For now there is only two synchronization wrappers:


For working with LogP6 library you need to use LogP6; module. Without any tags it provides only get-logger($trait) sub for retrieving a logger. :configure tag provides factory subroutines for configuring loggers from the code. Another option to configure logger is using configuration file.

Logger retrieve

For retrieve a logger you need to use LogP6 module and call get-logger($trait) sub with logger trait you need. Example:

use LogP6;

my $log = get-logger('main');
# using $log ...

If you did not configure a Cliche for specified logger trait ('main' in the example), the default logger will be returned (see Default logger). In other case the logger created by the cliche with matcher the trait satisfy will be returned.

Factory subroutines

LogP6 provides subroutines for configure loggers from the code dynamically. To get access to them you need to use LogP6 with :configure tag. There are subroutines for configuring filters, writers, cliches and any default values like writer pattern, logger wrapper or so. Concrete subroutines will be described in corresponding sections below. There is get-logger-pure($trait) sub for retrieve pure logger without any wrappers. Also five variables for five LogP6::Level enum values are exported as $trace, $debug, $info, $warn and $error. Example:

use LogP6 :configure;

set-default-wrapper(; # set default wrapper
set-default-level($debug);    # set default logger level as debug
my $log = get-logger('main'); # get wrapped logger
my $pure-log = get-logger-pure('main'); # this logger will not synchronize its configuration

Configuration file

Better alternative (especially for production using) of configuration by factory subroutines is configuration file. You can specify path to it through LOG_P6_JSON system environment variable. In case the variable is empty then standard path ./log-p6.json will be used (if it exists). Or you can initialize LogP6 library using init-from-file($config-path) factory subroutine.

Configuration file is json formatted file. Example:

  "default-pattern": "%msg",
  "default-level":  "trace",
  "default-handle": { "type": "std", "path": "err" },
  "writers": [{ "type": "std", "name": "w" }],
  "filters": [{ "type": "std", "name": "f" }],
  "cliches": [{ "name": "c2", "matcher": "main" }]

Concrete format for concrete objects will be described in corresponding sections below.

Some object like writers, wrappers or so have a type filed. Each object has its own list of available types. There are type which can be used in any object - custom. It uses to describe factory method or class which will be used to produce the object. It requires additional fields: - require - name of module with factory method or class; - fqn-method or fqn-class - full qualified name of method or class in require module; - args - list of named arguments which will be passed to fqn-method() or

For example, creating IO::Handle by create-handle subroutine in MyModule with arguments :path<out.txt>, :trait<rw>:

  "default-handle": {
    "type": "custom",
    "require": "MyModule",
    "fqn-method": "MyModule::EXPORT::DEFAULT::&create-handle",
    "args": { "path": "out.txt", "trait": "rw" }

## Writer configuration

### WriterConf

`WriterConf` is a configuration object which contains all necessary information
and algorithm for creating a concrete `writer` instance. For more
information please look at methods' declarators in `LogP6::WriterConf`.

### Standard WriterConf

Standard `WriterConf` (`LogP6::WriterConf::Std`) makes a writer which writes log
message to abstract `IO::Handle`. It has a `pattern` - string with special
placeholders for values like `ndc`, current `Thread` name, log message and so.
`Writer` will put all necessary values into `pattern` and write it to handle.
Also standard `WriterConf` has boolean `auto-exceptions` property - if it is
`True` then placeholder for exception will be concatenated to the `pattern`
automatically. Form of the exception placeholder can be configured separately
(see [Defaults](#defaults) and [Cliche](#cliche)).

### Pattern

Pattern placeholders starts with `%` symbol following placeholder name. If
placeholder has arguments they can be passed in curly brackets following
placeholder name.

Pattern can has the following placeholders:

- `%trait`, `%trait{short=[package-delimeter]number}`, `%trait{sprintf=pattern}` -
for name of logger trait. Additionally you can specify one of two options of
trait representation. `sprintf` option is useful for traits like `database`,
`audit` or so, when you want to represent all traits with the same length. For
example, `[%trait{sprintf=%s7}]` can be converted into `[  audit]`. `short`
option is useful for traits like `Module::Packge1::Package2::Class`. You can
specify package delimiter (instead of `::`) and how many packages will be
displayed. For example, `%trait{short=[.]1` can be converted into
`Class`, `%trait{short=[.]-1` - into `Packge1.Package2.Class` and
`%trait{short=[.]2.4` - into `Modu.Pack.Package2.Class`. If `number` is a
positive integer then only `number` right elements will be displayed. If
`number` is a negative integer then `|number|` left elements will be deleted. If
`number` is real then left elements will be cut to fractional symbols;
- `%tid` - for current `Thread` id;
- `%tname` - for current `Thread` name;
- `%msg` - for log message;
- `%ndc` - for full NDC array joined by space symbol;
- `%mdc{obj_key}` - for MDC value with `obj_key` key;
- `%x{$msg $name $trace}` - for exception. String in curly brackets is used as
subpattern. `$msg` - optional exception message, `$name` - optional exception
name, `$trace` - optional exception stacktrace. For example,
`'%x{($name "$msg") Trace: $trace}'` can be converted into
`'(X::AdHoc "test exception") Trace: ...'`;
- `%level{WARN=W DEBUG=D ERROR=E TRACE=T INFO=I length=2}` - log importance
level. By default logger will use level name in upper case but you can
specify synonyms for all or part of them in curly brackets in format
`<LEVEL_NAME>=<sysnonym>`. You can specify a fixed length of log level name.
Default length is 0 - write level as is. For example
`'[%level{WARN=hmm ERROR=alarm length=5}]'` can be converted into
`'[hmm  ]'`, `'[alarm]'`, `'[INFO ]'`, `'[DEBUG]'`;
- `%color{TRACE=yellow DEBUG=green INFO=blue WARN=magenta ERROR=red}` - colorize
log string after that placeholder. You can specify color for any log level.
Level you not specified color will be use its default color (as in example
above). For example, `%color{ERROR=green}` means
`%color{TRACE=yellow DEBUG=green INFO=blue WARN=magenta ERROR=green}`. You can
use `yellow`, `green`, `blue`, `magenta`, `green` color names or color code
(more [information](
For example `%color{TRACE=35 DEBUG=30;48;5;82 INFO=green}`. You can use `%color`
placeholder several times;
- `%color{reset}` or `%creset` - reset log string colorizing after that
- `%date{$yyyy-$yy-$MM-$MMM-$dd $hh:$mm:$ss:$mss $z}` - current date and time.
String in curly brackets is used as
    - `$yyyy`, `$yy` - year in 4 and 2 digits format;
    - `$MM`, `$MMM` - month in 2 digits and short name format;
    - `$dd` - day in 2 digits format;
    - `$hh`, `$mm`, `$ss`, `$mss` - hours, minutes, seconds and milliseconds
    - `$z` - timezone
- `%framefile` - for log caller frame file name. The same as `callframe().file`
in log call block;
- `%frameline` - for log caller frame file line. The same as `callframe().line`
at the same log call line;
- `%framename` - for log caller frame code name. The same as
`callframe()` in log call block;

Note that using `%framefile`, `%frameline` or `%framename` in the pattern will
slow your logging because it requires several `callframe()` calls on each
resultative log call;

### Async writing

`LogP6` provides writer and handle implementation for asynchronous writing.

You can use ` :$delegate!, Scheduler :$scheduler = $*SCHEDULER)`
as handle which will schedule `WRITE` method call of `delegate` handle.

If is it not enough to wrap a handle then you can wrap whole writer. Use
` :$delegate!, Scheduler :$scheduler = $*SCHEDULER), :$name, Bool :$need-callframe)`
as writer configuration of another configuration. Final writer will schedule 
`write` method call of `delegate` created writer with copy of current
`logger context`. If you miss a `:name` parameter then `delegate`'s name will
be used. Pass boolean parameter `need-callframe` if you plan to use callframe
information in wrapped writer. Note that using callframe will slow your logging
because it requires several `callframe()` calls on each resultative log call.

### Writer factory subroutines

`LogP6` module has the following subs for manage writers configurations:

- `get-writer(Str:D $name --> LogP6::WriterConf)` - gets writer with specified
- `writer(:$name, :$pattern, :$handle, :$auto-exceptions, :create, :update, :replace --> LogP6::WriterConf)` -
create, update or replace standard `WriterConf` with specified name. If you want
to `:update` only concrete fields in already created configuration then the rest
fields will not be changed. In case of `:replace` the new configuration will be
created and replaced the old one. You can create configuration without name -
then the configuration will not be stored, but only returned to you. The method
returns the old writer configuration (`:update`, `:replace`) and the new one
- `writer(LogP6::WriterConf:D $writer-conf, :create, :replace --> LogP6::WriterConf)` -
save or replace any implementation of `WriterConf`. The configuration name
will be retrieved from the `$writer-conf`. The method returns the old writer
configuration (`:replace`) and the new one (`:create`);
- `writer(:$name, :$remove --> LogP6::WriterConf)` - remove and return a
configuration with specified name.

### Writer configuration file

In configuration file writer configurations have to be listed in `writers`
array. Only `std` (for standard configuration) and `custom` types are supported.

In case of standard configuration all field are optional excepts `name`. Handle
can be:

- `file` type for output into file. You can specify `path`,`append` (`True`
by default) and `out-buffer` arguments;
- `std` type for output into `$*OUT` or `$*ERR`. You can specify `path` as `out`
or `err`.
- `custom` type.

In case of `custom` writer type the result writer configuration have to returns
not empty name.


  "writers": [
    {"type": "std", "name": "w1", "pattern": "%msg", "handle": {"type": "std", "path": "out"}},
    {"type": "std", "name": "w2", "handle": {"type": "file", "path": "log.txt", "append": false}},
    {"type": "custom", "require": "Module", "fqn-method": "Module::EXPORT::DEFAULT::&writer", "args": { "name": "w3" }}

Filter configuration


Filter creates by FilterConf - a configuration object which contains all necessary information and algorithm for creating a concrete filter instance. For more information please look at methods' declarators in LogP6::FilterConf.

Standard FilterConf

Standard FilterConf (LogP6::FilterConf::Std) has array for do-before subs and array for do-after subs. Filter made by standard FilterConf calls each do-before sub and stop at the first False returned value. If all do-before subs returned True, then the filter's do-before method returns True. The do-after work in the same way. Also there is first-level-check property. If it set to True then the sub for checking log level will be added automatically as the first element in do-before array; if the property set to False then the sub will be added automatically as the last element in do-before array.

Filter factory subroutines

LogP6 module has the following subs for manage filters configurations:

Filter configuration file

In configuration file filter configurations have to be listed in filters array. Only std (for standard configuration) and custom types are supported.

In case of standard configuration all field are optional excepts name. before-check and after-check are arrays with custom typed elements.

In case of custom filter type the result filter configuration have to returns not empty name.


  "filters": [
    {"type": "std", "name": "f1", "level": "error", "first-level-check": false},
    {"type": "std", "name": "f2", "level": "info", "before-check": [{ "require": "MyModule", "fqn-method": "MyModule::EXPORT::DEFAULT::&before-check" }]},
    {"type": "custom", "require": "MyModule", "fqn-class": "MyModule::MyFilter", "args": { "name": "f3" }}


Standard filters and writers has fields and options which affect their work. Some of them you can specify in factory subroutines or configuration file fields. If such arguments are omitted then the default values of it will be used. Another fields and options cannot be setter this way. For example, pattern for exception which will be concatenated to main pattern in standard writer when auto-exceptions sets to True (see Standard WriterConf). Such properties have default values too. All the defaults can be set through factory subroutines or fields in configuration file.

Configuring default values are useful in case you what to avoid many boilerplate configurations.

Defaults factory subroutines

There are the following factory subs for set defaults values:

Defaults configuration file

You can configure default values in configuration file through the following json fields of a root object:

Wrapper can be:


Cliche is a template for creating Logger. Each cliche has cliche's matcher - literal or regex field. When you what to get logger for some logger trait then logger system try to find a cliche with matcher the trait satisfies (by smartmatch). If there are more then one such cliche then the most resent created will be picked. The picked cliche's content will be used for creating the new logger.

Cliche contains writers and filters configurations pairs called grooves and own defaults values which overrides global defaults values (see Defaults). You can use the same writer and/or filter in several grooves. It the grooves list is empty or missed the created logger will drop the all logs you pass to it;

Cliche factory subroutines

LogP6 module has the following subs for manage cliches configurations:

Cliche configuration file

In configuration file cliches have to be listed in cliches array. It has the following fields:


  "cliches": [{
    "name": "c1", "matcher": "/bo .+ om/", "grooves": [ "w1", "f1", "w2", "f1" ],
    "wrapper": { "type": "transparent" }, "default-pattern": "%level %msg"

Default logger

In any way you configured your cliches by the factory routines or configuration file or did not use non of them the default cliche will be in the logger library. Default cliche corresponds the following configuration: cliche(:name(''), :matcher(/.*/), grooves => (writer(:name('')), filter(:name('')))). In another words, default cliche has empty string name, matches any trait, has only one groove with empty (uses all defaults) writer with empty string name and with empty (uses all defaults) filter with empty string name. It means, by default you do not need to configure nothing at all. But you can change the default cliche or default writer and filter by factory subroutines or in configuration file. Note that if LogP6 library will not find cliche with matcher logger trait satisfies then an exception will be thrown.

Change configuration

Sometimes you may need to change logger configuration in runtime execution. It can be simply done by factory subroutines. After calling any factory subroutine all loggers for already used logger traits will be recreated and you can get it by get-logger($trait) sub. If your already got logger use synchronisation wrapper then the wrapper will sync the logger himself correspond its algorithm.

Another way of change configuration is using configuration file modification. Changes in configuration file will be detected only if you already using any of synchronisation wrapper (in defaults or in one of cliches). After any change detection all already configured configuration will be dropped and created the new from the file.


Lets explore a few general use cases:

Use external library witch uses LogP6

LogP6 can be used during library development and a user of the library wants fully turn off any logs from the library. Lets imagine that all libraries loggers traits starts with LIBNAME letters. In such case we can create a cliche with corresponding matcher and empty grooves - all library logs will be dropped.

In Perl 6:

use LogP6 :configure;

cliche(:name('turn off LIBNAME'), :matcher(/^LIBNAME .*/), :wrapper(;

Or in configuration file:

{ "cliches": [{"name": "turn off LIBNAME", "matcher": "/^LIBNAME .*/", "wrapper": {"type": "transparent"}}] }

We use wrapper without synchronisation (transparent) because we do not plan to change configuration for the library loggers.

Change console application verbosity level

Lets imagine we are writing console application and we want to add flag --verbose for getting more detail output. Lets using special logger in purpose of application console output instead of using simple say and change filter level according user's choice:

In Perl 6:

use LogP6 :configure;

cliche(:name<output>, :matcher<say>, grooves => (writer(:pattern('%msg'), :handle($*OUT)), filter(:name<verbosity>, :level($info))));

sub MAIN(Bool :$verbose) {
  filter(:name<verbosity>, :level($debug), :update) if $verbose;
  my $say = get-logger('say');
  $say.debug('You set verbose flag to %s value', $verbose);

In that case we do not need to use configuration file. But if you want then you can remove line with cliche creation and add the following configuration file:

  "writers": [{ "type": "std", "name": "say", "pattern": "%msg", "handle": { "type": "std", "path": "out" }}],
  "filters": [{ "type": "std", "name": "verbosity", "level": "info"}],
  "cliches": [{ "name": "output", "matcher": "say", "grooves": [ "say", "verbosity" ]}]

Associate logs with concrete user

Lets imagine we write a server application. Many users at the same time can connect to the server and do some action which produces log messages in log file. If some exception will be caught and log we want to reconstruct user's execution flow to understand what went wrong. But needful logs in log file will be alongside with logs from other users actions. In such cases we need to associate each log entry with some user id. Then we just can grep log file for the user id. For that just use MDC.

In Perl 6:

use LogP6 :configure;

cliche(:name<logfile>, :matcher<server-log>, grooves => (
    :pattern('[%date{$hh:$mm:$ss:$mss}][user:%mdc{user-id}]: %msg'),

my $server-log = get-logger('server-log');

sub database-read() { # note we do not pass $user in the sub
  $'read from database'); # [23:35:43:1295][user:717]: read from database
  # read
  CATCH { default {
    $server-log.error('database fail', :x($_)); # [23:35:44:5432][user:717]: database fail Exception X::AdHoc "database not found" <trace> 

sub enter(User $user) {
  $server-log.mdc-put('user-id', $;
  $'connected');     # [23:35:43:1245][user:717]: connected


  $'disconnected');  # [23:35:44:9850][user:717]: disconnected
  $server-log.mdc-remove('user-id'); # it is not necessary to remove 'user-id' value from MDC 

The same configuration you can write in configuration file:

  "writers": [{ "type": "std", "name": "logfile", "pattern": "[%date{$hh:$mm:$ss:$mss}][user:%mdc{user-id}]: %msg",
    "handle": { "type": "file", "path": "logfile.log" }}],
  "filters": [{ "type": "std", "name": "logfile", "level": "info"}],
  "cliches": [{ "name": "logfile", "matcher": "server-log", "grooves": [ "logfile", "logfile" ]}]

Filter log by its content

Lets imagine we have an application which may writes sensible content to log file. For example, user passwords. And we want to drop such sensible logs. In such case we can use special sub in do-before action of log's filter.

In Perl 6:

unit module Main;
use LogP6 :configure;

sub drop-passwords($context) is export {...}

cliche(:name<sensible>, :matcher<log>, grooves => (
  writer(:pattern('%msg'), :handle('logfile.log',
  filter(:level($info), before-check => (&drop-passwords))

sub drop-passwords($context) {
  return False if $context.msg ~~ / password /;
  # If you want remove password from the log entry instead of drop it
  # you can remove the password from the message and store it in the context
  # like:
  # $context.msg-set(remove-password($context.msg));

sub connect(User $user) {
  get-logger('log').info('user with name %s and password %s connected', $, $user.passwd);

The same configuration you can write in configuration file:

  "writers": [{ "type": "std", "name": "writer", "pattern": "%msg",
    "handle": { "type": "file", "path": "logfile.log" }}],
  "filters": [{ "type": "std", "name": "pass-filter", "level": "info",
    "before-check": [{ "require": "Main", "fqn-method": "Main::EXPORT::DEFAULT::&drop-passwords" }]}],
  "cliches": [{ "name": "logfile", "matcher": "server-log", "grooves": [ "writer", "pass-filter" ]}]

Write one log in several outputs

Lets imagine we have an application which works with several types of databases. For example, Oracle and SQLite. We want to log of work with the databases. But we want store Oracle related logs in oracle.log and database.log files, but store SQLite related log only in database.log. In such case we need one simple logger for SQLite related logs and another one (with two grooves) for Oracle related logs.

In Perl 6:

use LogP6 :configure;

writer(:name<database>, :handle('database.log';
writer(:name<oracle>,   :handle(  'oracle.log';
filter(:name<filter>, :level($info));
cliche(:name<oracle>, :matcher<oracle>, grooves => ('database', 'filter', 'oracle', 'filter'));
cliche(:name<sqlite>, :matcher<sqlite>, grooves => ('database', 'filter'));

sub oracle-db-fetch() {
  get-logger('oracle').info('fetch data');
  # fetch

sub sqlite-db-fetch() {
  get-logger('sqlite').info('fetch data');
  # fetch

The same configuration you can write in configuration file:

  "default-pattern": "%msg",
  "writers": [
    { "name": "database", "type": "std", "handle": { "type": "file", "path": "database.log"}},
    { "name": "oracle",   "type": "std", "handle": { "type": "file", "path": "oracle.log"  }}
  "filters": [{ "name": "filter", "type": "std", "level": "info" }],
  "cliches": [
    { "name": "oracle", "matcher": "oracle", "grooves": [ "database", "filter", "oracle", "filter" ]},
    { "name": "sqlite", "matcher": "sqlite", "grooves": [ "database", "filter" ]}

Write logs in journald

Lets imagine you want to store logs in journald service. You can use LogP6::Writer::Journald module for that. For more information please look at writer module README. Example of configuration:

In Perl 6:

use LogP6 :configure;

  # name, pattern and auto-exceptions as in standard writer
  :name<to-journald>, :pattern('%msg'), :auto-exeptions
  # which additional information must be written
  :use-priority, # write 'PRIORITY=' field to journald automatically
  :use-mdc       # write all MDC contend as field to journald in 'key=value' format

The same configuration you can write in configuration file:

{"writers": [{
  "type": "custom",
  "require": "LogP6::WriterConf::Journald",
  "fqn-class": "LogP6::WriterConf::Journald",
  "args": {
    "name": "to-journald",
    "pattern": "%msg",
    "auto-exceptions": true,
    "use-priority": true,
    "use-mdc": true

Write custom writer handle for your need

Some time you may need to write log to some exotic place. In such cases you will need to implement your own Writer and its WriterConf. In simple cases it would be enough to implement your own IO::Handle for standard writer. For example, there are no de-facto must-use library for working with databases yet. That is why there are no special writer for it in LogP6. But lets try to write it now:

Imagine we decided to use SQLite database and DB::SQLite library. We can ask the standard writer to prepare sql insert expression for us. Therefore we can only write custom IO::Handle. Fortunately it is easy in Perl 6.d version:

unit module MyDBHandle;
use DB;
use DB::SQLite;

class DBHandle is IO::Handle {
  has Str $.filename is required;
  has DB $!db;

  submethod TWEAK() {
    self.encoding: 'utf8';
    # open database file and create table for logging
    $!db =$!filename);
    $!db.execute('create table if not exists logs (date text, level text, log text)');

  method WRITE(IO::Handle:D: Blob:D \data --> Bool:D) {
    # decode Blob data and execute. we expect the valid sql dml expression in data.

  method close() { $!db.finish } # close database

  method READ(|) { #`[do nothing] }

  method EOF { #`[do nothing] }

It is all we need. Now we can write LogP6 configuration. In Perl 6:

use MyDBModule;
use LogP6 :configure;

  # pattern provides us the valid sql dml expression
  :pattern('insert into logs (date, level, log) values (\'%date\', \'%level\', \'%msg%x{ - $name $msg $trace}\')'),
  # handle with corresponding database filename
  handle =><database-log.sqlite>),
  # turn off auto-exceptions because it will corrupt our sql dml expression
cliche(:name<db>, :matcher<db>, grooves => ('db', level($trace)));

my $log-db = get-logger('db');

$'database logging works well');

The same configuration you can write in configuration file:

  "writers": [{
    "type": "std",
    "name": "db",
    "pattern": "insert into logs (date, level, log) values ('%date', '%level', '%msg%x{ - $name $msg $trace}'",
    "handle": {
      "type": "custom",
      "require": "MyDBModule",
      "fqn-class": "MyDBModule::DBHandle",
      "args": {
        "filename": "database-log.sqlite"
    "auto-exceptions": false
  "filters": [{ "name": "filter", "type": "std", "level": "trace" }],
  "cliches": [{
    "name": "db",
    "matcher": "db",
    "grooves": [ "db", "filter" ]


Try to use good traits for your loggers. If you use loggers in your library then probably using one prefix in all your traits is the best option. It allows users of your library manage your loggers easily.

Try to choice logger trait according logger semantic or location. For example, you can use $?CLASS.^name as logger trait in any your classes or traits like database, user management or so.

If you use logger within a class then make the logger be a class field like has $!log = get-logger('$?CLASS.^name'); If you use logger withing a subroutines logic then make a special sub for retrieve logger like sub log() { state $log = get-logger('trait'); }. Then use it like'msg'); It prevents any side effects caused by precompilation.



Mikhail Khorkov

Source can be located at: github. Comments and Pull Requests are welcome.


Copyright 2019 Mikhail Khorkov

This library is free software; you can redistribute it and/or modify it under the Artistic License 2.0.