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qlogger v1.6.0

very fast easily customizable logger


Build Status Coverage Status

quick nodejs logging and newline delimited data transport

QLogger is a a very fast logger, also a toolkit for building very fast loggers. It can be used out of the box as-is, or it can be easily configured in new ways for custom loggers. It uses the standard Unix syslog(2) loglevels, and is lean, fast, very flexible, and easy to use.

The logger can log in any format, eg space-separated text or json bundles. Formatting and writing is done by pluggable filters that transform the log messages, and writers that record them. Use one of the several built in, or provide your own.

The built-in createWriter built writer of type file:// is multi-process safe, it does not let line fragments from one logger overwrite or interleave with line fragments of another logger; each line is guaranteed to be logged in its entirety.

And it's nice and fast. On my system I get 1450k 200 byte lines per second saved to a shared logfile under LOCK_EX mutex (writing with qfputs without filtering, or 850k/sec if also adding a timestamp and the loglevel.

A slow logger can report on the data being processed. A fast logger is a data streaming engine, and can itself process data.

    const qlogger = require('qlogger');
    const filters = require('qlogger/filters');

    const logger = qlogger('info');
    logger.addWriter(qlogger.createWriter('file:///var/log/myApp/app.log');'Hello, world.');

qlogger is a nodejs rewrite of the Quick_Logger PHP logging utility; see quicklib.


new QLogger( [loglevel], [writer] )

Create a logger that will log messages of loglevel importance or above. It is an error if the loglevel is not recognized.

Loglevel can be specified as a string 'error', 'info' or 'debug'. If omitted, it defaults to 'info'. Internally, loglevels are converted to the standard unix syslog(2) loglevels 3, 6 and 7. As of version v1.4.0, all syslog loglevels are recognized: 'emerg', 'alert', 'crit', 'err', 'warning', 'notice', 'info' and 'debug', along with 'all' and 'none'.

The optional writer may be a writerObject (see addWriter below), or a writer specification string. The latter will create one of the built-in writers using QLogger.createWriter(). If no writerSpec is given, the logger will be created without a writer. Writers can be added later at any time. It is an error if the writer specification is not recognized. The built-in writers are those supported by createWriter().

QLogger.createWriter( writerSpec )

This class method will create a new writer corresponding to the spec. The recognized writer specifications are:

    file://</path/to/file>          // absolute filepath
    file://<file/name>              // relative filename
    stdout://                       // process.stdout
    stderr://                       // process.stderr
    tcp://<host>:<port>             // net.connect() tcp connection
    udp://<host>:<port>             // datagram

The returned writer will have a method write(str, cb), and some also a method fflush(cb).

Logging Methods

log( message [, ...] )

Log a multi-argument message. The message is passed to the filters using the current loglevel (so a logger that has loglevel 'debug' will write log() messages as if they were from debug()). Multiple arguments are gathered into an array, and the filter is expected to convert them to a string. (This last allows printf-like formatted output).

error( message )

Log an error message. Error messages will be logged by all loglevels, 'error', 'info' and 'debug'.

info( message )

Log an informational message. The logger must have loglevel 'info' or 'debug'.

debug( message )

Log a debug message. The logger must have loglevel of 'debug'.

fflush( callback )

Tries to force all writers to write out any buffered data. Invokes the callback once the writes have all finished. This is a half-hearted implementation, since fflush can only flush write streams, tcp sockets and objects with an fflush(cb) method e.g. FileWriter (file://) objects.

loglevel( [newLoglevel] )

returns the current loglevel. The loglevel controls the log sensitivity; a loglevel of 'info' would write info() and error() messages but not debug(). If a new loglevel is specified, the logger will change the loglevel and returns the old loglevel.

Configuration Methods

addWriter( writerObject )

Have the logger write log messages with the writer object. The writerObject must have a method write( string, callback ). The writer will be called with the already formatted log line. Multiple writers are supported. Writers are run in the order added, but are not serialized, and writers may complete out of order. If the writer has a method fflush( callback ) it will be used when flushing the buffered data. Qlogger tries to snoop stream and socket objects to know whether they're busy, other objects should either have an fflush method or will not be checkpointable.

A writer can be any object that records the message, for example:

    const logger = qlogger();
        write: function(message, callback) {
            const timestamp = new Date().toISOString();
            const levelName = qlogger.LEVELNAMES[loglevel];
            process.stdout.write(timestamp + ' [' + levelName + '] ' + message + '\n', callback);
        fflush: function fflush(callback) {
            process.stdout.write("", callback);

getWriters( )

Return the array of writers added to this qlogger.

removeWriter( writerObject )

Remove the writer object from this qlogger. The writerObject should be the same as was added with addWriter.

addFilter( filterFunction( message, loglevel ) )

A filter is a function that modifies the message being logged before it is written. It is passed two arguments, the message and the current numeric loglevel, and is expected to return the message to log. If the final transformed message does not have a terminating newline, one will be added before writing. Filters are applied in the order they were added.

If there is more than one filter on a logger, they will be run in the order added.

    QLogger = require('qlogger');
    logger = new QLogger('info', process.stdout);
        function(msg, loglevel) {
            return new Date().toISOString() + " [" + QLogger.LEVELNAMES[loglevel]+ "] " + msg;
    );"Hello, world.");
    logger.debug("debug messages not enabled");
    // => 2014-10-18T12:34:56.667Z [info] Hello, world.
    // => 2014-10-18T12:34:56.668Z [error] Done.
    // => 

By using filters it is possible to daisy-chain or fan out loggers to have messages be observed by multiple loggers or logged by multiple agents. The very first filter added sees the raw unfiltered message, each subsequent filter modifies the result string returned by the previous filter.

    const logger = qlogger('info');
    logger.addFilter(function(message, loglevel) {
        return 'logger says: ' + message;
    logger.addFilter(function(message, loglevel) {
        return 'listen up, ' + message;
    })'hello, world.');
    // => "listen up, logger says: hello, world.\n'

A few simple filters are included with qlogger; see below.

getFilters( )

Return the array of filters added to this logger.

removeFilter( filterFunction )

Remove the filter function from this qlogger. The function should be the same as was added wtih addFilter.

Built-In Filters

filterBasic = require('qlogger/filters').BasicFilter.create()

BasicFilter produces a plaintext logline with a human-readable timestamp and the logelevel.

    var filter = require('qlogger/filters').BasicFilter.create();
    logger.addFilter(filter);"Hello, world.")
    // => "2014-10-19 01:23:45.678 [info] Hello, world.\n"

filterJson = require('qlogger/filters').JsonFilter.create( template [,opts] )

filterJson(message, level) logs a stringified json bundle with fields "time", "level" and possibly "message" (unless explicitly disabled by setting them to false). time is a millisecond timestamp, level is the name of the message loglevel. Other fields are copied from the message object being logged (unless not an object, in which case the bundle message property is set to the logged value).

The json filter can merge fields from a static template into each logline. The logged bundle fields will contain the template fields, the standard fields, then all other fields on the logged object, in that order. The template can be used to add static info to each logline (e.g. hostname, version) and to control the order of the fields in the output.

The standard fields "time", "level" and "message" (and "error" if logging an Error object), are replaced with run-time values. If the message itself contains time, level or message, the fields from the message will be the ones output.

To omit "time" or "level" from the logline, set them to false in the template. message cannot be disabled, since it is automatically set to the logged value when logging non-objects; is not used otherwise.

    var JsonFilter = require('qlogger/filters').JsonFilter.create();
    var loglineTemplate = {
        // the template defines the basic set of fields to log
        // and the order they will appear in.  If logging objects,
        // any additional fields from the object will be appended.
        // If logging Error objects, the message will be set to
        // the error message, and error:{code:, message:, stack:}
        // will be copied from the Error object.
        time: 'provide',
        level: 'provide',
        custom1: 123,
        message: 'will provide'
    filterJson = JsonFilter.create(loglineTemplate);
    logger.addFilter(filterJson);"Hello, world.");
    // {"time":1414627805981,"level":"info","custom1":123,"message":"Hello, world."} Error("oops"));
    // {"time":1414627805981,"level":"info","custom1":123,"message":"Error: oops",
    //  "error":{"code":undefined,"message":"oops","stack":"Error: oops\n    at ..."}}

The json encoding function to use can be specified in opts.encode. The default is JSON.stringify.


  • encode - encoding function to use to serialize. Default is JSON.stringify
  • timestamp - function to generate the timestamp value to include in the output, eg filters.formatJsDateIsoString(). Default is filters.getTimestamp().


qlogger exports its source of high-speed millisecond timestamps. These timestamps are also tuned for groups of reads close together.

    const filters = require('qlogger/filters');

filters.getTimestamp( )

Return the current millisecond timestamp, like new Date().getTime(). The current timestamp is cached and reused, with a setTimeout to invalidate it when it expires. If the event loop is blocked, the next few timestamps fetched may be stale (up to 50). Yielding to the event loop with an asynchronous callback, setTimeout or setImmediate will refresh the timestamp.

filters.getTimestampAsync( callback(err, ms) )

Return the current millisecond timestamp, guaranteed to not be stale. Freshness is ensured to within a millisecond of the actual wallclock time by wrapping the callback in a setImmediate, thus letting the timestamp expire first in case the event loop had been blocked.

Timestamp Formatting

QLogger exports a few simple timestamp formatters. The formatters take a millisecond timestamp as returned by, and convert it to a datetime string. The formatters are optimized to be esepcially fast for clustered timestamps, many close together.

filters.formatIsoDate( [timestamp] )

    var timestamp =;
    // => 1414627805981

    var time = formatIsoDate(timestamp);
    // => 2014-10-29 20:10:05

filters.formatIsoDateUtc( [timestamp] )

    // => 2014-10-30 00:10:05

filters.formatNumericDateUtc( [timestamp] )

    // => "20141030001005.981"

    // => "20190203194104.461"

filters.formatJsDateIsoString( [timestamp] )

    // => "2019-02-03T19:41:04.461Z"

filters.formatBasicDate( [timestamp] )

    // => "2019-02-03 19:41:04.461"


QLogger sends newline-delimited strings (messages) to writers. The strings may be edited in flight by filters. Filters return the modified string, and can annotate it with timestamp, loglevel, hostname, etc., or serialize objects for export. Writers deliver the strings to their detaination.

Newline delimited text is a universally compatible, easy to scan, and very very fast way to stream and process data.

Writers can be added to write to file, send over TCP/IP, send to syslog, etc. The messages can be modified in flight by filters, causing the altered message to be written. Common filters would be to add a timestap and the message loglevel. Writers and filters must be configured explicitly, there is no default. Each logger supports multiple filters and multiple writers.

Qlogger exports the full set of the syslog(2) log reporting levels, from emerg() to debug():

#define LOG_EMERG       0       /* system is unusable */
#define LOG_ALERT       1       /* action must be taken immediately */
#define LOG_CRIT        2       /* critical conditions */
#define LOG_ERR         3       /* error conditions */
#define LOG_WARNING     4       /* warning conditions */
#define LOG_NOTICE      5       /* normal but significant condition */
#define LOG_INFO        6       /* informational */
#define LOG_DEBUG       7       /* debug-level messages */QLogger.ERROR = 3;

Points to keep in mind when using logfiles for general-purpose data transport:

  • the logfile might have multiple writers and unix writes are not atomic, ie writes need a mutex (write-write mutex, eg flock(LOCK_EX))
  • the logfile might be consumed by simple readers that do not tolerate partial writes, so each write should be a complete newline terminated message (ie, hold the lock for the duration of the write)
  • the reader might itself modify the logfile (eg compact it), so writes need a mutex (read-write mutex, eg flock(LOCK_EX))
  • the logfile might get consumed (renamed or removed), ie cannot reuse the file handle indefinitely, must reopen the file periodically
  • the logfile could be used for low-latency buffering, so the reopen interval should be pretty short (all consumers of the logfile must wait out the reopen interval to ensure that activity has settled before moving on from the file)

The above safeguards are built into the file:// type writers, with a reopen frequency of 0.05 seconds


Log to stdout, formatting the log lines with the basic plaintext filter:

    qlogger = require('qlogger');
    filters = require('qlogger/filters');
    logger = qlogger('info', process.stdout);

And then"Hello, world.");
    // => 2014-11-22 15:03:38.482 [info] Hello, world.
    logger.debug("debug messages not on");
    // =>
    logger.error("Hello again.");
    // => 2014-11-22 15:03:38.483 [error] Hello again.

The above, step by step:

    QLogger = require('qlogger');
    logger = new QLogger();
    BasicFilter = require('qlogger/filters').BasicFilter;

Log to file using a write stream, formatting the log lines with a quick inline function (note: this is just as an example, file write streams are too slow to use where speed matters):

    fs = require('fs');
    QLogger = require('qlogger');
    logger = new QLogger('info', fs.createWriteStream('app.log', 'a'));
    logger.addFilter(function(msg, level) {
        return new Date().toISOString + " " + msg;

Log to file using a qfputs FileWriter, without any additional formatting. This can stream over 100MB/sec of data one line at a time to a mutex-controlled shared logfile.

    QLogger = require('qlogger');
    logger = new QLogger('info', QLogger.createWriter('file://app.log', 'a'));


For pure streaming line-oriented data transport, see qfputs for high-speed batched fputs(), and qfgets for batched fgets().


  • support a _printit(level, fmt, ...args) method for sprintf-formatted output
  • move the constants out into a separate file to not make filters load the dependencies


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