This article describes the Fluentd logging mechanism.

Fluentd has two logging layers: global and per plugin. Different log levels can be set for global logging and plugin level logging.

Log Level

Here is the list of supported levels in increasing order of verbosity:

  • fatal

  • error

  • warn

  • info

  • debug

  • trace

The default log level is info, and Fluentd outputs info, warn, error and fatal logs by default.

Global Logs

Global logging is used by Fluentd core and plugins that do not set their own log levels. The global log level can be adjusted up or down.

By Command Line Option

Increase Verbosity Level

The -v option sets the verbosity to debug while the -vv option sets the verbosity to trace.

$ fluentd -v ... # debug log level
$ fluentd -vv ... # trace log level

These options are useful for debugging purposes.

Decrease Verbosity Level

The -q option sets the verbosity to warn while the -qq option sets the verbosity to error:

$ fluentd -q ... # warn log level
$ fluentd -qq ... # error log level

By Config File

You can also configure the logging level in <system> section:

# same as -qq command line option
log_level error

Per Plugin Log

The @log_level option sets different levels of logging for each plugin. It can be set in each plugin's configuration file.

For example, in order to debug in_tail and to suppress all but fatal log messages for in_http, their respective @log_level options should be set as follows:

@type tail
@log_level debug
path /var/log/data.log
# ...
@type http
@log_level fatal

If you do not specify the @log_level parameter, the plugin will use the global log level.

Log Format

Following format are supported:

  • text (default)

  • json

The format can be configured through <log> directive under <system>:

format json
time_format %Y-%m-%d

With this setting, the following log line:

2017-07-27 06:44:54 +0900 [info]: #0 fluentd worker is now running worker=0

is changed to:

{"time":"2017-07-27","level":"info","message":"fluentd worker is now running worker=0","worker_id":0}

Suppress log/stacktrace messages

Fluentd provides two parameters to suppress log/stacktrace messages

  • ignore_repeated_log_interval (since v1.10.2)

  • ignore_same_log_interval (since v1.11.3)


ignore_repeated_log_interval 60s

Under high loaded environment, output destination sometimes becomes unstable and it causes lots of same log message. This parameter mitigates such situation.


ignore_same_log_interval 60s

This is similar to ignore_repeated_log_inteval but covers more usecases. For example, if the plugin generates several log messages in one action, logs are not repeated:

# Retry generates several type messages. ignore_repeated_log_interval can't suppress these messages
def write(chunk)
# process 1...
log.error "message1"
# process 2...
log.error "message2"

ignore_same_log_interval resolves these cases.

Output to Log File

By default, Fluentd outputs to the standard output. Use -o command line option to specify the file instead:

$ fluentd -o /path/to/log_file

Log Rotation Setting

By default, Fluentd does not rotate log files. You can configure this behavior via command-line options:

--log-rotate-age AGE

AGE is an integer or a string:

  • integer: Generations to keep rotated log files.

  • string: frequency of rotation. (Supported: daily, weekly, monthly)

NOTE: When --log-rotate-age is specified on Windows, log files are separated into log-supervisor-0.log, log-0.log, ..., log-N.log where N is generation - 1 due to the system limitation. Windows does not permit delete and rename files simultaneously owned by another process.

--log-rotate-size BYTES

The byte size to rotate log files. This is applied when --log-rotate-age is specified with integer:

Here is an example:

$ fluentd -c fluent.conf --log-rotate-age 5 --log-rotate-size 104857600

NOTE: When --log-rotate-size is specified on Windows, log files are separated into log-supervisor-0.log, log-0.log, ..., log-N.log where N is generation - 1 due to the system limitation. Windows does not permit delete and rename files simultaneously owned by another process.

Capture Fluentd logs

Fluentd marks its own logs with the fluent tag. You can process Fluentd logs by using <match fluent.**>(Of course, ** captures other logs) in <label @FLUENT_LOG>. If you define <label @FLUENT_LOG> in your configuration, then Fluentd will send its own logs to this label. This is useful for monitoring Fluentd logs.

For example, if you have the following configuration:

# omit other source / match
<label @FLUENT_LOG>
<match fluent.*>
@type stdout

Then, Fluentd outputs fluent.info logs to stdout like this:

2014-02-27 00:00:00 +0900 [info]: shutting down fluentd
2014-02-27 00:00:01 +0900 fluent.info: {"message":"shutting down fluentd"} # by <match fluent.*>
2014-02-27 00:00:01 +0900 [info]: process finished code = 0

Case 1: Send Fluentd Logs to Monitoring Service

You can send Fluentd logs to a monitoring service by plugins e.g. datadog, sentry, irc, etc.

# Add hostname for identifying the server
<label @FLUENT_LOG>
<filter fluent.*>
@type record_transformer
host "#{Socket.gethostname}"
<match fluent.*>
@type monitoring_plugin
# ...

Case 2: Use Aggregation/Monitoring Server

You can use out_forward to send Fluentd logs to a monitoring server. The monitoring server can then filter and send the logs to your notification system e.g. chat, irc, etc.

Leaf Server Example:

# Add hostname for identifying the server and tag to filter by log level
<label @FLUENT_LOG>
# If you want to capture only error events, use 'fluent.error' instead.
<filter fluent.*>
@type record_transformer
host "#{Socket.gethostname}"
original_tag ${tag}
<match fluent.*>
@type forward
# Monitoring server parameters

Monitoring Server Example:

@type forward
# Ignore trace, debug and info log. Of course, you can use strict matching
# like `<filter fluent.{warn,error,fatal}>` without grep filter.
<filter fluent.*>
@type grep
key original_tag
pattern fluent.(warn|error|fatal)
# your notification setup. This example uses irc plugin
<match fluent.*>
@type irc
host irc.domain
channel notify
message notice: %s [%s] @%s %s
out_keys original_tag,time,host,message
# Unlike v0.12, if `<label @FLUENT_LOG>` is defined,
# `<match fluent.*>` in root is not used for log capturing.
<label @FLUENT_LOG>
# Monitoring server's internal log

If an error occurs, you will get a notification message in your Slack notify channel:

01:01 fluentd: [11:10:24] notice: fluent.warn [2014/02/27 01:00:00] @leaf.server.domain detached forwarding server 'server.name'

Deprecate Top-Level Match

You can still use v0.12 way without <label @FLUENT_LOG> but this feature is deprecated. This feature will be removed in fluentd v2.

If this article is incorrect or outdated, or omits critical information, please let us know. Fluentd is an open-source project under Cloud Native Computing Foundation (CNCF). All components are available under the Apache 2 License.