The out_exec_filter Buffered Output plugin (1) executes an external program using an event as input and (2) reads a new event from the program output. It passes tab-separated values (TSV) to stdin and reads TSV from stdout by default.

Example Configuration

out_exec_filter is included in Fluentd's core. No additional installation process is required.
<match pattern>
@type exec_filter
command cmd arg arg
in_keys k1,k2,k3
out_keys k1,k2,k3,k4
tag_key k1
time_key k2
time_format %Y-%m-%d %H:%M:%S
Please see the Config File article for the basic structure and syntax of the configuration file.


type (required)

The value must be exec_filter.

command (required)

The command (program) to execute. The out_exec_filter plugin passes the incoming event to the program input and receives the filtered event from the program output.


The number of spawned process for command. Default is 1.
If the number is larger than 2, fluentd uses spawned processes by round robin fashion.


Respawn command when command exit. Default is disabled.
If you specify a positive number, try to respawn until specified times. If you specify inf or -1, try to respawn forever.


The format used to map the incoming event to the program input.
The following formats are supported:
  • tsv (default)
When using the tsv format, please also specify the comma-separated in_keys parameter.
in_keys k1,k2,k3
  • json
  • msgpack


The format used to process the program output.
The following formats are supported:
  • tsv (default)
When using the tsv format, please also specify the comma-separated out_keys parameter.
out_keys k1,k2,k3,k4
  • json
  • msgpack
When using the json format, this plugin uses the Yajl library to parse the program output. Yajl buffers data internally so the output isn't always instantaneous.


The name of the key to use as the event tag. This replaces the value in the event record.


The name of the key to use as the event time. This replaces the the value in the event record.


The format for event time used when the time_key parameter is specified. The default is UNIX time (integer).

Buffered Output Parameters

For advanced usage, you can tune Fluentd's internal buffering mechanism with these parameters.


The buffer type is memory by default (buf_memory) for the ease of testing, however file (buf_file) buffer type is always recommended for the production deployments. If you use file buffer type, buffer_path parameter is required.

buffer_queue_limit, buffer_chunk_limit

The length of the chunk queue and the size of each chunk, respectively. Please see the Buffer Plugin Overview article for the basic buffer structure. The default values are 64 and 8m, respectively. The suffixes "k" (KB), "m" (MB), and "g" (GB) can be used for buffer_chunk_limit.


The interval between data flushes. The default is 60s. The suffixes "s" (seconds), "m" (minutes), and "h" (hours) can be used.


If set to true, Fluentd waits for the buffer to flush at shutdown. By default, it is set to true for Memory Buffer and false for File Buffer.

retry_wait, max_retry_wait

The initial and maximum intervals between write retries. The default values are 1.0 seconds and unset (no limit). The interval doubles (with +/-12.5% randomness) every retry until max_retry_wait is reached.
Since td-agent will retry 17 times before giving up by default (see the retry_limit parameter for details), the sleep interval can be up to approximately 131072 seconds (roughly 36 hours) in the default configurations.

retry_limit, disable_retry_limit

The limit on the number of retries before buffered data is discarded, and an option to disable that limit (if true, the value of retry_limit is ignored and there is no limit). The default values are 17 and false (not disabled). If the limit is reached, buffered data is discarded and the retry interval is reset to its initial value (retry_wait).


The number of threads to flush the buffer. This option can be used to parallelize writes into the output(s) designated by the output plugin. Increasing the number of threads improves the flush throughput to hide write / network latency. The default is 1.


The threshold for checking chunk flush performance. The default value is 20.0 seconds. Note that parameter type is float, not time.
If chunk flush takes longer time than this threshold, fluentd logs warning message like below:
2016-12-19 12:00:00 +0000 [warn]: buffer flush took longer time than slow_flush_log_threshold: elapsed_time = 15.0031226690043695 slow_flush_log_threshold=10.0 plugin_id="foo"

log_level option

The log_level option allows the user to set different levels of logging for each plugin. The supported log levels are: fatal, error, warn, info, debug, and trace.
Please see the logging article for further details.

Script example

Here is an example writtein in ruby.
require 'json'
require 'msgpack'
while line = STDIN.gets # continue to read a event from stdin
# Input format depends on exec_filter's in_format setting
json = JSON.parse(line)
# main processing. You can do anything, mutate record, access to database and etc.
json['new_field'] = "Hey from exec_filter script!"
# Write data to stdout. Output format depends on exec_filter's out_format setting
STDOUT.print MessagePack.pack(json)
# Call flush to avoid buffering events
rescue Interrupt # Ignore Interrupt exception because it happens during exec_filter shutdown
Corresponding configuration is below:
<match test.**>
@type exec_filter
command ruby /path/to/ruby_script.rb
in_format json
out_format msgpack
flush_interval 10s
tag filtered.exec
If you want to use other language, translate above script example into your language.
If this article is incorrect or outdated, or omits critical information, please let us know. Fluentd is a open source project under Cloud Native Computing Foundation (CNCF). All components are available under the Apache 2 License.
Last modified 4yr ago