The filter_record_transformer filter plugin mutates/transforms incoming event streams in a versatile manner. If there is a need to add/delete/modify events, this plugin is the first filter to try.

Example Configurations

filter_record_transformer is included in Fluentd's core. No installation required.

  @type record_transformer
    hostname "#{Socket.gethostname}"
    tag ${tag}

The above filter adds the new field "hostname" with the server's hostname as its value (It is taking advantage of Ruby's string interpolation) and the new field "tag" with tag value. So, an input like

{"message":"hello world!"}

is transformed into

{"message":"hello world!", "hostname":"", "tag":""}

Here is another example where the field "total" is divided by the field "count" to create a new field "avg":

  @type record_transformer
    avg ${record["total"] / record["count"]}

It transforms an event like

{"total":100, "count":10}


{"total":100, "count":10, "avg":"10"}

With the enable_ruby option, an arbitrary Ruby expression can be used inside ${...}. Note that the "avg" field is typed as string in this example. You may use auto_typecast true option to treat the field as a float.

You can also use this plugin to modify your existing fields as

  @type record_transformer
    message yay, ${record["message"]}

An input like

{"message":"hello world!"}

is transformed into

{"message":"yay, hello world!"}

Finally, this configuration embeds the value of the second part of the tag in the field "service_name". It might come in handy when aggregating data across many services.

<filter web.*>
  @type record_transformer
    service_name ${tag_parts[1]}

So, if an event with the tag "web.auth" and record {"user_id":1, "status":"ok"} comes in, it transforms it into {"user_id":1, "status":"ok", "service_name":"auth"}.


<record> directive

Parameters inside <record> directives are considered to be new key-value pairs:


For NEW_FIELD and NEW_VALUE, a special syntax ${} allows the user to generate a new field dynamically. Inside the curly braces, the following variables are available:

  • The incoming event's existing values can be referred by their field

    names. So, if the record is {"total":100, "count":10}, then

    record["total"]=100 and record["count"]=10.

  • tag refers to the whole tag.

  • time refers to stringanized event time.

  • hostname refers to machine's hostname. The actual value is result



You can also access to a certain potion of a tag using the following notations:

  • tag_parts[N] refers to the Nth part of the tag.

  • tag_prefix[N] refers to the [0..N] part of the tag.

  • tag_suffix[N] refers to the [N..] part of the tag.

All indices are zero-based. For example, if you have an incoming event tagged, then tag_parts[1] will represent "my". Also in this case, tag_prefix[N] and tag_suffix[N] will work as follows:

tag_prefix[0] = debug          tag_suffix[0] =
tag_prefix[1] =       tag_suffix[1] =
tag_prefix[2] =   tag_suffix[2] = app

enable_ruby (optional)

When set to true, the full Ruby syntax is enabled in the ${...} expression. The default value is false.

With true, additional variables could be used inside ${}.

  • record refers to the whole record.

  • time refers to event time as Time object, not stringanized event


Here is the examples:

jsonized_record ${record.to_json}
avg ${record["total"] / record["count"]}
formatted_time ${time.strftime('%Y-%m-%dT%H:%M:%S%z')}
escaped_tag ${tag.gsub('.', '-')}
last_tag ${tag_parts.last}
foo_${record["key"]} bar_${record["value"]}

auto_typecast (optional)

Automatically cast the field types. Default is false.

LIMITATION: This option is effective only for field values comprised of a single placeholder.

Effective Examples:

foo ${record["foo"]}

Non-Effective Examples:

foo ${record["foo"]}${record["bar"]}
foo ${record["foo"]}bar
foo 1

Internally, this keeps the original value type only when a single placeholder is used.

renew_record (optional)

By default, the record transformer filter mutates the incoming data. However, if this parameter is set to true, it modifies a new empty hash instead.

renew_time_key (optional, string type)

renew_time_key foo overwrites the time of events with a value of the record field foo if exists. The value of foo must be a unix time.

keep_keys (optional, array type)

A list of keys to keep. Only relevant if renew_record is set to true.

remove_keys (optional, array type)

A list of keys to delete.

Need more performance?

filter_record_modifier is light-weight and faster version of filter_record_transformer. filter_record_modifier doesn't provide several filter_record_transformer features, but it covers popular cases. If you need better performace for mutating records, consider filter_record_modifier instead.


What are the differences between ${record["key"]} and ${key}?

${key} is short-cut for ${record["key"]}. This is error prone because ${tag} is unclear for event tag or record["tag"]. So the ${key} syntax is now deprecated for avoiding this problem. Don't use ${key} short-cut syntax on the production.

Since v0.14, ${key} short-cut syntax is removed.

Learn More

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.

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