record_transformer

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.

It is included in the Fluentd's core.

Example Configurations

<filter foo.bar>
  @type record_transformer
  <record>
    hostname "#{Socket.gethostname}"
    tag ${tag}
  </record>
</filter>

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":"db001.internal.example.com", "tag":"foo.bar"}

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

<filter foo.bar>
  @type record_transformer
  enable_ruby
  <record>
    avg ${record["total"] / record["count"]}
  </record>
</filter>

It transforms an event like

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

into

{"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 a 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:

<filter foo.bar>
  @type record_transformer
  <record>
    message yay, ${record["message"]}
  </record>
</filter>

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
  <record>
    service_name ${tag_parts[1]}
  </record>
</filter>

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"}.

Parameters

See Common Parameters.

@type

The value must be record_transformer.

<record> directive

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

<record>
  NEW_FIELD NEW_VALUE
</record>

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 stringified event time.

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

    Socket.gethostname.

You can also access to a certain portion 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 debug.my.app, 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] = debug.my.app
tag_prefix[1] = debug.my       tag_suffix[1] = my.app
tag_prefix[2] = debug.my.app   tag_suffix[2] = app

enable_ruby

type
default
version

bool

false

0.14.0

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 stringified event time.

Here is an example:

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"]}
nested_value ${record["payload"]["key"]}

By historical reason, enable_ruby true is too slow. If you need this option, consider record_modifier filter instead. See also Need more performance? section.

auto_typecast

type
default
version

bool

false

0.14.0

Automatically casts 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

type
default
version

bool

false

0.14.0

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

type
default
version

string

nil

0.14.0

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 timestamp.

keep_keys

type
default
version

array

nil

0.14.0

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

keep_keys has been supported since 0.14.0.

remove_keys

type
default
version

array

nil

0.14.0

A list of keys to delete.

This parameter supports nested field via record_accessor syntax since v1.1.0.

Example:

Given the configuration:

<filter foo.bar>
  @type record_transformer
  remove_keys hostname,$.kubernetes.pod_id
</filter>

And a message:

{
  "hostname":"db001.internal.example.com",
  "kubernetes":{
    "pod_name":"mypod-8f6bb798b-xxddw",
    "pod_id":"b1187408-729a-11ea-9a13-fa163e3bcef1"
  }
}

The output would be:

{
  "kubernetes":{
    "pod_name":"mypod-8f6bb798b-xxddw"
  }
}

Need more performance?

filter_record_modifier is lightweight and faster version of filter_record_transformer. filter_record_modifier does not provide several filter_record_transformer features, but it covers popular cases. If you need better performance for mutating records, consider filter_record_modifier instead.

Tips

Use dig Method for Nested Field

Users sometimes need to access a nested field. In this case, you can use [] to create a chain like this:

${record["top"]["nest1"]["nest2"]}

But, this approach has a problem. When record["top"] or record["top"]["nest1"] does not exist, you hit an unexpected error.

Here is the log example:

error_class=RuntimeError error="failed to expand `record[\"top\"][\"nest1\"][\"nest2\"]` : error = undefined method `[]' for nil:NilClass

dig method resolves this problem. If field is not found, it returns nil instead of raising error:

${record.dig("top", "nest1", "nest2")}

FAQ

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

${key} was a shortcut for ${record["key"]}. This is error-prone because ${tag} is unclear, event tag or record["tag"]. So the ${key} syntax was removed since v0.14. v0.12 still supports ${key} but it is not recommended.

I got unknown placeholder ${record['msg']} found error, why?

Without enable_ruby, ${} placeholder supports only double-quoted string for record field access. So, use ${record["key"]} instead of ${record['key']}. This could also happen when the input does not contain key.

Learn More

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.

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