in_dummy has been renamed in_sample since Fluentd v1.11.12.
See sample input plugin article for more details.
You can keep to use following configuration in v1:
<source>
@type dummy
dummy {"hello":"world"}
</source>
Fluentd v2 will remove this plugin name.
If this article is incorrect or outdated, or omits critical information, please let us know.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.