# How to Write Buffer Plugin

TODO: Write

## Methods

### evacuate\_chunk

You can override this method to add feature to evacuate chunks before clearing the queue when reaching the retry limit. See [Buffer - Handling Successive Failures](https://docs.fluentd.org/buffer#handling-successive-failures) for details.

```ruby
def evacuate_chunk(chunk)
  unless chunk.is_a?(Fluent::Plugin::Buffer::FileChunk)
    raise ArgumentError, "The chunk must be FileChunk, but it was #{chunk.class}."
  end

  backup_dir = File.join(backup_base_dir, 'buffer', safe_owner_id)
  FileUtils.mkdir_p(backup_dir, mode: system_config.dir_permission || Fluent::DEFAULT_DIR_PERMISSION) unless Dir.exist?(backup_dir)

  FileUtils.copy([chunk.path, chunk.meta_path], backup_dir)
  log.warn "chunk files are evacuated to #{backup_dir}.", chunk_id: dump_unique_id_hex(chunk.unique_id)
rescue => e
  log.error "unexpected error while evacuating chunk files.", error: e
end
```

If this article is incorrect or outdated, or omits critical information, please [let us know](https://github.com/fluent/fluentd-docs-gitbook/issues?state=open). [Fluentd](http://www.fluentd.org/) is an open-source project under [Cloud Native Computing Foundation (CNCF)](https://cncf.io/). All components are available under the Apache 2 License.


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