ACK of network transfer is available ("require_ack_response" option). This option ends up choice of at-most-once semantics vs. at-least-once semantics. You need to choose and you can choose.

Fuentd provides "buffer_type file" to buffer records on disk. Shutting down won't loose data. If you need to choose memory buffer for performance reasons, fluentd enables "flush_at_shutdown" option by default.

You would also want to use <secondary> feature. This lets you to write a buffer chunk to another storage if the primary destination is not available "retry_limit" times.

Those concerns would be solved by the document: http://docs.fluentd.org/articles/out_forward#buffered-output...