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To solve this problem, we propose introducing a new idempotence mode in 1.0.0, ie. enable.idempotence=saferequested. The other modes would be enable.idempotence=off and enable.idempotence=required. In the safe mode, idempotence would be best effort, ie. you get it if the broker supports it, otherwise no error is raised. In required mode, requests would fail if the broker did not support idempotence. In off mode, there is no idempotence (similar to the false value today).

The new enable.idempotence=saferequested mode would be introduced in the 1.0.0 release. In addition, the 1.0.0 broker will automatically downgrade messages sent in safe mode 'requested' mode from the 0.11.0 message format if the topic being written to is configured to use an older message format version. In required mode, the broker would raise an error.

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Additionally, if a 1.0.0 client with enable.idempotence=safe is requested is writing to an 0.11.0 broker, idempotence will be disabled.

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