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Here is the detailed strategy:

1. Zstd will only be allowed with magic = 2 format. That is,
   • Instantiating MemoryRecords with magic < 2 is disallowed.
   • Down-conversion of zstd-compressed records will not be supported. So if the requested partition uses 'producer' compression codec and the client requests magic < 2, the broker will down-convert the batch until before using zstd and return with a dummy oversized record in place of the zstd-compressed batch. When the client attempts to fetch from then on, it will receive a UNSUPPORTED_COMPRESSION_TYPE error.
     
2. Bump produce and fetch request versions. It will give the old clients a message to update their version.
3. Zstd will only be allowed for the bumped produce API. That is, for older version clients (=below KAFKA_2_1_IV0), we return UNSUPPORTED_COMPRESSION_TYPE regardless of the message format.

4. Zstd will only be allowed for the bumped fetch API. That is, if the requested partition uses zstd and the client version is below KAFKA_2_1_IV0,  we return UNSUPPORTED_COMPRESSION_TYPE regardless of the message format. Down-conversion of zstd-compressed records will not be supported.

The following section explains why we chose this strategy.

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