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When the group lost all members and offsets.retention.minutes expires, the group metadata tombstone tombstones is written to remove the records.

StreamsGroupPartitionMetadataKey

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The target assignment is stored in N + 1 records, where N is the number of members in the group. When a new target assignment is computed, the group coordinator will compare it with the current target assignment and only write the difference between the two assignments to the __consumer_offsets  partition. That also means if a member left the group, a corresponding tombstone record is written. The assignment must be atomic, so the group coordinator will ensure that all the records are written in a single batch, limiting the size to the maximum batch size (default 1MB), as in consumer groups. When a member leaves the group, a new target assignment is calculated and the target assignment member valWhen the group lost all members and offsets.retention.minutes expires, the group metadata tombstone is written.

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