The batch reply: each op’s own reply bytes, in request order, exactly what
a standalone round trip would have returned (including a typed error
reply for an item that failed, so the caller decodes each slot with the
item’s own reply type).
The mixed-operation batch request
(AGDX_BATCH_CODE): up to
MAX_BATCH_OPS managed requests in one
round trip, each carrying its own command code and encoded payload. The
batch amortizes the round trip and nothing else: items execute
independently in order, each yields its own result, and a failed item
fails alone (explicitly NOT a transaction). A nested batch (an item whose
code is the batch code) is rejected.