Note that the messageId is a concatenation of various elements and does not have a determinant length. You can generally rely on it to be less than 64 bytes - however there is no guarantee that this is safe.
Also note that messageId is generally unique but is not guaranteed to be unique over a long temporal range as the restart of a message processor resets to a zero value a component of the messageId.
If your intent is to use the messageId as unique key in secondary data store (say you want to collect analytics data for offline analysis), we recommend instead using a sequence number or a UUID.
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