SBE

When I parse/write a string from/to a fixed length SBE field

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how

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do I determine/signal where to truncate?

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The string with either be null-terminated or will be of the size of the fixed length field. So if the string is smaller than the field length; it will end with a \0. If the string is the same length as the field length then there is no terminator. 
When writing you do not have to ensure the remainder of the array is all \0 values - although that would work. The string just needs to be terminated with a single \0; or if the string is exactly as large as the field length - you don't need to do anything.

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In addition, SBE is a performance-oriented wire protocol. If checksums are included as for the FIX protocol, application-level checksum computation becomes a significant component of the overall cost.

Is there no way to define groups once rather than have them repeatedly defined for each message type that uses them in the SBE schema?

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Unlike FIXML, there is no definition of an attributeGroup component that is defined once and referenced multiple times (e.g. in a complexType). In SBE each group has to be an explicit component of it's parent group or message. To minimise the amount of 'duplicate' code, there are two broad avenues to consider:

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