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The Development Guide has a chapter 7.4. Clustered HA Singleton Service.
The EJB Development Guide has a chapter 8.9. Implement an HA Singleton.
They are both on the same topic, but are vastly different. The chapter in EJB Dev Guide has more detailed examples, but the text seems wrong (it speaks about "wrapping a service with the SingletonService decorator", which is how things worked in EAP 6, not in EAP 7). On the other hand, the chapter in Dev Guide has a more precise text but only partial example.
This should be reconciled. I'd suggest starting with the example from EJB Dev Guide and the text from Dev Guide.
If the resulting topic will be reused in both guides, I'm fine with that.