Cover by Francesco Cesarini, Simon Thompson

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Chapter 8. Software Upgrade

You receive a bug report that in one of your instant messaging (IM) servers, the euro symbol is reaching its final destination in a garbled form. You find the error in the library module that maps special characters, correct the bug, recompile the code, and test it. When it’s validated, you transfer the patch to the live servers and load it in the Erlang runtime system. The next time a euro symbol is received, the patched module will be used and the euro symbol will be mapped correctly.

You achieve all of this without strange or complex workarounds, without having to restart your system, and most importantly, without affecting any of the other IM-related events currently being handled. If all of this sounds simple, well, it is. And not only is it simple, but it is also really, really cool! Originally inspired by the Smalltalk language, the software upgrade capability is a feature very rarely found in modern programming.

The ability to load new and updated modules during runtime allows systems to run without interruption, not only when errors are being fixed but also when new functionality is added. It also reduces the turnaround time of bugs and facilitates testing, as in most cases, systems do not have to be restarted for patches to be validated and deployed. The software upgrade mechanism relies on a set of simple but powerful constructs on which more powerful tools are built. These upgrade tools are used in pretty much every Erlang-based system where ...

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