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# Upgrade when Erlang/OTP has Changed

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## Introduction

As of Erlang/OTP 17, most applications deliver a valid application upgrade file (`appup`). Many of the applications use the `restart_application` instruction. These are applications for which it is not crucial to support real soft upgrade, for example, tools and library applications. The `restart_application` instruction ensures that all modules in the application are reloaded and thereby running the new code.

## Upgrade of Core Applications

The core applications ERTS, Kernel, STDLIB, and SASL never allow real soft upgrade, but require the Erlang runtime system to be restarted. This is indicated to the `release_handler` by the upgrade instruction `restart_new_emulator`. This instruction is always the very first instruction executed, and it restarts the runtime system with the new versions of the previously mentioned core applications and the old versions of all other applications. When the node is back up, all other upgrade instructions are executed, making sure each application is finally running its new version.

It might seem strange to do a two-step upgrade instead of just restarting the runtime system with the new version of all applications. The reason for this design decision is to allow `code_change` functions to have side effects, for example, changing data on disk. It also guarantees that the upgrade mechanism for non-core applications does not differ depending on whether or not core applications are changed at the same time.

If, however, the more brutal variant is preferred, the release upgrade file can be handwritten using only the single upgrade instruction `restart_emulator`. This instruction, in contrast to `restart_new_emulator`, causes the runtime system to restart with the new versions of all applications.

Note: If other instructions are included before `restart_emulator` in the handwritten `relup` file, they are executed in the old runtime system. This is a big risk since there is no guarantee that new BEAM code can be loaded into the old runtime system. Adding instructions after `restart_emulator` has no effect as the `release_handler` will not execute them.

For information about the release upgrade file, see relup in SASL. For more information about upgrade instructions, see appup in SASL.

## Applications that Still do Not Allow Code Upgrade

A few applications, such as Erl_interface, do not support upgrade. This is indicated by an application upgrade file containing only `{Vsn,[],[]}`. Any attempt at creating a release upgrade file with such input fails. The only way to force an upgrade involving applications like this is to handwrite the file `relup`, preferably as described above with only the `restart_emulator` instruction.

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