# 4 Upgrade when Erlang/OTP has Changed

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4.1 Introduction

As of Erlang/OTP 17, most applications deliver a valid application upgrade file (appup). In earlier releases, a majority of the applications in Erlang/OTP did not support upgrade. 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.

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4.2 Upgrade of Core Applications

The core applications ERTS, Kernel, STDLIB, and SASL never allow real soft upgrade, but require the Erlang emulator 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 emulator with the new versions of the above 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 emulator 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 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 emulator 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 emulator. This is a big risk since there is no guarantee that new beam code can be loaded into the old emulator. Adding instructions after restart_emulator has no effect as the release_handler will not execute them.

For information about the release upgrade file, see the relup(4) manual page in SASL. For more information about upgrade instructions, see the appup(4) manual page in SASL.

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4.3 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.