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Erlang / Erlang Advanced Interview questions

Explain the internal working of a relup-based hot upgrade?

A relup (release upgrade) file is a generated script describing exactly how to move a running system from one release version to another without a restart: which applications to stop/start, which modules to load, in what order, and which processes need their internal state transformed via code_change/3.

flowchart TD
    A[release_handler:install_release] --> B[Suspend affected processes]
    B --> C[Load new module code]
    C --> D[Call code_change/3 on running gen_server state]
    D --> E[Resume processes with upgraded code + transformed state]
    E --> F{Upgrade successful?}
    F -->|No| G[Roll back using matching downgrade instructions]

release_handler:install_release/1 reads the relup script and executes its instructions in order, pausing affected gen_servers just long enough to swap in new code and let code_change/3 reshape their existing state to match the new version's expectations. Because the relup also contains the reverse (downgrade) instructions, a failed upgrade can be rolled back to the previous known-good release rather than leaving the system in a broken intermediate state.

What OTP callback lets a running gen_server adapt its existing state to a new code version during an upgrade?
What safety net does a relup file provide if an upgrade fails partway through?

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