34 lines
1.8 KiB
Markdown
34 lines
1.8 KiB
Markdown
# Quasi idempotence
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Idempotence means you can execute a function multiple times and always get the same result.
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In real world idempotence typically is difficult because of three aspects:
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1. Equality hard to define
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2. There are unreflected changes in functions input
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3. Function has side effects
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For intentionally side effect executing functions we introduce "quasi idempotence". To avoid ambiguousness we have to define on which attribute we recognize former executions.
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## Status
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Approved.
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## Context
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Software installation with provs. In case of temporal broken external dependencies we want to be able to do another installation over the previous partially broken one.
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We follow the principle "build from scratch", so we leave the question of software version updates unconsidered here.
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## Consequence
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1. add / change (a line) in existing files
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* Solution: Query for changed location respecting the file type / format and skip if it was already performed. We try to avoid this case and use separate files instead of modifying existing ones.
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2. installation of an single / directory file
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* Solution: If the file/directory exists, we consider installation successful and skip this step in another installation run
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3. installation of many files/directories
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* Solution: If the last file/directory exists, we consider installation successful and skip this step in another installation run - this is useful in order to skip time consuming steps. For fast / trivial steps we can choose solution 2.
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4. install apt/deb package
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* Solution: Query whether package (name without version) is installed (either implicitly e.g. in case of apt or explicitly in other cases to avoid unnecessary downloads etc)
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5. running docker containers
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* Solution: Query whether container with name (we have to provide the name on container start) is already running
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