add adr quasi-idempotence.md

adr-provs/quasi-idempotence.md

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+# Quasi idempotence
+
+Idempotence means you can execute a function multiple times and always get the same result.
+
+In real world idempotence typically is difficult because of three aspects:
+
+1. Equality hard to define
+2. There are unreflected changes in functions input
+3. Function has side effects
+
+For intentionally side effect executing functions we introduce "quasi idempotence". To avoid ambiguousness we have to define on which attribute we recognize former executions.
+
+## Status
+
+Approved.
+
+## Context
+
+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.
+
+We follow the principle "build from scratch", so we leave the question of software version updates unconsidered here.
+
+## Consequence
+
+1. add / change (a line) in existing files
+   1. 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.
+2. installation of an single / directory file
+   1. Solution: If the file/directory exists, we consider installation successful and skip this step in another installation run
+3. installation of many files/directories
+   1. 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.
+4. install apt/deb package
+   1. 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)
+5. running docker containers
+   1. Solution: Query whether  container with name (we have to provide the name on container start) is already running