35 lines
1.8 KiB
Markdown
35 lines
1.8 KiB
Markdown
|
# 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
|
