Add HIP for hooks with equivalent weight run in parallel

hips/hip-9999.md

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+---
+hip: 9999
+title: "Hook Parallelism"
+authors: [ "Jeff van Dam <[email protected]>" ]
+created: "2023-02-28"
+type: "feature "
+status: "draft"
+
+---
+
+## Abstract
+This feature would allow for hooks to be run in parallel instead of serially
+only for lifecycle hooks e.g pre-update, post-delete, pre-rollback, etc. Since
+Helm does not do this already this will save a significant amount of time
+during release operations with many hooks of the same weight.
+
+This feature will NOT work for test hooks!
+
+##  Motivation
+The motivation behind this feature is to save time during release lifecycle
+operations. Many hooks can run in parallel as they are not in any way tied to
+each other, so these hooks can be given the same weight. However even hooks
+marked with the same weight are not ran in parallel in helm. This can lead to a
+significant amount of time during the install/upgrade/rollback/uninstall,
+running hooks in order. Many hooks may only take a few milliseconds to run,
+however helm seems to have a built in timer which it seems to use to fire each
+hook in order, check that it completes, run the next hook. Many minutes are
+lost because of this.
+
+##  Rationale
+The proposed design is to add a flag called "hook-parallelism", which defaults
+to 1. When hooks are run they are then run in go routines, up to, but not
+exceeding the value of the flag concurrently. This will allow for users to only
+use the flag if they require their hooks to run in parallel. Not including the
+flag, or including the flag with a value of 1 or less will be the same as
+running serially.
+
+This feature will not allow test hooks to be run in parallel because it is up
+to the Application Distributor to write tests that they think should run in
+parallel or serially.
+
+##  Specification
+The flag (hook-parallelism) will be added to all applicable commands (install,
+rollback, uninstall, upgrade), and will allow for any hooks attached to those
+commands to be run in parallel when their weight is equivalent. Not including a
+flag will have the hooks run serially. The actual implementation will use a
+semaphore based on the value of the flag to limit the number of hooks being
+run. While the number of go routines run will be equal to the number of hooks
+(sorted by weight), only the value of the flag number of go routines will be
+run at the same time. This will allow the user flexibility in their usage in
+relation to their system constraints.
+
+##  Backwards compatibility
+This should have no backward compatibility issues. Not including the flag will
+have the value default to 1. This will only allow one hook to run at a time,
+and the user should notice no difference in the execution of their hooks.
+
+##  Security implications
+A malicious user could provide an unreasonably high number for the value of the
+hook parallelism flag. This would set the length of the semaphore. This should
+not be something that should affect the normal user.
+
+##  How to teach this
+A good example would be to have two separate test hooks written. Run initially
+and watch one pod spin up and run, then a second after the first has completed.
+Next the flag could be added, and shown that both test pods are spun up at the
+same time. It might be helpful to mention a general estimate of how many hooks
+you want to run in parallel compared to the number of hooks you have associated
+with a command.
+
+##  Reference implementation
+Currently there is an open Github Pull Request [here](https://github.com/helm/helm/pull/11804) which was based on
+[this review](https://github.com/helm/helm/pull/8946) that was based on [this review](https://github.com/helm/helm/pull/7792) which was made from [this issue](https://github.com/helm/helm/issues/7763).
+
+The reviews that this HIP is based on includes the abilty to parallelize test
+hooks which is not the case for this HIP.
+
+
+##  Rejected ideas
+This HIP is based on [another HIP](https://github.com/helm/community/pull/165) which stalled after several rounds of reviews
+pertaining to running test hooks in parallel which this HIP does not want to 
+accomplish.
+
+
+##  Open issues
+N/A
+
+##  References
+ - https://github.com/helm/community/pull/165
+ - https://github.com/helm/helm/pull/8946
+ - https://github.com/helm/helm/pull/7792
+ - https://github.com/helm/helm/issues/7763
+ - https://github.com/helm/helm/pull/11804