StructuredStateManagement
Contents
Structured state management
Please note that this is a PROPOSAL ONLY. This is not yet 100% implemented.
Goal: Move away from ad-hoc states and state transitions for resource acquisition and modification to a more concrete structured & organized state management system in nova. This new state management system will have advanced new & shiny features such as greater stability, automatic recover mechanisms and greater scalability than what currently exists in nova (and more!).
Definitions
- State
- The particular condition that someone or something is in at a specific time: "the state of the instance request".
- State transition
- Altering a state by applying a function on-top of that state (of which said function may take inputs and provide outputs) resulting in a new state.
- Task
- The application of a state transition on a given state.
- Workflow
- The sequence of administrative/other processes through which a piece of work passes from initiation to completion.
- Orchestrator
- An individual or entity that arranges or control the elements of, as to achieve a desired overall effect.
What problems does this solve in general
- Increases the [stability, extendability, reliability, recoverability] of states and state transitions in nova.
- Makes it easier to [debug, test, understand, verify, review] states and state transitions in nova.
- Removes hard to discover state+transition dependencies and interactions.
- Ensures state transitions are done reliably and correctly by isolating those transitions to a single place/entity.
- Fixes a variety of problems that previously had piecemeal like patches applied to attempt to solve them.
- Eliminates the inherent fragility of the current ad-hoc workflows that exist in nova.
- They are by there ad-hoc nature hard to debug, hard to verify, hard to adjust, hard to understand (just hard in general)...
- Makes it possible to audit & track the state transitions performed on a given resource.
- This kind of functionality has started to appear in nova, but the ad-hoc nature was preserved :-(
- Addresses the underlying key point of http://www.slideshare.net/harlowja/nova-states-summit/9 where states will now be fully & automatically recovered from on cutting events (node failure, resource failure, network failure..).
What problems does this solve in nova (on-top of the general ones)
- Removes the need for periodic tasks to cleanup garbage (orphaned instances, orphaned resources...) left by nova's ad-hoc states.
- Creates the path for smart resource scheduling by allowing the altering and/or replacement of the scheduling workflow with a more complex workflow.
- Makes it possible to do [resizing, live migration] in a more secure and manageable manner.
- Discussion about how this can be done correctly require a intermediary to orchestrate this ownership transfer.
- Makes it possible for nova to have multi-stage booting where an instances and its dependent resources are first reserved, the resources configured, the instance configured, and then finally the instance is powered-on (thus completing the instance provisioning process).
Issues that would likely not have happened with a better state management system
- https://blueprints.launchpad.net/nova/+spec/compute-instance-cleanup-service
- https://bugs.launchpad.net/nova/+bug/1050979
- https://bugs.launchpad.net/nova/+bug/1061024
- https://bugs.launchpad.net/nova/+bug/1082414
- ...
Blueprints
Related wikis
https://wiki.openstack.org/wiki/Convection
Potential Requirements
https://etherpad.openstack.org/task-system
Summit Discussions
Havana summit: https://etherpad.openstack.org/the-future-of-orch
Plan of record
Step 1
Create prototype
- Create core workflow/task library and prototype using said library in nova for run_instance action.
- Split this other action (refactored) into small atomic task chunks (don't aim for perfection just yet, since its a prototype).
- Organize chunks into a workflow and test workflow.
- Adjust unit tests for each small chunks (depending on what it changes) and add new ones.
- Show working prototype at summit session (with associated docs...)
Step 2
- Get feedback on prototype from people involved in making it.
- Get feedback from summit session.
- Get more feedback from email list + other interested parties.
Step 3
- Adjust nova prototype as needed from feedback.
- Split nova prototype into small chunks.
- Adjust tests for each small chunks (depending on what it changes) and add new ones for new functionality.
- Submit chunks into http://review.openstack.org (disabling whole/pieces component until ready to turn on?).
Step 4
- Pick another nova action and refactor it to use design from prototype.
- Split this other action (refactored) into small atomic task chunks.
- Organize chunks into a workflow and test workflow.
- Adjust unit tests for each small chunks (depending on what it changes) and add new ones for new functionality.
- Submit chunks into http://review.openstack.org (disabling whole/pieces component until ready to turn on?).
- Rinse & repeat.
Prototype
https://github.com/Yahoo/NovaOrc