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1805: docs: add overview and migrate existing to github r=tiagolobocastro a=tiagolobocastro
feat: parse human size for malloc and null bdevs
Parse a size with unit post-fix for the malloc and null bdevs.
This makes it much easier to use, example:
size=1TiB vs size_mb=1048576
Signed-off-by: Tiago Castro <tiagolobocastro@gmail.com>
---
docs: move older design docs into the git repo
Signed-off-by: Tiago Castro <tiagolobocastro@gmail.com>
---
docs: add overview for arch and improve csi
Adds an overview png for the README
Improves slight CSI wording and adds a CSI diagram
Signed-off-by: Tiago Castro <tiagolobocastro@gmail.com>
Co-authored-by: Tiago Castro <tiagolobocastro@gmail.com>- Loading branch information
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| # Control Plane Behaviour | ||
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| This document describes the types of behaviour that the control plane will exhibit under various situations. By | ||
| providing a high-level view it is hoped that the reader will be able to more easily reason about the control plane. \ | ||
| <br> | ||
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| ## REST API Idempotency | ||
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| Idempotency is a term used a lot but which is often misconstrued. The following definition is taken from | ||
| the [Mozilla Glossary](https://developer.mozilla.org/en-US/docs/Glossary/Idempotent): | ||
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| > An [HTTP](https://developer.mozilla.org/en-US/docs/Web/HTTP) method is **idempotent** if an identical request can be | ||
| > made once or several times in a row with the same effect while leaving the server in the same state. In other words, | ||
| > an idempotent method should not have any side-effects (except for keeping statistics). Implemented correctly, the `GET`, | ||
| `HEAD`,`PUT`, and `DELETE` methods are idempotent, but not the `POST` method. | ||
| > All [safe](https://developer.mozilla.org/en-US/docs/Glossary/Safe) methods are also ***idempotent***. | ||
| OK, so making multiple identical requests should produce the same result ***without side effects***. Great, so does the | ||
| return value for each request have to be the same? The article goes on to say: | ||
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| > To be idempotent, only the actual back-end state of the server is considered, the status code returned by each request | ||
| > may differ: the first call of a `DELETE` will likely return a `200`, while successive ones will likely return a`404`. | ||
| The control plane will behave exactly as described above. If, for example, multiple `create volume` calls are made for | ||
| the same volume, the first will return success (`HTTP 200` code) while subsequent calls will return a failure status | ||
| code (`HTTP 409` code) indicating that the resource already exists. \ | ||
| <br> | ||
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| ## Handling Failures | ||
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| There are various ways in which the control plane could fail to satisfy a `REST` request: | ||
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| - Control plane dies in the middle of an operation. | ||
| - Control plane fails to update the persistent store. | ||
| - A gRPC request to Mayastor fails to complete successfully. \ | ||
| <br> | ||
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| Regardless of the type of failure, the control plane has to decide what it should do: | ||
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| 1. Fail the operation back to the callee but leave any created resources alone. | ||
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| 2. Fail the operation back to the callee but destroy any created resources. | ||
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| 3. Act like kubernetes and keep retrying in the hope that it will eventually succeed. \ | ||
| <br> | ||
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| Approach 3 is discounted. If we never responded to the callee it would eventually timeout and probably retry itself. | ||
| This would likely present even more issues/complexity in the control plane. | ||
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| So the decision becomes, should we destroy resources that have already been created as part of the operation? \ | ||
| <br> | ||
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| ### Keep Created Resources | ||
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| Preventing the control plane from having to unwind operations is convenient as it keeps the implementation simple. A | ||
| separate asynchronous process could then periodically scan for unused resources and destroy them. | ||
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| There is a potential issue with the above described approach. If an operation fails, it would be reasonable to assume | ||
| that the user would retry it. Is it possible for this subsequent request to fail as a result of the existing unused | ||
| resources lingering (i.e. because they have not yet been destroyed)? If so, this would hamper any retry logic | ||
| implemented in the upper layers. | ||
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| ### Destroy Created Resources | ||
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| This is the optimal approach. For any given operation, failure results in newly created resources being destroyed. The | ||
| responsibility lies with the control plane tracking which resources have been created and destroying them in the event | ||
| of a failure. | ||
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| However, what happens if destruction of a resource fails? It is possible for the control plane to retry the operation | ||
| but at some point it will have to give up. In effect the control plane will do its best, but it cannot provide any | ||
| guarantee. So does this mean that these resources are permanently leaked? Not necessarily. Like in | ||
| the [Keep Created Resources](#keep-created-resources) section, there could be a separate process which destroys unused | ||
| resources. \ | ||
| <br> | ||
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| ## Use of the Persistent Store | ||
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| For a control plane to be effective it must maintain information about the system it is interacting with and take | ||
| decision accordingly. An in-memory registry is used to store such information. | ||
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| Because the registry is stored in memory, it is volatile - meaning all information is lost if the service is restarted. | ||
| As a consequence critical information must be backed up to a highly available persistent store (for more detailed | ||
| information see [persistent-store.md](./persistent-store.md)). | ||
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| The types of data that need persisting broadly fall into 3 categories: | ||
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| 1. Desired state | ||
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| 2. Actual state | ||
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| 3. Control plane specific information \ | ||
| <br> | ||
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| ### Desired State | ||
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| This is the declarative specification of a resource provided by the user. As an example, the user may request a new | ||
| volume with the following requirements: | ||
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| - Replica count of 3 | ||
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| - Size | ||
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| - Preferred nodes | ||
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| - Number of nexuses | ||
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| Once the user has provided these constraints, the expectation is that the control plane should create a resource that | ||
| meets the specification. How the control plane achieves this is of no concern. | ||
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| So what happens if the control plane is unable to meet these requirements? The operation is failed. This prevents any | ||
| ambiguity. If an operation succeeds, the requirements have been met and the user has exactly what they asked for. If the | ||
| operation fails, the requirements couldn’t be met. In this case the control plane should provide an appropriate means of | ||
| diagnosing the issue i.e. a log message. | ||
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| What happens to resources created before the operation failed? This will be dependent on the chosen failure strategy | ||
| outlined in [Handling Failures](#handling-failures). | ||
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| ### Actual State | ||
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| This is the runtime state of the system as provided by Mayastor. Whenever this changes, the control plane must reconcile | ||
| this state against the desired state to ensure that we are still meeting the users requirements. If not, the control | ||
| plane will take action to try to rectify this. | ||
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| Whenever a user makes a request for state information, it will be this state that is returned (Note: If necessary an API | ||
| may be provided which returns the desired state also). \ | ||
| <br> | ||
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| ## Control Plane Information | ||
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| This information is required to aid the control plane across restarts. It will be used to store the state of a resource | ||
| independent of the desired or actual state. | ||
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| The following sequence will be followed when creating a resource: | ||
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| 1. Add resource specification to the store with a state of “creating” | ||
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| 2. Create the resource | ||
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| 3. Mark the state of the resource as “complete” | ||
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| If the control plane then crashes mid-operation, on restart it can query the state of each resource. Any resource not in | ||
| the “complete” state can then be destroyed as they will be remnants of a failed operation. The expectation here will be | ||
| that the user will reissue the operation if they wish to. | ||
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| Likewise, deleting a resource will look like: | ||
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| 1. Mark resources as “deleting” in the store | ||
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| 2. Delete the resource | ||
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| 3. Remove the resource from the store. | ||
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| For complex operations like creating a volume, all resources that make up the volume will be marked as “creating”. Only | ||
| when all resources have been successfully created will their corresponding states be changed to “complete”. This will | ||
| look something like: | ||
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| 1. Add volume specification to the store with a state of “creating” | ||
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| 2. Add nexus specifications to the store with a state of “creating” | ||
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| 3. Add replica specifications to the store with a state of “creating” | ||
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| 4. Create replicas | ||
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| 5. Create nexus | ||
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| 6. Mark replica states as “complete” | ||
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| 7. Mark nexus states as “complete” | ||
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| 8. Mark volume state as “complete” |
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