Added new WebhookKernelSessionManager for Kernel Persistence

[amazingarman]

Added a new way to persist kernels to any database. You must create an API to interact with your database. The API should have 4 endpoints:

• 1 DELETE that will delete all kernel sessions from a list of kernel ids
• 1 POST that will take kernel id as a path variable and kernel session in body and save it to a database
• 1 GET that will take the kernel id as a path variable and retrieve that information from a database
• 1 GET that will retrieve a list of all kernel sessions from a database

In order to setup the WebhookKernelSessionManager, you must set --EnterpriseGatewayApp.kernel_session_manager_class to enterprise_gateway.services.sessions.kernelsessionmanager.WebhookKernelSessionManager, --WebhookKernelSessionManager.webhook_url to the endpoint url of your API and --WebhookKernelSessionManager.enable_persistence=True to enable kernel persistence.

[kevin-bates]

This is very interesting @amazingarman! I find this to be a clever (and flexible) way to integrate with a persistent store of any kind. Very nice.

Here are some next steps that should probably be included in this pull request.

1. Extend the list of classes to include WebhookKernelSessionManager. This will enable the configuration options to appear when running jupyter enterprisegateway --help-all.
2. Extend the command-line options portion of the doc to include the WebhookKernelSessionManager options following the existing FileKernelSessionManager options.
3. We should probably add a section in the Operators Guide for "Kernel Session Persistence" that includes bothFileKernelSessionManager and WebhookKernelSessionManager. This will have some overlap with the Availability Modes PR Availability modes #1095 where it defines the modes and talks a bit about session persistence.
4. I'm wondering if we should introduce version info into the URL, but I suppose that would be up to the provider and built into the web-url. The concern being any extensions to the payload (like the addition of tenant ID in Add support for Tenant ID #1097).

On that "modes" topic, this PR implies you must be using some form of availability and I'm curious what your thoughts are about where we're headed with that? What kinds of issues are you running into with the current code base? (Mostly curious about re-connects and culling, but any other experiences would be very helpful.)

[amazingarman]

@kevin-bates updated the PR based off of your comments.

4. I'm wondering if we should introduce version info into the URL, but I suppose that would be up to the provider and built into the web-url. The concern being any extensions to the payload (like the addition of tenant ID in Add support for Tenant ID #1097).

I agree that the version info in the URL is up to the provider to figure out.

On that "modes" topic, this PR implies you must be using some form of availability and I'm curious what your thoughts are about where we're headed with that? What kinds of issues are you running into with the current code base? (Mostly curious about re-connects and culling, but any other experiences would be very helpful.)

I don't have any availability set up currently and in the early stages of building this out. The modes PR is something that will definitely be used and agree on the direction it seems to be heading in. The code base is easy to read and follow, although the documentation is sparse in areas. We found solutions reading through past PRs and Issues raised.

[kevin-bates]

@amazingarman - thanks so much for the update. There are just a few minor touchups necessary, but this looks good!

We will have some heavy overlap with the documentation in #1095, but I'm happy to tackle that once this PR is merged (following the additional updates).

Thanks again!

[kevin-bates]

@amazingarman - Thank you for the updates and this nice feature!

[kevin-bates]

Will merge Friday morning barring any objections.

[welcome]

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