014 - Proposal to refactor frontend handler and centralize session context

[hrishabhg]

Refactor frontend handler architecture to improve separation of concerns, introduce a centralized session context, fix HAProxy message memory leak, and simplify the state machine by eliminating unnecessary states.

[tombentley]

This seems mostly reasonable. Thanks.

As a more general point, please don't feel the need to write a proposal for a refactoring which doesn't touch public APIs (which is the case here). A proposal can serve as a place to discuss a larger change without actually having to implement it in its entirety so you can open a PR for it.

I've also been thinking about a few other changes which would function as either clean up of accumulated tech debt (which we should do anyway) or enablers for routing-like functionality:

• Currently we initialize the backend pipeline in the frontend handler. That seems ripe for moving to the backend handler itself. I hope to open a PR for this next week.
• I think we should be able to factor out some kind of abstraction for tracking reasons and managing the autoRead flag which we use for backpressure. I hope to open a PR for this next week.
• I don't think the PCSM can really survive in its current form if we want to support routing. It seems to me there's a state machine associated with the frontend (tracking the building up of your ClientSession), and one associated with a backend. A router could only forward (most) requests to a backend once we have TCP, TLS?, SASL?. At that point the Route is in the FORWARDING state, and requests can pass down the pipeline associated with that Route. Does this fit whatever thoughts you might have had about routing?

[tombentley]

This seems like low-hanging fruit which doesn't depend on the rest. We should just fix it. Would you be willing to open a PR for this?

[hrishabhg]

I will take that up soon with the HAProxyMessage fix.

[k-wall]

Hi! We've updated the proposal numbering system to use PR numbers as proposal identifiers.

Action required: Please rebase your PR on main.

Once you rebase, you'll need to rename your proposal file and update the title:

git mv proposals/014-frontend-handler-refactoring-and-centralised-session-context.md proposals/088-handler-refactoring-and-centralised-session-context.md
# Update title: remove any old number prefix and add PR number
sed -i.bak '0,/^# /{s/^# \([0-9]\{3\}\|xxx\|nnn\|000\) - /# 88 - /; t; s/^# /# 88 - /}' proposals/088-handler-refactoring-and-centralised-session-context.md && rm proposals/088-handler-refactoring-and-centralised-session-context.md.bak
git add proposals/088-handler-refactoring-and-centralised-session-context.md
git commit -m "Rename proposal to use PR number"
git push

The GitHub workflow will automatically check your proposal file naming after you push and update this PR description if any corrections are still needed.

See proposals/README.md for the updated workflow.

[k-wall]

Correction: The previous notification had an incorrect filename. Here are the correct commands:

git mv proposals/014-frontend-handler-refactoring-and-centralised-session-context.md proposals/088-frontend-handler-refactoring-and-centralised-session-context.md
# Update title: remove any old number prefix and add PR number
sed -i.bak '0,/^# /{s/^# \([0-9]\{3\}\|xxx\|nnn\|000\) - /# 88 - /; t; s/^# /# 88 - /}' proposals/088-frontend-handler-refactoring-and-centralised-session-context.md && rm proposals/088-frontend-handler-refactoring-and-centralised-session-context.md.bak
git add proposals/088-frontend-handler-refactoring-and-centralised-session-context.md
git commit -m "Rename proposal to use PR number"
git push

Sorry for the confusion!