012 - Hot reload feature proposal (#83)

* hot reload feature proposal

Signed-off-by: Urjit Patel <105218041+Uzziee@users.noreply.github.com>
Signed-off-by: Urjit Patel <upatel@confluent.io>

* replace file watcher with HTTP based approach

Signed-off-by: Urjit Patel <105218041+Uzziee@users.noreply.github.com>
Signed-off-by: Urjit Patel <upatel@confluent.io>

* Restructure proposal to reflect discussion consensus

Rewrite the hot reload proposal to focus on architectural decisions
rather than implementation detail. The PR discussion has established
consensus on several key points that the document didn't reflect:

- Reframe around applyConfiguration(Configuration) as the core API,
  decoupled from trigger mechanisms (HTTP, file watcher, operator)
- Remove all Java class implementations and handler chains — these
  belong in the code PR where they're reviewable in context
- Call out remove+add with brief per-cluster downtime as deliberate
- Call out all-or-nothing rollback as the initial default, consistent
  with startup behaviour
- Move ReloadOptions to deployment-level static configuration rather
  than per-call parameters
- Identify plugin resource tracking as a known gap with pointer to
  separate proposal
- Flag open questions (config granularity, failure behaviour options,
  drain timeout configurability)
- Defer trigger mechanism design as explicit future work

Assisted-by: Claude claude-opus-4-6 <noreply@anthropic.com>
Signed-off-by: Sam Barker <sam@quadrocket.co.uk>
Signed-off-by: Urjit Patel <upatel@confluent.io>

* Address review feedback on restructured proposal

- Fix summary to read as proposed behaviour, not existing
- Use "administrators" instead of "operators" for humans to avoid
  confusion with the Kubernetes operator process
- Fix filter config examples (KMS endpoint, key selection pattern)
- Clarify failure behaviour is consistent across trigger mechanisms
- Note thundering herd as a known trade-off of remove+add
- Fix "original proposal" to "earlier iterations"

Assisted-by: Claude claude-opus-4-6 <noreply@anthropic.com>
Signed-off-by: Sam Barker <sam@quadrocket.co.uk>
Signed-off-by: Urjit Patel <upatel@confluent.io>

* updated hot reload proposal as per the new virtual lifecycle state proposal

This proposal introduces a mechanism for applying configuration changes to a running Kroxylicious proxy without a full restart. It defines a core `applyConfiguration(Configuration)` operation that detects changes and restarts only affected virtual clusters.

Signed-off-by: Urjit Patel <105218041+Uzziee@users.noreply.github.com>
Signed-off-by: Urjit Patel <upatel@confluent.io>

* Refine hot reload proposal with new policies and layers

Signed-off-by: Urjit Patel <105218041+Uzziee@users.noreply.github.com>
Signed-off-by: Urjit Patel <upatel@confluent.io>

* Fix wording as per comments

Signed-off-by: Urjit Patel <105218041+Uzziee@users.noreply.github.com>
Signed-off-by: Urjit Patel <upatel@confluent.io>

* refactor

Signed-off-by: Urjit Patel <105218041+Uzziee@users.noreply.github.com>
Signed-off-by: Urjit Patel <upatel@confluent.io>

* Added javadoc to public applyConfiguration API

Signed-off-by: Urjit Patel <105218041+Uzziee@users.noreply.github.com>
Signed-off-by: Urjit Patel <upatel@confluent.io>

* Rename proposal to use PR number

Signed-off-by: Urjit Patel <upatel@confluent.io>

* Updated design doc as per latest discussion

Signed-off-by: Urjit Patel <upatel@confluent.io>

* Added concurrency section, PR comments

Signed-off-by: Urjit Patel <upatel@confluent.io>

* Updated Compatibility section

Signed-off-by: Urjit Patel <upatel@confluent.io>

* Update document to reflect latest PR comments

Signed-off-by: Urjit Patel <upatel@confluent.io>

* handled ConcurrentApplyException in trigger examples

Signed-off-by: Urjit Patel <upatel@confluent.io>

* Fix indentation

Signed-off-by: Urjit Patel <upatel@confluent.io>

* Rename applyConfiguration to reconfigure and rename associated classes

Signed-off-by: Urjit Patel <upatel@confluent.io>

---------

Signed-off-by: Urjit Patel <105218041+Uzziee@users.noreply.github.com>
Signed-off-by: Urjit Patel <upatel@confluent.io>
Signed-off-by: Sam Barker <sam@quadrocket.co.uk>
Co-authored-by: Sam Barker <sam@quadrocket.co.uk>

Author: Uzziee