Tracking: ongoing refactors — case-config finalization and hardening

Summary

Tracking issue for the ongoing refactor work on case configuration handling in VectorDBBench. The goal is to make case-config construction and validation deterministic, mutation-free, and consistent across all task entry points (CLI and web UI), then harden the config model so classes can be made immutable.

Background / Why

db_case_config was touched in four layers — CLI construction, run() FTS routing, assembler in-place mutation, and task-runner/client reads — and the CLI-only routing was skipped entirely on the web UI path. This made config-handling changes easy to break, and the two entry points could diverge. A follow-up audit of the config model also found self-assigning cached-field methods in several backends that prevent immutable config classes.

Scope (work items)

1. Finalize db_case_config at a single choke point — PR #872 (WIP)

• New vectordb_bench/backend/db_case_config.py with finalize_db_case_config(db, case_type, base_config, *, parameters, dataset).
• Routes both entry points (cli.run() and web UI generate_tasks()) through one resolver; the assembler no longer mutates the config; select_cli_db_case_config remains as a thin backward-compatible wrapper.
• Cross-PR note: the refactor moves the FTS-compatible field whitelist (including force_merge_target_size_mb) into the resolver, subsuming the one-line whitelist fix in PR #871 (cli/cli.py). When both merge, drop #871's cli/cli.py hunk.

2. Enforce frozen=True across all case-config classes (approved, deferred — separate PR)

• ~55 root classes across 44 backend config files.
• Rework the self-assigning cached-field methods in:
  • aws_opensearch / oss_opensearch parse_metric()
  • hologres
  • pgvector
• Cross-backend regression sweep after the change.

Definition of done

• Both task entry points produce identical, mutation-free case configs.
• No in-place mutation of db_case_config remains outside the resolver.
• All case-config classes are frozen=True with no self-assigning cached-field methods.
• Full backend regression sweep passes (unit tests + ruff/black clean).

References

• PR #872 — refactor: finalize db_case_config at a single choke point
• PR #871 — feat: add CLI control for Milvus force-merge size and toggle (whitelist overlap)
• Issue #825 — force-merge compaction concerns that motivated the force_merge_target_size_mb whitelist entry

Start with PR #872 and the proposed backend/db_case_config.py resolver, then trace cli.run() and web UI generate_tasks() to compare both entry points. Review the listed backend config files and cached-field methods in aws_opensearch, oss_opensearch, hologres, and pgvector. Done means mutation-free identical configs, frozen case-config classes, and a passing backend regression sweep with unit tests, ruff, and black.