Stefan
ebdb92713a
Assesses the proposed server-held edit state + SignalR live-edit channel against the current stateless design. Recommendation: don't do the full rewrite now (it adds stateful-server/scaling/reconnect complexity for a single-editor back-office flow); instead add a stateless inv/calc endpoint that reuses the backend pricing (InvoiceSetPricing/VAT) so the editor stops duplicating the math — capturing the real value without sockets. Add SignalR later only as transport if real-time co-editing becomes a genuine requirement. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
99 lines
5.0 KiB
Markdown
99 lines
5.0 KiB
Markdown
# Evaluation — Backend-cached invoice editing over SignalR
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**Idea (as proposed):** hold invoices that users are editing in a **server-side
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cache**, keep a **SignalR / WebSocket** connection open, apply each front-end
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change **in the backend**, and **push the recomputed state back** to the browser.
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The backend becomes the single source of truth for the in-progress invoice.
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This note evaluates that against the current design and recommends a path.
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---
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## 1. How invoice editing works today
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- **Stateless.** The browser holds the editor state. It posts the whole `invc`
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JSON to `req/sprep|sedit|save`; the server computes/persists and returns a PDF
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preview (image collection). No per-user editing state lives on the server.
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- Totals/§13b/§35a and now **set pricing** are computed from posted data; the
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invoice **total comes from the registration balance**, not summed lines.
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- Auth is cookie-based (`OCORECookieAuthenticationEvents`), SQL-first data access,
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controller is stateless and DI-scoped.
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**Implication:** the server is horizontally scalable and crash-tolerant for
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editing — there is nothing to lose if a node restarts mid-edit.
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---
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## 2. What the proposal would buy
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| Benefit | Real for Fuchs? |
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| Server-authoritative calculation (one place for pricing/VAT/set rules) | **Partly already true** — the PDF/totals are server-computed; the editor only previews. The duplicated logic is the *live* line math in JS. |
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| Real-time multi-user co-editing | **Low value** — invoices are edited by one back-office user at a time; concurrent editing of the same draft is rare. |
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| Live validation / instant recompute without full round-trips | **Some value** — smoother UX than re-posting the whole `invc` for each tweak. |
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| Reduced payload (deltas vs whole invoice) | Marginal — invoices are small. |
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---
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## 3. Costs and risks
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- **Server-side edit state.** Per-user/per-draft cache with lifetime management
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(idle expiry, explicit discard, max size), or the server leaks memory. Needs a
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distributed cache if scaled out (Redis), or **sticky sessions** for SignalR.
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- **Concurrency/locking.** Two tabs or two users on the same draft → need
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optimistic concurrency / locking semantics that don't exist today.
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- **Connection lifecycle.** Reconnect, replay, and "lost update" handling;
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offline/flaky networks; auth on the socket (cookie works, but token refresh and
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disconnect-on-logout must be handled).
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- **Scaling.** SignalR with multiple instances needs a backplane (Redis/Azure
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SignalR). Today the app has none.
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- **Big rewrite of the editor.** The 1,200-line `fis.inv_shared.js` becomes an
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event-driven client of server state — a substantial, risky rewrite of working
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code, with new failure modes (desync between optimistic UI and server truth).
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- **Testing surface** grows (connection states, races) far beyond the current
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request/response model.
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For a single-tenant back-office app with one editor at a time, this is a lot of
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**accidental complexity** for modest UX gains.
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---
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## 4. Recommendation
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**Do not do a full SignalR rewrite now.** It optimises a problem (real-time
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collaboration, server-held edit state) the business doesn't strongly have, while
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adding stateful-server, scaling, and reconnection complexity to an app that is
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currently simple and robust.
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Prefer an **incremental, lower-risk path** that captures most of the value:
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1. **Single source of pricing truth (highest value, do first).**
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Add a stateless endpoint `inv/calc` that takes the `invc` JSON and returns the
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computed lines + totals + set-mode resolution using the **same backend code**
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(`InvoiceSetPricing`, VAT, §13b/§35a). The editor calls it on change (debounced)
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to recompute, instead of duplicating the math in JS. This removes the
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front/back duplication — the actual pain — **without** sockets or server state.
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It also makes the interplay fully unit-testable (the endpoint is pure).
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2. **Keep preview as-is** (post → PDF image) but allow it to reuse `inv/calc`.
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3. **If/when live UX is still wanted**, add SignalR **only as a transport** on top
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of the stateless calc (push recompute results), keeping persistence stateless.
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Defer server-held draft state until genuine multi-user co-editing is required.
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4. **If server-held drafts are truly needed**, scope a pilot: one entity (invoice
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draft), `IDistributedCache`-backed, explicit acquire/release lock, idle TTL,
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and a reconnect/replay protocol — behind a feature flag, measured against the
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current flow before rollout.
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### Why this fits the codebase
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It aligns with the just-completed DI service layer: the calc lives in
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`IInvoiceService`/`InvoiceSetPricing` (already tested), reused by both the preview
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and a future socket. We get "changes computed by the backend, reflected in the
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frontend" — the stated goal — **without** turning a stateless, scalable web app
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into a stateful real-time system prematurely.
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**Suggested next step:** implement `inv/calc` (item 1) and migrate the editor's
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line math to it; revisit SignalR only if real-time/co-editing becomes a real
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requirement.
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