# Technical Architecture

## 1. Overview

Single docker container Go application built on PocketBase as an embedded framework (https://pocketbase.io/). The binary contains:

- PocketBase's HTTP/API server and admin dashboard;
- the custom review dashboard;
- the S3 poller and the PDF processing pipeline, executed through PocketBase's standard job scheduler (`app.Cron()`);
- the declarative rule engine (JSONC config + `expr-lang/expr` expressions);
- access to the self-hosted S3 storage via the MinIO Go SDK.

Everything runs offline on the server infrastructure: no cloud APIs, no outbound calls other than to the S3 cluster.

## 2. Stack

| Component | Choice |
|---|---|
| Framework / runtime | PocketBase (Go framework, single binary) |
| Language | Go |
| Scheduling | `app.Cron()` (S3 poller, periodic tasks) |
| S3 | MinIO Go SDK (`github.com/minio/minio-go/v7`) |
| PDF text extraction | `github.com/ledongthuc/pdf` (native text, no OCR) |
| Rule expressions | `github.com/expr-lang/expr` |
| Regex | stdlib `regexp` (RE2); `regexp2` only if a lookaround proves indispensable |
| Config validation | `santhosh-tekuri/jsonschema` (schema generated from structs via `invopop/jsonschema`) |
| Database | SQLite (embedded in PocketBase) |
| Full-text search | SQLite FTS5 index synchronized via hooks |

## 3. S3 Layout

A single versioned bucket:

```
inbox/       # drop zone for new PDFs (via Nextcloud / S3 client)
pending/     # files awaiting human review
files/       # renamed and filed files (configured prefix structure)
```

Nextcloud mounts the bucket as external storage: it is the user-facing interface for uploading, downloading, and sharing. The application only manipulates S3 keys, never a local filesystem for documents.

## 4. Data Model (PocketBase Collections)

### `documents`
One row per processed PDF file:

- `sha256`: the sha256 is a primary key as the system will reject as duplicate any file with the same hash
- `original_filename`: original file name in `inbox/`
- `file_key`: current S3 key of the file
- `extracted_text`: full text extracted from all pages (base of the FTS5 index)
- `first_page_text`: first-page text (extraction base)
- `size`: file size
- `status`: `pending` | `processed` | `validated`
+ all the fields declared by the user in the configuration file

The append-only `document_events` collection is the single source of truth for a document's history; no history is stored on the `documents` record itself.

### `document_events`
Append-only history: `document`, `event_type` (`classified`, `moved`, `corrected`, `validated`), `payload` as JSON, `timestamp`.


## 5. Configuration File (JSONC)

A `config.jsonc` file kept alongside the binary, reloaded on restart. Three sections:

1. `fields`: declaration of fields to extract from the files:
   - `name`: name of the field
   - `type`: `text` | `number` | `date` | `boolean` | `enum`
   - `mandatory`: true | false

   Type semantics in rule expressions: `number` and `boolean` expose native values, `date` exposes a native Go `time.Time` (comparable, arithmetic via `duration()`, usable with the `date()`/`now()` builtins), `text` and `enum` expose strings. `enum` does not declare its values: the application builds the field's vocabulary from the constants the rules assign to it; the dashboard correction form offers this vocabulary as a dropdown, with free entry allowed. Assigning a value that does not match the field type (e.g. a raw string to a `date` field, a non-numeric value to a `number` field) is a rule error and routes the document to `pending/`.

2. `rules`: an ordered tree of rules. Each rule contains four keys:
   - `when` (required): an `expr-lang/expr` expression evaluated against the first-page text and the current field values; the rest of the rule runs only if it returns true (e.g. `firstPageText contains "Relevé de compte"`, or `emittingCompany in ["BRED", "Fortunéo"] && documentType == "Relevé de compte"`)
   - `set`: list of assignments, evaluated in order — `{ "field": ..., "value": ... }` for a constant, `{ "field": ..., "eval": ... }` for an expression evaluated in the same context
   - `delete`: list of field names to unset (they become undefined again for later rules and template rendering)
   - `rules`: child rules, evaluated in order only if the parent `when` matched; nesting is recursive

   The application registers two built-in helpers in the expression environment. `firstMatch(regex)` returns the first match of the regex in the first-page text (first capture group when present). `period(value, unit, shift, layout?)` renders a period instead of a raw date: `value` is a native date (e.g. `emissionDate`), `unit` is `month` | `quarter` | `half` | `year`, `shift` is `-1` (previous period), `0` (period containing the date) or `+1` (next period) — e.g. a rent receipt issued mid-month uses the containing month, a service-charge call issued at the end of a quarter uses the next quarter, a bank statement uses the previous month. It returns a French string: `Janvier 2026`, `T2 2026`, `S1 2026` or `2026`. The optional `layout` argument is a standard Go reference-date layout applied to the period's first day, with month names rendered in French by the application (default layouts: `January 2006` for `month`, `2006` for `year`); it is ignored for `quarter` and `half`, whose formats (`T<quarter> <year>`, `S<half> <year>`) are fixed. A missing or invalid date is a rule error and routes the document to `pending/`. Parsing strings into native dates uses expr's standard `date(value, layout)` builtin (Go date layout, e.g. `date(`15/01/2026`, `02/01/2006`)`) — the format is carried by the parsing rule, so it can differ per document type.

3. `templates`: named naming templates (e.g. `bank_statement: {emittingCompany}/{emissionDate.Format("2006-01-02")}_{documentType}_{bankAccount}.pdf`). A template is a literal string containing `{...}` placeholders: the inside of each placeholder is an `expr-lang/expr` expression evaluated in the document context (fields, helpers, builtins — e.g. `{emissionDate.Format("2006-01-02")}` or `{upper(emittingCompany)}`) and replaced by its result; the static text between placeholders stays literal. Unset optional fields evaluate to an empty string; a missing mandatory field or an evaluation error renders the result invalid and routes the file to `pending/`. All placeholder expressions are compiled at config load; "/" must not appear in the result (it is the S3 key delimiter). The template is chosen by the rules themselves: `template` is a field declared in the `fields` section like any other and set with `set` (e.g. `set template = "bank_statement"` when `emittingCompany in ["BRED", "Fortunéo"]` and `documentType == "Relevé de compte"`). If no rule sets `template`, the special `default` template is used. A `template` value with no matching entry renders the result invalid and routes the file to `pending/`.

Validation on load: JSON schema, regex compilation, `expr` expression compilation (`when`, `eval` and template placeholder expressions) against a typed context (`Document{FirstPageText, Fields}`) with the `firstMatch` and `period` helpers, regex precompilation and caching. Full rejection if invalid, no partial processing.

## 6. Processing Pipeline

Executed by the PocketBase cron job (configurable interval, e.g. every minute):

1. **Poll**: list objects in `inbox/`; an object is a candidate only if no `documents` record references its key yet, and its size and ETag are stable across two consecutive polls (write-completion wait).
2. **Download**: fetch the object into memory (streamed), no disk writes. The sha256 is computed during download; if a `documents` record with the same hash already exists, the object is a duplicate: the `inbox/` key is deleted (delete marker, data recoverable), a message is logged in the application logs, and processing stops there.
3. **Extraction**: first-page text (for rule evaluation) + full text (for FTS) via `ledongthuc/pdf`, including headers and footers
4. **Rule evaluation**: rules evaluated in order against the first-page text and the fields set so far; full evaluation log written to the application logs (built-in dry-run mode for debugging from the dashboard).
5. **Validation**: a result is valid if and only if: all rules have been processed with no error, all mandatory fields are identified, the selected template exists in the configuration, and the naming template evaluated with no error. Otherwise: route directly to `pending/`.
6. **Record**: create the `documents` record (sha256, original filename, current inbox key, extracted texts, fields, status) and add a `classified` event including the original filename. The record exists before any physical move, so an interrupted filing leaves a recoverable state.
7. **Filing**:
   - valid → resolve the target name first: if a file with the same name already exists in the target prefix, append a number to the filename; then copy to the new key (`files/` + rendered template — the template renders the full object path, there is no separate prefix), verify the target object exists, and delete the `inbox/` key only after the copy is confirmed; finally update the `documents` record with the final, conflict-resolved filename (`file_key`, status `processed`, `moved` event).
   - invalid → move to `pending/` (incremental suffix on name conflict); update `file_key`; status `pending`.

The bucket is versioned: any `inbox/` object deletion creates a delete marker, the data remains recoverable.

## 7. Review Dashboard

Custom pages on the PocketBase instance:

- File list: all files with status, filters, combined search over full text (FTS5) + metadata/fields.
- Detail: PDF preview streamed from S3 through a dedicated route, along with extracted fields; debug mode adds detailed log lines explaining every rule evaluation and action performed.
- Correction: forms to edit the extracted fields. A save is only possible if the selected template can be evaluated without error; otherwise the form refuses the save. A successfully corrected file is re-filed (copy to the new key, delete the old one, incremental suffix on conflict) and takes the status `validated`, whatever its previous status; a corrected `pending` file is thereby removed from `pending/`. Enum fields are offered as a dropdown built from the rules' vocabulary, with free entry allowed; date fields use a native date picker. History is preserved.
- Validation: transition from `processed` to `validated`. A `pending` file can only leave the review queue through a correction that can be saved, which itself sets the status to `validated`.
- Rule management: config display and dry-run on an existing document. The configuration is reloaded only on restart; there is no runtime reload.

Authentication: single account via PocketBase's standard auth.

## 8. Error Handling

- Transient S3 error: retry with backoff, the object stays in `inbox/` and will be polled again.
- Unreadable PDF: `pending` status, reason recorded in the payload of the `classified` event.
- Target key conflict (two files renamed identically, whether filing to `files/` or moving to `pending/`): automatic incremental suffix, event logged.
- Invalid config: error message displayed and the application does not start.