# DWH Execution Manual

> **Purpose:** A reusable playbook for building an `extract → blob → load` data warehouse bronze layer using n8n (or any equivalent orchestrator) + object storage + PostgreSQL/PostGIS. Generalised from the Fireside Tracksolid DWH pipeline (2026-04-24). Apply this pattern to future data projects to skip re-deriving the same decisions.
>
> **Reference implementation:** `dwh/26100{1..4}.sql` + `docs/DWH_PIPELINE.md` + the two `n8n-workflows/dwh_*.json` files. Treat those as the copy-paste template for the next project.

---

## 1. When to Use This Pattern

**Use it when all of these are true:**
- You need an analytical replica of a production OLTP DB without letting analytical load hit the OLTP.
- Source and target are separate PostgreSQL instances (possibly separate networks).
- Data volumes are moderate: millions of rows per day, not billions.
- You already have an orchestrator (n8n, Airflow, Prefect) and object storage (rustfs, S3, MinIO) in the stack.
- Latency tolerance is hours, not minutes.

**Don't use it when:**
- Sub-minute latency is required → use logical replication or CDC (Debezium, pg_logical, AWS DMS).
- Volumes exceed ~100 GB/day → step up to Spark/Flink + columnar store (Iceberg, Delta).
- Source and target are the same DB → just use materialized views or scheduled SQL.
- You need exactly-once streaming semantics → this pattern is at-least-once + idempotent load.

---

## 2. The Core Pattern

```
┌──────────────┐       ┌──────────────┐       ┌──────────────┐       ┌──────────────┐
│  Source DB   │──(a)──▶  Orchestr.  │──(b)──▶ Object Store │──(c)──▶  Target DB   │
│   (OLTP)     │       │  (extract)   │       │   (CSVs)     │       │   (bronze)   │
└──────────────┘       └──────────────┘       └──────────────┘       └──────────────┘
                              │                                              ▲
                              └──────────────(d)─────────────────────────────┘
                                     (load workflow, per CSV)

(a) Watermarked SELECT, closed upper bound
(b) Atomic CSV upload with timestamped path
(c) CSV stays until load confirms success
(d) Load = BEGIN → INSERT ON CONFLICT → UPDATE watermark → UPDATE run log → COMMIT → move CSV
```

**Why three stages and not two:**
- **Audit trail** — every extracted CSV is a point-in-time snapshot you can replay.
- **Decoupling** — target DB downtime doesn't lose data; the CSV waits.
- **Observability** — blob listings are a second source of truth independent of the DB.

---

## 3. Pre-flight Checklist

Before writing any SQL or workflow JSON, confirm in writing:

- [ ] Source DB reachable from orchestrator (internal network preferred, VPN/public IP with `sslmode=require` otherwise).
- [ ] Target DB reachable; you hold a superuser credential for one-time DDL.
- [ ] Object storage bucket exists; credentials are configured in the orchestrator.
- [ ] For each source table to extract, you have identified:
  - A **DB-insertion timestamp column** (not device/user-reported time), or "it's a snapshot table".
  - A **natural unique key** that already has a `PRIMARY KEY` or `UNIQUE` constraint on source (for the `ON CONFLICT` target on bronze).
  - Any **unit/column drift** between source and target (e.g., `distance_m` vs. `distance_km`).
- [ ] Acceptable end-to-end latency (to calibrate cron cadence).
- [ ] Security baseline: who writes bronze, who reads it, SSL requirement, password rotation cadence.

If any row is unchecked, pause and resolve it. Skipping this step is the #1 cause of pipelines that "worked in test but lose data in prod."

---

## 4. Phase-by-Phase Execution

Execute in order. Phases are independent of each other within their phase, but phases have strict dependencies.

### Phase A — Target DB preparation

Apply three types of migrations, in numeric order:

1. **Bronze DDL** — one table per source table. Use `IF NOT EXISTS`; make it idempotent.
2. **Control schema** — `dwh_control.extract_watermarks` + `dwh_control.extract_runs`.
3. **Assertion migrations** — verify roles exist, verify every `ON CONFLICT` target is backed by a PK/UNIQUE (fail loudly if not).

Template files: `dwh/260423_dwh_ddl_v1.sql`, `dwh/261001_dwh_control.sql`, `dwh/261002_bronze_constraints_audit.sql`, `dwh/261003_dwh_roles.sql`.

**Role model:**
- `<proj>_owner` — owns schemas, writes bronze + control tables.
- `<proj>_ro` (or `grafana_ro`) — reads everything, writes nothing.
- Never use `postgres` or another superuser from the orchestrator.

**Watermark seed:** set `last_extracted_at` to a date before any real data (`'2000-01-01T00:00:00Z'` is safe) so the first run back-fills all history in a single CSV per table.

### Phase B — Object storage

Create two prefixes per table:

```
s3://<bucket>/<project>/exports/{table}/     # active CSVs, in-flight
s3://<bucket>/<project>/processed/{table}/   # loaded CSVs, never deleted (audit)
```

Naming convention: `{YYYYMMDD_HHMM}_{TZ}.csv` (e.g., `20260424_1400_EAT.csv`). Timezone in the filename because "08:00" means nothing a year from now without it.

Retention: match whatever backup retention is already in the stack (e.g., 30 days). `processed/` should outlive `exports/`.

### Phase C — Orchestrator credentials

Three credentials:

| Credential | Role | Purpose |
|---|---|---|
| `<proj>_source` | Read-only role on source DB | Extract queries |
| `<proj>_dwh_target` | `<proj>_owner` on target DB | Bronze writes + control updates |
| `<proj>_s3` | IAM user with `s3:PutObject`, `s3:GetObject`, `s3:ListBucket`, `s3:DeleteObject` on the prefix | CSV upload/download/move |

**Always** `sslmode=require` on any public-IP DB connection. Test each credential with the orchestrator's "Test connection" button before proceeding.

### Phase D — Load workflow (build this BEFORE the extract workflow)

Building load first lets you iterate with hand-crafted CSVs in blob storage before wiring up extract. Much faster feedback loop.

Load workflow input (parameters):
```json
{
  "table": "position_history",
  "csv_path": "s3://bucket/project/exports/position_history/20260424_1400_EAT.csv",
  "run_id": 12345,
  "run_started_at": "2026-04-24T11:00:00Z"
}
```

Load workflow steps:
1. Download CSV from blob storage.
2. Parse CSV into rows.
3. **Open transaction.**
4. `INSERT INTO bronze.<table> (...) VALUES (...) ON CONFLICT (<natural_key>) DO NOTHING;`
5. `UPDATE dwh_control.extract_watermarks SET last_extracted_at = :run_started_at, last_loaded_at = NOW(), rows_loaded_last_run = <count> WHERE table_name = :table;`
6. `UPDATE dwh_control.extract_runs SET status = 'loaded', run_finished_at = NOW(), rows_loaded = <count> WHERE run_id = :run_id;`
7. **Commit.**
8. Move CSV from `exports/` to `processed/` (copy-then-delete; never delete before copy confirms).

**Non-negotiable invariants:**
- Steps 3–7 are one transaction. If any fails, all rollback.
- Step 8 only runs after commit. If step 8 fails, the next run will re-load the CSV (idempotent via ON CONFLICT) — not a data loss event.

### Phase E — Extract workflow

Extract workflow steps, per table:

1. Read current watermark: `SELECT last_extracted_at FROM dwh_control.extract_watermarks WHERE table_name = :table;`
2. Capture `run_started_at = NOW()` (in the target DB's clock, not the orchestrator's — reduces clock-skew bugs).
3. `INSERT INTO dwh_control.extract_runs (table_name, run_started_at, status) VALUES (:table, :run_started_at, 'extracting') RETURNING run_id;`
4. Query source with closed upper bound:
   ```sql
   SELECT <cols>
   FROM <source_schema>.<table>
   WHERE <watermark_col> >  :last_extracted_at
     AND <watermark_col> <= :run_started_at
   ORDER BY <watermark_col>;
   ```
5. Render rows as CSV. For geometry columns: `CASE WHEN geom IS NULL THEN NULL ELSE ST_AsEWKT(geom) END`.
6. Upload CSV to `s3://bucket/project/exports/{table}/{YYYYMMDD_HHMM}_{TZ}.csv`.
7. `UPDATE dwh_control.extract_runs SET status = 'uploaded', rows_extracted = <count>, csv_path = :path WHERE run_id = :run_id;`
8. Call load workflow with `{table, csv_path, run_id, run_started_at}`.

### Phase F — Schedule + observability

**Cron cadence:** start with 6–8 runs/day during active hours. Fold the overnight gap where traffic is low. Example: `0 5,8,11,14,17,20,23 * * *` TZ `Africa/Nairobi`.

**Three observability views** (readable by the RO role):

- `v_table_freshness` — per-table lag from last successful load. Drives the freshness alert.
- `v_recent_failures` — failed runs in last 24h. Zero rows = healthy.
- `v_watermark_lag` — extract vs. load lag per table. Distinguishes "nothing to extract" from "stuck".

Template file: `dwh/261004_dwh_observability_views.sql`.

**Grafana panels** (add at minimum):
1. Freshness panel — red if any row in `v_table_freshness` has `lag > 4h`.
2. Failures panel — red if `v_recent_failures` has any row.
3. Row counts panel — daily bar chart from `extract_runs`.

---

## 5. Design Principles (Do Not Skip)

### 5.1 Watermark on DB insertion time, not source-reported time

The watermark column must be "when the target DB got the row", not "when the device/user said it happened". Device clocks skew, webhooks arrive late, and batch imports backdate records. A source-reported watermark will silently drop rows that arrive out of order. Use `recorded_at`, `created_at`, `updated_at` (with `DEFAULT NOW()`), or `ingested_at` — never `gps_time` / `event_time` / `timestamp`.

### 5.2 Closed upper bound

Extract uses `>  last_extracted_at AND <= run_started_at`. The closed upper bound prevents "row committed at `NOW()` during the extract query" from appearing in two adjacent runs. Without it, some rows are double-extracted (wasteful) or missed (data loss).

### 5.3 Idempotent load via natural unique keys

Every incremental bronze table needs a PRIMARY KEY or UNIQUE that matches the source's natural unique key. `ON CONFLICT DO NOTHING` makes re-running a CSV harmless. **Do not invent surrogate keys on bronze** — they defeat the ON CONFLICT guarantee. If the source has no natural key, fix the source or accept the table as a snapshot.

### 5.4 Transactional load boundary

Insert + watermark update + run-log update are one transaction. Splitting them creates "ghost" states where watermark advanced but rows didn't load, causing silent holes.

### 5.5 CSV audit trail — never delete

Moved-to-`processed/` CSVs are cheap ($0.023/GB/month on S3-class storage). They pay for themselves the first time you need to replay a window or debug a row-count mismatch.

### 5.6 PostGIS round-trip via EWKT

`ST_AsEWKT(geom)` on extract, `ST_GeomFromEWKT(ewkt)` on load. Preserves SRID inline. Do NOT store `ST_AsText` + separate SRID column — it doubles the chance of mismatch. Guard NULLs: `CASE WHEN geom IS NULL THEN NULL ELSE ST_AsEWKT(geom) END`.

### 5.7 Fail loud, fail early

Audit migrations (roles, constraints) should `RAISE EXCEPTION` with a bullet list of what's missing. Silent success is worse than noisy failure — a missing PK surfaces three months later as "why are there duplicate trips?".

---

## 6. Snapshot vs. Incremental Decision Matrix

| Signal | Snapshot (TRUNCATE + reload) | Incremental (watermark + append) |
|---|---|---|
| Row count | < ~10k | > ~10k |
| Meaning of "current state" | Matters | Doesn't matter; history matters |
| Deletes in source | Common | Rare |
| Update frequency per row | High | Low (append-mostly) |
| Natural unique key | May not exist | Must exist |
| Example | `devices`, `live_positions`, `geofences` | `position_history`, `trips`, `alarms`, event logs |

When in doubt: **snapshot is simpler**. Only escalate to incremental when the snapshot CSV would exceed a few MB per run.

---

## 7. Observability Contract

Every pipeline adds these three views to its control schema — no exceptions:

```sql
CREATE OR REPLACE VIEW <control>.v_table_freshness AS
SELECT table_name,
       MAX(run_finished_at) AS last_loaded_at,
       NOW() - MAX(run_finished_at) AS lag,
       COUNT(*) FILTER (WHERE run_started_at > NOW() - INTERVAL '24 hours') AS loads_last_24h
FROM <control>.extract_runs
WHERE status = 'loaded'
GROUP BY table_name;

CREATE OR REPLACE VIEW <control>.v_recent_failures AS
SELECT run_id, table_name, run_started_at, run_finished_at, csv_path, error_message
FROM <control>.extract_runs
WHERE status = 'failed' AND run_started_at > NOW() - INTERVAL '24 hours'
ORDER BY run_started_at DESC;

CREATE OR REPLACE VIEW <control>.v_watermark_lag AS
SELECT table_name, last_extracted_at, last_loaded_at, rows_loaded_last_run,
       NOW() - last_loaded_at    AS load_lag,
       NOW() - last_extracted_at AS extract_lag
FROM <control>.extract_watermarks;
```

Wire a Grafana alert on each view. Test the alert by manually failing a run before go-live.

---

## 8. Schema Drift Handling

Schema drift between source and bronze is inevitable. Two rules:

1. **Detect at design time.** Diff source DDL against bronze DDL before writing any extract SQL. Unit changes (metres vs. km), renamed columns, and added nullable columns are the usual suspects.
2. **Fix in the extract query, not the load.** Put all transformations in the SELECT so the CSV on disk already matches the bronze column names and units. The load workflow should be dumb — CSV column N goes to bronze column N.

Document every drift in the runbook (§5 of the operations runbook). Future developers WILL hit them.

---

## 9. Verification Gates

### Pre-deploy (before first cron tick)

- [ ] Every migration applied successfully.
- [ ] Control tables seeded (one watermark row per incremental table).
- [ ] Every credential's "Test connection" passes.
- [ ] Blob storage prefixes exist.
- [ ] Manual workflow trigger succeeds end-to-end for one table.

### First run (manual trigger of extract workflow)

- [ ] Every processed table has a row in `extract_runs` with `status='loaded'`.
- [ ] Row-count parity with source (± in-flight writes): `SELECT COUNT(*) FROM <source>` vs. `SELECT COUNT(*) FROM bronze.<table>`.
- [ ] Geometry columns round-trip cleanly: `SELECT ST_AsText(geom) FROM bronze.<table> LIMIT 5` returns valid POINTs.
- [ ] All CSVs moved from `exports/` to `processed/`.

### Steady-state (after 24h / first full schedule cycle)

- [ ] `v_table_freshness` shows lag < cadence × 2 for every table.
- [ ] `v_recent_failures` is empty.
- [ ] Row counts in bronze growing at expected rate.

Only declare "done" after all three gates pass.

---

## 10. Scheduling Calibration

Tradeoffs:

| Cadence | Pros | Cons |
|---|---|---|
| Every 15 min | Low lag, small CSVs | High orchestrator churn, noisy alerts |
| Every 3 h (recommended) | Predictable, fits ops windows, tolerable lag | Overnight backlog carries to morning |
| Nightly (once/day) | Cheap, simple | Unacceptable for real-time panels |

Rule of thumb: cadence = 25–50% of your latency tolerance. 4h latency budget → 1-2h cadence.

Fold cadence around traffic patterns. Don't run 24× at 1-hour intervals if the source generates zero rows between midnight and 05:00.

---

## 11. Common Failure Modes & Recovery

| Failure | Symptom | Fix |
|---|---|---|
| CSV stuck in `exports/` | `v_recent_failures` has a row; CSV never moved | Next scheduled run retries automatically (idempotent). If persistent, open orchestrator logs by `run_id`. |
| Table marked `loading` for >1 cadence | n8n executor crashed mid-transaction | DB rolled back. Next run retries. If stuck >2 cadences, manually re-trigger the extract. |
| Row counts diverge > 1% | CSV parse error silently dropped rows | `rows_extracted != rows_loaded` in `extract_runs` — inspect the CSV for malformed rows. |
| Geometry loads as NULL | EWKT serialisation broke | Check for missing `CASE WHEN geom IS NULL` guard in extract SQL. |
| Distance/units 1000× wrong | Schema drift not caught | Check extract SQL for the unit conversion (see §8). |

**Back-fill a window:**
```sql
UPDATE <control>.extract_watermarks
SET last_extracted_at = NOW() - INTERVAL '24 hours'
WHERE table_name = '<table>';
```
Next run re-extracts the gap. `ON CONFLICT DO NOTHING` filters duplicates.

**Full reseed (nuclear):**
```sql
UPDATE <control>.extract_watermarks
SET last_extracted_at = '2000-01-01T00:00:00Z'
WHERE table_name = '<table>';
```
Next run back-fills all history in one very large CSV. Expected; it moves to `processed/` on success.

---

## 12. Security Baseline

- Two roles minimum: owner (writes) and RO (reads). Never use superuser from the orchestrator.
- `sslmode=require` on every public-IP DB connection.
- Passwords never in committed SQL — use placeholder tokens (`CHANGE_ME_BEFORE_APPLY`) and swap in-session during apply. Document rotation in the runbook.
- Blob storage credentials scoped to the project's prefix, not the whole bucket.
- Rotate all credentials before go-live (don't reuse the ones that were flying around in design conversations).

---

## 13. Reusability Checklist (Applying to a New Project)

When starting a new data project, copy the Tracksolid DWH layout and edit these points:

- [ ] Rename schemas: `<proj>_control` instead of `dwh_control` if multiple DWHs share a DB.
- [ ] Adjust `<proj>_owner` / `<proj>_ro` role names.
- [ ] Update bucket prefix: `s3://<bucket>/<project>/exports|processed/`.
- [ ] Re-do the snapshot/incremental decision for every source table (§6).
- [ ] Identify watermark columns and natural unique keys for every incremental table (§5.1, §5.3).
- [ ] Map schema drift before writing extract SQL (§8).
- [ ] Calibrate cadence to the new project's latency budget (§10).
- [ ] Ship the three observability views (§7) — even if nobody will look at them in week one.
- [ ] Write the runbook from the template: follow `docs/DWH_PIPELINE.md` section-for-section.
- [ ] Run the verification gates (§9) before declaring done.

---

## 14. Reference Implementation (Tracksolid DWH)

These files are the copy-paste template:

| File | Purpose |
|---|---|
| `dwh/260423_dwh_ddl_v1.sql` | Bronze DDL + roles + schemas |
| `dwh/261001_dwh_control.sql` | Control schema (watermarks + run log) |
| `dwh/261002_bronze_constraints_audit.sql` | ON CONFLICT key assertion |
| `dwh/261003_dwh_roles.sql` | Role contract assertion |
| `dwh/261004_dwh_observability_views.sql` | Freshness/failure/watermark views |
| `docs/DWH_PIPELINE.md` | Operations runbook (troubleshooting, manual re-run, rotation) |
| `docs/superpowers/specs/2026-04-24-n8n-dwh-bronze-pipeline-design.md` | Design spec (why each decision) |
| `docs/superpowers/plans/2026-04-24-n8n-dwh-bronze-pipeline.md` | Task-by-task implementation plan |
| `n8n-workflows/dwh_extract.json` | Extract workflow (reference) |
| `n8n-workflows/dwh_load_bronze.json` | Load workflow (reference) |

**For the next project, fork this manual first, then adapt.** Do not re-design from scratch — the seven design principles in §5 are the parts people keep getting wrong.