Demo 11 — `cost-governor` plugin

Demo 11 — cost-governor plugin

Module brief: §Module 11

UVP

Per-tenant query cost budgets with auto-block at the proxy. Stops a runaway tenant from eating the shared database without killing legit traffic.

Use cases

• SaaS noisy-neighbour control. Free-tier tenant runs a Cartesian join — gets blocked at minute-budget without taking down paid tenants.
• Fair-share for analytics workloads. Each tenant gets a daily compute budget; runaway dashboards stop when they exhaust it.

What this demo shows

1. Seed acme with a tight minute-budget (1 cost-unit).
2. Run 5 quick queries → no problem.
3. Run a SELECT * FROM events (~6.4 MB response, ≈ 6.4 cost-units).
4. The 6th query → ERROR: Query blocked by plugin: tenant exceeded minute budget (6.40/1.00) (retry in 60s).
5. Wait 60s → next query succeeds.

Run it

cd demos/v0.4.0/11-cost-governor
./demo.sh

Sample run:

=== cost-governor demo ===
[1/5] Starting proxy + Postgres + cost-governor.wasm
[2/5] Seeding acme budget: minute=1.0, hour=10.0, day=100.0
[3/5] Running 5 small queries — all succeed
[4/5] Running 1 large query (SELECT * FROM events) — exhausts budget
   ✓ usage now: minute=6.4, hour=6.4, day=6.4
[5/5] Running 6th query — blocked
   ERROR:  Query blocked by plugin: tenant exceeded minute budget
           (6.40/1.00) (retry in 60s)

Implementation pointer

Plugin source: HDB-HeliosDB-Proxy-Plugins/cost-governor/src/lib.rs. Hook entry points: pre_query (decides) + post_query (records). KV layout documented at top of the file.

HeliosDB compatibility

Backend-agnostic.