67244223ec
Add 10 - Services/ docs for all sub-projects: backend, frontend, scanner, deployment (new), update amanat-assist. Update Scanner Architecture, Telegram Mini App flow, and Activity Log. Add payment safety edge cases.
178 lines
11 KiB
Markdown
178 lines
11 KiB
Markdown
---
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title: Scanner Architecture
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tags: [architecture, scanner, payment]
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created: 2026-05-30
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updated: 2026-06-08
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---
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# Scanner Architecture
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AMN Pay Scanner is a standalone Go microservice that watches on-chain payment events and notifies the backend via signed webhook when a payment is confirmed. It replaces the Request Network integration with an in-house polling scanner that supports EVM chains, Tron, and TON.
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> [!info]
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> Repo: `scanner/` within the escrow monorepo. Binary: `scanner`. Written in Go 1.25. SQLite (WAL mode) for state. No external dependencies beyond the chain APIs.
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>
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> For operational how-it-works detail (API, webhook payloads, config vars, direct balance checks) see [[scanner]] in 10 - Services.
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---
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## 1. Responsibilities
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- Accept payment **intents** from the backend (`POST /intents`)
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- Watch the relevant chain for matching on-chain transfers
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- Track confirmation depth (EVM) or rely on API-reported finality (Tron, TON)
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- Deliver a signed webhook to the backend callback URL when confirmed
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- Retry failed webhook deliveries with exponential back-off
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- Expire stale pending intents on a configurable TTL
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- Read an EVM ERC-20 balance on demand (`POST /balances/check`)
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- Watch an EVM address/token pair for balance changes with age-decayed polling cadence (`POST /balance-watches`)
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---
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## 2. Supported chains
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Chains are defined in `supported-chains.json`. A worker is spawned only for chains with `"verified": true` (or listed in `SCANNER_ENABLED_CHAINS`).
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| Chain | Chain ID | Type | Proxy address | Conf. threshold | Active by default |
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|---|---|---|---|---|---|
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| BNB Smart Chain | 56 | EVM | `0x0DfbEe143b42B41eFC5A6F87bFD1fFC78c2f0aC9` | 200 blocks | Yes |
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| Ethereum Mainnet | 1 | EVM | `0x370DE27fdb7D1Ff1e1BaA7D11c5820a324Cf623C` | 50 blocks | Yes |
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| BSC Testnet | 97 | EVM | `0x0DfbEe143b42B41eFC5A6F87bFD1fFC78c2f0aC9` | 5 blocks | Yes (testnet) |
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| Arbitrum One | 42161 | EVM | `0x0DfbEe143b42B41eFC5A6F87bFD1fFC78c2f0aC9` | 2400 blocks | No |
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| Polygon | 137 | EVM | `0x0DfbEe143b42B41eFC5A6F87bFD1fFC78c2f0aC9` | 300 blocks | No |
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| Base | 8453 | EVM | `0x1892196E80C4c17ea5100Da765Ab48c1fE2Fb814` | 300 blocks | No |
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| Tron Mainnet | 728126428 | Tron | `TR7NHqjeKQxGTCi8q8ZY4pL8otSzgjLj6t` (USDT TRC20) | 200 (API-confirmed) | No |
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| TON Mainnet | 1100 | TON | `EQCxE6mUtQJKFnGfaROTKOt1lZbDiiX1kCixRv7Nw2Id_sDs` (USDT Jetton master) | 120 (API-finalized) | No |
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> [!note] Proxy address variations
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> Ethereum mainnet uses a v0.1.0 proxy (`0x370DE...`). Base uses a non-canonical CREATE2 address (`0x189219...`). All other EVM chains use the canonical v0.2.0 address (`0x0DfbEe...`). The memory note [[RN proxy addresses per chain]] has background on why CREATE2 canonical-address claims should not be trusted without verification.
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To enable a disabled chain without a rebuild: set `SCANNER_ENABLED_CHAINS=56,1,42161` (overrides the JSON `verified` flags).
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---
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## 3. Architecture decisions
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### Why a standalone Go service
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The scanner runs a tight polling loop that needs to hold open TCP connections to multiple RPC endpoints, manage per-chain checkpoints, and retry webhook delivery independently of backend restarts. A dedicated process with its own SQLite state is simpler and more reliable than embedding this into the Node.js backend.
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### Why SQLite
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Single-node deployment. WAL mode gives concurrent reads during writes. The state set is small (one row per intent, one checkpoint per chain). No operational overhead of a separate DB process inside the container.
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### Two payment rails
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The scanner supports two fundamentally different payment models:
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1. **Proxy-contract rail (EVM)**: funds flow through `ERC20FeeProxy`; the scanner matches by `paymentReference` embedded in the contract event. No unique destination address required; the reference is the discriminator.
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2. **Direct-address rail (Tron, TON, and EVM balance-watch)**: each payment gets a unique HD-derived destination address. The scanner matches by `to` address and validates amount. This is the only model available on Tron and TON because no fee-proxy contract exists there.
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### Confirmation thresholds
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EVM confirmation depths are conservative to handle reorgs:
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- **BSC (200)**: BSC has had historical reorg incidents; 200 blocks (~10 min) provides a practical safety margin.
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- **ETH (50)**: ~10 min at 12 s/block; Ethereum finality is probabilistic post-merge but 50 blocks is well past economic finality.
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- **Arbitrum (2400)**: Arbitrum uses optimistic rollup; 2400 blocks (~54 min) covers the challenge window.
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- **Polygon (300)**: polygon reorgs have occurred at depth >100; 300 blocks gives headroom.
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- **Base (300)**: Base is an OP Stack chain; same rationale as Polygon.
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Tron and TON do not use block-depth confirmation — TronGrid and TonCenter only surface confirmed/finalized transactions, so status goes directly to `confirmed`. The scanner reports the chain's acceptance floor (200 / 120) in the webhook for backend use.
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### Reorg protection (EVM)
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The EVM worker re-scans `3 × confirmationThreshold` blocks before the checkpoint (clamped 20–500) on every tick. This `ReorgBuffer()` ensures that a log in a block that was reorganised off the canonical chain will be re-evaluated when the chain reorganises. The window is wide enough to cover any realistic reorg depth for the chains the scanner targets.
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### Startup reconciliation
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On startup, `confirmed` intents with `webhook_delivered_at IS NULL` created within the last 7 days have their webhook re-delivered. This recovers from a crash between `finalizeIntent` and `deliverWebhook` without requiring a manual retry trigger.
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---
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## 4. Component map
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```
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┌─────────────────────────────────────────────────────────┐
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│ scanner binary │
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│ │
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│ main.go │
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│ ├── loadConfig() config.go │
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│ ├── initDB() intent.go (SQLite schema) │
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│ ├── startup reconcile intent.go │
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│ ├── newServer() api.go │
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│ │ └── startWorkers() api.go │
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│ │ ├── ChainWorker chain.go (EVM) │
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│ │ ├── TronChainWorker tron_chain.go (Tron) │
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│ │ └── TonChainWorker ton_chain.go (TON) │
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│ ├── HTTP routes api.go / main.go │
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│ ├── intent TTL expiry main.go + intent.go │
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│ ├── webhook retry loop main.go + webhook.go │
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│ └── BalanceWatchScheduler balance_watch.go │
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│ │
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│ reference.go — payment reference / topic hash │
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│ webhook.go — delivery, HMAC signing, retry │
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│ balance.go — EVM ERC-20 balanceOf reads │
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│ balance_watch.go — balance_watches state + webhooks │
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└─────────────────────────────────────────────────────────┘
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```
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One worker goroutine is spawned per active chain. All three chain types implement a common `Worker` interface (`start()`, `stop()`, `getHead()`). Workers poll on `POLL_INTERVAL_SEC` (default 15 s).
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---
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## 5. Backend integration points
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| Direction | Endpoint | When |
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|---|---|---|
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| Backend → Scanner | `POST /intents` | New payment initiated; returns `checkoutBlock` with `paymentReference` and proxy address |
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| Backend → Scanner | `GET /intents/{id}` | Poll intent status (optional; webhook is primary) |
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| Scanner → Backend | `POST <callbackUrl>` | Payment confirmed; signed with `X-AMN-Signature` HMAC-SHA256 |
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| Backend → Scanner | `POST /balances/check` | Synchronous ERC-20 balance read (direct-address rail) |
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| Backend → Scanner | `POST /balance-watches` | Start async balance watch (direct-address rail) |
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| Scanner → Backend | `POST <callbackUrl>` | Balance changed; `X-AMN-Event-Type: balance_changed` |
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| Backend → Scanner | `DELETE /balance-watches/{id}` | Stop watch after payment accepted or cancelled |
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| Backend → Scanner | `GET /scanner/status` | Chain lag + pending counts (ops/monitoring) |
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| Backend → Scanner | `POST /admin/webhooks/retry` | Force re-delivery of `webhook_failed` intents |
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All non-health endpoints require `Authorization: Bearer <SCANNER_API_KEY>`. Webhooks are HMAC-SHA256 signed; backend must verify `X-AMN-Signature` before crediting any payment.
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The `amn.scanner` backend provider wires intent creation, webhook receipt, and balance-watch lifecycle. See memory note [[amn scanner pay-in wiring + env]] for the 6 required env vars and the dispatcher registration.
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---
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## 6. Intent lifecycle
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```
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pending ──(tx seen)──► confirming ──(enough blocks)──► confirmed ──(webhook ok)──► [done]
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│ │ │
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│ │ (deep reorg / TTL) │ (all retries fail)
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└───────────────────────┴──────────► expired webhook_failed
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```
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- **Tron / TON** skip `confirming` and go directly to `confirmed` (API only surfaces finalized txns).
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- `webhook_failed` intents are retried every `WEBHOOK_RETRY_HOURS` (default 6 h) and on `POST /admin/webhooks/retry`.
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- Retry schedule on first delivery attempt: 5 s → 30 s → 2 min → 10 min → 1 h → `webhook_failed`.
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---
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## 7. Security model
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- All non-health endpoints require `Authorization: Bearer <SCANNER_API_KEY>` (constant-time compare).
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- Unset `SCANNER_API_KEY` logs a warning and allows all requests — local dev only.
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- Webhooks signed with HMAC-SHA256: `X-AMN-Signature: hex(hmac(body, callbackSecret))`.
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- `callbackSecret` stored in DB but excluded from all JSON responses (`json:"-"`).
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- Request bodies limited to 64 KB.
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- `SCANNER_CALLBACK_ALLOWED_HOSTS` env var restricts allowed webhook target hosts (SSRF guard).
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---
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## 8. Known limitations and open items
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| Item | Detail |
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| TON O(n) API calls | Per-intent polling — one TonCenter call per pending TON intent per scan cycle. Fine at low volume; needs batching for scale. |
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| Direct balance reads: EVM only | `POST /balances/check` and `POST /balance-watches` only support EVM ERC-20 (`eth_call` balanceOf). Tron/TON balance reads are future scope. |
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| Arbitrum / Polygon / Base / Tron / TON disabled | `verified: false` in `supported-chains.json`. Enable via `SCANNER_ENABLED_CHAINS` env var without a code change. |
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| Ethereum proxy version | Chain 1 uses the v0.1.0 proxy (`0x370DE...`). A v0.2.0 proxy is also deployed but checkout still uses the v0.1.0 ABI. |
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