// project
An Integration Lab for the Android payments ecosystem: every gateway behind one abstraction, with a live look at what actually happens on each transaction.
The brief
Payments is the hardest integration surface on Android: every gateway ships a different SDK, most of them are Activity-callback-era, the client can lie about the outcome, and the interesting logic (signatures, webhooks, idempotency, recovery) lives on the server. PaymentsLab-KMP runs real payment flows across a 66-gateway catalog behind a single PaymentGateway abstraction, and visualizes them step by step. A Ktor server does the order creation, signature verification and webhook reconciliation a real integration requires. Beyond one-shot pay-in it models five money-movement rails.
What shipped
// case study
The problem
Every payment gateway ships a different, mostly Activity-callback-era SDK, and the client can lie about the outcome: the logic that actually decides whether money moved (signatures, webhooks, idempotency, recovery) has to live somewhere the client can't fake it.
The decision
Put a Ktor server between the client and the truth: order creation, signature verification and webhook reconciliation happen there, a Room journal is written before the SDK even launches so a process death mid-payment is always recoverable, and a redaction layer keeps every secret and PII out of logs and screens.
The result
66 cataloged gateways now sit behind one PaymentGateway contract, with a client-side Success read only as a hint until the server confirms it. The same discipline now extends to five money-movement rails beyond one-shot checkout, every one of them MOCK_MODE-honest until real sandbox keys are set.
66 gateways cataloged
Watch the two-minute tour ↓
// guided tour
A storyboarded walkthrough of the real app — tap the speaker for the voiceover, or read along with the captions.
// shipping
Every line below is checkable. Add the repository in an F-Droid client and the app installs, signed with the same key its listing pins.
// pipeline
Read live from GitHub Actions and the F-Droid index. Not a badge: the actual runs, what shipped, and the certificate you can check the download against.
reading the pipeline
44
Gradle modules (15 local + 29 composed)
66
gateways cataloged
5
money-movement rails
1
PaymentGateway contract
// multiplatform
The real screens (and, where it's live, the running build) per platform — not a mockup.
// design notes
A client-side Success is a hint, never proof. Only the server decides the true state, after signature verification and webhook reconciliation. A server that owns price and truth, a client that always confirms before trusting, a journal written to Room before the SDK launches so a process death mid-payment is always recoverable, and a redaction layer so no secret or PII ever renders or logs.
One Gradle module per native-SDK provider is contributed into a registry via Koin's getAll<PaymentGateway>(), so adding gateway N+1 touches no existing code. There are 15 local modules plus 25 composed from kmp-toolkit (19 of them standalone provider gateway modules). The in-app catalog spans 66 registered gateways: 7 native-SDK integrations, 47 hosted-webview gateways behind one archetype, 8 mobile-money flows and 4 catalog-only / KYC-gated entries, each with its own status badge and region.
Beyond one-shot checkout the server models payouts (/payouts: money out to a beneficiary), mandates & subscriptions (/mandates + scheduled debits and cancel), a card vault (/vault: tokenize once, charge later by id), marketplace Connect onboarding (/connect: sub-merchant KYC + split payouts) and an internal double-entry wallet ledger (/wallet: seed / debit / refund against a real running balance), plus split payments, a two-leg orchestration that compensates if one leg fails. Ten provider modules ride these rails (Paystack, Flutterwave, Paytm, Xendit, M-Pesa, Peach, NMI, Stripe Connect, plus wallet and a record-only cash gateway), every one MOCK_MODE-honest until real sandbox keys are set.
Razorpay, Cashfree, Stripe (+ Google Pay), Square, Omise and a raw UPI intent flow all implement the same tiny PaymentGateway interface. The Activity-callback SDKs are bridged into suspending coroutines by a PaymentHost that never leaks an Activity upward. A generic hosted-webview archetype covers the whole class of gateways with no native SDK behind the same contract. Env-backed credentials auto-degrade from SANDBOX_READY to MOCK_MODE honestly instead of silently pretending to work.
The lifecycle is a pure (State, Event) → Effects reducer, zero coroutines/DI/IO, with the orchestrator just executing its effects. A payment's path is a recorded event log that replays byte-for-byte identically, the auditing property money movement wants. The MVI base comes from my own kmp-toolkit library, shared with other apps.
core:security holds real Android Keystore AES-256-GCM at-rest encryption, FLAG_SECURE + recursive tapjacking protection, device-integrity checks (root, emulator, debugger, Frida/Xposed hook detection, SSL-pinning-bypass detection), and a certificate-pinning config, with detection kept deliberately separate from enforcement policy.
// architecture
// under the hood
// directions
Pick a treatment to see it whole. Where one ships a dark counterpart, drag the divider to compare the two faces of it — same layout, same content, only the light changes.
// gallery
Swipe or use the arrows · tap a screen to enlarge
frequently asked
SDE-2, Android & Product Owner at Dice.tech (June 2023 - Present), platform owner of the app behind 50k+ monthly active users (22k+ daily, platform owner at Dice.tech). See the source.
Took GPS accuracy to 95%: up from 50%, by predictive dead reckoning. See the source.
80% crash reduction: Crashlytics + structured concurrency fixes. See the source.
~87% of the UI layer: 455k of 523k UI-layer LOC, verified screen by screen against the legacy XML. See the source.
B.Tech, Computer Science & Engineering, NIT Bhopal (MANIT) (2017 - 2021). See the source.
Consulting Engineer, Platform & AI at Neev Consulting (April 2026 - Present). Built the LLM assistant layer of an ERPNext/Frappe consulting ERP: business-context resolution, capability discovery, and an AI capability gate that defaults OFF with a test proving it. Models client to project to PO to milestone to GST invoice to payment end to end. See the source.
Doori: Offline-first mileage, travel & expense tracker on one Kotlin codebase across Android, iOS, Wear OS, watchOS & Desktop. See the source.
Gaddi: A Hinglish social-deduction bluffing game of power, satire & second chances. Gaddi ke liye kuch bhi karega. See the source.
PaymentsLab-KMP: An Integration Lab for the Android payments ecosystem: every gateway behind one abstraction, with a live look at what actually happens on each transaction. See the source.
Candidai: A native, multiplatform AI career-intelligence engine, and the open-source project it's built on. See the source.
Yes: The Loopdown, his field-notes writing hub, where the recurring bug characters are named after real production incidents. See the source.
Answered by his own teammates, not by him: see the EB Profiles, one question per member, in their own words. See the source.
Open to remote (worldwide / India) and hybrid in Pune / Bengaluru. Email siddharthpandalai990@gmail.com. See the source.
Shared Kotlin Multiplatform libraries used across his own apps, plus merged upstream pull requests on career-ops: see the repos and the running count. See the source.
Yes: see the Play Store shelf for the apps that are actually live and installable, not just source. See the source.
Every route on the site, as a tile you can open: 3D builds, labs and canvases, not screenshots. See the source.
