Middle/Senior Backend Engineer

About the project

Xsolla ZK is building a cross-game infrastructure that empowers gamers to freely trade and use their digital assets without barriers. The backend team develops services for:

Xsolla Bazaar — NFT marketplace for trading digital assets across Ethereum, Polygon, and Xsolla ZK chains

Web3 Backpack — Smart wallet backend with account abstraction (ZK SSO 4337)

Blockchain Services — Event scanners, transaction processors, and Web3 connectors

BFF Layer — Backend-for-Frontend services orchestrating complex Web3 operations

Tech Stack:

Language: Go (go-ethereum, ethkit, idiomatic concurrency patterns)

Databases: MongoDB, PostgreSQL, Redis

Message Queue: NATS, RabbitMQ, Kafka

API: REST, WebSocket, gRPC, OpenAPI (Swagger)

Blockchain Infrastructure:

ZK Stack / ZKsync Era: EraVM, zksolc/zkvyper compilers, system contracts, bootloader, native Account Abstraction on zkSync-family chains

Account Abstraction (ERC-4337): EntryPoint (v0.6–v0.8), UserOperation lifecycle (prepare→sign→submit→confirm), bundler and paymaster roles, ERC-7562 validation rules, ERC-6492 signature validation for undeployed accounts

Ethereum L1: go-ethereum, abigen for smart contract bindings, RPC and indexer semantics

Smart Contracts: Solidity, ERC-20/721/1155, modular account standards (ERC-7579 / ERC-6900), session keys (ERC-7710 / ERC-7715)

Signature Schemes: EIP-712 typed data, ERC-1271 contract signatures, WebAuthn/passkey (P-256)

Infrastructure: Docker, Kubernetes, GitLab CI/CD, Graylog

Key Technical Challenges:

ZK Stack Integration: Working with EraVM execution environment, native Account Abstraction on zkSync-family chains, double resource accounting (EE gas + native computational resources), L1↔L2 communication via ZKsync Gateway, finality and reorg behavior

Account Abstraction Backend Services: Implementing the ERC-4337 UserOperation lifecycle, integrating with bundlers and paymasters, handling gas estimation failure modes and counterfactual deployment

Smart Contract Interaction: Generating Go bindings via abigen, handling factory dependencies for contract deployment, implementing EIP-712 transaction signing and ERC-1271 signature verification

Session Keys & Delegated Permissions: Supporting scoped, time-limited, contract-allowlisted authority (ERC-7710 / ERC-7715) for gasless, prompt-free gameplay transactions

Event Processing: Building blockchain event scanners for multiple chains (Ethereum, Polygon, Xsolla ZK), implementing reorg handling and transaction finality tracking

High-Availability Architecture: Designing resilient microservices for NFT marketplace and wallet backends with real-time WebSocket updates

Responsibilities

Develop backend services using Go with ethkit and go-ethereum for Ethereum interactions and RPC integration

Integrate with ZK Stack infrastructure: work with EraVM, system contracts, and L1↔L2 messaging

Build and maintain wallet backend indexing integrations for real-time token balance tracking and webhooks (via internal indexer or third-party providers)

Design and implement APIs for NFT marketplace and orderbook services, including on-chain order matching and settlement flows

Implement smart contract interaction layer using abigen-generated Go bindings for ERC-20/721/1155 tokens

Build Account Abstraction (ERC-4337) backend support: UserOperation submission and monitoring, paymaster gas-sponsorship logic, session-key issuance and revocation

Develop blockchain event scanners with reorg handling for Ethereum, Polygon, and Xsolla ZK chains

Implement BFF (Backend-for-Frontend) services orchestrating complex multi-chain operations

Design efficient caching and indexing strategies for on-chain data using Redis and MongoDB

Write comprehensive tests including integration tests with local ZKsync test environment

Document APIs using OpenAPI/Swagger specification

Monitor and troubleshoot production systems, handle blockchain-specific edge cases (stuck transactions, gas estimation)

Nice to Have

• Required:
• 2+ years of commercial experience with Go

Strong understanding of MongoDB and PostgreSQL (query optimization, indexing)

Experience with Redis for caching and session management

Knowledge of message queues (NATS, RabbitMQ, or similar)

• Experience designing and implementing REST APIs with OpenAPI/Swagger
• Understanding of microservices architecture patterns
• Experience with Docker and container orchestration basics
• Proficiency with Git and GitLab CI/CD
• Solid understanding of distributed systems concepts
• Preferred:
• Experience with Ethereum/EVM blockchain integration :
• Working with go-ethereum library and RPC nodes
• Smart contract interaction via ABI bindings (abigen)

Understanding of transaction lifecycle, gas estimation, nonce management

Knowledge of ZK Stack / ZKsync Era :

EraVM execution model and differences from EVM, including native Account Abstraction on zkSync-family chains

System contracts (ContractDeployer, NonceHolder, etc.)

L1↔L2 communication, bridging, and finality/reorg behavior

Working knowledge of ERC-4337 Account Abstraction :

• EntryPoint contract and the UserOperation lifecycle

Bundler and paymaster roles, gas estimation failure modes

Session keys and delegated permissions (ERC-7710 / ERC-7715)

Familiarity with signature schemes : EIP-712 typed data, ERC-1271 contract signatures, WebAuthn/passkey signers

Exposure to AA infrastructure providers (any of: Sequence, thirdweb, Privy, Dynamic, Turnkey, Pimlico, Alchemy, Biconomy, ZeroDev, Safe{Core}) for wallet, indexer, or transaction-relay integrations

Knowledge of token standards (ERC-20, ERC-721, ERC-1155) and their implementation details

Experience with blockchain event processing : event scanning, reorg handling, finality

Understanding of DeFi concepts and NFT marketplace mechanics

Experience with Kubernetes for container orchestration