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

Benefits

We are passionate about fostering a supportive environment for our team, so we prioritize the physical, mental, and emotional well-being of our employees and their families through a comprehensive Benefits Program. This includes medical, dental, and vision, PTO, and a personalized career roadmap for each employee. By investing in professional development through training and educational opportunities, we ensure that our team thrives both personally and professionally. Together, we’re not just building a business; we’re cultivating a community that values creativity, collaboration, and the transformative power of play.