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