Building Gas-less Experiences with Sui Paymaster: Architecture and Implementation Guide
Imagine a world where users can interact with your dApp seamlessly, without needing to hold any native tokens (SUI). This is no longer a distant dream. With Sui's Gas Station (also known as a Paymaster), developers can cover gas fees on behalf of their users, completely removing one of the biggest barriers for new entrants to Web3 and enabling a truly frictionless on-chain experience.
This article provides a complete guide to upgrading your dApp to be gas-less. We'll dive deep into the core concepts of the Sui Paymaster, its architecture, implementation patterns, and best practices.
1. Background and Core Concepts: What is a Sponsored Transaction?
In the world of blockchain, every transaction requires a network fee, or "gas." For users accustomed to the seamless experiences of Web2, this is a significant cognitive and operational hurdle. Sui addresses this challenge at the protocol level with Sponsored Transactions.
The core idea is simple: allow one party (the Sponsor) to pay the SUI gas fees for another party's (the User) transaction. This way, even if a user has zero SUI in their wallet, they can still successfully initiate on-chain actions.
Paymaster ≈ Gas Station
In the Sui ecosystem, the logic for sponsoring transactions is typically handled by an off-chain or on-chain service called a Gas Station or Paymaster. Its primary responsibilities include:
- Evaluating the Transaction: It receives a user's gas-less transaction data (
GasLessTransactionData). - Providing Gas: It locks and allocates the necessary gas fee for the transaction. This is usually managed through a gas pool composed of many SUI Coin objects.
- Generating a Sponsor Signature: After approving the sponsorship, the Gas Station signs the transaction with its private key (
SponsorSig), certifying its willingness to pay the fee. - Returning the Signed Transaction: It sends back the
TransactionData, which now includes the gas data and the sponsor's signature, to await the user's final signature.
In short, a Gas Station acts as a refueling service for your dApp's users, ensuring their "vehicles" (transactions) can travel smoothly on the Sui network.
2. High-Level Architecture and Interaction Flow
A typical gas-less transaction involves coordination between the user, the dApp frontend, the Gas Station, and a Sui Full Node. The interaction sequence is as follows:
Flow Breakdown:
- The User performs an action in the dApp UI, which constructs a transaction data package without any gas information.
- The dApp sends this data to its designated Gas Station to request sponsorship.
- The Gas Station verifies the request's validity (e.g., checks if the user is eligible for sponsorship), then populates the transaction with a Gas Coin and its signature, returning the semi-complete transaction to the dApp.
- The User sees the full transaction details in their wallet (e.g., "Purchase one NFT") and provides the final signature. This is a crucial step that ensures the user maintains consent and control over their actions.
- The dApp broadcasts the complete transaction, containing both the user's and the sponsor's signatures, to a Sui Full Node.
- After the transaction is finalized on-chain, the Gas Station can confirm this by listening for on-chain events or receipts, then notify the dApp backend via a webhook to close the loop on the business process.
3. Three Core Interaction Models
You can use the following three interaction models individually or in combination to suit your business needs.
Model 1: User-Initiated → Sponsor-Approved (Most Common)
This is the standard model, suitable for the vast majority of in-dApp interactions.
- User constructs
GasLessTransactionData: The user performs an action within the dApp. - Sponsor adds
GasDataand signs: The dApp backend sends the transaction to the Gas Station, which approves it, attaches a Gas Coin, and adds its signature. - User reviews and gives final signature: The user confirms the final transaction details in their wallet and signs it. The dApp then submits it to the network.
This model strikes an excellent balance between security and user experience.
Model 2: Sponsor-Initiated Airdrops/Incentives
This model is perfect for airdrops, user incentives, or batch asset distributions.
- Sponsor pre-fills
TransactionData+ signs: The Sponsor (typically the project team) pre-constructs most of the transaction (e.g., airdropping an NFT to a specific address) and attaches its sponsorship signature. - User's second signature makes it effective: The user only needs to sign this "pre-approved" transaction once for it to be executed.
This creates an extremely smooth user experience. With just one click to confirm, users can claim rewards or complete tasks, dramatically increasing the conversion rates of marketing campaigns.
Model 3: Wildcard GasData (Credit Line Model)
This is a more flexible and permission-based model.
- Sponsor transfers a
GasDataobject: The Sponsor first creates one or more Gas Coin objects with a specific budget and transfers ownership directly to the user. - User spends freely within the budget: The user can then freely use these Gas Coins to pay for any transactions they initiate within the budget's limits and validity period.
- Gas Coin is returned: Once depleted or expired, the Gas Coin object can be designed to be automatically destroyed or returned to the Sponsor.
This model is equivalent to giving the user a limited-time, limited-budget "gas fee credit card," suitable for scenarios requiring a high degree of user autonomy, such as offering a free-to-play experience during a game season.
4. Typical Application Scenarios
The power of the Sui Paymaster lies not just in solving the gas fee problem, but also in its ability to deeply integrate with business logic to create new possibilities.
Scenario 1: Paywalls
Many content platforms or dApp services require users to meet certain criteria (e.g., hold a VIP NFT, reach a certain membership level) to access features. The Paymaster can implement this logic perfectly.
- Flow: A user requests an action → the dApp backend verifies the user's qualifications (e.g., NFT ownership) → if eligible, it calls the Paymaster to sponsor the gas fee; if not, it simply denies the signing request.
- Advantage: This model is inherently resistant to bots and abuse. Since the sponsorship decision is made on the backend, malicious users cannot bypass the qualification check to drain gas funds.
Scenario 2: One-Click Checkout
In e-commerce or in-game purchase scenarios, simplifying the payment process is critical.
- Flow: The user clicks "Buy Now" on a checkout page. The dApp constructs a transaction that includes the business logic (e.g.,
transfer_nft_to_user). The user only needs to sign to approve the business transaction in their wallet, without worrying about gas. The gas fee is covered by the dApp's Sponsor. - Advantage: You can encode business parameters like an
order_iddirectly into theProgrammableTransactionBlock, enabling precise on-chain attribution for backend orders.
Scenario 3: Data Attribution
Accurate data tracking is fundamental to business optimization.
- Flow: When constructing the transaction, write a unique identifier (like an
order_hash) into the transaction's parameters or into an event that will be emitted upon execution. - Advantage: When the Gas Station receives the on-chain receipt for a successful transaction, it can easily extract this
order_hashby parsing the event or transaction data. This allows for a precise mapping between on-chain state changes and specific backend orders or user actions.
5. Code Skeleton (Based on the Rust SDK)
Here is a simplified code snippet demonstrating the core interaction steps.
// Assume tx_builder, sponsor, and wallet have been initialized
// Step 1: On the user or dApp side, construct a gas-less transaction
let gasless_transaction_data = tx_builder.build_gasless_transaction_data(false)?;
// Step 2: On the Sponsor (Gas Station) side, receive the gasless_transaction_data,
// fill it with a Gas Coin, and return the transaction data with the Sponsor's signature.
// The sponsor_transaction_block function handles gas allocation and signing internally.
let sponsored_transaction = sponsor.sponsor_transaction_block(gasless_transaction_data, user_address, gas_budget)?;
// Step 3: The dApp sends the sponsored_transaction back to the user,
// who signs and executes it with their wallet.
let response = wallet.sign_and_execute_transaction_block(&sponsored_transaction)?;
For a complete implementation, refer to the official Sui documentation's Gas Station Tutorial which offer out-of-the-box code examples.
6. Risks and Protection
While powerful, deploying a Gas Station in a production environment requires careful consideration of the following risks:
- Equivocation (Double-Spending): A malicious user might try to use the same Gas Coin for multiple transactions in parallel, which would cause the Gas Coin to be locked by the Sui network. This can be effectively mitigated by assigning a unique Gas Coin per user or transaction, maintaining a blacklist, and rate-limiting signing requests.
- Gas Pool Management: In high-concurrency scenarios, a single large-value Gas Coin can become a performance bottleneck. The Gas Station service must be capable of automatically splitting large SUI Coins into many smaller-value Gas Coins and efficiently reclaiming them after use. Professional Gas Station providers like Shinami offer mature, managed solutions for this.
- Authorization and Rate Limiting: You must establish strict authorization and rate-limiting policies. For instance, manage sponsorship limits and frequencies based on user IP, wallet address, or API tokens to prevent the service from being drained by malicious actors.
7. Ecosystem Tools
The Sui ecosystem already offers a rich set of tools to simplify Paymaster development and deployment:
- Official SDKs (Rust/TypeScript): Include high-level APIs like
sponsor_transaction_block(), significantly reducing integration complexity. - Shinami Gas Station: Provides an all-in-one managed service, including automated Gas Coin splitting/reclaiming, detailed metrics monitoring, and webhook notifications, allowing developers to focus on business logic.
- Enoki / Mysten Demos: The community and Mysten Labs also provide open-source Paymaster implementations that can be used as a reference for building your own service.
8. Implementation Checklist
Ready to upgrade your dApp to the gas-less era? Go through this checklist before you start:
- Plan Your Funding Flow: Define the Sponsor's funding source, budget, and replenishment strategy. Set up monitoring and alerts for key metrics (e.g., gas pool balance, consumption rate).
- Reserve Attribution Fields: When designing your transaction parameters, be sure to reserve fields for business identifiers like
order_idoruser_id. - Deploy Anti-Abuse Policies: You must implement strict authorization, rate-limiting, and logging mechanisms before going live.
- Rehearse on Testnet: Whether building your own service or integrating a third-party Gas Station, always conduct thorough concurrency and stress testing on a testnet or devnet first.
- Continuously Optimize: After launch, continuously track transaction success rates, failure reasons, and gas costs. Fine-tune your budget and strategies based on the data.
Conclusion
The Sui Paymaster (Gas Station) is more than just a tool for covering user gas fees. It's a powerful paradigm that elegantly combines a "zero SUI on-chain" user experience with the business need for "order-level on-chain attribution" within a single, atomic transaction. It paves the way for Web2 users to enter Web3 and provides developers with unprecedented flexibility for business customization.
With an increasingly mature ecosystem of tools and the current low gas costs on the Sui network, there has never been a better time to upgrade your dApp's payment and interaction flows to the gas-less era.
