Interview Questions for Solana Development

Solana employs a novel consensus mechanism called “Leaderless Proof-of-Stake” (LPoS). Unlike traditional Proof-of-Stake (PoS) systems where a single validator is chosen to lead each epoch, Solana distributes leadership amongst a rotating set of validators. This leader rotation is crucial for its high throughput. It leverages a combination of PoH, Proof-of-Stake, and a specific consensus algorithm called Tower BFT (Byzantine Fault Tolerance) to ensure transaction finality and security.

Advantages over other protocols: Compared to PoW protocols like Bitcoin, Solana’s LPoS eliminates the energy-intensive mining process, resulting in significantly lower energy consumption. Compared to other PoS systems, its distributed leader approach and innovative use of PoH dramatically increase transaction throughput and reduce latency. This makes it ideal for high-frequency trading and decentralized applications requiring fast and reliable transactions.

Proof of History (PoH) is a fundamental component of Solana’s architecture. It’s a cryptographic mechanism that creates a verifiable, globally-consistent, and tamper-proof record of time. Think of it as a high-resolution clock that’s shared by all nodes on the network. Instead of relying on network communication for time synchronization, PoH uses a verifiable delay function (VDF) to generate timestamps. This VDF ensures that generating a timestamp takes a predictable amount of time, making it computationally infeasible to manipulate the timeline.

Role in Solana’s architecture: PoH provides a single source of truth for time across the network, enabling Solana to reach consensus much faster than traditional consensus protocols. By providing a synchronized view of time, PoH significantly improves the efficiency and speed of transaction processing, which is a key reason for Solana’s high throughput.

Solana and Ethereum are both prominent blockchain platforms, but they differ significantly in their architecture and approach to scalability. Ethereum, a pioneer in smart contract functionality, utilizes a Proof-of-Stake (PoS) consensus mechanism and relies heavily on smart contracts written in Solidity. It’s known for its robust developer ecosystem and wide adoption but struggles with scalability challenges.

• Consensus Mechanism: Solana uses a Leaderless Proof-of-Stake (LPoS) with PoH; Ethereum uses Proof-of-Stake (PoS).
• Scalability: Solana prioritizes high throughput and low latency; Ethereum faces scalability limitations.
• Programming Language: Solana uses Rust primarily for smart contracts (though other languages are supported); Ethereum primarily uses Solidity.
• Transaction Fees: Solana generally has lower transaction fees than Ethereum.
• Smart Contract Functionality: Both support smart contracts but have different programming paradigms.

In essence, Solana prioritizes speed and scalability, while Ethereum prioritizes its mature ecosystem and smart contract functionality.

Solana achieves impressive transaction throughput and scalability through a multi-faceted approach:

• Leaderless Proof-of-Stake (LPoS): The distributed leadership model ensures that no single point of failure limits throughput.
• Proof of History (PoH): Provides a highly efficient mechanism for time synchronization and consensus, dramatically reducing the time required to reach consensus.
• Parallel Transaction Processing: Solana processes transactions in parallel across multiple validators, significantly improving efficiency.
• Turbine: A specialized network layer that enhances data transmission and reduces latency.
• Sealevel: A high-performance transaction processing engine which handles thousands of transactions per second.

This combination of architectural innovations allows Solana to handle thousands of transactions per second, far exceeding the capabilities of many other blockchain networks.

In the context of Solana, RPC (Remote Procedure Call) is the primary method for interacting with the Solana network from external applications or clients. It allows developers to access data and functionality provided by Solana nodes without needing to directly manage the node itself.

How it works: RPC uses a client-server model. A client (e.g., a web application, a mobile app, or a command-line tool) sends requests to a Solana RPC node, and the node processes the request and sends back the results. These requests can include various operations, such as querying account balances, submitting transactions, and accessing blockchain data.

Example: A developer might use an RPC call to get the balance of a specific Solana account. They’d send a JSON-RPC request to an RPC endpoint, and the node would return a JSON response containing the account’s balance.

I have extensive experience using the Solana CLI (Command-Line Interface). I’ve used it for various tasks, including:

• Account Management: Creating, deleting, and managing Solana accounts.
• Transaction Submission: Sending and receiving SOL, and interacting with smart contracts.
• Program Deployment: Deploying and updating smart contracts on the Solana network.
• Data Retrieval: Querying the Solana blockchain for specific information.
• Network Monitoring: Checking the health of the network and monitoring transaction status.

I find the Solana CLI to be a powerful and efficient tool for interacting with the network, and I regularly use it in my development workflow. For example, I’ve used it to troubleshoot issues by directly examining the network state and transaction logs, which is often much more efficient than using a graphical interface.

Deploying and managing smart contracts on Solana involves several key steps:

1. Development: Write the smart contract using Rust (or another supported language). This involves defining the contract’s functionality, data structures, and interfaces.
2. Compilation: Compile the Rust code into a Solana program using the Solana compiler (solana-program).
3. Deployment: Use the Solana CLI or a similar tool to deploy the compiled program to a Solana cluster (e.g., devnet or mainnet). This involves sending a transaction to the network that creates a new program account containing the compiled program’s code.
4. Testing: Thoroughly test the deployed contract on a testnet to ensure its functionality and security before deploying it to mainnet.
5. Management: Once deployed, you can manage the contract using the CLI or other tools. This includes updating the contract code, interacting with it using client applications and monitoring its performance.

Example CLI command to deploy a program: solana program deploy dist/program/myprogram.so

Managing smart contracts efficiently often involves using tools that automate processes like deployment, testing, and monitoring. Using a robust testing framework is crucial to ensure the reliability of your smart contracts.

Solana, while boasting impressive speed and scalability, isn’t immune to security vulnerabilities. Like any blockchain, it faces risks stemming from both its core architecture and the smart contracts built upon it. Key considerations include:

• Software Bugs: Bugs in the Solana runtime or in smart contracts can lead to exploits, allowing malicious actors to steal funds or manipulate the network. This is a common vulnerability across all blockchain platforms.
• Transaction Ordering: Solana’s unique transaction ordering mechanism, while efficient, can be complex. Improper handling of transaction ordering within smart contracts can create vulnerabilities exploitable by attackers.
• Oracle Manipulation: Smart contracts often rely on external data feeds (oracles). If these oracles are compromised or manipulated, it can lead to incorrect contract execution and potentially significant financial losses.
• DoS Attacks: Denial-of-service attacks can target the network, making it unavailable for legitimate users. These attacks can be computationally expensive to launch, but their impact can be substantial.
• Key Management: Users’ private keys are crucial. Losing access to them means losing control of funds. Secure key management practices are absolutely essential for every user.
• Smart Contract Vulnerabilities: Similar to other blockchain platforms, poorly written or audited smart contracts can introduce various vulnerabilities such as reentrancy, overflow/underflow errors, and logic flaws. Thorough audits are necessary.

Mitigating these risks requires a multi-pronged approach, including rigorous code audits, regular security updates, and user education regarding best practices for key management and interacting with the network.

Debugging Solana smart contracts, written primarily in Rust, requires a multifaceted approach. It’s different from traditional debugging because you’re dealing with a distributed, immutable ledger. Here’s how I’d tackle it:

• Runtime Logging: Embed println! statements or equivalent logging mechanisms within the Rust code of your smart contract. These logs will output information during contract execution. You’ll need to use a Solana client to view these logs, often using command-line tools.
• Solana Test Suite: The Solana ecosystem offers a testing framework allowing for unit and integration testing. This allows me to isolate and identify bugs before deploying to the mainnet.
• Remote Debugging: Specialized Solana debugging tools and integrations with IDEs (Integrated Development Environments) may allow remote debugging. Though limited, it can sometimes provide vital insights.
• Transaction Tracing: Analyze the transaction logs on the Solana blockchain explorer. This provides valuable information about the execution flow of your smart contract and can pinpoint issues in the program logic.
• Static Analysis Tools: Use static analysis tools (linters) to scan the Rust code for potential issues. This can identify issues like potential vulnerabilities and style inconsistencies before they become runtime problems.
• Post-Mortem Analysis: If a failure occurs on mainnet, meticulously examine the transaction details, event logs, and the contract’s state to pinpoint the root cause.

For example, if a transaction fails with an error message, I’d meticulously examine that message in conjunction with the transaction details and logs to understand the point of failure within the smart contract’s logic.

I’ve worked extensively with various Solana wallets, each offering unique functionalities:

• Phantom Wallet: User-friendly interface, good for both beginners and experienced users, supports various Solana tokens and NFTs.
• Sollet Wallet: A browser-based wallet, offering a relatively simpler interface, good for basic transactions.
• Slope Wallet: Similar to Phantom in terms of user experience, supporting DeFi integrations and NFT management.
• Trust Wallet: A multi-chain wallet that supports Solana along with many other blockchains. This is convenient if you manage assets across multiple networks.
• Ledger Hardware Wallets: These provide maximum security by keeping your private keys offline. They integrate with Solana via software wallets that act as interfaces.

My experience includes using these wallets for transactions, managing NFTs, interacting with decentralized exchanges (DEXs) on Solana, and staking SOL. The choice of wallet often depends on the user’s level of technical expertise and security preferences.

There are several ways to interact with a Solana smart contract:

• Using a Solana Client Library: These libraries (available in various languages like JavaScript, Rust, etc.) allow developers to programmatically interact with the network, sending transactions to invoke functions within the smart contract.
• Using a Wallet: Most Solana wallets provide interfaces to interact with smart contracts. This often involves approving transactions and signing them, simplifying the interaction for non-programmatic use.
• Using a Decentralized Application (DApp): Many DApps (decentralized applications) are built on top of Solana and provide user-friendly interfaces to interact with underlying smart contracts. This abstracts away much of the technical complexity.
• Command-Line Interface (CLI): Using the Solana CLI tools allows for direct interaction with the network, though this is more geared towards developers and requires a deeper understanding of the network’s structure and RPC calls.

For example, using the JavaScript library, you could create a transaction that calls a specific function in your smart contract, specifying the necessary parameters, and then send it to the network. The wallet would then prompt the user to sign the transaction to confirm the action.

My familiarity with Rust in the context of Solana development is extensive. Rust is the primary language for writing Solana smart contracts. I’m proficient in using its features for memory management, concurrency, and building robust and secure smart contracts. I’m comfortable with:

• The Solana Rust SDK: I have practical experience using the Solana Rust SDK for creating, deploying, and interacting with smart contracts.
• Memory Safety: Rust’s focus on memory safety is crucial for preventing common smart contract vulnerabilities like buffer overflows and dangling pointers.
• Concurrency and Parallelism: Solana’s high transaction throughput requires understanding Rust’s concurrency features to write efficient and scalable contracts.
• Error Handling: I understand how to implement robust error handling mechanisms within Rust smart contracts, essential for maintaining application stability and security.

I’ve personally developed and deployed several Solana smart contracts using Rust, including [mention a project or two if you have experience to showcase].

Solana’s tokenomics revolve around its native token, SOL. SOL has several key roles:

• Transaction Fees: SOL is used to pay for transaction fees on the Solana network.
• Staking: Users can stake their SOL to participate in network validation and earn rewards. This is crucial for the network’s security and decentralization.
• Governance: SOL holders can participate in the governance of the network, influencing its future development.
• Ecosystem Funding: A portion of transaction fees and inflation might be used to fund the Solana ecosystem and development initiatives.

Understanding Solana’s inflation model, staking mechanics, and governance processes is critical for anyone participating in the ecosystem. I’m familiar with the complexities of SOL’s tokenomics and how they impact the overall network health and stability. For example, changes in staking rewards can affect network participation and security.

Solana offers both on-chain and off-chain data storage solutions. Each has its strengths and weaknesses:

• On-Chain Storage: Data stored directly on the Solana blockchain is highly secure and tamper-proof. However, it’s expensive and limited in capacity. It’s ideal for storing critical data that needs to be verifiable and immutable, such as account balances or smart contract state.
• Off-Chain Storage: Data stored off-chain, often using IPFS (InterPlanetary File System) or Arweave, is more cost-effective and can handle larger datasets. However, it introduces trust assumptions because the integrity of the data depends on the off-chain storage provider. It’s suitable for storing larger amounts of data like images or large files associated with NFTs.

Choosing between on-chain and off-chain storage depends on the specific requirements of your application. For instance, a simple counter app might benefit from purely on-chain storage, while an NFT marketplace would likely rely on both on-chain (metadata pointers) and off-chain (actual image or video files) storage. I have experience implementing strategies to optimize the use of both storage mechanisms in Solana-based projects.

Managing transaction costs on Solana is crucial for building sustainable dApps. Unlike Ethereum’s gas model based on computational complexity, Solana’s fee structure is primarily determined by the size of the transaction itself. Each transaction incurs a small, fixed fee in SOL, regardless of the complexity of the operation. This makes it predictable and generally cheaper for users. However, complex smart contracts might still lead to higher costs due to the increased data involved.

To effectively manage these costs, developers can employ several strategies:

• Batching Transactions: Combining multiple transactions into a single batch reduces the overall number of transactions and thus the associated fees.
• Optimizing Smart Contracts: Writing efficient code that minimizes data usage directly impacts the transaction size, hence reducing the fees. This involves careful consideration of data structures and program logic.
• Using Transaction Fee Payers: For applications where users might not have sufficient SOL for transaction fees, a central account can act as a fee payer, covering costs on behalf of users. This is common in many dApps.
• Off-chain computations: Offloading computationally expensive tasks from the blockchain to an off-chain system (like using a centralized server to perform verification before submitting data to the chain) can significantly reduce transaction costs and improve performance.

For example, if you’re building a social media dApp, you might use an off-chain solution to store user posts, only sending hashes or metadata to the blockchain to verify ownership and authenticity. This reduces the amount of data written to the chain, thus reducing fees significantly.

Creating a Solana dApp involves several key steps. It’s akin to building a house; you need a strong foundation and careful planning.

1. Choose a Development Framework: Solana offers multiple frameworks, including Anchor, Solana Program Library (SPL), and others. Anchor is particularly popular for its ease of use and high-level abstractions. The choice depends on your project’s specific needs and your familiarity with different programming languages (Rust is commonly used for Solana smart contracts).
2. Design the Smart Contract: This is the core of your dApp, defining its functionality and data structures. It’s written in Rust (primarily) and carefully designed to be secure and efficient. Think about the data your dApp needs to store, how it will interact with users, and the logic governing its operations.
3. Develop the Frontend: This is what users interact with. Technologies like React, Vue, or Angular are commonly used, along with libraries for interacting with the Solana network (like the Solana Web3.js library). This communicates with the smart contract on the blockchain.
4. Testing: Rigorous testing is essential to ensure the correctness and security of both the smart contract and the frontend. Unit tests, integration tests, and end-to-end tests should be conducted thoroughly.
5. Deployment: Once thoroughly tested, you’ll deploy your smart contract to the Solana network using tools like Anchor or command-line interfaces provided by Solana. You’ll also deploy the frontend to a hosting platform.
6. Monitoring: After deployment, it is crucial to monitor your dApp’s performance and security, including transaction rates, network latency, and potential vulnerabilities.

For example, a simple DeFi application might involve a smart contract that manages tokens, with a frontend to enable users to deposit, withdraw, and transfer those tokens.

Solana development, while exciting, comes with its set of challenges. These can range from technical hurdles to ecosystem-specific issues.

• Rust Expertise: Solana smart contracts are primarily written in Rust, requiring developers to be proficient in this systems programming language. Rust has a steeper learning curve than other languages like Solidity (used in Ethereum).
• Network Congestion: During periods of high network activity, transaction processing times can increase, leading to delays and potential disruptions in your dApp’s functionality. This is a characteristic of any high-throughput blockchain.
• RPC Node Reliability: Your dApp relies on RPC (Remote Procedure Call) nodes to interact with the Solana network. If these nodes experience outages or delays, your application might become unavailable.
• Security Concerns: As with any blockchain platform, vulnerabilities in smart contracts can be exploited, leading to potential loss of funds or data. Rigorous code reviews and audits are essential.
• Debugging Complexity: Debugging smart contracts written in Rust can be more challenging compared to other environments, requiring specialized tooling and techniques.

For instance, handling network congestion might involve implementing strategies to queue transactions, retry failed transactions after a delay, and provide informative feedback to users about network status.

Error and exception handling in Solana smart contract development is crucial to prevent unexpected behaviour and potential vulnerabilities. Solana programs, being written primarily in Rust, benefit from its robust error handling mechanisms.

The key is to anticipate potential issues and implement graceful degradation when things go wrong. This includes:

• Result Type: Leverage Rust’s Result type to explicitly handle potential errors. This returns either a successful value (Ok(T)) or an error (Err(E)). Each function should explicitly define what errors it can return and how these errors are communicated to the caller.
• Custom Error Types: Define custom error types to provide more meaningful information about the nature of errors. This aids debugging and allows for more sophisticated error handling.
• Logging: Implement logging to record errors and program events. This information is crucial for monitoring, debugging and post-mortem analysis.
• Panic Handling (with Caution): While Rust’s panic! macro can halt program execution, it should be used sparingly in Solana smart contracts as it can lead to resource loss and state inconsistency. Most errors are best handled via the Result type.

Example:

fn transfer_sol(from: Pubkey, to: Pubkey, amount: u64) -> Result<(), SolanaError> {     // ... some code ...     if amount > account.lamports {         return Err(SolanaError::InsufficientFunds); //Custom Error     }     // ... more code ...     Ok(()) } 

This example shows a function that handles insufficient funds by returning a custom SolanaError, allowing the caller to react accordingly.

Ensuring data security and privacy in Solana dApps is paramount. Several strategies are critical here:

• Secure Smart Contract Design: Employ secure coding practices to prevent vulnerabilities like reentrancy attacks, integer overflows, and access control issues. Regular code audits are essential.
• Data Minimization: Only store the necessary data on the blockchain. Avoid storing sensitive information directly on the chain if possible. Consider using on-chain hashes or commitments to refer to off-chain data.
• Encryption: For sensitive data that must be stored, employ robust encryption techniques both at rest and in transit. End-to-end encryption is preferable whenever feasible.
• Access Control: Implement strong access control mechanisms to restrict access to sensitive data based on roles and permissions. Solana’s accounts and their associated capabilities can help you enforce this.
• Zero-Knowledge Proofs (ZKPs): For situations where you need to prove the possession of information without revealing it, ZKPs offer a strong privacy solution. This is an advanced technique though.
• Off-chain Storage: Store non-critical data off-chain (using IPFS, Arweave, or other decentralized storage systems) and use on-chain references for verifiable access. This significantly reduces the cost and security concerns associated with on-chain storage.

For example, a dApp handling user identities might not store the user’s personal information directly on-chain but use a secure hashing mechanism to link the identity to an on-chain representation, preserving privacy while maintaining accountability.

My experience with testing and deploying Solana smart contracts involves a comprehensive approach to ensure both functionality and security. The testing process includes multiple layers:

• Unit Testing: This involves testing individual functions and modules of the smart contract in isolation, often employing tools like the Rust testing framework. This focuses on verifying the core logic.
• Integration Testing: This tests the interaction between different parts of your smart contract and, importantly, its interaction with the Solana runtime. It’s crucial to simulate real-world conditions.
• End-to-End Testing: These tests involve the whole system, encompassing the smart contract and its frontend interaction. This stage includes testing the user experience.

Deployment usually goes through several stages:

1. Local Development Network: First, the contract is tested thoroughly on a local development network, allowing for easy iterations and debugging.
2. Testnet Deployment: Before deploying to the mainnet, I always deploy to a testnet like Solana’s Devnet or Testnet. This allows for testing in a real network environment with real transactions (without incurring actual costs).
3. Mainnet Deployment: Once satisfied, the contract is deployed to the Solana mainnet, marking the actual release. This usually involves tools provided by the chosen framework.

I have experience with various deployment strategies, from simple command-line interactions to more sophisticated Continuous Integration/Continuous Deployment (CI/CD) pipelines to automate the process.

I’m familiar with a wide range of Solana ecosystem projects across various sectors. My knowledge spans several key areas:

• DeFi (Decentralized Finance): I have experience with projects such as Solana-based decentralized exchanges (DEXs), lending protocols, and stablecoin implementations. This includes understanding the unique challenges of building financial applications on a high-performance blockchain like Solana.
• NFTs (Non-Fungible Tokens): I’m familiar with various NFT marketplaces and projects built on Solana, understanding the technical aspects of creating, minting, and managing NFTs on the platform.
• Web3 Applications: I’ve worked with various web3 applications built on Solana, from decentralized social media platforms to gaming dApps. I understand how these applications leverage the unique capabilities of the Solana ecosystem.
• Infrastructure Projects: I’m aware of significant Solana infrastructure projects, including RPC node providers, wallets, and tooling that support the broader ecosystem.

My knowledge extends beyond just the technical aspects, encompassing the business models, community dynamics, and the strengths and weaknesses of different projects within the Solana ecosystem. I actively follow the latest developments and innovations within the space.

My Solana development workflow relies heavily on a robust toolset. For smart contract development, I primarily use Rust, leveraging its memory safety and performance advantages. The Solana CLI is indispensable for interacting with the network, deploying programs, and managing accounts. For testing, I extensively use tools like Anchor, which simplifies the process of writing, testing, and deploying smart contracts. Solana Test Validator is crucial for local development and testing, allowing me to simulate the Solana network environment without impacting the mainnet. Finally, for debugging and analysis, I utilize the Solana JSON-RPC API to inspect the state of the blockchain and troubleshoot issues. I also find using IDEs like VS Code with appropriate extensions invaluable for code completion and debugging.

Optimizing a Solana smart contract involves a multi-pronged approach focusing on reducing computational complexity and minimizing data usage. Firstly, I prioritize using efficient data structures. Instead of using large arrays, I employ more compact representations whenever possible, like using bitmaps for managing flags or opting for accounts with specific layouts for data retrieval. Secondly, I minimize the use of computationally expensive operations. This means avoiding nested loops and using optimized algorithms whenever possible. For instance, instead of iterating through a large data set within the smart contract, I’d prefer to perform that processing off-chain and only transmit relevant results to the contract. Thirdly, I focus on reducing account size by storing only the necessary data. Unnecessary data bloat increases storage costs and processing time. Lastly, I leverage features like accounts.key and pre-computed hashes where appropriate for more efficient data access and comparisons.

For example, consider an auction program. Instead of storing the entire bid history in a single account, it’s more efficient to only maintain a summary of the top bids, reducing storage costs and improving the speed of processing new bids. Regular profiling using the Solana JSON-RPC API to monitor transaction execution times is key to identify performance bottlenecks.

I have extensive experience with Solana’s various APIs and libraries. The JSON-RPC API is my primary interface for interacting with the blockchain programmatically. I regularly use it for tasks such as retrieving account data, submitting transactions, and monitoring network events. I’m proficient with Anchor, which provides a higher-level framework that simplifies smart contract development in Rust. This significantly streamlines the process of defining accounts, writing instructions, and handling transactions. I’ve also used the Solana Web3.js library for interacting with Solana from JavaScript-based applications, particularly in the context of frontend development for decentralized applications (dApps). Understanding the intricacies of these tools allows me to build robust and efficient Solana applications.

Staying updated in the rapidly evolving Solana ecosystem is paramount. I actively follow the official Solana blog and developer documentation for announcements on new features, updates, and best practices. I regularly participate in the Solana community forums and Discord servers, engaging in discussions and learning from other developers. Following key figures in the Solana ecosystem on social media platforms such as Twitter provides valuable insights into emerging trends and technologies. Furthermore, I consistently review and experiment with new tools and libraries that emerge, ensuring my skills remain cutting-edge. Attending conferences and workshops related to Solana further enhances my knowledge.

My experience with integrating Solana with other blockchains and services primarily involves using cross-chain communication protocols. For instance, I’ve worked on projects that leverage Wormhole for transferring tokens and data between Solana and Ethereum. This involves understanding the nuances of the protocol, including the security implications and the limitations of cross-chain transactions. Other integrations have included connecting Solana smart contracts to off-chain services like oracles for accessing external data, and using IPFS for decentralized storage. Understanding the capabilities and constraints of different protocols is essential for choosing the most appropriate solution for a given integration task. Security remains the primary concern in such integrations, and I always prioritize secure and robust implementation.

One particularly challenging project involved building a decentralized exchange (DEX) on Solana that needed to handle a high volume of trades with minimal latency. The complexity arose from the need to efficiently manage order books and ensure atomicity in trade executions to prevent race conditions. My approach involved designing a custom order book data structure that optimized for fast lookups and updates. Instead of a traditional centralized order book, we employed a more distributed approach utilizing multiple accounts to manage different parts of the order book, distributing the workload and enhancing scalability. To ensure atomicity, we leveraged Solana’s built-in transaction capabilities and carefully crafted instructions to manage state changes. We also implemented a robust testing framework, utilizing Solana Test Validator extensively, to simulate high-volume trading scenarios and detect potential race conditions. Through rigorous testing and iterative refinements, we successfully developed a highly performant and reliable DEX.