Shardeum’s Replay App: Debugging, Gas Efficiency, and More

Shardeum’s Replay App: Debugging, Gas Efficiency, and More

Learn more about Shardeum's Replay App, an advanced tool for debugging, gas efficiency, and network analysis, ensuring high performance and...

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We are excited to introduce an advanced feature of Shardeum—Replay App, a crucial development by our engineering team in Q4 of 2023, designed to ensure feature-readiness of the network. Initially conceptualized as an internal tool, this application efficiently accumulates data across all the shards, enabling the replay of historical transactions to scrutinize and debug intricate outputs, especially those hard-to-reproduce issues. Before we unpack its use cases with more details, let’s unpack some of the core features of Shardeum which are relevant to understanding the full suite of functionalities of the Replay App!

EVM and Opcodes

What is the EVM and what are Opcodes? Opcodes are the low-level operations that the Ethereum Virtual Machine (EVM) can perform. The EVM is the runtime environment for smart contracts in Ethereum. When you write a smart contract in a high-level language like Solidity, it gets compiled down to bytecode, which is a series of these opcodes that the EVM can understand and execute. Why is this relevant? Well, since Shardeum is a sharded, EVM compatible network and the Replay App replays historical transactions which initially occur via the EVM, it’s worth understanding the initial context the transaction occurred in.

Introducing Shardeum’s Replay App

So what does the Replay App actually do? At its core, the Replay App has multiple functional use cases. One of the major use cases is recreating historical states and executing prior transactions to debug, troubleshoot, or just to provide general information on transactions. The Replay App executes these prior transactions in its own independent isolated environment that does not interfere with any other parallel networks running on Shardeum – be it a testnet or mainnet – ensuring that there is no interference or consensus required from any validators in the network, and therefore minimal additional burden on the network.

Features of Replay App

1. Historical States at Your Fingertips

The Replay App’s standout feature is its ability to reconstruct historical states and re-run old transactions. What’s even more impressive? All this occurs in a self-contained environment, ensuring that the primary validator network remains untouched, uninterrupted and unencumbered.

2. Deeper Analytical Insights

Revisit previous transactions in style! Unlike traditional blockchain explorers that primarily offer transaction tracking and block visualization, the Replay App’s comprehensive overviews deliver deeper analytical insights, enabling users to understand the nuanced dynamics of transaction patterns, gas costs, and network interactions on a granular level.

3. Advanced Debugging for Developers

The Replay App allows developers to delve into complex scenarios, streamlining the debugging process. Additionally, improved debugging translates to an optimized experience for developers who can troubleshoot and debug rapidly and efficiently with the Replay App in their toolkit. Gone are the days of sifting through lines of code without direction. With Replay App, developers can employ breakpoint debugging in integrated development environments (IDEs) like Remix. This feature ensures step-by-step oversight and control during transaction execution, enhancing efficiency and precision.

4. Accurate Gas Estimation

Estimating gas limits will be precise and swift! By pre-executing transactions away from the validator network, the Replay App calculates the exact gas required, serving rapid and pinpoint estimations. Notably, this process does not add any additional strain to the Shardeum network or nodes! The Replay App is also being used in the RPC server to provide super accurate gas estimations. This is done by pre-executing the transaction (completely separate from the validator network, as the RPC has its own VM now which is the Replay App) and calculating the exact gas required. This provides quick and accurate gas limit estimations without adding any load to the validator network. Previously all gas estimations would happen from the actual validators, thus using the Replay App for the same estimations shifts a significant load away from the network!

Beyond its present capabilities, the Replay App is laden with innumerable potential. Currently it is used in RPC servers for impeccable gas estimations among other use cases mentioned above, but we have no doubt it will become a standard feature for other future applications. The tool promises to be a cornerstone in Shardeum’s technological suite.

Benefits of Replay App

Understanding every action and decision made in a transaction can be crucial at times. The Replay App delivers on this front by offering meticulous and in-depth logging capabilities for every replayed transaction, logging down to the opcode level. Interested in gas requirements? Replay provides comprehensive details about gas utilization at every stage. Not only that, but Replay’s new logging system brings forth a multitude of major advantages:

  1. Debugging dApps: If a transaction or smart contract isn’t behaving as expected, examining the executed opcodes via the Replay App’s logs could help pinpoint where things are going wrong.
  2. Understanding Gas Costs: Every opcode has an associated gas cost. By examining the opcodes, developers can get a clearer picture of why certain operations in their smart contract are expensive in terms of gas.
  3. Optimization & Smart Contract Fine Tuning: By allowing comprehensive logging so that developers can examine the opcodes to see their dApps level of gas usage, developers can now rapidly identify inefficiencies in their smart contract code. For example, they might discover that certain operations are being executed more times than necessary, thus allowing them to fine-tune their contracts to guarantee more aggregate efficiency. 
  4. Security Analysis: The Replay Apps logging features could be a boon for security researchers and auditors, as they can examine the opcodes to ensure that a smart contract doesn’t have any vulnerabilities. This is especially important in the world of smart contracts, where bugs can lead to significant financial losses.
  5. Educational and Testing Purposes: The Replay App can be used for training, demonstrations, or educational purposes, allowing users to see the effects of transactions without risking current network operations.


With the network’s archive nodes storing complete snapshots of the blockchain state at every block or cycle (in the case of Shardeum), they provide immediate answers to historical queries. Complementing this, the Replay App enables dynamic interaction with past transactions, offering advanced tools for complex investigations and optimizations. This feature underscores Shardeum’s commitment to creating a developer-centric network that combines ease of use with robust security.

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