Understanding ETH Simulation Bundles and When to Use Them

Written by Dan Marzec | May 11, 2022

Blocknative recently added bundle support for simulating transactions on Ethereum Mainnet. This capability enables users to fully harness the power of the Ethereum mempool, our network of Ethereum nodes, and our existing decoding and transaction lifecycle knowledge to accurately simulate their transactions.

By adding support for simulation bundles, Blocknative has enhanced the ability of wallets, protocols, and traders to preview net-balance changes and smart contract calls, reduce failed transactions, and optimize arbitrage opportunities.

But how?

To answer this, we first must go over the basics of how transaction simulations work on EVM-based chains.

Simulation Basics

From a birds-eye view, Ethereum is a transaction-based distributed ‘world’ computer. One way to know how your transaction affects that computer is to have your transaction included in a block. But, at that point, there’s no turning back and the state-change is on-chain and permanent.

However, if you are unsure about whether or not you want your transaction processed, you can “fork” the computer and run your transaction on top of it as if it were the real network—this is a transaction simulation. Forking creates a temporary copy of the chain state upon which transactions can be applied without impacting the true chain. Depending on the speed of your set-up, you can quickly simulate scenarios on the most current state of the chain.

When a transaction itself is the outcome of the smart contracts, just knowing the end-state of the transaction is not enough as key details about the transaction can be lost by the time the transaction makes it on-chain. These missing steps are known as internal transactions.

Internal transactions aren’t stored on-chain, making them invisible to anyone that isn’t collecting this data in-flight. This is why, in addition to simulating your transaction through the Ethereum Virtual Machine (EVM), Blocknative also attaches a logger so users can see all the steps in-between submission and execution. As such, Blocknative is able to provide users with a list of decoded internal transactions on top of accurate slippage, net-balance changes and smart contract calls before authorizing transactions.

Simple transactions such as wallet-to-wallet transfers or an NFT mint can be done with a single simulation, Transacting with more complicated protocols, such as initiating a swap on a DEX or executing an arbitrage, will typically require simulating multiple transactions in one bundle.

What Makes Simulating Bundles Different Than Simulating Single Transactions

Because we need to fork the chain, there is a large infrastructure requirement to simulate at scale. Furthermore, the stock version of the most popular Ethereum client, go-ethereum, does not include the ability to simulate more than one transaction on top of this new forked Ethereum state.

In order to simulate more than one transaction, we have added the ability to take the intermediate state that your initial simulation has created and feed it into the simulation of the next transaction. This allows for multiple transactions to be executed sequentially in the EVM, as well as collecting their internal transaction logs and accumulating the net balance changes of impacted accounts.

Simulation Bundles in Action

Transaction Preview comes in the form of a REST endpoint, to which users can send the details of one or more transactions. In return, the user will receive a payload describing the execution of the transaction(s) in the latest block height. A few examples of simulated bundles can be found below.

Simple bundle: approve and swap

One of the biggest limitations with single transaction simulations, as available via RPC on stock go-ethereum, is that they do not work well with contracts interacting with the ERC20 approve() pattern. This pattern can be seen in most decentralized exchanges. When using a DEX like Uniswap, the Uniswap router is the contract initiating the token transfers (via internal transactions), so the user must grant approval to that contract to transfer. In a single simulation, you will not be able to approve and swap, but now you can! To see an example, we are going to approve a transfer of Balancer tokens from Vitalik’s account, and we will then swap those tokens for ETH.

Request:


curl --location --request POST 'https://api.blocknative.com/simulate' \
--header 'credentials: :' \
--header 'Content-Type: application/json' \
--data-raw '{
  "system":"ethereum",
  "network":"main",
  "transactions":[
      {
    "to":"0xba100000625a3754423978a60c9317c58a424e3D",
    "from":"0xd8dA6BF26964aF9D7eEd9e03E53415D37aA96045",
    "gas":12407056,
    "maxFeePerGas":82306635634,
    "maxPriorityFeePerGas":2.14,
    "value":0,
    "input":"0x095ea7b300000000000000000000000068b3465833fb72a70ecdf485e0e4c7bd8665fc45000000000000000000000000000000000785ee10d5da46d900f436a000000000"
  },
      {
    "to":"0x68b3465833fb72A70ecDF485E0e4C7bD8665Fc45",
    "from":"0xd8dA6BF26964aF9D7eEd9e03E53415D37aA96045",
    "gas":12407056,
    "maxFeePerGas":82306635634,
    "maxPriorityFeePerGas":2.14,
    "value":0,
    "input":"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"
  }
  ]
}'