What is a Block Engine / Relayer?
In Jito’s decentralized stack, a relayer acts as a proxy to receive and temporarily queue transactions to the validator. It is the first point of contact for transactions from outside the Solana network. A relayer deduplicates and filters transactions, then sends them to validators for processing. The block engine is the software that runs an off-chain auction of bundles. It accepts bundles from a network of searchers, runs simulation of these bundles, and orders valid transactions that maximize tips before sending the block to a leader.
Think of this as an airport analogy. The relayer is akin to the security checkpoint where all inbound transactions are cleared, sorted, and held in an orderly queue. Meanwhile, the block engine functions like an air traffic control tower that receives all inbound flights requesting access to the runway—Solana’s validator leader has only a 400-ms block slot—and sequences the highest priority ones onto the runway.
Technical Details
With four 400-millisecond slots per leader, Solana leaders are easily predictable for searchers. This predictability allowed searchers to flood the network with millions of duplicates of transactions prior to a block leader slot. This spam activity was the culprit behind the congestion events of 2022-2024. Now Jito’s relayer terminates these connections for a validator to reduce load, deduplicates, and filters them, holding them briefly so that the validator does not have to use compute for connection-spam sorting but instead does the actual work of processing consensus. Jito’s block engine then converts this spam race into a sealed auction. Searchers provide transactions and tips in the form of bundles. The block engine then simulates the bundles on-chain and forwards only the highest tip bundle to a leader. This means that if a whale swap happens on a DEX, instead of millions of spam duplicates flying around the network, a single transaction pair is processed.
For instance: A whale swap on Jupiter moves a Raydium pool 0.2% away from Orca in one slot. Five searchers send similar arbitrage opportunities to Jito, and five bundles land in the block engine. In one slot, Jito orders and simulates the highest tip bundle which is then forwarded to a leader. In this case, the bundle with the 0.008 SOL tip was forwarded over a rival bundle with 0.005 SOL.
Comparing to Ethereum
Ethereum faced a similar MEV and congestion problem. In Ethereum, this was solved with Flashbots relays and block builders, known as a “proposer-builder separation” (PBS), where a validator outsources the block construction process to a builder. Validators then only need to order a bundle of transactions in their slot and relay them to the block engine. The architecture then is that validators send their bundles to the relayer, which orders them. Relayers then send a final ordered set of transactions to the leader in 12-second slots. Jito’s architecture is a much smaller version of Flashbots, and since Jito is on Solana, it is 60x more granular (400ms) and the entire architecture sits atop a chain that lacks a public mempool. Because of this, the Jito-Solana client runs on a large percentage of validators and total stake. This is similar to Ethereum Flashbots, where the vast majority of ETH staking was on Flashbots post-merge.
The issue with Jito’s architecture is that Jito is not a decentralizing protocol at its roots. In fact, putting another company’s infrastructure in the middle of a user to validator relationship on a chain that claims to be neutral does create trust risk and potential for censorship. Jito’s solution to these trust risks is to build a system called BAM. BAM will allow transaction ordering to be handled in trusted execution environments and not Jito’s servers. It will not need to rely on trust, but only on trusted execution. BAM is still in its infancy, and we won’t know the results of it until it’s used at scale.
Why the Block Engine and Relayer matter?
You will never interact with these systems directly, but they affect each time a trade is executed during a block during times of congestion, and every time your wallet is waiting for a transaction on a congested network. Whenever pump.fun has a vertical launch and everyone wants to buy it, the order of transactions is decided by Jito’s block engine. Without the block engine and relayer, the 2022-2024 congestion would still be prevalent. Jito’s block engine converts MEV to tips and puts tips into validators, which also pay JitoSOL holders, which keeps congestion and spam at bay.
Does the relayer see your transaction before the validator does?
Jito’s relayer sees a transaction before the validator does, which is why we believe in the need for trusted, audited infrastructure and BAM to remove trust issues from Jito’s systems.
