## Centralized Sequencers: The Single Point of Failure in Blockchain Rollups In the world of blockchain rollups, the sequencer is a critical component responsible for maintaining the flow and order of transactions. While essential for efficiency, the current implementation in most rollups presents a significant risk to the network and its users. This is because, in their current state, most sequencers are centralized. A sequencer is a specialized operator whose primary job is to **order transactions** submitted by users. In some cases, it also performs the secondary role of bundling these ordered transactions together before they are submitted to the main blockchain. When this powerful role is controlled by a single entity, we call it a centralized sequencer. This concentration of power introduces two fundamental problems that strike at the core principles of blockchain technology: censorship and reliability. ### The Risk of Censorship A centralized sequencer controlled by a single party has the ultimate power to decide which transactions are included in a block and in what order. This opens the door for malicious behavior and censorship. A dishonest sequencer could simply refuse to include transactions from specific users, effectively blocking them from using the network. Imagine trying to withdraw your funds from a rollup, only to have a malicious sequencer repeatedly ignore your transaction request. In this scenario, your assets are trapped, held hostage by a single entity. Furthermore, a centralized sequencer can manipulate the order of transactions to extract financial value for itself, a practice known as Maximal Extractable Value (MEV). By reordering transactions, it can front-run trades or take advantage of arbitrage opportunities at the expense of ordinary users. ### The Problem of Reliability: A Single Point of Failure Beyond malicious intent, centralization creates a critical vulnerability: a single point of failure. The entire rollup network depends on this one sequencer to function. If that single entity goes offline for any reason—be it a technical glitch, a targeted attack, or a regulatory shutdown—the entire rollup halts. When the sequencer is down, no new transactions can be processed. This means no deposits, no transfers, and, most importantly, no withdrawals. The network becomes completely unusable. Consider the implications. What happens if you can't access your funds for a day? What if the outage lasts a week? A year? What if the sequencer never comes back online? If you cannot access or move your assets, their value is effectively zero. This existential threat undermines the very promise of user-owned assets that blockchain technology is supposed to guarantee. ### The Solution: The Path to Decentralization The clear and necessary solution to the risks of centralization is to decentralize the sequencer role. The end goal for any mature rollup is to have a system with multiple, independent sequencers who work together to order and process transactions. In a decentralized model, if one sequencer goes offline or acts maliciously, others can step in to ensure the network remains live, operational, and fair. This redundancy eliminates the single point of failure and drastically reduces the risk of censorship, as no single party has ultimate control. Projects in the space, such as zkSync, are actively working on this transition. Many rollups launch with a centralized sequencer for reasons of simplicity and rapid development in their early stages. However, the long-term roadmap for any reputable project involves a strategy of progressive decentralization. The state of a rollup's sequencer—whether it is centralized or has moved to a decentralized model—is one of the most important indicators of its maturity, security, and commitment to the core principles of web3.