Sequencing is the mechanism by which incoming transactions are ordered into atomic bundles or blocks to be processed and finalized on a blockchain. All sequencing mechanisms are rooted in the concept of the write-lock, which is a fundamental concurrency control mechanism used to manage access to a shared blockchain state. Most economic value associated with sequencing flows to the actors that control the components of the write-lock, namely the execution, inclusion, and ordering guarantees of transaction data.

Traditional sequencing methods delegate ownership of the write-lock to the off-chain operator(s) of the underlying network. However, new mechanisms enable verifiable sequencer rules that are flexible, programmable, and can exist at any infrastructure layer. These mechanisms make protocols sequencing-aware: value that flowed out-of-protocol can instead be internalized by the protocol, thereby enabling alternative ways of distributing value at the individual application and user levels.

There are three main types of sequencing:

Traditional based sequencing delegates the write-lock to a set of off-chain L1 actors that are part of the transaction supply chain.

Centralized sequencing is the status quo for most rollups today, where the write-lock is owned by a single centralized operator. 

Application-specific sequencing is arguably the most customizable mechanism in which the application or appchain owns the write-lock, thereby implementing another set of sequencing rules on top of those implemented by the underlying networks.

Interestingly, there are also solutions that are fluid across these mechanisms. Generic programmable sequencing layers like Astria can simultaneously serve as based sequencers for a certain settlement layer (Ethereum or otherwise) and shared or application-specific sequencers. They do so by implementing static, well-known, and repeatable sequencing rules embedded within consensus at the level of individual rollups or applications, thereby allowing for a spectrum of configurations, each with its own implications for decentralization, performance, and value distribution. 

Outlined below are visualizations of different sequencing and their value accrual mechanisms.