Definition
Block availability is a consensus-critical property describing whether a proposed block and its complete data can be obtained by nodes in the network. A block is considered available when its contents, not just its header, are distributed widely enough that independent validators can download and inspect it. This concept ensures that a block’s transactions and other payload data are actually present and verifiable, rather than merely referenced. Without block availability, nodes cannot reliably determine the validity of a block or safely extend the chain with further blocks.
Block availability is closely related to, but distinct from, data availability, which focuses more broadly on whether the underlying data of a system can be retrieved and checked. In the context of a block, availability specifically concerns the timely and complete propagation of that block’s data across the network. It underpins secure block inclusion in the canonical chain and supports meaningful finality guarantees, because blocks that are not available cannot be confidently treated as permanent history.
Context and Usage
In practice, block availability is evaluated during block propagation, as nodes attempt to download and reconstruct the full block from peers. If a block’s data fails to reach a sufficient portion of the network, that block may be treated as unavailable, even if its header has been seen. This condition helps prevent situations where a block appears to advance the chain but hides data that cannot be independently verified.
Block availability is a foundational concept for robust finality, because finality assumes that all finalized blocks are fully known and checkable by the network. When block availability is reliably maintained, participants can trust that block inclusion in the chain reflects data that has been widely disseminated and is open to verification. As a result, block availability acts as a safeguard against opaque or selectively shared blocks that could undermine the integrity of the blockchain’s history.