Definition
A Block DAG is a data structure used in some cryptocurrencies where blocks are organized as a directed acyclic graph rather than a single linear chain. In a Block DAG, each block can reference multiple previous blocks, and multiple new blocks can be added at roughly the same time without being immediately discarded as conflicts. This structure is designed to generalize the idea of a blockchain while still preserving a clear, non-circular ordering of blocks. It aims to support higher throughput and more flexible block inclusion than a strictly linear chain.
Because a Block DAG is acyclic, blocks always point from newer to older blocks, preventing loops and preserving a consistent history. Consensus rules built on top of a Block DAG determine how the network interprets the graph to derive a final ordering of transactions and decide which blocks are considered confirmed. This concept is often used as an alternative to traditional chain-based designs for coins that want to handle more frequent block creation or higher transaction volume.
Context and Usage
In the context of cryptocurrencies, a Block DAG underlies how some networks record and relate blocks that contain transactions. Instead of forcing the network to choose a single winning block at each height, the Block DAG structure allows several blocks to be accepted into the history, with their relationships encoded as graph links. This can reduce the number of blocks that are discarded as orphans in more traditional chain-based systems.
Block DAGs are typically discussed when comparing different designs for securing and scaling digital coins. They remain conceptually related to blockchains because they still group transactions into blocks and maintain an ordered, tamper-resistant record, but they relax the requirement that this record must be a single, unbranched chain. As a result, Block DAGs represent an alternative foundational concept for how a coin’s ledger can be structured and agreed upon by the network.