A blockDAG is a structure that lets blocks link to several earlier blocks, rather than only one. Kaspa uses this structure to keep blocks created in parallel, then uses its GHOSTDAG consensus rules to put them into an order that nodes can agree on. This allows fast block creation without treating every simultaneous block as a competing branch that must be discarded.[1]
What the name means
DAG stands for directed acyclic graph. In a blockDAG, the blocks are the items in the graph, and references to earlier blocks are the links between them. The links have a direction, toward the past, and cannot form a loop.[1]
A new block can reference several earlier blocks, called its parents. In the paper’s mining rules, miners reference the latest blocks they can see, called tips, and broadcast their new block as quickly as possible.[1]
Two miners can therefore produce blocks without either having seen the other’s block. Later blocks can reference both, bringing their histories together.[1]
Why Kaspa uses one
Bitcoin’s blocks each reference one predecessor. Its consensus selects a single main chain, leaving competing blocks outside that chain. Its security depends on blocks spreading through the network faster than new blocks are usually created.[1]
Kaspa takes a different approach: allow parallel block production, keep those blocks in a shared structure, and solve the problem of agreeing on their order. The GHOSTDAG paper presents this as a generalization of Bitcoin’s Nakamoto consensus, designed to support higher block rates.[1]
In his Bitcoin Takeover interview, Yonatan Sompolinsky explains the central challenge: parallel blocks may contain conflicting transactions, so nodes need an ordering that every node ends up agreeing on.[2]
How the network agrees
A blockDAG is the structure, not the complete consensus mechanism. GHOSTDAG supplies the rules that turn that structure into an agreed order.[1]
The reasoning works in three stages:
- Find well-connected blocks. Miners following the protocol share blocks promptly and reference what they have seen. Their blocks therefore tend to form a well-connected group.[1]
- Use that group to guide ordering. GHOSTDAG builds this group with a practical, step-by-step selection method. The paper calls the selected blocks *blue* and the others *red*. These are protocol classifications, not proof that an individual miner is honest or malicious.[1]
- Process transactions in order. Nodes accept transactions one by one and exclude transactions that conflict with ones already accepted. Keeping two blocks does not mean accepting two conflicting spends.[1]
This is why “Kaspa keeps parallel blocks” does not mean “every transaction in every block succeeds.” Agreement still requires transaction validation and conflict resolution.[1]
Why a chain can still appear inside a DAG
A lesser-known detail is that GHOSTDAG uses a selected chain inside the wider graph to organize its calculations. That does not turn the ledger back into a single-chain system: blocks outside that selected chain also enter the ordering.[1]
Sompolinsky explains that this selected chain is not simply the longest chain. It serves as the organizing center of the well-connected block structure.[3]
For the formal explanation, read the PHANTOM GHOSTDAG paper, especially its mining and ordering sections.[1] For the plain-language explanation, watch Sompolinsky’s Bitcoin Takeover interview from 6:12, where he explains multiple parent references, parallel blocks, and why ordering is necessary.[2]