Qitmeer adopts a hybrid consensus that combines SPECTRE and GHOSTDAG in order to achieve fast confirmation and high throughput. The Hybrid Consensus is;
• Speedy • Transaction confirmation time dependent only on the actual network delay. • Transactions confirmed usually within seconds • Economical •Combining a blockchain protocol with a SPECTRE and GHOSTDAG that help to confirms transactions is what Qitmeer network does. • To obtain the efficiency of permissioned protocols while having the decentralized quality of permissionless protocol.
SPECTRE is a BlockDAG based protocol that achieves fast confirmation and high throughput with 50% attack resilience. SPECTRE guarantees safety, which means the transaction is unlikely to be reversed once it is accepted. However, the SPECTRE guarantees fast conrmations for honest users rather than all users which is weak liveness.
There is a trade-off between liveness and fast confirmation, whereby SPECTRE prioritizes the latter due to weak liveness only affects malicious users, that enables SPECTRE to be a suitable protocol for payment model. In the event malicious users launch double spending attack, their transactions are likely to be delayed indefinitely.
SPECTRE is a stateless transaction model, whereby there is no need to gain a total ordering over all the blocks. Only when two blocks conflicting that a pairwise ordering is needed. SPECTRE is a stateless transaction model, whereby there is no need to gain a total ordering over all the blocks. Only when two blocks conflicting that a pairwise ordering is needed. SPECTRE employs a voting algorithm to decide which block wins when there is a conflict between two blocks.
Qitmeer is designed for decentralized payment network. In most scenarios, it is suffcient to provide only partial ordering or pairwise ordering for blocks in the ledger. However, sometimes it may still need to obtain a total (linear) ordering of all the blocks, especially in the event of rewarding blocks based on their ordering. Obtaining total ordering for a DAG ledger is not so intuitive as it is for blockchains due to a DAG ledger contains forks, which are caused by various factors, such as, network propagation delay, concurrent block creations, faulty miners.
Therefore, as a supplement to the consensus protocol of Qitmeer, it uses GHOSTDAG to obtain the total ordering to reward blocks which appears earlier in the ordering. In addition to total ordering, GHOSTDAG also provides Strong Liveness guarantee to make the consensus protocol more robust, which means both honest blocks and malicious blocks can be confirmed within a definite time, though it may take a long time to confirm malicious blocks.