Liveness (Blockchain)

Liveness, in blockchains and other distributed systems, is the guarantee that the system keeps moving forward: new blocks keep being produced and every valid transaction is eventually included and confirmed. It is usually paired with safety, the guarantee that nodes never finalise conflicting histories. In Byzantine fault tolerant designs, researchers have shown that under severe network delays a system cannot always guarantee both, so each consensus mechanism chooses which to favour. Bitcoin and other longest-chain systems prioritise liveness and keep producing blocks even during network splits, at the cost of possible reorganisations. Many BFT-style chains prioritise safety and will halt if too many validators go offline. Ethereum combines both approaches, with a fork choice rule that keeps the chain growing and a separate finality gadget. Liveness also covers censorship resistance: a chain that produces blocks but excludes certain users' transactions is not fully live for them.