Weak subjectivity: How PoS nodes confirm the valid chain
This guide explains objectivity, subjectivity, and weak subjectivity in blockchains, and why newly joining or long-offline PoS nodes need to rely on other nodes to confirm the currently valid chain.
Understanding the issue through multiple valid paths
As distributed systems, blockchains may have multiple valid paths extending from the genesis block to the latest block. Nodes must determine which one is the currently active chain. Different networks use different methods for making this determination, so whether a new node can quickly synchronize to a valid state using only on-chain data depends on the network's design and consensus mechanism.
The difference between objectivity and subjectivity
In networks with objectivity, a new node can determine the active chain according to clear rules. For example, Bitcoin nodes typically choose the chain with the greatest accumulated proof of work. Subjectivity means that deterministic rules alone are insufficient to make the determination, so nodes need to consult information provided by other nodes; the discussion here concerns identifying the active chain, not the consensus-formation process itself.
When weak subjectivity occurs
Weak subjectivity Mainly used to describe a requirement of PoS blockchains. Continuously online nodes can generally track the evolution of the chain, but nodes newly joining the network, or reconnecting after being offline for a long time, need to rely on a recent, trusted state provided by other nodes to identify the active chain. Because this dependence occurs only in specific circumstances, it is called “weak” subjectivity.
Original Tutorial
https://www.binance.com/en/academy/glossary/weak-subjectivity
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