In a proof of work blockchain, syncing with the chain is the same process as staying up-to-date with the consensus: download blocks, and look for the one with the most total work. In proof-of-stake, the consensus process is more complex, as it involves rounds of communication between the nodes to determine what block should be committed next. Using this process to sync up with the blockchain from scratch can take a very long time. It's much faster to just download blocks and check the merkle tree of validators than to run the real-time consensus gossip protocol.
To support faster syncing, tendermint offers a fast-sync mode, which
is enabled by default, and can be toggled in the config.toml or via
--fast_sync=false.
In this mode, the tendermint daemon will sync hundreds of times faster
than if it used the real-time consensus process. Once caught up, the
daemon will switch out of fast sync and into the normal consensus mode.
After running for some time, the node is considered caught up if it
has at least one peer and it's height is at least as high as the max
reported peer height. See the IsCaughtUp
method.
If we're lagging sufficiently, we should go back to fast syncing, but this is an open issue.