# Block Sync
Formerly known as Fast Sync
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.
# Using Block Sync
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
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 Block 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 (opens new window).
Note: There are two versions of Block Sync. We recommend using v0 as v2 is still in beta.
If you would like to use a different version you can do so by changing the version in the
If we're lagging sufficiently, we should go back to block syncing, but this is an open issue (opens new window).
# The Block Sync event
When the tendermint blockchain core launches, it might switch to the
mode to catch up the states to the current network best height. the core will emits
a fast-sync event to expose the current status and the sync height. Once it catched
the network best height, it will switches to the state sync mechanism and then emit
another event for exposing the fast-sync
complete status and the state
The user can query the events by subscribing
Please check types (opens new window) for the details.