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, CometBFT offers a block-sync mode, which is enabled by default, and can be toggled in the config.toml or via --block_sync=false.

In this mode, the CometBFT 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 its height is at least as high as the max reported peer height. See the IsCaughtUp method.

Note: While there have historically been multiple versions of blocksync, v0, v1, and v2, all versions other than v0 have been deprecated in favor of the simplest and most well understood algorithm.

#######################################################
###       Block Sync Configuration Options          ###
#######################################################
[blocksync]

# Block Sync version to use:
# 
# In v0.37, v1 and v2 of the block sync protocols were deprecated.
# Please use v0 instead.
#
#   1) "v0" - the default block sync implementation
version = "v0"

If we’re lagging sufficiently, we should go back to block syncing, but this is an open issue.