tendermint/docs/tendermint-core/block-sync.md

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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 --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 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.

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 config.toml:

#######################################################
###       Block Sync Configuration Connections       ###
#######################################################
[fastsync]

# Block Sync version to use:
#   1) "v0" (default) - the legacy Block Sync implementation
#   2) "v2" - complete redesign of v0, optimized for testability & readability
version = "v0"

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

The Block Sync event

When the tendermint blockchain core launches, it might switch to the block-sync 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 height.

The user can query the events by subscribing EventQueryBlockSyncStatus Please check types for the details.