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  1. Fast Sync
  2. =========
  3. Background
  4. ----------
  5. In a proof of work blockchain, syncing with the chain is the same
  6. process as staying up-to-date with the consensus: download blocks, and
  7. look for the one with the most total work. In proof-of-stake, the
  8. consensus process is more complex, as it involves rounds of
  9. communication between the nodes to determine what block should be
  10. committed next. Using this process to sync up with the blockchain from
  11. scratch can take a very long time. It's much faster to just download
  12. blocks and check the merkle tree of validators than to run the real-time
  13. consensus gossip protocol.
  14. Fast Sync
  15. ---------
  16. To support faster syncing, tendermint offers a ``fast-sync`` mode, which
  17. is enabled by default, and can be toggled in the ``config.toml`` or via
  18. ``--fast_sync=false``.
  19. In this mode, the tendermint daemon will sync hundreds of times faster
  20. than if it used the real-time consensus process. Once caught up, the
  21. daemon will switch out of fast sync and into the normal consensus mode.
  22. After running for some time, the node is considered ``caught up`` if it
  23. has at least one peer and it's height is at least as high as the max
  24. reported peer height. See `the IsCaughtUp
  25. method <https://github.com/tendermint/tendermint/blob/b467515719e686e4678e6da4e102f32a491b85a0/blockchain/pool.go#L128>`__.
  26. If we're lagging sufficiently, we should go back to fast syncing, but
  27. this is an open issue:
  28. https://github.com/tendermint/tendermint/issues/129