Syncing is the process by which Geth catches up to the latest Ethereum block and current global state. There are several ways to sync a Geth node that differ in their speed, storage requirements and trust assumptions. Now that Ethereum uses proof-of-stake based consensus, a consensus client is required for Geth to sync.
Snap sync starts from a relatively recent block and syncs from there to the head of the chain, keeping only the most recent 128 block states in memory. The block header to sync up to is provided by the consensus client. Between the initial sync block and the 128 most recent blocks, the node stores occasional snapshots that can be used to rebuild any intermediate state "on-the-fly". The difference between the snap-synced node and a full block-by-block synced node is that a snap synced node started from an initial checkpoint that was more recent than the genesis block. Snap sync is much faster than a full block-by-block sync from genesis. To start a node with snap sync pass `--syncmode snap` at startup.
Snap sync works by first downloading the headers for a chunk of blocks. Once the headers have been verified, the block bodies and receipts for those blocks are downloaded. In parallel, Geth also sync begins state-sync. In state-sync, Geth first downloads the leaves of the state trie for each block without the intermediate nodes along with a range proof. The state trie is then regenerated locally.
The state download is the part of the snap-sync that takes the most time to complete and the progress can be monitored using the ETA values in the log messages. However, the blockchain is also progressing at the same time and invalidating some of the regenerated state data. This means it is also necessary to have a 'healing' phase where errors in the state are fixed. It is not possible to monitor the progress of the state heal because the extent of the errors cannot be known until the current state has already been regenerated. Geth regularly reports `Syncing, state heal in progress` during state healing - this informs the user that state heal has not finished. It is also possible to confirm this using `eth.syncing` - if this command returns `false` then the node is in sync. If it returns anything other than `false` then syncing is still in progress.
The healing has to outpace the growth of the blockchain, otherwise the node will never catch up to the current state. There are some hardware factors that determine the speed of the state healing (speed of disk read/write and internet connection) and also the total gas used in each block (more gas means more changes to the state that have to be handled).
**Note** Snap sync is the default behaviour, so if the `--syncmode` value is not passed to Geth at startup, Geth will use snap sync. A node that is started using `snap` will switch to block-by-block sync once it has caught up to the head of the chain.
A full block-by-block sync generates the current state by executing every block starting from the genesis block. A full sync independently verifies block provenance as well as all state transitions by re-executing the transactions in the entire historical sequence of blocks. Only the most recent 128 block states are stored in a full node - older block states are pruned periodically and represented as a series of checkpoints from which any previous state can be regenerated on request. 128 blocks is about 25.6 minutes of history with a block time of 12 seconds.
An archive node is a node that retains all historical data right back to genesis. There is no need to regenerate any data from checkpoints because all data is directly available in the node's own storage. Archive nodes are therefore ideal for making fast queries about historical states. At the time of writing (September 2022) a full archive node that stores all data since genesis occupies nearly 12 TB of disk space (keep up with the current size on [Etherscan](https://etherscan.io/chartsync/chainarchive)). Archive nodes are created by configuring Geth's garbage collection so that old data is never deleted: `geth --syncmode full --gcmode archive`.
It is also possible to create a partial/recent archive node where the node was synced using `snap` but the state is never pruned. This creates an archive node that saves all state data from the point that the node first syncs. This is configured by starting Geth with `--syncmode snap --gcmode archive`.
A light node syncs very quickly and stores the bare minimum of blockchain data. Light nodes only process block headers, not entire blocks. This greatly reduces the computation time, storage and bandwidth required relative to a full node. This means light nodes are suitable for resource-constrained devices and can catch up to the head of the chain much faster when they are new or have been offline for a while. The trade-off is that light nodes rely heavily on data served by altruistic full nodes. A light client can be used to query data from Ethereum and submit transactions, acting as a locally-hosted Ethereum wallet. However, because they don't keep local copies of the Ethereum state, light nodes can't validate blocks in the same way as full nodes - they receive a proof from the full node and verify it against their local header chain. To start a node in light mode, pass `--syncmode light`. Be aware that full nodes serving light data are relatively scarce so light nodes can struggle to find peers. **Light nodes are not currently working on proof-of-stake Ethereum**.
Now that Ethereum has switched to proof-of-stake, all consensus logic and block propagation is handled by consensus clients. This means that syncing the blockchain is a process shared between the consensus and execution clients. Blocks are downloaded by the consensus client and verified by the execution client. In order for Geth to sync, it requires a header from its connected consensus client. Geth does not import any data until it is instructed to by the consensus client. **Geth cannot sync without being connected to a consensus client**. This includes block-by-block syncing from genesis. The consensus client is required to provide a header from the tip of the chain that Geth can sync towards - without it, Geth cannot know that it has followed the right sequence of blocks.
Once a header is available to use as a syncing target, Geth retrieves all headers between that target header and the local header chain in reverse chronological order. These headers show that the sequence of blocks is correct because the parenthashes link one block to the next right up to the target block. Eventually, the sync will reach a block held in the local database, at which point the local data and the target data are considered 'linked' and there is a very high chance the node is syncing the correct chain. The block bodies are then downloaded and then the state data. The consensus client can update the target header - as long as the syncing outpaces the growth of the blockchain then the node will eventually get in sync.
Optimistic sync downloads blocks before the execution client has validated them. In optimistic sync the node assumes the data it receives from its peers is correct during the downloading phase but then retroactively verifies each downloaded block. Nodes are not allowed to attest or propose blocks while they are still 'optimistic' because they can't yet guarantee their view of the head of the chain is correct.
Alternatively, the consensus client can grab a checkpoint from a trusted source which provides a target state to sync up to, before switching to full sync and verifying each block in turn. In this mode, the node trusts that the checkpoint is correct. There are many possible sources for this checkpoint - the gold standard would be to get it out-of-band from another trusted friend, but it could also come from block explorers or [public APIs/web apps](https://eth-clients.github.io/checkpoint-sync-endpoints/). Checkpoint sync is very fast - a consensus cleint should be able to sync in a few minutes using this method.
There are several ways to sync a Geth node. The default is to use snap sync to create a full node. Full nodes can be created by syncing block-by-block from genesis (full-sync) or by starting at an intermediate checkpoint block (snap-sync). By default, these modes prune state data older than 128 blocks, keeping only checkpoints that enable on-request regeneration of historical states. For rapid queries of historical data an archive node is required. Archive nodes keep local copies of all historical data right back to genesis - currently about 12 TB and growing. Partial archive nodes can be created by snap-syncign a node and turning off state-pruning to create a node that keeps all states since the initial sync block. Currently, due to the transition to proof-of-stake, light-sync does not work (new light client protocols are being developed).
