State Synchronization

Nodes in an Aptos network (e.g., validators, VFNs and PFNs) must always be synchronized to the latest blockchain state. The state synchronization (state sync) component that runs on each node is responsible for this. State sync identifies and fetches new blockchain data from the peers, validates the data and persists it to local storage. This document explains how state sync works and how to configure it.

Manual configuration Most users should not need to configure state sync. State sync will automatically select the best configuration for your node. You should only manually configure state sync if you have a specific use case that requires it. Selecting the wrong configuration will lead to slower syncing times and degraded performance.

State sync

At a high-level, state sync operates in two phases. First, all nodes will bootstrap on startup (in the bootstrapping phase). This allows the node to catch up to the latest state in the Aptos blockchain. Next, the node will stay up-to-date with the blockchain by continuously syncing (in the continuous syncing phase). Each of these phases has different modes of operation.

Need to start a node quickly? If you need to start a node quickly, here’s what we recommend by network and use case:

• Devnet: To sync the entire blockchain history, use intelligent syncing. Otherwise, use fast sync.

• Testnet: To sync the entire blockchain history, restore from a backup. Otherwise, download a snapshot or use fast sync.

• Mainnet: To sync the entire blockchain history, restore from a backup. Otherwise, use fast sync.

Bootstrapping phase

When the node starts, state sync will perform bootstrapping by using the configured bootstrapping mode. There are several bootstrapping modes:

• Execute all the transactions since genesis. In this mode, the node will retrieve from the Aptos network all the transactions since genesis, i.e., since the start of the blockchain’s history, and re-execute those transactions. Naturally, this mode takes the longest amount of time because it must process all transactions since the network began.

• Apply transaction outputs since genesis. In this mode, the node will retrieve all the transactions since genesis, but it will skip transaction re-execution and instead apply the outputs of the transactions that were previously produced by validator execution. This mode reduces the amount of CPU time required, but still processes all transactions since the network began.

• Intelligent syncing since genesis. In this mode, the node will retrieve all the transactions since genesis and will either execute the transactions, or apply the transaction outputs, depending on whichever is faster, per data chunk. This allows the node to adapt to CPU and network resource constraints more efficiently. This mode is the default mode for devnet and other testing environments.

• Fast syncing. In this mode, the node will skip the transaction history in the blockchain and will download only the latest blockchain state directly. As a result, the node will not have the historical transaction data, but it will be able to catch up to the Aptos network much more rapidly. This mode is the default mode for testnet and mainnet.

Bootstrapping defaults? The default bootstrapping mode depends on the network type:

• Testnet and Mainnet: The default bootstrapping mode is fast sync, because these networks are long-lived and have a large amount of historical data.

• Devnet and other networks: The default bootstrapping mode is intelligent syncing, because these networks are typically short-lived and have a small amount of historical data.

Continuous syncing phase

After the node has bootstrapped and caught up to the Aptos network initially, state sync will then move into the continuous syncing phase to stay up-to-date with the blockchain. There are several continuous syncing modes:

• Executing transactions. This mode will keep the node up-to-date by executing new transactions as they are committed to the blockchain.

• Applying transaction outputs. This mode will keep the node up-to-date by skipping the transaction execution and only applying the outputs of the transactions as previously produced by validator execution.

• Intelligent syncing. This mode will keep the node up-to-date by either executing the transactions, or applying the transaction outputs, depending on whichever is faster, per data chunk. This allows the node to adapt to CPU and network resource constraints more efficiently. This is the default mode for all environments.

Continuous syncing defaults? The default continuous syncing mode is always intelligent syncing, because this mode is the most performant.

Configuring state sync

The snippets below provide instructions for configuring state sync on your nodes for different use cases. These configurations can be added to your node’s configuration file, e.g., fullnode.yaml or validator.yaml.

Manual configuration You should only manually configure state sync if you have a specific use case that requires it. Selecting the wrong configuration will lead to slower syncing times and degraded performance.

Executing all transactions

To execute all the transactions since genesis and continue to execute new transactions as they are committed, add the following to your node configuration file (for example,fullnode.yaml or validator.yaml):

fullnode.yaml

state_sync:  
  state_sync_driver:    
    bootstrapping_mode: ExecuteTransactionsFromGenesis    
    continuous_syncing_mode: ExecuteTransactions

Verify node syncing While your node is syncing, you’ll be able to see the aptos_state_sync_version{type="synced"} metric gradually increase.

Applying all transaction outputs

To apply all transaction outputs since genesis and continue to apply new transaction outputs as transactions are committed, add the following to your node configuration file:

fullnode.yaml

state_sync:  
  state_sync_driver:    
    bootstrapping_mode: ApplyTransactionOutputsFromGenesis    
    continuous_syncing_mode: ApplyTransactionOutputs

Verify node syncing While your node is syncing, you’ll be able to see the aptos_state_sync_version{type="synced"} metric gradually increase.

Intelligent syncing

To execute or apply all transactions and outputs since genesis (and continue to do the same as new transactions are committed), add the following to your node configuration file:

fullnode.yaml

state_sync:  
  state_sync_driver:    
    bootstrapping_mode: ExecuteOrApplyFromGenesis    
    continuous_syncing_mode: ExecuteTransactionsOrApplyOutputs

Verify node syncing While your node is syncing, you’ll be able to see the aptos_state_sync_version{type="synced"} metric gradually increase.

Fast syncing

Proceed with caution Fast sync should only be used as a last resort for validators and VFNs. This is because fast sync skips the blockchain history, and: (i) reduces data availability in the network (as the blockchain history is truncated on the fast synced nodes); and (ii) hinders validator consensus performance (if too much data has been skipped). Validators that fast sync may require additional running time before they are eligible to participate in consensus.`

To download the latest blockchain state and continue to process new transactions as they are committed, add the following to your node configuration file:

fullnode.yaml

state_sync:  
  state_sync_driver:    
    bootstrapping_mode: DownloadLatestStates    
    continuous_syncing_mode: ExecuteTransactionsOrApplyOutputs

Verify node syncing While your node is syncing, you’ll be able to see the aptos_state_sync_version{type="synced_states"} metric gradually increase. However, aptos_state_sync_version{type="synced"} will only increase once the node has bootstrapped. This may take several hours depending on the amount of data, network bandwidth and node resources available.

Note: If aptos_state_sync_version{type="synced_states"} does not increase then do the following:

1. Double-check the node configuration file has correctly been updated.

2. Make sure that the node is starting up with an empty storage database (i.e., that it has not synced any state previously).

Archival PFNs

To operate an archival PFN (which is a PFN that contains all blockchain data since the start of the network, i.e., genesis), you should:

1. Make sure that your PFN is not using fast syncing as the bootstrapping mode. Fast syncing will skip the transaction history. Instead, using a mode that syncs from genesis, e.g., intelligent syncing from genesis.

2. Disable the ledger pruner, as described in the Data Pruning document. This will ensure that no data is deleted and the PFN contains all blockchain data.

Following these two steps together will ensure that your PFN fetches all data since genesis, and continues to synchronize without pruning any data.

Archival nodes are deprecated Running and maintaining archival nodes is expensive and slow, as the amount of data being stored on the node will grow endlessly. As a result, archival nodes have officially been deprecated. If you wish to store and maintain the entire blockchain history, we recommend using an Indexer.

Security implications and data integrity

Each of the different state sync syncing modes perform data integrity verifications to ensure that the data being synced to the node has been correctly produced and signed by the validators. This occurs slightly differently for each syncing mode:

1. Executing transactions: Executing transactions from genesis is the most secure syncing mode. It will verify that all transactions since the beginning of time were correctly agreed upon by consensus and that all transactions were correctly executed by the validators. All resulting blockchain state will thus be re-verified by the syncing node.

2. Applying transaction outputs: Applying transaction outputs from genesis is faster than executing all transactions, but it requires that the syncing node trusts the validators to have executed the transactions correctly. However, all other blockchain state is still manually re-verified, e.g., consensus messages, the transaction history and the state hashes are still verified.

3. Intelligent syncing: Intelligent syncing will either execute or apply the transaction outputs depending on whichever is faster, per data chunk. Thus, the security implications of using this mode are identical to those of applying transaction outputs.

4. Fast syncing: Fast syncing skips the transaction history and downloads the latest blockchain state before continuously syncing. To do this, it requires that the syncing node trust the validators to have correctly agreed upon all transactions in the transaction history as well as trust that all transactions were correctly executed by the validators. However, all other blockchain state is still manually re-verified, e.g., epoch changes and the resulting blockchain states.

All the syncing modes get their root of trust from the validator set and cryptographic signatures from those validators over the blockchain data. For more information about how this works, see the state synchronization blog post.