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ADR 104: State sync from local application snapshot

Changelog

  • 2023-05-05: Initial Draft (@cason)
  • 2023-24-05: Added description of SDK-based & a CometBFT based solution (@jmalicevic)

Status

Accepted

Context

  1. What is state sync?

From CometBFT v0.34, synchronizing a fresh node with the rest of the network can benefit from state sync, a protocol for discovering, fetching, and installing application snapshots. State sync is able to rapidly bootstrap a node by installing a relatively recent state machine snapshot, instead of replaying all historical blocks.

  1. What is the problem?

With the widespread adoption of state sync to bootstrap nodes, however, what should be one of its strengths - the ability to discover and fetch application snapshots from peers in the p2p network - has turned out to be one of its weaknesses. In fact, while downloading recent snapshots is very convenient for new nodes (clients of the protocol), providing snapshots to multiple peers (as servers of the protocol) is bandwidth-consuming, especially without a clear incentive for node operators to provide this service. As a result, the number of nodes in production CometBFT networks offering the state sync service (i.e., servers offering snapshots) has been limited, which has rendered the service fragile (from the client's point of view). In other words, it is very hard to find a node with good snapshots, leading nodes to often block during sync up.

  1. High level idea behind the solution

This ADR stems from the observation that state sync is more than a protocol to discover and fetch snapshots from peers in the network. In fact, in addition to installing a snapshot, state sync also checks the consistency of the installed application state with the state (appHash) recorded on the blockchain at the same height, and bootstraps CometBFT's block store and state store accordingly. As a result, once state sync is completed the node can safely switch to Block sync and/or Consensus to catch up with the latest state of the network.

The purpose of this ADR is to provide node operators with more flexibility in defining how or where state sync should look for application snapshots. More precisely, it enables state sync to support the bootstrap of nodes from application snapshots obtained out-of-band by operators, available to the node locally. Applications dump snapshots into an exportable format, which can be then obtained by the operators and placed on the syncing node. The node can then sync locally without transferring snapshots via the network. The goal is to provide an alternative to the mechanism currently adopted by state sync, discovering and fetching application snapshots from peers in the network, in order to address the above mentioned limitations, while preserving most of state sync's operation.

The ADR presents two solutions:

  1. The first solution is implemented by the application and was proposed and implemented by the Cosmos SDK (PR #16067 and #16061 ). This ADR describes the solution in a general way, proividing guidlines to non-SDK based applications if they wish to implement their own local state sync.
  2. The second part of the ADR proposes a more general solution, that uses ABCI to achieve the same behavior provided by the SDK's solution.

Alternative Approaches

Strengthen p2p-based state sync

This ADR proposes a workaround for some limitations of the existing state sync protocol, briefly summarized above. A more comprehensive and sustainable long-term solution would require addressing such limitations, by improving the current p2p based state sync solution.

From a protocol design point of view, probably the major limitation of state sync is the lack of mechanisms to incentivize and reward established nodes that provide "good" (updated and consistent) snapshots to peers. The existing protocol is essentially altruistic: it assumes that established nodes will support the bootstrap of fresh nodes without receiving anything in return, other than having new peers joining their network.

From an operational point of view, the design of the p2p layer should take into consideration the fact that not every node in the network is a "good" state sync server. Fresh nodes, which need support for bootstrapping, should then be able to discover peers in the network that are able or willing to provide application snapshots. The implementation of this feature would require a more complex content-based peer discovery mechanism.

However, while relevant, strengthening p2p-based state sync should be seen as orthogonal to the solution proposed in this ADR, which should have relevant use cases even for an enhanced version of state sync.

Manual application state transfer

The proposed ADR 083/ADR 103 inspired the discussions that led to this ADR. At first glance, their proposal might look identical to the solution discussed here. The main distinction is that these previous proposals do not involve installing application snapshots, but rely on node operators to manually synchronize the application state.

More precisely, their proposal is to "allow applications to start with a bootstrapped state (i.e., the application has already a state before the node is started) alongside an empty CometBFT instance (i.e., the node's block store is empty and the state store is at genesis state) using a subsystem of the state sync protocol".

Starting from an "empty CometBFT instance" is a requirement for running state sync, which is maintained in the current proposal. The mentioned "subsystem of the state sync protocol" is the light client, responsible for verifying the application state and for bootstrapping CometBFT's block store and state store accordingly.

The distinction, therefore, lies in the way which the state of the application from a running node is transferred to the new node to be bootstrapped. Instead of relying on application snapshots, produced by the application but handled by state sync, ADR 083/ADR 103 assumes that node operators are able to manually synchronize the application state from a running node (it might be necessary to stop it) to a not-yet-started fresh node.

The main limitation of the approach in ADR 083/ADR 103 is that it relies on the ability of node operators to properly synchronize the application state between two nodes. While experienced node operators are likely able to perform this operation in a proper way, we have to consider a broader set of users and emphasize that it is an operation susceptible to errors. Operators need to know the format of the application's stored state, export the state properly and guarantee consistency. That is, they have to make sure that nobody is trying to read the exported state while they are exporting it. Furthermore, it is an operation that is, by definition, application-specific: applications are free to manage their state as they see fit, and this includes how and where the state is persisted. Node operators would therefore need to know the specifics of each application in order to adopt this solution.

Decision

State sync should support the bootstrap of new nodes from application snapshots available locally. Implementing this option does not mean that networked State sync should be removed, but not both should be enabled at the same time.

In other words, the following operation should be, in general terms, possible:

  1. When configuring a new node, operators should be able to define where (e.g., a local file system location) the client side of state sync should look for application snapshots to install or restore;
  2. Operators should be able to instruct the server side of State sync on a running node to export application snapshots, when they are available, to a given location (e.g., at the local file system);
  3. It is up to node operators to transfer application snapshots produced by the running node (server side) to the new node (client side) to be bootstrapped using this new State sync feature.

As Cosmos SDK implemented a solution for all applications using it, and we do not have non-SDK in production users requesting this feature, at the moment we will not implement the generally applicable solution.

This ADR will outline both the solution implemented by the SDK and a design proposal of a generally applicable solution, that can be later on picked up and implemented in CometBFT.

Detailed Design

Application-implemented local state sync

This section describes the solution to local state sync implemented by the SDK. An application can chose to implement local state sync differently, or implement only a subset of the functionalities implemented by the SDK.

This solution exposes a command line interface enabling a node to manipulate the snapshots including dumping existing snapshots to an exportable format, loading, restoring and deleting exported snapshots, as well as a command to bootstrap the node by resetting CometBFT's state and block store.

The SDK exposes the following commands for snapshot manipulation:


delete      Delete a snapshot from the local snapshot store
dump        Dump the snapshot as portable archive format
export      Export app state to snapshot store
list        List local snapshots
load        Load a snapshot archive file into snapshot store
restore     Restore app state from a snapshot stored in the local snapshot store

and the following command to bootstrap the state of CometBFT upon installing a snapshot:


comet bootstrap-state          Bootstrap the state of CometBFT's state and block store from the
                               application's state

These commands enable the implementation of both the client and server side of statesync. Namely, a statesync server can use dump to create a portable archive format out existing snapshots, or trigger snapshot creation using export.

The client side, restores the application state from a local snapshot that was previously exported, using restore. Before restore can be called, the client has to load an archived snapshot into its local snapshot store. Upon successful completion of the previous sequence of commands, the state of CometBFT is bootstrapped using bootstrap-state and CometBFT can be launched.

There are three prerequisites for this solution to work when a node is syncing:

  1. The application has access to the snapshot store (usually as a separate database used by applications)
  2. CometBFT's state and block stores are empty or reset
  3. CometBFT is not running while the node is state syncing

The server side of state sync (snapshot generation and dumping), can be performed while the node is running. The application has to be careful not to interfere with normal node operations, and to use a snapshot store and dumping mechanism that will mitigate the risk of requesting snapshots while they are being dumped to an archive format.

In order to be able to dump or export the snapshots, the application must have access to the snapshot store.

We describe the main interface expected from the snapshot database and used by the above mentioned CLI commands for snapshot manipulation. The interface was derived from the SDK's implementation of local State sync.

// Delete a snapshot for a certain height
func (s *Store) Delete(height uint64, format uint32) error

// Retrieves a snapshot for a certain height
func (s *Store) Get(height uint64, format uint32) (*Snapshot, error) 

// List recent snapshots, in reverse order (newest first)
func (s *Store) List() ([]*Snapshot, error)

// Loads a snapshot (both metadata and binary chunks). The chunks must be consumed and closed.
// Returns nil if the snapshot does not exist.
func (s *Store) Load(height uint64, format uint32) (*Snapshot, <-chan io.ReadCloser, error)

// LoadChunk loads a chunk from disk, or returns nil if it does not exist. The caller must call
// Close() on it when done.
func (s *Store) LoadChunk(height uint64, format, chunk uint32) (io.ReadCloser, error)

// Save saves a snapshot to disk, returning it.
func (s *Store) Save(height uint64, format uint32, chunks <-chan io.ReadCloser) (*Snapshot, error) 

// PathChunk generates a snapshot chunk path.
func (s *Store) PathChunk(height uint64, format, chunk uint32) string

In order to dump a snapshot, an application needs to retrieve all the chunks stored at a certain path.

CometBFT state bootstrap

In addition to managing snapshots, it is necessary to bootstrap (setup) the state and block store of CometBFT before starting up the node. Upon a successful start, CometBFT performs block sync and consensus. At the moment of writing this ADR, there is no command line in CometBFT that supports this, but an issue has been opened to address this. Until it has been resolved, the application developers have to, within their bootstraping command:

  • Create a state and block store
  • Launch a light client to obtain and verify the block header for the snapshot's height.
  • Use the light client's State to verify that the AppHash on the retrieved block header matches the AppHash obtained via the ABCI Info call to the application (the same procedure is performed by the node on startup).
  • Use Commit to retrieve the last commit for the snapshot's height.
  • Save the retrieved values into the state and block stores.

This code essentially mimics what CometBFT does as part of node setup, once state sync is complete.

CometBFT based local state sync

Given that snapshot manipulation is entirely application defined, and to avoid pulling this functionality into CometBFT, we propose a solution using ABCI, that mimics the behaviour described in the previous section.

On the client side, the main difference between local State sync done by the application and CometBFT is that the application has to perform the sync offline, in order to properly set up CometBFT's initial state. Furthermore, the application developer has to manually bootstrap CometBFTs state and block stores. With support for local State sync, a node can simply load a snapshot from a predefined location and offer it to the application as is currently done via networked State sync.

On the server side, without any support for local State sync, an operator has to manually instruct the application to export the snapshots into a portable format (via dump).

Having support for this within CometBFT, the app can automatically perform this export when taking snapshots.

In order to support local State sync, the following changes to CometBFT are necessary:

  1. Adding new configuration options to the config file.
  2. Introduce a CLI command that can explicitly tell the application to create a snapshot export, in case operators decide not to generate periodical exports.
  3. Extract a snapshot from the exported format.
  4. Potentially alter existing ABCI calls to signal to the application that we want to create a snapshot export periodically.
  5. Allow reading a snaphsot from a compressed format into CometBFT and offer it to the application via the existing OfferSnapshot ABCI call.

At a very high level, there are two possible solutions and we will present both:

  1. The application will export snapshots into an exportable format on the server side. When a node syncs up, CometBFT will send this as a blob of bytes to the application to uncompress and apply the snapshot.

  2. CometBFT creates a snapshot using existing ABCI calls and exports it into a format of our choosing. CometBFT is then in charge of reading in and parsing the exported snapshot into a snapshot that can be offered to the application via the existing OfferSnapshot and ApplyChunk methods. This option does not alter any current APIs and is a good candidate for an initial implementation.

Config file additions

[statesync]
# State syncing from a local snapshot
local_sync=false 
# Path to snapshot, will be ignored if local_sync=false
snapshot_load_path=""
# Periodically dump snapshots into archive format (optional)
auto_snapshot_dump=false
# If dumping nodes into archive format, set the path to where the file is dumped
# and the file format
# This can be changed when using a CLI command
snapshot_dump_path=""
snapshot_dump_format=""

CLI command for application snapshot management

CometBFT exposes a CLI command to instruct the application to dump the existing snapshots into an exportable format:

dump      Dump existing snapshots to exportable file format.

We could expand the CLI interface to allow for additional operations, similar to the CLI designed for Cosmos SDK. However, as snapshot generation, dumping and loading a local snapshot would be built into CometBFT, while the node is running, we do not need to rely on a CLI for this functionality.

The dump command can be implemented in two ways:

  1. Rely on the existing ABCI functions ListSnapshots and LoadChunks to retrieve the snapshots and chunks from a peer. This approach requires no change to the current API and is easy to implement. Furthermore, CometBFT has complete control over the format of the exported snapshot. It does however involve more than one ABCI call and network data transfer.
  2. Extend RequestListSnapshots with a flag to indicate that we want an exportable snapshot format and extend ResponseListSnapshot to return a path and format of the exported snapshots.

An improvement to the second option mentioned above would be to add path parameters to the command, and include the path into RequestListSnapshots instructing the application to generate a snapshot export at a given location.

A third option is the introduction of a new ABCI call: ExportSnapshots, which will have the same effect as option 2 above.

Automatic snapshot exporting

Applications generate snapshots with a regular cadence (fixed time intervals or heights). The application itself measures the time or number of heights passed since the last snapshot, and CometBFT has no role in instructing the application when to take snapshots.

The State sync reactor currently retrieves a list of snapshots from a peer, who obtains these snapshots from the local instance of the application using the ABCI RequestListSnapshots call.

Applications can thus themselves be in charge of dumping the snapshots into a given file format, the same way they generate snapshots. If auto_snapshot_dump is true, CometBFT instructs the application to export the snapshots periodically.

An alternative solution is that CometBFT, itself, using the implementation of the dump command, whichever is chosen at the time, creates or asks the application to create snapshot exports. This is not forcing the application to create a snapshot at the time of he request, rather dumps existing snapshots into an exportable file format.

Export file consistency

The same caveat of making sure the file into which the snapshot is exported is not corrupted by concurrent reads and writes applies here as well. If we decide to simply export snapshots into a compressed file, we need to make sure that we do not return files that with ongoing writes. An alternative is for CometBFT to provide a Snapshot export store with snapshot isolation. This store would essentially be pruned as soon as a new snapshot is written, thus not taking too much more space.

If it is the application that exports the snapshots, it is something the application developer has to be aware of.

Syncing a node using local snapshots

On startup, if local_sync is set to true, CometBFT will look for an existing snapshot at the path given by the operator. If a snapshot is available, it will be loaded and state restored as if it came from a peer in the current implementation.

Note that, if it is not CometBFT that created the snapshot export from the data retrieved via ABCI (a combination of ListSnapshots and LoadChunks), CometBFT might not be aware of how the snapshot was exported, and needs to ask the application to restore the snapshot.

If a snapshot was created using option 1 (by CometBFT) from the previous section, or the export format is known to CometBFT (like tar, gzip etc.), CometBFT can extract the snapshot itself, and offer it to the application via RequestOfferSnapshot without any API changes.

If CometBFT is not the one who created the exported file, we introduce a new ABCI call UnpackSnapshot to send the exported snapshot as a blob of bytes to the application, which uncompresses it, installs it and responds whether the snapshot is accepted (as in OfferSnapshot) and the chunks application has passed (as in LoadChunk).

  • Request:

    Name Type Description Field Number
    exportedSnapshot [] byte Snapshot export created by the application. 1

    Commit signals the application to persist application state. It takes no parameters.

  • Response:

    Name Type Description Field Number
    result Result The result of the snapshot offer. 1
    resultChunk ResultC The result of applying the chunks. 2 
  enum ResultC {
    UNKNOWN         = 0;  // Unknown result, abort all snapshot restoration
    ACCEPT          = 1;  // The chunks were accepted.
    ABORT           = 2;  // Abort snapshot restoration, and don't try any other snapshots.
    RETRY_SNAPSHOT  = 3;  // Restart this snapshot from `OfferSnapshot`, reusing chunks unless instructed otherwise.
    REJECT_SNAPSHOT = 4;  // Reject this snapshot, try a different one.
  }

Unlike networked state sync, we do not re-fetch individual chunks, thus if the application of a chunk fails, then the application of the whole snapshot fails.

Consequences

Adding the support for a node to sync up using a local snapshot can speed up the syncing process, especially as network based State sync has proven to be fragile.

Positive

  • There is a possibility to implement this in a non-breaking manner: providing an alternative and complementary implementation for State sync
  • Node operators will not need to use workarounds to make State sync to download application snapshots from specific nodes in the network, in particular from nodes that are controlled by the same operators

Negative

  • Implementing additional ABCI functions is API breaking and might not be backwards compatible.

Neutral

  • Additional complexity, with additional parameters for State sync's configuration and the bootstrap of a node
  • Additional complexity, with the possible addition of a CLI command to save application snapshots to the file system

References