Snapshots¶
When a node in the Mavryk network is started, it has to catch up the current state of the blockchain (a process known as “bootstrapping”), making sure that this state is coherent with the whole history of the ledger. As the chain invariably grows every day, retrieving a full chain from the peer-to-peer network can be a very long process. Thanks to the implementation of history modes, it is now possible to propose an import/export feature: snapshots. This procedure allows to gather all the data necessary to bootstrap a node from a single file within few minutes.
Importing a snapshot¶
When bootstrapping from a snapshot, the first thing that you want to be sure of is that the loaded state is not fake.
The snapshot format does not (and cannot) provide any evidence that the imported block is actually a real point in the history of the Mavryk network blockchain. To avoid to be fooled by a fake chain, it is necessary to carefully check that the block hash of the imported block is included in the main chain. (Note that all the websites providing snapshots mention the hash corresponding to the head block in each snapshot.) This check can be done by comparing the hash to one provided by another node under the user’s control, or by relying on social cues to obtain a hash from a large number of trusted parties which are unlikely to be colluding.
For instance, one may use one or more Mavryk block explorers to check whether this block hash matches an existing block.
After that careful selection and verification of the imported block hash, you can trust the node with the rest of the procedure. In particular, you do not need to trust the source of the file. The snapshot format contains everything necessary for the node to detect any inconsistency, malicious or not.
This safety comes from the fact that block headers are designed to make sure that applying a block has the same result for everyone in the network. To achieve this, a block header includes hashes of its operations and predecessor, as well as the resulting chain state. The import process does the same checks, recomputing and checking all the hashes it encounters in the snapshot.
To bootstrap an Mavkit node from a file to an empty Mavryk node directory (running this command from an already synchronised node will not work), run:
mavkit-node snapshot import <FILE> --block <BLOCK_HASH> [--data-dir <NODE_DIR>]
The --block <BLOCK_HASH> option argument aims to verify that the
block contained in the snapshot is the one that you are expecting to
import.
Warning
While importing a snapshot, many checks are performed to ensure the
consistency of the imported data. In order to speed up the process
and only if the snapshot’s source is highly trusted (or exported by
yourself), it is possible to disable some checks. The validity of
the target block will be, of course, ensured. However, the rest of
the data will be copied directly, without additional consistency
checks. To do so, use the --no-check option.
Snapshot information¶
When retrieving a snapshot, it can be useful to check the actual
content of the snapshot. To do so, the node’s snapshot info
command can be used to display snapshot’s information such as:
snapshot’s version
chain name
history mode
targeted block hash, level and timestamp
…
This information is displayed by the following command:
mavkit-node snapshot info <FILE>
As can be seen in the snapshot information, a snapshot contains historical data corresponding to a given history mode, which can be: Full, Archive, or Rolling (see history modes).
Storage reconstruction from a snapshot¶
When importing a full snapshot you can optionally trigger an
archive storage reconstruction using the --reconstruct
option. After importing the snapshot’s data, all the chain data will
be recomputed. This operation may take a couple of days to complete.
Exporting a snapshot¶
To export a snapshot, we first select a block hash which will represent the point in history at which consumers of this snapshot will start bootstrapping. By default, if no block hash is provided, we automatically choose a block corresponding to the last checkpoint. This is important as nodes bootstrapped from this snapshot will not be able to reorganise their chain below this block (they will set their checkpoint to this block).
Depending on the snapshot export option, additional history may also
be put in the snapshot file. By default, the snapshot export command
will create a full snapshot. Such a snapshot will contain all the
blocks from a given block hash back to the genesis. Thus, the whole chain
will be exported into the snapshot, from the beginning to the selected
point. This kind of snapshot can only be created from a full or an
archive node.
mavkit-node snapshot export --block <BLOCK>
The <BLOCK> hint can be given as a block hash, a block level,
an alias (head, savepoint or checkpoint) and a relative block
target using the ~, - or + notation (such as head~42).
If no --block <BLOCK> option is given, the checkpoint level will
be chosen as the default block to export.
By default, the snapshot will be exported into a file with a name
following this pattern
<NETWORK>-<BLOCK_HASH>-<BLOCK_LEVEL>.<SNAPSHOT_KIND>. A specific
snapshot file name can be given as an additional argument. For example:
mavkit-node snapshot export recent_head_snapshot.full --block head
Note
If your node’s current storage was created with Mavkit v13.0 (or earlier versions), it is not compatible with the context pruning storage layout, and therefore it is no longer possible to export a storage snapshot.
In order to benefit from recent storage features and optimizations, it is strongly recommended to restart your node with the latest Mavkit version and with a fresh, up-to-date storage – either by importing a recent snapshot (that is, exported by an Mavkit node version >= v13.0), or by bootstrapping from scratch if this is not possible.
Rolling export¶
Rolling snapshots are useful to quickly deploy a node
or for testing and experimentation purposes (such as in a classroom
setting), as they are much smaller. However, to bootstrap a long
running node on the network, we recommend using full snapshots to
participate into the network-wide preservation and sharing of chain
history.
mavkit-node snapshot export <FILE>.rolling --block <BLOCK_HASH> --rolling
Snapshot file format and IPFS¶
By default, the snapshot is exported as an archive file (in the
.tar format). Such a single archive file is suitable for applying
compression mechanisms. However, the compression of a snapshot file is
not handled by the node.
If one prefers not to export the snapshot as a single archive file, it
is possible to add the flag --export-format raw to the export
command. The snapshot is then exported as a folder containing all the
necessary files. As the structure of the snapshot follows the storage
representation which is based on the network’s cycles, the major part
of the raw snapshot’s data is canonical. The block history is thus
represented as cemented cycles and will stay as it is for ever. Only
the rest of the file contains data relative to the snapshot’s target
block, such as the current incomplete cycle and the block’s associated
ledger state. This canonical representation is well suited for
distributing snapshots through IPFS.
Export capabilities¶
The following table recapitulates the different kinds of snapshots that can be exported from a given history mode node.
From/To |
Full snapshot |
Rolling snapshot |
|---|---|---|
Archive |
Yes |
Yes |
Full |
Yes |
Yes |
Rolling |
No |
Yes |
Snapshot providers¶
There are several services providing node snapshots. They create snapshots of their nodes on a regular basis (usually daily) and make them available for download. These include: