Module snapps_txn_reference_impl::event

Auxiliary information exposed by snapp smart contract execution.

The format of this data is opaque to the Mina protocol, and is defined by each individual smart contract. A smart contract may emit zero or more ‘events’, which are communicated with the transaction when it is broadcast over the Mina network.

These ‘events’ are collapsed into a single hash for the purposes of the zero-knowledge proofs, and this hash is unused by the transaction logic.

An event may be used to expose information that is useful or important to make publically available, but without storing the data in accounts on-chain.

Nodes may make this data available for transactions in their pool or from within blocks. Archive nodes and similar services may make older historical event data available.

Example

A snapp smart contract could use a merkle tree to store more data than it can fit in its 8 general purpose registers. Usually, when a snap account updates the root of its merkle tree, there will be no way to know what the new contents of the merkle tree are; only the cryptographic ‘root hash’ is publically available.

In order to update the merkle tree while making the new contents publically known, a snapp developer can emit an event containing the new data and its position as an event, and any other member of the network can look at this event to discover the new contents.

Consider for example the tree containing 8 values A, B, C, D, E, F, G, H.

                            H7=Hash(H5, H6)
                           /               \
            H5=Hash(H1, H2)                 H6=Hash(H3, H4)
           /               \               /               \
  H1=Hash(A, B)     H2=Hash(C, D)     H3=Hash(E, F)     H4=Hash(G, H)
 /             \   /             \   /             \   /             \
A               B C               D E               F G               H

H7 is used as the ‘root hash’ of the tree.

A snapp might update the tree at the 5th position (0-indexed, replacing F with I), generating the resulting tree

                            H7'=Hash(H5, H6')
                           /               \
            H5=Hash(H1, H2)                 H6'=Hash(H3', H4)
           /               \               /               \
  H1=Hash(A, B)     H2=Hash(C, D)     H3'=Hash(E, I)     H4=Hash(G, H)
 /             \   /             \   /             \   /             \
A               B C               D E               I G               H

and storing its new ‘root hash’ H7' in its app_state.

The snapp can emit an event that encodes the new data to be placed in the tree, for example as {data: I, position: 5}, and then any user who previously knew the contents of the tree can update their local copy to match, and can continue to interact with the snapp’s data by querying/updating the new tree.

Structs

Event

A single event emitted by a snapp. See crate::event for more.

EventsHash

A hash representing a list of events.

Type Definitions

Events

A list of zero or more events, which may be emitted by a snapp. See crate::event for more.