net.corda.flows / FetchDataFlow

# FetchDataFlow

`abstract class FetchDataFlow<T : NamedByHash, in W : Any> : FlowLogic<Result<T>>`

An abstract flow for fetching typed data from a remote peer.

Given a set of hashes (IDs), either loads them from local disk or asks the remote peer to provide them.

A malicious response in which the data provided by the remote peer does not hash to the requested hash results in DownloadedVsRequestedDataMismatch being thrown. If the remote peer doesnt have an entry, it results in a HashNotFound exception being thrown.

By default this class does not insert data into any local database, if you want to do that after missing items were fetched then override maybeWriteToDisk. You must override load. If the wire type is not the same as the ultimate type, you must also override convert.

### Parameters

`T` - The ultimate type of the data being fetched.
`W` - The wire type of the data being fetched, for when it isnt the same as the ultimate type.

### Types

Request | `data class Request`
Result | `data class Result<out T : NamedByHash>`

### Exceptions

BadAnswer | `open class BadAnswer : Exception`
DownloadedVsRequestedDataMismatch | `class DownloadedVsRequestedDataMismatch : BadAnswer`
HashNotFound | `class HashNotFound : BadAnswer`

### Constructors

<init> | `FetchDataFlow(requests: Set<SecureHash>, otherSide: Party)` An abstract flow for fetching typed data from a remote peer.

### Properties

otherSide | `val otherSide: Party`
requests | `val requests: Set<SecureHash>`

### Inherited Properties

fsm | `lateinit var fsm: FlowStateMachine<*>` Reference to the Fiber instance that is the top level controller for the entire flow.
logger | `val logger: <ERROR CLASS>` This is where you should log things to.
progressTracker | `open val progressTracker: ProgressTracker?` Override this to provide a ProgressTracker. If one is provided and stepped, the framework will do something helpful with the progress reports. If this flow is invoked as a sub-flow of another, then the tracker will be made a child of the current step in the parent. If its null, this flow doesnt track progress.
serviceHub | `val serviceHub: ServiceHub` Provides access to big, heavy classes that may be reconstructed from time to time, e.g. across restarts. It is only available once the flow has started, which means it cannnot be accessed in the constructor. Either access this lazily or from inside call.

### Functions

call | `open fun call(): Result<T>` This is where you fill out your business logic.
convert | `open fun convert(wire: W): T`
load | `abstract fun load(txid: SecureHash): T?`
maybeWriteToDisk | `open fun maybeWriteToDisk(downloaded: List<T>): Unit`

### Inherited Functions

getCounterpartyMarker | `open fun getCounterpartyMarker(party: Party): Class<*>` Return the marker Class which party has used to register the counterparty flow that is to execute on the other side. The default implementation returns the class object of this FlowLogic, but any Class instance will do as long as the other side registers with it.
receive | `fun <T : Any> receive(receiveType: Class<T>, otherParty: Party): UntrustworthyData<T>` `fun <T : Any> receive(otherParty: Party): UntrustworthyData<T>`
send | `fun send(otherParty: Party, payload: Any): Unit`
sendAndReceive | `fun <T : Any> sendAndReceive(otherParty: Party, payload: Any): UntrustworthyData<T>` `fun <T : Any> sendAndReceive(receiveType: Class<T>, otherParty: Party, payload: Any): UntrustworthyData<T>`
subFlow | `fun <R> subFlow(subLogic: FlowLogic<R>, shareParentSessions: Boolean = false): R` Invokes the given subflow by simply passing through this FlowLogics reference to the FlowStateMachine and then calling the call method.
track | `fun track(): <ERROR CLASS><String, <ERROR CLASS><String>>?`

### Inheritors

FetchAttachmentsFlow | `class FetchAttachmentsFlow : FetchDataFlow<Attachment, ByteArray>` Given a set of hashes either loads from from local storage or requests them from the other peer. Downloaded attachments are saved to local storage automatically.
FetchTransactionsFlow | `class FetchTransactionsFlow : FetchDataFlow<SignedTransaction, SignedTransaction>` Given a set of tx hashes (IDs), either loads them from local disk or asks the remote peer to provide them.