Package com.apple.foundationdb.record.cursors

Class AutoContinuingCursor<T>

  • Type Parameters:
    T - the type of elements returned by this cursor
    All Implemented Interfaces:
    RecordCursor<T>, AutoCloseable, Iterator<T>
    @API(EXPERIMENTAL)
public class AutoContinuingCursor<T>
extends Object
implements RecordCursor<T>
    A cursor that can iterate over a cursor across transactions. It is provided a generator that produces a cursor (referred to as underlying cursor). The underlying cursor is iterated over until it is either exhausted or until one of the scan limits is reached:
    • If the underlying cursor is exhausted, the AutoContinuingCursor is also exhausted.
    • If scan limit properties of the underlying cursor are reached, the generator is asked for a new underlying cursor which (1) is in the context of a new transaction and (2) takes the continuation from the previous underlying cursor. Then the process is repeated.

    AutoContinuingCursor is responsible for all FDBRecordContext management, so all reads it does are in the scope of transactions that it controls.

    The AutoContinuingCursor has no visibility into the ScanProperties of the underlying cursor and, therefore, will not be involved in enforcing any limits that may be individually applied to the underlying cursor. For example, if the generator returns an underlying cursor that specified, say, a record scan limit of 10 records, the AutoContinuingCursor will scan all data until it is exhausted, at most 10 records at a transaction.

    • Method Detail

      • onNext

        @Nonnull
public CompletableFuture<RecordCursorResult<T>> onNext()
        Description copied from interface: RecordCursor
        Asynchronously return the next result from this cursor. When complete, the future will contain a RecordCursorResult, which represents exactly one of the following:
        1. The next object of type T produced by the cursor. In addition to the next record, this result includes a RecordCursorContinuation that can be used to continue the cursor after the last record returned. The returned continuation is guaranteed not to be an "end continuation" representing the end of the cursor: specifically, RecordCursorContinuation.isEnd() is always false on the returned continuation.
        2. The fact that the cursor is stopped and cannot produce another record and a RecordCursor.NoNextReason that explains why no record could be produced. The result include a continuation that can be used to continue the cursor after the last record returned. If the result's NoNextReason is anything other than RecordCursor.NoNextReason.SOURCE_EXHAUSTED, the returned continuation must not be an end continuation. Conversely, if the result's NoNextReason is SOURCE_EXHAUSTED, then the returned continuation must be an an "end continuation".
        In either case, the returned RecordCursorContinuation can be serialized to an opaque byte array using RecordCursorContinuation.toBytes(). This can be passed back into a new cursor of the same type, with all other parameters remaining the same.
        Specified by:
        onNext in interface RecordCursor<T>
        Returns:
        a future for the next result from this cursor representing either the next record or an indication of why the cursor stopped
        See Also:
        RecordCursorResult, RecordCursorContinuation

      • getContinuation

        @Nullable
@Deprecated
public byte[] getContinuation()
        Deprecated.
        Description copied from interface: RecordCursor
        Get a byte string that can be used to continue a query after the last record returned.
        Specified by:
        getContinuation in interface RecordCursor<T>
        Returns:
        opaque byte array denoting where the cursor should pick up. This can be passed back into a new cursor of the same type, with all other parameters remaining the same. Returns null if the underlying source is completely exhausted, independent of any limit passed to the cursor creator. Since such creators generally accept null to mean no continuation, that is, start from the beginning, one must check for null from getContinuation to keep from starting over. Result is not always defined if called before onHasNext or before next after onHasNext has returned true. That is, a continuation is only guaranteed when called "between" records from a while (hasNext) next loop or after its end.

      • accept

        public boolean accept​(@Nonnull
                      RecordCursorVisitor visitor)
        Description copied from interface: RecordCursor
        Accept a visit from hierarchical visitor, which implements RecordCursorVisitor. By contract, implementations of this method must return the value of visitor.visitLeave(this), which determines whether or not subsequent siblings of this cursor should be visited.
        Specified by:
        accept in interface RecordCursor<T>
        Parameters:
        visitor - a hierarchical visitor
        Returns:
        true if the subsequent siblings of the cursor should be visited, and false otherwise