AvroSource.AvroReader (Google Cloud Dataflow SDK 1.9.1 API)

Google Cloud Dataflow SDK for Java, version 1.9.1

com.google.cloud.dataflow.sdk.io

Class AvroSource.AvroReader<T>

  • Type Parameters:
    T - The type of records contained in the block.
    All Implemented Interfaces:
    AutoCloseable
    Enclosing class:
    AvroSource<T>


    @Experimental(value=SOURCE_SINK)
    public static class AvroSource.AvroReader<T>
    extends BlockBasedSource.BlockBasedReader<T>
    A BlockBasedSource.BlockBasedReader for reading blocks from Avro files.

    An Avro Object Container File consists of a header followed by a 16-bit sync marker and then a sequence of blocks, where each block begins with two encoded longs representing the total number of records in the block and the block's size in bytes, followed by the block's (optionally-encoded) records. Each block is terminated by a 16-bit sync marker.

    • Constructor Detail

      • AvroReader

        public AvroReader(AvroSource<T> source)
        Reads Avro records of type T from the specified source.
    • Method Detail

      • getCurrentSource

        public AvroSource<T> getCurrentSource()
        Description copied from class: BoundedSource.BoundedReader
        Returns a Source describing the same input that this Reader currently reads (including items already read).

        Usage

        Reader subclasses can use this method for convenience to access unchanging properties of the source being read. Alternatively, they can cache these properties in the constructor.

        The framework will call this method in the course of dynamic work rebalancing, e.g. after a successful BoundedSource.BoundedReader.splitAtFraction(double) call.

        Mutability and thread safety

        Remember that Source objects must always be immutable. However, the return value of this function may be affected by dynamic work rebalancing, happening asynchronously via BoundedSource.BoundedReader.splitAtFraction(double), meaning it can return a different Source object. However, the returned object itself will still itself be immutable. Callers must take care not to rely on properties of the returned source that may be asynchronously changed as a result of this process (e.g. do not cache an end offset when reading a file).

        Implementation

        For convenience, subclasses should usually return the most concrete subclass of Source possible. In practice, the implementation of this method should nearly always be one of the following:
        • Source that inherits from a base class that already implements BoundedSource.BoundedReader.getCurrentSource(): delegate to base class. In this case, it is almost always an error for the subclass to maintain its own copy of the source.
          
             public FooReader(FooSource<T> source) {
               super(source);
             }
          
             public FooSource<T> getCurrentSource() {
               return (FooSource<T>)super.getCurrentSource();
             }
           
        • Source that does not support dynamic work rebalancing: return a private final variable.
          
             private final FooSource<T> source;
          
             public FooReader(FooSource<T> source) {
               this.source = source;
             }
          
             public FooSource<T> getCurrentSource() {
               return source;
             }
           
        • BoundedSource.BoundedReader that explicitly supports dynamic work rebalancing: maintain a variable pointing to an immutable source object, and protect it with synchronization.
          
             private FooSource<T> source;
          
             public FooReader(FooSource<T> source) {
               this.source = source;
             }
          
             public synchronized FooSource<T> getCurrentSource() {
               return source;
             }
          
             public synchronized FooSource<T> splitAtFraction(double fraction) {
               ...
               FooSource<T> primary = ...;
               FooSource<T> residual = ...;
               this.source = primary;
               return residual;
             }
           
        Overrides:
        getCurrentSource in class FileBasedSource.FileBasedReader<T>
      • getCurrentBlockSize

        public long getCurrentBlockSize()
        Description copied from class: BlockBasedSource.BlockBasedReader
        Returns the size of the current block in bytes as it is represented in the underlying file, if possible. This method may return 0 if the size of the current block is unknown.

        The size returned by this method must be such that for two successive blocks A and B, offset(A) + size(A) <= offset(B). If this is not satisfied, the progress reported by the BlockBasedReader will be non-monotonic and will interfere with the quality (but not correctness) of dynamic work rebalancing.

        This method and BlockBasedSource.Block.getFractionOfBlockConsumed() are used to provide an estimate of progress within a block (getCurrentBlock().getFractionOfBlockConsumed() * getCurrentBlockSize()). It is acceptable for the result of this computation to be 0, but progress estimation will be inaccurate.

        Specified by:
        getCurrentBlockSize in class BlockBasedSource.BlockBasedReader<T>
      • getSplitPointsRemaining

        public long getSplitPointsRemaining()
        Description copied from class: BoundedSource.BoundedReader
        Returns the total amount of parallelism in the unprocessed part of this reader's current BoundedSource (as would be returned by BoundedSource.BoundedReader.getCurrentSource()). This corresponds to all unprocessed split point records (see RangeTracker), including the last split point returned, in the remainder part of the source.

        This function should be implemented only in addition to BoundedSource.BoundedReader.getSplitPointsConsumed() and only if an exact value can be returned.

        Consider the following examples: (1) An input that can be read in parallel down to the individual records, such as CountingSource.upTo(long), is called "perfectly splittable". (2) a "block-compressed" file format such as AvroIO, in which a block of records has to be read as a whole, but different blocks can be read in parallel. (3) An "unsplittable" input such as a cursor in a database.

        Assume for examples (1) and (2) that the number of records or blocks remaining is known:

        • Any reader for which the last call to Source.Reader.start() or Source.Reader.advance() has returned true should should not return 0, because this reader itself represents parallelism at least 1. This condition holds independent of whether the input is splittable.
        • A finished reader (for which Source.Reader.start() or Source.Reader.advance()) has returned false should return a value of 0. This condition holds independent of whether the input is splittable.
        • For example 1: After returning record #30 (starting at 1) out of 50 in a perfectly splittable 50-record input, this value should be 21 (20 remaining + 1 current) if the total number of records is known.
        • For example 2: After returning a record in block 3 in a block-compressed file consisting of 5 blocks, this value should be 3 (since blocks 4 and 5 can be processed in parallel by new readers produced via dynamic work rebalancing, while the current reader continues processing block 3) if the total number of blocks is known.
        • For example (3): a reader for any non-empty unsplittable input, should return 1 until it is finished, at which point it should return 0.
        • For any reader: After returning the last split point in a file (e.g., the last record in example (1), the first record in the last block for example (2), or the first record in the file for example (3), this value should be 1: apart from the current task, no additional remainder can be split off.

        Defaults to BoundedSource.BoundedReader.SPLIT_POINTS_UNKNOWN. Any value less than 0 will be interpreted as unknown.

        Thread safety

        See the javadoc on BoundedSource.BoundedReader for information about thread safety.
        Overrides:
        getSplitPointsRemaining in class OffsetBasedSource.OffsetBasedReader<T>
        See Also:
        BoundedSource.BoundedReader.getSplitPointsConsumed()
      • startReading

        protected void startReading(ReadableByteChannel channel)
                             throws IOException
        Description copied from class: FileBasedSource.FileBasedReader
        Performs any initialization of the subclass of FileBasedReader that involves IO operations. Will only be invoked once and before that invocation the base class will seek the channel to the source's starting offset.

        Provided ReadableByteChannel is for the file represented by the source of this reader. Subclass may use the channel to build a higher level IO abstraction, e.g., a BufferedReader or an XML parser.

        If the corresponding source is for a subrange of a file, channel is guaranteed to be an instance of the type SeekableByteChannel.

        After this method is invoked the base class will not be reading data from the channel or adjusting the position of the channel. But the base class is responsible for properly closing the channel.

        Specified by:
        startReading in class FileBasedSource.FileBasedReader<T>
        Parameters:
        channel - a byte channel representing the file backing the reader.
        Throws:
        IOException


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