org.apache.beam.sdk.coders

Interface Coder<T>

Type Parameters:

T - the type of the values being transcoded

All Superinterfaces:

Serializable

All Known Implementing Classes:

AtomicCoder, AvroCoder, BigDecimalCoder, BigEndianIntegerCoder, BigEndianLongCoder, BigIntegerCoder, ByteArrayCoder, ByteCoder, ByteStringCoder, CoGbkResult.CoGbkResultCoder, CollectionCoder, CustomCoder, DelegateCoder, DeterministicStandardCoder, DoubleCoder, DurationCoder, GlobalWindow.Coder, InstantCoder, IterableCoder, IterableLikeCoder, JAXBCoder, KvCoder, ListCoder, MapCoder, NullableCoder, PaneInfo.PaneInfoCoder, ProtoCoder, SerializableCoder, SetCoder, StandardCoder, StringDelegateCoder, StringUtf8Coder, TableRowJsonCoder, TextualIntegerCoder, TimestampedValue.TimestampedValueCoder, UnionCoder, VarIntCoder, VarLongCoder, VoidCoder

public interface Coder<T>
extends Serializable

A Coder<T> defines how to encode and decode values of type T into byte streams.

Coder instances are serialized during job creation and deserialized before use, via JSON serialization. See SerializableCoder for an example of a Coder that adds a custom field to the Coder serialization. It provides a constructor annotated with JsonCreator, which is a factory method used when deserializing a Coder instance.

Coder classes for compound types are often composed from coder classes for types contains therein. The composition of Coder instances into a coder for the compound class is the subject of the CoderFactory type, which enables automatic generic composition of Coder classes within the CoderRegistry. With particular static methods on a compound Coder class, a CoderFactory can be automatically inferred. See KvCoder for an example of a simple compound Coder that supports automatic composition in the CoderRegistry.

The binary format of a Coder is identified by getEncodingId(); be sure to understand the requirements for evolving coder formats.

All methods of a Coder are required to be thread safe.

Nested Class Summary

Nested Classes

Modifier and Type	Interface and Description
static class	Coder.Context The context in which encoding or decoding is being done.
static class	Coder.NonDeterministicException Exception thrown by verifyDeterministic() if the encoding is not deterministic, including details of why the encoding is not deterministic.

Method Summary

Modifier and Type	Method and Description
org.apache.beam.sdk.util.CloudObject	asCloudObject() Returns the CloudObject that represents this Coder.
boolean	consistentWithEquals() Returns true if this Coder is injective with respect to Object.equals(java.lang.Object).
T	decode(InputStream inStream, Coder.Context context) Decodes a value of type T from the given input stream in the given context.
void	encode(T value, OutputStream outStream, Coder.Context context) Encodes the given value of type T onto the given output stream in the given context.
Collection<String>	getAllowedEncodings() A collection of encodings supported by decode(java.io.InputStream, org.apache.beam.sdk.coders.Coder.Context) in addition to the encoding from getEncodingId() (which is assumed supported).
List<? extends Coder<?>>	getCoderArguments() If this is a Coder for a parameterized type, returns the list of Coders being used for each of the parameters, or returns null if this cannot be done or this is not a parameterized type.
String	getEncodingId() An identifier for the binary format written by encode(T, java.io.OutputStream, org.apache.beam.sdk.coders.Coder.Context).
boolean	isRegisterByteSizeObserverCheap(T value, Coder.Context context) Returns whether registerByteSizeObserver(T, org.apache.beam.sdk.util.common.ElementByteSizeObserver, org.apache.beam.sdk.coders.Coder.Context) cheap enough to call for every element, that is, if this Coder can calculate the byte size of the element to be coded in roughly constant time (or lazily).
void	registerByteSizeObserver(T value, org.apache.beam.sdk.util.common.ElementByteSizeObserver observer, Coder.Context context) Notifies the ElementByteSizeObserver about the byte size of the encoded value using this Coder.
Object	structuralValue(T value) Returns an object with an Object.equals() method that represents structural equality on the argument.
void	verifyDeterministic() Throw Coder.NonDeterministicException if the coding is not deterministic.

Method Detail

encode

void encode(T value,
            OutputStream outStream,
            Coder.Context context)
     throws CoderException,
            IOException

Encodes the given value of type T onto the given output stream in the given context.

Throws:

IOException - if writing to the OutputStream fails for some reason

CoderException - if the value could not be encoded for some reason

decode

T decode(InputStream inStream,
         Coder.Context context)
  throws CoderException,
         IOException

Decodes a value of type T from the given input stream in the given context. Returns the decoded value.

Throws:

IOException - if reading from the InputStream fails for some reason

CoderException - if the value could not be decoded for some reason

getCoderArguments

List<? extends Coder<?>> getCoderArguments()

If this is a Coder for a parameterized type, returns the list of Coders being used for each of the parameters, or returns null if this cannot be done or this is not a parameterized type.

asCloudObject

org.apache.beam.sdk.util.CloudObject asCloudObject()

Returns the CloudObject that represents this Coder.

verifyDeterministic

void verifyDeterministic()
                  throws Coder.NonDeterministicException

Throw Coder.NonDeterministicException if the coding is not deterministic.

In order for a Coder to be considered deterministic, the following must be true:

• two values that compare as equal (via Object.equals() or Comparable.compareTo(), if supported) have the same encoding.
• the Coder always produces a canonical encoding, which is the same for an instance of an object even if produced on different computers at different times.

Throws:

Coder.NonDeterministicException - if this coder is not deterministic.

consistentWithEquals

boolean consistentWithEquals()

Returns true if this Coder is injective with respect to Object.equals(java.lang.Object).

Whenever the encoded bytes of two values are equal, then the original values are equal according to Objects.equals(). Note that this is well-defined for null.

This condition is most notably false for arrays. More generally, this condition is false whenever equals() compares object identity, rather than performing a semantic/structural comparison.

structuralValue

Object structuralValue(T value)
                throws Exception

Returns an object with an Object.equals() method that represents structural equality on the argument.

For any two values x and y of type T, if their encoded bytes are the same, then it must be the case that structuralValue(x).equals(@code structuralValue(y).

Most notably:

• The structural value for an array coder should perform a structural comparison of the contents of the arrays, rather than the default behavior of comparing according to object identity.
• The structural value for a coder accepting null should be a proper object with an equals() method, even if the input value is null.

See also consistentWithEquals().

Throws:

Exception

isRegisterByteSizeObserverCheap

boolean isRegisterByteSizeObserverCheap(T value,
                                        Coder.Context context)

Returns whether registerByteSizeObserver(T, org.apache.beam.sdk.util.common.ElementByteSizeObserver, org.apache.beam.sdk.coders.Coder.Context) cheap enough to call for every element, that is, if this Coder can calculate the byte size of the element to be coded in roughly constant time (or lazily).

Not intended to be called by user code, but instead by PipelineRunner implementations.

registerByteSizeObserver

void registerByteSizeObserver(T value,
                              org.apache.beam.sdk.util.common.ElementByteSizeObserver observer,
                              Coder.Context context)
                       throws Exception

Notifies the ElementByteSizeObserver about the byte size of the encoded value using this Coder.

Not intended to be called by user code, but instead by PipelineRunner implementations.

Throws:

Exception

getEncodingId

@Experimental(value=CODER_ENCODING_ID)
String getEncodingId()

An identifier for the binary format written by encode(T, java.io.OutputStream, org.apache.beam.sdk.coders.Coder.Context).

This value, along with the fully qualified class name, forms an identifier for the binary format of this coder. Whenever this value changes, the new encoding is considered incompatible with the prior format: It is presumed that the prior version of the coder will be unable to correctly read the new format and the new version of the coder will be unable to correctly read the old format.

If the format is changed in a backwards-compatible way (the Coder can still accept data from the prior format), such as by adding optional fields to a Protocol Buffer or Avro definition, and you want Dataflow to understand that the new coder is compatible with the prior coder, this value must remain unchanged. It is then the responsibility of decode(java.io.InputStream, org.apache.beam.sdk.coders.Coder.Context) to correctly read data from the prior format.

getAllowedEncodings

@Experimental(value=CODER_ENCODING_ID)
Collection<String> getAllowedEncodings()

A collection of encodings supported by decode(java.io.InputStream, org.apache.beam.sdk.coders.Coder.Context) in addition to the encoding from getEncodingId() (which is assumed supported).

This information is not currently used for any purpose. It is descriptive only, and this method is subject to change.

See Also:

getEncodingId()