Prompt: What is a coder in Apache Beam?

Response: In Apache Beam, a Coder is an object responsible for specifying how elements in a given PCollection are encoded and decoded into byte strings. The Coder objects provide type consistency and safety during distributed processing between workers, ensuring the correct interpretation of data based on its original types.

Every PCollection in an Apache Beam pipeline requires a corresponding Coder. While the Apache Beam SDKs can automatically infer a Coder for a PCollection based on its element type or the producing transform, there are cases where explicit specification or custom Coder development is necessary. It is important to note that multiple Coder objects can exist for a single data type.

The Apache Beam SDKs use various mechanisms to automatically infer the Coder for a PCollection. Each pipeline object contains a CoderRegistry object representing a mapping of language types to the default coder for those types.

In the Apache Beam SDKs for Python and Java, the Coder type provides the necessary methods for encoding and decoding data. The SDKs offer various Coder subclasses working with standard Python and Java types, available in the apache_beam.coders package for Python and the Coder package for Java.

By default, the Beam SDKs use the typehints (Python) or the type parameters (Java) from the transform's function object (such as DoFn) to infer the Coder for elements in a PCollection. For example, in Apache Beam Python SDK, a ParDo annotated with the typehints @beam.typehints.with_input_types(int) and @beam.typehints.with_output_types(str) indicates that it accepts int inputs and produces str outputs. The Python SDK automatically infers the default Coder for the output PCollection, in this case, BytesCoder (based on the default CoderRegistry).

You can use the CoderRegistry to look up the default coder for a given type or register a new default coder. The following tables show the default mappings of coders to standard types for any pipeline created using the Beam SDK for Java and Python:

Java Type	Default Coder
Double	DoubleCoder
Instant	InstantCoder
Integer	VarIntCoder
Iterable	IterableCoder
KV	KvCoder
List	ListCoder
Map	MapCoder
Long	VarLongCoder
String	StringUtf8Coder

Python Type	Default Coder
int	VarIntCoder
float	FloatCoder
str	BytesCoder
bytes	StrUtf8Coder
Tuple	TupleCoder