Encoder

An Encoder encodes a Scala value of type T into a JVM value suitable for use with Avro.

For example, an encoder could encode a String as an instance of Utf8, or it could encode it as an instance of GenericFixed.

Alternatively, given a Scala enum value, the enum could be encoded as an instance of GenericData.EnumSymbol or as a String.

An encoder is invoked with an Avro schema, and a FieldMapper and returns a reusable function that then encodes values of type T into Avro types.

It is possible to configure encoders entirely through annotations, which is fine if your system is self contained. But if your schemas are generated outside of avro4s, or even in another language, you may need to use these "third-party" schemas to influence the encoding process.

Some encoders use the schema to determine the encoding function to return. For example, strings can be encoded as UTF8s, GenericFixeds, ByteBuffers or java.lang.Strings. Therefore the Encoder[String] typeclass instances uses the schema to select which of these implementations to use.

Other types may not require the schema at all. For example, the default Encoder[Int] always returns a java.lang.Integer regardless of any schema input.

The second parameter to an encoder is the field mapper. This is used to derive the field names used when generating record or error types. By default, the field mapper will use the field names as they are defined in the type itself (from the case class). However for interop with other systems you may wish to customize this, for example, by writing out field names in snake_case or adding a prefix.