Codecs

Codec is the central concept in the library. Codecs represent an Avro schema, together with an encode and decode function for converting between Scala types and types recognized by the Apache Avro library. There are default Codecs defined for many standard library types. For example, we can check what the default Option[Instant] encoding is by asking for a Codec instance.

import java.time.Instant
import vulcan.Codec

Codec[Option[Instant]]
// res1: Codec[Option[Instant]] = WithTypeName(
//   codec = Validated(
//     codec = OptionCodec(
//       codec = WithTypeName(
//         codec = Validated(
//           codec = WithLogicalType(
//             codec = Validated(
//               codec = ImapErrors(
//                 codec = Codec("long"),
//                 f = vulcan.Codec$WithValidSchema$$Lambda$15834/0x0000000104409040@421b0c60,
//                 g = vulcan.Codec$WithValidSchema$$Lambda$15835/0x000000010440a040@7467195f
//               ),
//               validSchema = "long"
//             ),
//             logicalType = org.apache.avro.LogicalTypes$TimestampMillis@403b57f8
//           ),
//           validSchema = {"type":"long","logicalType":"timestamp-millis"}
//         ),
//         typeName = "Instant"
//       )
//     ),
//     validSchema = ["null",{"type":"long","logicalType":"timestamp-millis"}]
//   ),
//   typeName = "Option"
// )

In some cases, it's not possible to generate Avro schemas. This is why Codec schemas are wrapped in Either with error type AvroError. For example, Avro doesn't support nested unions, so what would happen when we try to ask for a Codec for Option[Option[Instant]]?

Codec[Option[Option[Instant]]]
// res2: Codec[Option[Option[Instant]]] = Fail(
//   error = AvroError(org.apache.avro.AvroRuntimeException: Duplicate in union:null)
// )

Encoding and decoding with a Codec might also be unsuccessful, so results are wrapped in Either with error type AvroError. Encoding accepts a value to be encoded according to the schema defined by the Codec, and decoding accepts a value and a schema to decode the value against. For example, what happens if we try to decode Ints using a Boolean schema?

import org.apache.avro.SchemaBuilder

Codec[Int].decode(10, SchemaBuilder.builder.booleanType)
// res3: Either[vulcan.AvroError, Int] = Left(
//   value = ErrorDecodingType(
//     decodingTypeName = "Int",
//     cause = AvroError(Got unexpected schema type BOOLEAN, expected schema type INT)
//   )
// )

Since the Apache Avro library encodes and decodes using Object, Codecs encode and decode between Scala types and Any. This means type safety is lost and tests should be used to ensure Codecs work as intended. This becomes important when we define Codecs from scratch. Note Schemas are treated as effectively immutable, even though they're in fact mutable.

Codecs form invariant functors, which means you can easily use an existing Codec to encode and decode a different type, by mapping back-and-forth between the Codec's existing type argument and the new type. This becomes useful when dealing with newtypes, like InstallationTime in the following example.

final case class InstallationTime(value: Instant)

Codec[Instant].imap(InstallationTime(_))(_.value)
// res4: Codec.Aux[Codec.instant.AvroType, InstallationTime] = ImapErrors(
//   codec = WithTypeName(
//     codec = Validated(
//       codec = WithLogicalType(
//         codec = Validated(
//           codec = ImapErrors(
//             codec = Codec("long"),
//             f = vulcan.Codec$WithValidSchema$$Lambda$15834/0x0000000104409040@421b0c60,
//             g = vulcan.Codec$WithValidSchema$$Lambda$15835/0x000000010440a040@7467195f
//           ),
//           validSchema = "long"
//         ),
//         logicalType = org.apache.avro.LogicalTypes$TimestampMillis@403b57f8
//       ),
//       validSchema = {"type":"long","logicalType":"timestamp-millis"}
//     ),
//     typeName = "Instant"
//   ),
//   f = vulcan.Codec$WithValidSchema$$Lambda$15834/0x0000000104409040@56ce3626,
//   g = vulcan.Codec$WithValidSchema$$Lambda$15835/0x000000010440a040@5292cb1b
// )

When we have a newtype where we ensure values are valid, we can use imapError instead.

import vulcan.AvroError

sealed abstract case class SerialNumber(value: String)

object SerialNumber {
  def apply(value: String): Either[AvroError, SerialNumber] =
    if(value.length == 12 && value.forall(_.isDigit))
      Right(new SerialNumber(value) {})
    else Left(AvroError(s"$value is not a serial number"))
}

Codec[String].imapError(SerialNumber(_))(_.value)
// res5: Codec.Aux[Codec.string.AvroType, SerialNumber] = ImapErrors(
//   codec = Codec("string"),
//   f = <function1>,
//   g = vulcan.Codec$$Lambda$15905/0x0000000104454840@438c6ec5
// )

Decimals

Avro decimals closely correspond to BigDecimals with a fixed precision and scale.

Codec.decimal can be used to create a Codec for BigDecimal given the precision and scale. When encoding, the Codec checks the precision and scale of the BigDecimal to make sure it matches the precision and scale defined in the schema. When decoding, we check the precision is not exceeded.

Codec.decimal(precision = 10, scale = 2)
// res6: Codec.Aux[vulcan.Avro.Bytes, BigDecimal] = DecimalCodec(
//   precision = 10,
//   scale = 2,
//   validSchema = {"type":"bytes","logicalType":"decimal","precision":10,"scale":2}
// )

Enumerations

Avro enumerations closely correspond to sealed traits with case objects.

We can use Codec.enumeration​ to specify an encoding.

sealed trait Fruit
case object Apple extends Fruit
case object Banana extends Fruit
case object Cherry extends Fruit

Codec.enumeration[Fruit](
  name = "Fruit",
  namespace = "com.example",
  doc = Some("A selection of different fruits"),
  symbols = List("apple", "banana", "cherry"),
  encode = {
    case Apple  => "apple"
    case Banana => "banana"
    case Cherry => "cherry"
  },
  decode = {
    case "apple"  => Right(Apple)
    case "banana" => Right(Banana)
    case "cherry" => Right(Cherry)
    case other    => Left(AvroError(s"$other is not a Fruit"))
  },
  default = Some(Banana)
)
// res7: Codec.Aux[vulcan.Avro.EnumSymbol, Fruit] = WithTypeName(
//   codec = Validated(
//     codec = Codec({
//   "type" : "enum",
//   "name" : "Fruit",
//   "namespace" : "com.example",
//   "doc" : "A selection of different fruits",
//   "symbols" : [ "apple", "banana", "cherry" ],
//   "default" : "banana"
// }),
//     validSchema = {"type":"enum","name":"Fruit","namespace":"com.example","doc":"A selection of different fruits","symbols":["apple","banana","cherry"],"default":"banana"}
//   ),
//   typeName = "com.example.Fruit"
// )

Derivation for enumeration types can be partly automated using the generic or enumeratum modules, although these will not support Scala 3 for the foreseeable future.

Fixed

Avro fixed types correspond to Array[Byte]s with a fixed size.

We can use Codec.fixed to define a codec.

sealed abstract case class Pence(value: Byte)

object Pence {
  def apply(value: Byte): Either[AvroError, Pence] =
    if(0 <= value && value < 100) Right(new Pence(value) {})
    else Left(AvroError(s"Expected pence value, got $value"))
}

Codec.fixed[Pence](
  name = "Pence",
  namespace = "com.example",
  size = 1,
  encode = pence => Array[Byte](pence.value),
  decode = bytes => Pence(bytes.head),
  doc = Some("Amount of pence as a single byte")
)
// res8: Codec.Aux[vulcan.Avro.Fixed, Pence] = WithTypeName(
//   codec = Validated(
//     codec = Codec({
//   "type" : "fixed",
//   "name" : "Pence",
//   "namespace" : "com.example",
//   "doc" : "Amount of pence as a single byte",
//   "size" : 1
// }),
//     validSchema = {"type":"fixed","name":"Pence","namespace":"com.example","doc":"Amount of pence as a single byte","size":1}
//   ),
//   typeName = "com.example.Pence"
// )

Records

Avro records closely correspond to case classes.

We can use Codec.record to specify a record encoding.

import cats.implicits._

final case class Person(firstName: String, lastName: String, age: Option[Int])

Codec.record[Person](
  name = "Person",
  namespace = "com.example",
  doc = Some("Person with a full name and optional age")
) { field =>
  field("fullName", p => s"${p.firstName} ${p.lastName}") *>
  (
    field("firstName", _.firstName),
    field("lastName", _.lastName, doc = Some("the last name")),
    field("age", _.age, default = Some(None))
  ).mapN(Person(_, _, _))
}
// res9: Codec.Aux[vulcan.Avro.Record, Person] = WithTypeName(
//   codec = Validated(
//     codec = Codec({
//   "type" : "record",
//   "name" : "Person",
//   "namespace" : "com.example",
//   "doc" : "Person with a full name and optional age",
//   "fields" : [ {
//     "name" : "fullName",
//     "type" : "string"
//   }, {
//     "name" : "firstName",
//     "type" : "string"
//   }, {
//     "name" : "lastName",
//     "type" : "string",
//     "doc" : "the last name"
//   }, {
//     "name" : "age",
//     "type" : [ "null", "int" ],
//     "default" : null
//   } ]
// }),
//     validSchema = {"type":"record","name":"Person","namespace":"com.example","doc":"Person with a full name and optional age","fields":[{"name":"fullName","type":"string"},{"name":"firstName","type":"string"},{"name":"lastName","type":"string","doc":"the last name"},{"name":"age","type":["null","int"],"default":null}]}
//   ),
//   typeName = "com.example.Person"
// )

For generic derivation support, refer to the generic module.

Unions

Avro unions closely correspond to sealed traits.

We can use Codec.union to specify a union encoding.

sealed trait FirstOrSecond

final case class First(value: Int) extends FirstOrSecond
object First {
  implicit val codec: Codec[First] =
    Codec[Int].imap(apply)(_.value)
}

final case class Second(value: String) extends FirstOrSecond
object Second {
  implicit val codec: Codec[Second] =
    Codec[String].imap(apply)(_.value)
}

Codec.union[FirstOrSecond] { alt =>
  alt[First] |+| alt[Second]
}
// res10: Codec.Aux[Any, FirstOrSecond] = WithTypeName(
//   codec = Validated(
//     codec = UnionCodec(
//       alts = Append(
//         leftNE = Singleton(a = vulcan.Codec$Alt$$anon$6@651fee47),
//         rightNE = Singleton(a = vulcan.Codec$Alt$$anon$6@1822a0da)
//       )
//     ),
//     validSchema = ["int","string"]
//   ),
//   typeName = "union"
// )

For generic derivation support, refer to the generic module.

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