Apache Avro - Introduction

In this post, we will discuss following items

  • What is Apache Avro?
  • What is Avro schema and how to define it?
  • Serialization in Apache Avro.

What is Apache Avro?

"Apache Avro is data serialization library" That's it, huh. This is what you will see when you open their official page.Apache Avro is:

  • Schema based data serialization library.
  • RPC framework (support).
  • Rich data structures (Primary includes null, string, number, boolean and Complex includes Record, Array, Map etc.).
  • A compact, fast and binary data format.

What is Avro schema and how to define it?

Apache Avro serialization concept is based on Schema. When you write data, schema is written along with it. When you read data, schema will always be present. The schema along with data makes it fully self describing.

Schema is representation of AVRO datum(Record). It is of two types: Primitive and Complex.

Primitive types

These are the basic type supported by Avro. It includes null, int, long, bytes, string, float and double. One quick example:

{"type": "string"}

Complex types

Apache Avro support six complex types i.e. record, enum, array, map, fixed and union.

RECORD

Record uses the name type 'record' and has following attributes.

  • name: a JSON string, providing the name of the record (required).
  • namespace: A JSON string that qualifies the name.
  • doc: A JSON string representing the documentation for the record.
  • aliases: A JSON array, providing alternate name for the record
  • fields: A JSON array, listing fields (required). It has own attributes.
    • name: A JSON string, providing the name of the field (required).
    • type: A JSON object, defining a schema or record definition (required).
    • doc: A JSON string, providing documentation for the field.
    • default: A default value for the field if the instance lack recognition of the field value.
{
  "type": "record",
  "name": "Node",
  "aliases": ["SinglyLinkedNodes"],
  "fields" : [
    {"name": "value", "type": "string"},
    {"name": "next", "type": ["null", "Node"]}
  ]
}
ENUM

Enum uses the type "enum" and support attributes i.e. name, namespace, aliases, doc and symbols (A JSON array).

{ 
  "type": "enum",
  "name": "Move",
  "symbols" : ["LEFT", "RIGHT", "UP", "DOWN"]
}
ARRAYS

Array uses the type "array" and support single attribute item.

{"type": "array", "items": "string"}
MAPS

Map uses the type "map" and support one attribute values. Its key by default are of type string.

{"type": "map", "values": "long"}
UNIONS

Unions are represented by JSON array as ["null", "string"] which means the value type could be null or string.

FIXED

Fixed uses type "fixed" and support two attributes i.e. name and size.

{"type": "fixed", "size": 16, "name": "md5"}

Serialization in Apache Avro

Apache Avro data is always serialized with its schema. It supports two types of encoding i.e. Binary and JSON . You can read more on serialization on their official specification and/ or can see the example usage here.

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