Avro使用JSON定义数据类型及通信协议，使用压缩二进制来序列化数据。在 Hadoop 的其他项目中，例如 HBase 和 Hive 的 Client 端与服务端的数据传输也采用了这个工具。Avro的schema定义示例如下：
{"namespace": "com.abc.avro",
"type": "record",
"name": "User",
"fields": [
{"name": "name", "type": "string"},
{"name": "favorite_number", "type": ["int", "null"]},
{"name": "favorite_color", "type": ["string", "null"]}
]
}
它的优点有：
缺点：
Schema 通过 JSON 对象表示。Schema 定义了简单数据类型(primitive types)和复杂数据类型(complex types)，其中复杂数据类型包含不同属性。通过各种数据类型用户可以自定义丰富的数据结构。
简单数据类型有：
| 类型 | 说明 |
|---|---|
| null | no value |
| boolean | a binary value |
| int | 32-bit signed integer |
| long | 64-bit signed integer |
| float | single precision (32-bit) IEEE 754 floating-point number |
| double | double precision (64-bit) IEEE 754 floating-point number |
| bytes | sequence of 8-bit unsigned bytes |
| string | unicode character sequence |
复杂数据类型：Avro定义了六种复杂数据类型，每一种复杂数据类型都具有独特的属性。六种复杂数据类型如下：
| Type | encoding |
|---|---|
| Records | encoded just the concatenation of the encodings of its fields |
| Enums | a int representing the zero-based position of the symbol in the schema |
| Arrays | encoded as series of blocks. A block with count 0 indicates the end of the array. block:{long,items} |
| Maps | encoded as series of blocks. A block with count 0 indicates the end of the map. block:{long,key/value pairs}. |
| Unions | encoded by first writing a long value indicating the zero-based position within the union of the schema of its value. The value is then encoded per the indicated schema within the union. |
| fixed | encoded using number of bytes declared in the schema |
Record类型字段说明如下：
name(必填)：record的名字
namespace：名称空间(可选)，相当于java中的包名
doc：这个类别的文件说明(可选)
aliases：record类别的别名(可选)
fields(必填)：每个字段需要以下属性：
例如，下面是employee.avsc的Record定义：
{
"type": "record",
"namespace": "com.aaa",
"name": "Employee",
"doc": "Employee avro schema "
"fields": [
{ "name": "id", "type": "string", "doc":"employee's name"},
{ "name": "first_name", "type": "string", "default": "", "doc":"employee first name"},
{ "name": "last_name", "type": "string", "default":""},
{ "name": "age", "type": "int"},
]
}
Logical Type在简单数据类型或复杂数据类型后面增加一个额外的字段，用于补充字段的属性
例如：
{
"type": "int",
"logicalType": "date"
}
参考： https://avro.apache.org/docs/current/spec.html#Logical+Types