binlog
A binary log file begins with a 4-byte magic number followed by an initial descriptor event that identifies the format of the file.

• In v1 and v3, this event is called a "start event."
• In v4, it is called a "format description event."

binlog文件会使用一个４字节的魔术数字去标识该文件
在v4版本中．他叫做format description event

以下事件图包含使用以下约定编写的字段说明：
字段行具有描述字段内容的名称。
名称后跟两个数字，以 offset：ｌｅｎｇｔｈ 格式，其中offset是事件中字段的基于0的偏移量（位置），length是字段的长度。两个值都以字节表示。

 v4 format description event (size ≥ 91 bytes; the size is 76 + the number of event types):

+=====================================+
| event  | timestamp         0 : 4    |
| header +----------------------------+
|        | type_code         4 : 1    | = FORMAT_DESCRIPTION_EVENT = 15
|        +----------------------------+
|        | server_id         5 : 4    |
|        +----------------------------+
|        | event_length      9 : 4    | >= 91
|        +----------------------------+
|        | next_position    13 : 4    |
|        +----------------------------+
|        | flags            17 : 2    |
+=====================================+
| event  | binlog_version   19 : 2    | = 4
| data   +----------------------------+
|        | server_version   21 : 50   |
|        +----------------------------+
|        | create_timestamp 71 : 4    |
|        +----------------------------+
|        | header_length    75 : 1    |
|        +----------------------------+
|        | post-header      76 : n    | = array of n bytes, one byte per event
|        | lengths for all            |   type that the server knows about
|        | event types                |
+=====================================+