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[skip ci] Add doc to describe extra information of binlog (#9649) 

Signed-off-by: dragondriver <jiquan.long@zilliz.com>
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dragondriver 2021-10-11 17:56:29 +08:00 committed by GitHub
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docs/developer_guides/chap08_binlog.md
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@ -62,6 +62,25 @@ Binlog file consists of 4 bytes magic number and a series of events. The first e
+=====================================+=====================================================================|
```
`ExtraBytes` is in json format.
`ExtraBytes` stores the extra information of the binlog file.
In binlog file, we have stored many common fields in fixed part, such as `CollectionID`, `PartitionID` and etc.
However, different binlog files have some other different information which differs from each other.
So, `ExtraBytes` was designed to store these different information.
For example, for index binlog file, we will store `indexID`, `indexBuildID`, `indexID` and other index-related
information to `ExtraBytes`.
In addition, `ExtraBytes` was also designed to extend binlog. Then we can add new features to binlog file without
breaking the compatibility.
For example, we can store the memory size of original content(before encode) to `ExtraBytes`.
The key in `ExtraBytes` is `original_size`. For now, `original_size` is required, not optional.
### 8.3 Type code
```