021 — 变更日志和升级
对于所有已发布的包，我们都有详细的变更日志和升级指南。
变更日志
每个发布的版本都会编写变更日志。文件名为 CHANGELOG.md。格式如下：
markdown
# My package Change Log
## 1.0.2 under development
- no changes in this release.
## 1.0.1 March 23, 2021
- Bug #42: Short description of the change (@author1, @author2)
## 1.0.0 February 02, 2021
- Initial release.其中“My package”是包的名称，1.0.1 是发布的版本，后面跟着发布日期。对于每个版本，都有多行列出变更内容。"Bug" 指的是变更类型。使用以下类型：
- New — 新功能。
- Chg — 一般变更。
- Enh — 现有功能增强。
- Bug — Bug 修复。
在变更日志文件中，行应按 New、Chg、Enh、Bug 的顺序排列。
上面的“#42”是对应变更的 issue 或 pull request 编号。“author1”是代码作者的 GitHub 昵称。“author2”是额外的作者。作者的昵称必须以 @ 为前缀。
升级
当有新的主版本与之前的版本不兼容时，会创建升级指南。它描述了升级应用程序代码所需的步骤。
文件名为 UPGRADE.md。格式如下：
markdown
# Upgrading Instructions for my package
This file contains the upgrade notes. These notes highlight changes that could break your
application when you upgrade the package from one version to another.
> **Important!** The following upgrading instructions are cumulative. That is, if you want
> to upgrade from version A to version C and there is version B between A and C, you need
> to follow the instructions for both A and B.
## Upgrade from 2.x
- Public method `test()` was removed. Use `perform()` instead.
## Upgrade from 1.x
- Clean up the cache after upgrading. Old cache is not compatible with new code.