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# 6.1.1 Secret概述

## 引入背景

当容器中的应用需要访问容器以外的服务（比如数据库、代码仓库和`kube-apiserver`等）时，那么不可避免地需要使用相应服务的帐号、密码等凭据（取决于具体服务的要求）。所以如何为容器提供凭据、以及容器中的应用如何使用这些凭据就成了不得不思考的问题。

在没有`Secret`之前，为容器提供凭据主要有如下几种方式：

* 将凭据硬编码；
* 将凭据保存到文件，并最终放置到镜像；
* 将凭据保存到文件中，并通过volume的形式挂载到容器；

首先，将凭据硬编码的方式当然不可取，每次凭据变更都需要重新生成二进制文件。其次将私密的凭据放入镜像， 会在镜像分发过程中泄露凭据，同时凭据变更也需要重新制做镜像，变更周期过长。最后，将凭据使用volume的形式挂载到容器这种方式，需要保证volume在多个集群节点共享（或每个节点拥有一个拷贝副本），难以保证凭据的私密性，同时在集群扩容时还需要保证新增节点上能够访问到凭据，维护成本比较大。

针对以上问题，`Kubernetes`设计了一个新的名为`Secrets`的资源类型，专门用来保存私密的凭据，它以键值方式存储凭据，并支持在 Pod 资源中通过环境变量或volume引用。

## 延伸阅读

* 《Secrets设计提案》<https://github.com/kubernetes/community/blob/1c3dcb393d96618ba59bde57cee4e3b212e42c33/contributors/design-proposals/auth/secrets.md>
* 《Secrets官方文档》<https://kubernetes.io/docs/concepts/configuration/secret/>


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