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## What is the difference between NixOS rollback capability and btrfs/zfs system snapshot rollback?
The difference lies in the nature of the snapshots. System snapshots created with btrfs/zfs are non-reproducible, meaning they do not include the "knowledge" of how to build the snapshot from scratch and are therefore **unexplainable**.
The difference lies in the nature of the snapshots. System snapshots created with btrfs/zfs does not contain the 'knowledge' of how to build this snapshot from scratch, it is **uninterpretable**, and its content is strongly correlated with the current hardware environment, making it difficult to reproduce on other machines.
On the other hand, NixOS configuration is a piece of "knowledge" that can build an identical OS from scratch. It is **explainable** and can be automatically built with just a few simple commands. The NixOS configuration serves as documentation of all the changes made to your OS and is also used to automatically build the OS itself.
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When compared to widely used traditional tools like Ansible, Nix has the following main advantages:
1. Nix's declarative configuration shields users from underlying details, allowing them to focus on their core requirements and providing a highly convenient system customization capability. Traditional tools like Ansible require users to handle all implementation details themselves.
1. If you have experience with declarative configuration tools such as Terraform or Kubernetes, you should easily understand this point. The benefits of declarative configuration become more significant as the complexity of requirements increases.
2. Nix declares its target state through declarative configuration, and Nix Flakes locks all dependency hashes, version numbers, data sources, and other information in a version lock file called `flake.lock`. This greatly enhances the reproducibility of the system. In contrast, traditional tools like Ansible have poor reproducibility, which is why Docker is so popular—it provides a **completely consistent runtime environment** at a lower cost than traditional operational tools like Ansible.
1. One of the biggest problems with this Anisble is that each deployment is based on incremental changes to the current state of the system. The current state of the system, like the snapshots mentioned above, is not interpretable and is difficult to reproduce. NixOS declares the target state of the system through its configuration files, so that the deployment result is independent of the current state of the system, and repeated deployments will not cause any problems.
2. Nix Flakes uses a version lock file `flake.lock` to lock the hash value, version number, data source and other information of all dependencies, which greatly improves the reproducibility of the system. Traditional tools like Ansible don't have this feature, so they're not very reproducible.
1. This is why Docker is so popular - it provides, at a fraction of the cost, a **reproducible system environment on a wide range of machines** that traditional Ops tools like Ansible don't. 1.
1. Nix provides a high degree of ease of system customization by shielding the underlying implementation details with a layer of declarative abstraction so that users only need to care about their core requirements. Tools like Ansible have much weaker abstractions.
1. If you've ever used a declarative configuration tool like terraform/kubernetes, this should be easy to understand. The more complex the requirements, the greater the benefit of declarative configuration.
## What are the advantages of Nix compared to Docker container technology?

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很多使用 Arch Linux / Ubuntu 等常规 Linux 发行版的用户，习惯于使用 btrfs / zfs 等文件系统提供的快照作为「后悔药」，这样系统出了问题能回滚修复。
而本书介绍的 NixOS 也提供了系统状态回滚能力，因此很容易会有这样的疑问：这两种系统回滚功能有何不同？
这里简单解释下，主要区别在于，系统快照的内容是不可复现的，快照内容不包含如何从零构建这个快照的「知识」，是**不可解释的**。
这里简单解释下，主要区别在于，快照内容不包含如何从零构建这个快照的「知识」，是**不可解释的**，而且其内容与当前硬件环境强相关，很难在其他机器上复现。
而 NixOS 的配置是一份从零构建出一个一模一样的 OS 的「知识」，是**可解释的**，而且可以通过简单几行命令就自动完成这个构建。NixOS 配置本身既是一份记录你的 OS 都做过哪些变更的文档，也是用于自动构建出这个 OS 的配置。
而 NixOS 的配置是一份从零构建出一个一模一样的 OS 的「知识」，是**可解释的**，而且可以通过简单几行命令就自动完成这个构建。NixOS 配置本身既是一份记录你的 NixOS 系统都做过哪些变更的文档，也可直接用于自动构建出你当前的 NixOS 系统。
NixOS 的配置文件就像是程序的**源代码**，只要源代码没丢，修改程序、审查程序，或者重新构建出一个一模一样的程序都很简单。
而系统快照就像是源代码编译出来的二进制程序，要对它做修改、审查，都要难得多。而且这个快照很大，分享或者迁移它的成本都要比源代码高得多。
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Nix 与 Ansible 这类被广泛应用的传统工具比，主要优势就在：
1. Nix 的声明式配置屏蔽了底层细节，使用户只需要关心自己最核心的需求，从而带来了高度便捷的系统自定义能力。而使用 Ansible 这类传统工具，用户需要自己去处理所有的实现细节。
1. Anible 这类工具一个最大的问题就是，它每次部署都是基于系统当前状态的增量修改。而系统的当前状态就如同前面提到的系统快照，是不可解释的，也很难复现。而 NixOS 是通过配置文件声明系统的目标状态，可以做到部署结果与系统当前状态无关，重复部署也不会导致任何问题。
2. Nix Flakes 通过一个版本锁文件 `flake.lock` 锁定了所有依赖的 hash 值、版本号、数据源等信息，这极大地提升了系统的可复现能力。而传统的 Ansible 等工具没有此功能，所以它们的可复现能力很差。
1. 这也是为什么 Docker 这么受欢迎的原因——它以较低的代价提供了 Ansible 等传统运维工具提供不了的**可在各种机器上复现的系统环境**。
1. Nix 通过一层声明式的抽象屏蔽了其底层的实现细节，使用户只需要关心自己最核心的需求，从而带来了高度便捷的系统自定义能力。而 Ansible 这类工具的抽象能力要弱得多。
1. 如果你有使用过 terraform/kubernetes 等声明式配置工具，应该很容易理解这一点。需求越是复杂的情况下，声明式配置带来的好处就越大。
2. Nix 通过声明式配置声明了自己的目标状态，而 Nix Flakes 通过一个版本锁文件 `flake.lock` 锁定了所有依赖的 hash 值、版本号、数据源等信息，这极大地提升了系统的可复现能力。而传统的 Ansible 等工具的可复现能力是很差的，这也是为什么 Docker 这么受欢迎的原因——它以较低的代价提供了 Ansible 等传统运维工具提供不了的**完全一致的运行环境**。
## Nix 与 Docker 容器技术相比有何优势？