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[翻译完成&等待校对]: 20180705 Building a Messenger App: Schema
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[#]: collector: (lujun9972)
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[#]: translator: (PsiACE)
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[#]: reviewer: ( )
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[#]: publisher: ( )
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[#]: url: ( )
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[#]: subject: (Building a Messenger App: Schema)
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[#]: via: (https://nicolasparada.netlify.com/posts/go-messenger-schema/)
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[#]: author: (Nicolás Parada https://nicolasparada.netlify.com/)
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Building a Messenger App: Schema
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======
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New post on building a messenger app. You already know this kind of app. They allow you to have conversations with your friends. [Facebook Messenger][1], [WhatsApp][2] and [Skype][3] are a few examples. Tho, these apps allows you to send pictures, stream video, record audio, chat with large groups of people, etc… We’ll try to keep it simple and just send text messages between two users.
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We’ll use [CockroachDB][4] as the SQL database, [Go][5] as the backend language, and JavaScript to make a web app.
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In this first post, we’re getting around the database design.
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```
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CREATE TABLE users (
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id SERIAL NOT NULL PRIMARY KEY,
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username STRING NOT NULL UNIQUE,
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avatar_url STRING,
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github_id INT NOT NULL UNIQUE
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);
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```
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Of course, this app requires users. We will go with social login. I selected just [GitHub][6] so we keep a reference to the github user ID there.
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```
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CREATE TABLE conversations (
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id SERIAL NOT NULL PRIMARY KEY,
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last_message_id INT,
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INDEX (last_message_id DESC)
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);
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```
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Each conversation references the last message. Every time we insert a new message, we’ll go and update this field. (I’ll add the foreign key constraint below).
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… You can say that we can group conversations and get the last message that way, but that will add much more complexity to the queries.
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```
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CREATE TABLE participants (
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user_id INT NOT NULL REFERENCES users ON DELETE CASCADE,
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conversation_id INT NOT NULL REFERENCES conversations ON DELETE CASCADE,
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messages_read_at TIMESTAMPTZ NOT NULL DEFAULT now(),
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PRIMARY KEY (user_id, conversation_id)
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);
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```
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Even tho I said conversations will be between just two users, we’ll go with a design that allow the possibility to add multiple participants to a conversation. That’s why we have a participants table between the conversation and users.
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To know whether the user has unread messages we have the `messages_read_at` field. Every time the user read in a conversation, we update this value, so we can compare it with the conversation last message `created_at` field.
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```
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CREATE TABLE messages (
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id SERIAL NOT NULL PRIMARY KEY,
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content STRING NOT NULL,
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user_id INT NOT NULL REFERENCES users ON DELETE CASCADE,
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conversation_id INT NOT NULL REFERENCES conversations ON DELETE CASCADE,
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created_at TIMESTAMPTZ NOT NULL DEFAULT now(),
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INDEX(created_at DESC)
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);
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```
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Last but not least is the messages table, it saves a reference to the user who created it and the conversation in which it goes. Is has an index on `created_at` too to sort messages.
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```
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ALTER TABLE conversations
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ADD CONSTRAINT fk_last_message_id_ref_messages
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FOREIGN KEY (last_message_id) REFERENCES messages ON DELETE SET NULL;
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```
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And yep, the fk constraint I said.
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These four tables will do the trick. You can save those queries to a file and pipe it to the Cockroach CLI. First start a new node:
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```
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cockroach start --insecure --host 127.0.0.1
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```
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Then create the database and tables:
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```
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cockroach sql --insecure -e "CREATE DATABASE messenger"
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cat schema.sql | cockroach sql --insecure -d messenger
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```
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* * *
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That’s it. In the next part we’ll do the login. Wait for it.
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[Souce Code][7]
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--------------------------------------------------------------------------------
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via: https://nicolasparada.netlify.com/posts/go-messenger-schema/
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作者：[Nicolás Parada][a]
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选题：[lujun9972][b]
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译者：[译者ID](https://github.com/译者ID)
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校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
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[a]: https://nicolasparada.netlify.com/
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[b]: https://github.com/lujun9972
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[1]: https://www.messenger.com/
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[2]: https://www.whatsapp.com/
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[3]: https://www.skype.com/
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[4]: https://www.cockroachlabs.com/
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[5]: https://golang.org/
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[6]: https://github.com/
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[7]: https://github.com/nicolasparada/go-messenger-demo
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115
translated/tech/20180705 Building a Messenger App- Schema.md
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115
translated/tech/20180705 Building a Messenger App- Schema.md
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@ -0,0 +1,115 @@
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[#]: collector: "lujun9972"
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[#]: translator: "PsiACE"
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[#]: reviewer: " "
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[#]: publisher: " "
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[#]: url: " "
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[#]: subject: "Building a Messenger App: Schema"
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[#]: via: "https://nicolasparada.netlify.com/posts/go-messenger-schema/"
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[#]: author: "Nicolás Parada https://nicolasparada.netlify.com/"
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# 构建一个即时消息应用：模式
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这是一系列关于构建「即时消息」应用的新帖子。你应该对这类应用并不陌生。有了它们的帮助，我们才可以与朋友畅聊无忌。[Facebook Messenger][1], [WhatsApp][2] 和 [Skype][3] 就是其中的几个例子。正如你所看到的那样，这些应用允许我们发送图片、传输视频、录制音频、以及和一大帮子人聊天等等。当然，我们的教程应用将会尽量保持简单，只在两个用户之间发送文本消息。
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我们将会用 [CockroachDB][4] 作为 SQL 数据库，用 [Go][5] 作为后端语言，并且用 JavaScript 来制作 web 应用。
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这是第一篇帖子，我们将会讲述数据库的设计。
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```
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CREATE TABLE users (
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id SERIAL NOT NULL PRIMARY KEY,
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username STRING NOT NULL UNIQUE,
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avatar_url STRING,
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github_id INT NOT NULL UNIQUE
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);
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```
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显然，这个应用需要一些用户。我们这里采用社交登录的形式。由于我选用了 [GitHub][6]，所以这里需要保存一个对 GitHub 用户 ID 的引用。
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```
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CREATE TABLE conversations (
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id SERIAL NOT NULL PRIMARY KEY,
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last_message_id INT,
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INDEX (last_message_id DESC)
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);
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```
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每个对话都会引用最近一条消息。每当我们输入一条新消息时，我们都会更新这个字段。我会在后面添加外键约束。
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… 你可能会想，我们可以先对对话进行分组，然后再通过这样的方式获取最近一条消息。但这样做会使查询变得更加复杂。
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```
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CREATE TABLE participants (
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user_id INT NOT NULL REFERENCES users ON DELETE CASCADE,
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conversation_id INT NOT NULL REFERENCES conversations ON DELETE CASCADE,
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messages_read_at TIMESTAMPTZ NOT NULL DEFAULT now(),
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PRIMARY KEY (user_id, conversation_id)
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);
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```
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尽管之前我提到过对话只会在两个用户之间进行，但我们还是采用了允许向对话中添加多个参与者的设计。因此，在对话和用户之间有一个参与者表。
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为了知道用户是否有未读消息，我们在消息表中添加了「读取时间」`messages_read_at` 字段。每当用户在对话中读取消息时，我们都会更新它的值，这样一来，我们就可以将它与对话中最后一条消息的「创建时间」`created_at` 字段进行比较。
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```
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CREATE TABLE messages (
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id SERIAL NOT NULL PRIMARY KEY,
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content STRING NOT NULL,
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user_id INT NOT NULL REFERENCES users ON DELETE CASCADE,
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conversation_id INT NOT NULL REFERENCES conversations ON DELETE CASCADE,
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created_at TIMESTAMPTZ NOT NULL DEFAULT now(),
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INDEX(created_at DESC)
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);
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```
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尽管我们将消息表放在最后，但它在应用中相当重要。我们用它来保存对创建它的用户以及它所出现的对话的引用。而且还可以根据「创建时间」`created_at` 来创建索引以完成对消息的排序。
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```
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ALTER TABLE conversations
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ADD CONSTRAINT fk_last_message_id_ref_messages
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FOREIGN KEY (last_message_id) REFERENCES messages ON DELETE SET NULL;
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```
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我在前面已经提到过这个外键约束了，不是吗:D
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有这四张表就足够了。你也可以将这些查询保存到一个文件中，并将其通过管道传送到 Cockroach CLI。
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首先，我们需要启动一个新节点：
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```
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cockroach start --insecure --host 127.0.0.1
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```
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然后创建数据库和这些表：
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```
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cockroach sql --insecure -e "CREATE DATABASE messenger"
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cat schema.sql | cockroach sql --insecure -d messenger
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```
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---
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这篇帖子就到这里。在接下来的部分中，我们将会介绍「登录」，敬请期待。
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[Souce Code][7]
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---
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via: https://nicolasparada.netlify.com/posts/go-messenger-schema/
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作者：[Nicolás Parada][a]
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选题：[lujun9972][b]
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译者：[PsiACE](https://github.com/PsiACE)
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校对：[校对者 ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux 中国](https://linux.cn/) 荣誉推出
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[a]: https://nicolasparada.netlify.com/
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[b]: https://github.com/lujun9972
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[1]: https://www.messenger.com/
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[2]: https://www.whatsapp.com/
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[3]: https://www.skype.com/
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[4]: https://www.cockroachlabs.com/
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[5]: https://golang.org/
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[6]: https://github.com/
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[7]: https://github.com/nicolasparada/go-messenger-demo
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