Add documentation for backend development (#17512)

* Add documentation for backend development

* Update backend guidline

* More sections

* Add modules/setting and modules/git

* Uniform gitea as Gitea

* some improvements

* some improvements

docs/content/doc/developers.zh-cn.md

@@ -0,0 +1,13 @@
+---
+date: "2016-12-01T16:00:00+02:00"
+title: "开发者"
+slug: "developers"
+weight: 40
+toc: false
+draft: false
+menu:
+  sidebar:
+    name: "开发者"
+    weight: 50
+    identifier: "developers"
+---

docs/content/doc/developers/guidelines-backend.md

@@ -0,0 +1,115 @@
+---
+date: "2021-11-01T23:41:00+08:00"
+title: "Guidelines for Backend Development"
+slug: "guidelines-backend"
+weight: 20
+toc: false
+draft: false
+menu:
+  sidebar:
+    parent: "developers"
+    name: "Guidelines for Backend"
+    weight: 20
+    identifier: "guidelines-backend"
+---
+
+# Guidelines for Backend Development
+
+**Table of Contents**
+
+{{< toc >}}
+
+## Background
+
+Gitea uses Golang as the backend programming language. It uses many third-party packages and also write some itself. 
+For example, Gitea uses [Chi](https://github.com/go-chi/chi) as basic web framework. [Xorm](https://xorm.io) is an ORM framework that is used to interact with the database. 
+So it's very important to manage these packages. Please take the below guidelines before you start to write backend code.
+
+## Package Design Guideline
+
+### Packages List
+
+To maintain understandable code and avoid circular dependencies it is important to have a good code structure. The Gitea backend is divided into the following parts:
+
+- `build`: Scripts to help build Gitea.
+- `cmd`: All Gitea actual sub commands includes web, doctor, serv, hooks, admin and etc. `web` will start the web service. `serv` and `hooks` will be invoked by git or openSSH. Other sub commands could help to mantain Gitea.
+- `integrations`: Integration tests
+- `models`: Contains the data structures used by xorm to construct database tables. It also contains functions to query and update the database. Dependencies to other Gitea code should be avoided. You can make exceptions in cases such as logging.
+  - `models/db`: Basic database operations. All other `models/xxx` packages should depend on this package. The `GetEngine` function should only be invoked from `models/`.
+  - `models/fixtures`: Sample data used in unit tests and integration tests. One `yml` file means one table which will be loaded into database when beginning the tests.
+  - `models/migrations`: Stores database migrations between versions. PRs that change a database structure **MUST** also have a migration step.
+- `modules`: Different modules to handle specific functionality in Gitea. Work in Progress: Some of them should be moved to `services`.
+  - `modules/setting`: Store all system configurations read from ini files and has been referenced by everywhere. But they should be used as function parameters when possible.
+  - `modules/git`: Package to interactive with `Git` command line or Gogit package.
+- `public`: Compiled frontend files (javascript, images, css, etc.)
+- `routers`: Handling of server requests. As it uses other Gitea packages to serve the request, other packages (models, modules or services) shall not depend on routers.
+  - `routers/api` Conatins routers for `/api/v1` aims to handle RESTful API requests. 
+  - `routers/install` Could only reponse when system is INSTALL mode. 
+  - `routers/private` will only be invoked by internal sub commands, especially `serv` and `hooks`. 
+  - `routers/web` will handle HTTP requests from web browsers or Git SMART HTTP protocols.
+- `services`: Support functions for common routing operations or command executions. Uses `models` and `modules` to handle the requests.
+- `templates`: Golang templates for generating the html output.
+
+### Package Dependencies
+
+Since Golang don't support import cycles, we have to decide the package dependencies carefully. There are some levels between those packages. Below is the ideal package dependencies direction.
+
+`cmd` -> `routers` -> `services` -> `models` -> `modules`
+
+From left to right, left packages could depend on right packages, but right packages MUST not depend on left packages. The sub packages on the same level could depend on according this level's rules.
+
+**NOTICE**
+
+Why do we need database transactions outside of `models`? And how?
+Some actions should allow for rollback when database record insertion/update/deletion failed. 
+So services must be allowed to create a database transaction. Here is some example,
+
+```go
+// servcies/repository/repo.go
+func CreateXXXX() error {\
+  ctx, committer, err := db.TxContext()
+	if err != nil {
+		return err
+	}
+	defer committer.Close()
+
+  // do something, if return err, it will rollback automatically when `committer.Close()` is invoked.
+  if err := issues.UpdateIssue(ctx, repoID); err != nil {
+      // ...
+  }
+
+  // ......
+
+  return committer.Commit()
+}
+```
+
+You should **not** use `db.GetEngine(ctx)` in `services` directly, but just write a function under `models/`. 
+If the function will be used in the transaction, just let `context.Context` as the function's first parameter.
+
+```go
+// models/issues/issue.go
+func UpdateIssue(ctx context.Context, repoID int64) error {
+    e := db.GetEngine(ctx)
+
+    // ......
+}
+```
+
+### Package Name
+
+For the top level package, use a plural as package name, i.e. `services`, `models`, for sub packages, use singular,
+i.e. `servcies/user`, `models/repository`.
+
+### Import Alias
+
+Since there are many package levels and sub packages, so you will find `modules/user`, `models/user`, `services/user`. When these packages are import into one Go file, it's difficult to know which package we are using and if it's a variable name or an import name. So we recommand to always use import alias. To differ from package variables which are commonly use camelCase, just use **snake_case** as import package alias.
+i.e. `import user_service "code.Gitea.io/Gitea/services/user"`
+
+### Future Tasks
+
+Currently, we are creating some refactors to do the following things:
+
+- Correct that codes which doesn't follow the rules.
+- There are too many files in `models`, so we are moving some of them into a sub package `models/xxx`.
+- Some `modules` sub packages should be moved to `services` because they depends on `models`.