> For the complete documentation index, see [llms.txt](https://docs.prestd.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.prestd.com/plugins/middleware-plugin.md).

# Middleware Plugin

With prestd's middleware plugin system it is possible to create new middlewares to process before reaching the http handler (endpoint).

### Naming patterns

The prestd configuration file receives two parameters:

* **File name:** name of the `.so` file to be loaded into the library path (`file` in prestd config file), the root directory of middleware is the direct libraries (by default is `./lib`) **+** `/middlewares` folder
* **Function name:** function name which will be loaded (`{func}MiddlewareLoad`, is actually the prefix of the function name)

#### Filename

The file name will be used in the middleware to identify which library will be loaded **when the server (prestd) loads**.

> After the **server loads, the library is not loaded again**, it is just executed, i.e. if the file (`.so`) is changed after the server loads, the changes are not applied.

#### Function name

When talking about a compiled *library* we have no way of identifying its functions. Given this characteristic we have defined some name and behavior patterns to develop *libraries* for ***prestd***.

**function name:** `{Function Name}MiddlewareLoad`

* `{Function Name}`: The name of the function that will be called
* `MiddlewareLoad`: The suffix of the function name - the function returns `negroni.Handler` (an interface; use `negroni.HandlerFunc` as adapter)

> `fmt.Sprintf("%sMiddlewareLoad", funcName)`

### Example

* **Source code name:** `./lib/src/middlewares/hello.go`
* **Library file name:** `./lib/middlewares/hello.so`
* **Function name:** `HelloMiddlewareLoad`

```go
// nolint
// all plugins must have their package name as `main`
// each plugin is isolated at compile time
package main

import (
	"net/http"

	"github.com/urfave/negroni/v3"
)

// BUILD:
// go build -mod=vendor -trimpath -ldflags "-s -w" \
//   -o ./lib/middlewares/hello.so \
//   -buildmode=plugin ./lib/src/middlewares/hello.go
func HelloMiddlewareLoad() negroni.Handler {
	return negroni.HandlerFunc(func(rw http.ResponseWriter, rq *http.Request, next http.HandlerFunc) {
		rw.Header().Add("X-Hello-Middleware", "Hello Middleware")
		next(rw, rq)
	})
}
```