How does nodemon work?

I have always had this question because I wanted to know how nodemon detects changes made in a file system or a file tree and then restarts the whole node process.

How does it get the entry point program's process id? Does it use the "node" command internally?

When Googling, I came across a great article that portrays a good view of how nodemon works under the hood, or maybe how it's supposed to work. Before reading the article, I had some prejudice and was wondering how it worked. I had a rough idea that obviously it's calling the "node" binary underneath, but how does it detect changes to the file system, and how does it reload or restart the server or node process based on an event?

Well, all of those questions and more were answered once I read the previously mentioned article, and obviously my mind was blown. I knew that it was calling the node process but didn't have any idea that it was actually forking a node child_process. So basically, nodemon acts as a wrapper around the node process, and on any relevant changes to the file system, it restarts the server or process by killing the existing child process and then spawning a new child process.

Nodemon watches on specific directories and files by default and also ignores some obvious ones like node_modules, its config can also be expanded upon by providing a nodemon.json file, which nodemon uses internally. It also uses the package.json for gathering the project info or other types of relevant information.

It uses the chokidar npm library for detecting file or directory changes because apparently the inbuilt fs.watch in node is faulty as it fires unnecessary events, which are sometimes not appropriate. I also found out that chokidar is itself a wrapper around fs.watch, exposing an optimized and appropriate API for detecting event-based file system changes.

To set out on an adventure

Sneaking in a little bit of One Piece reference

In order to explore and learn more, I set out on an adventure to make a nodemon equivalent tool for Go, so that I don't have to manually restart, say a server or a process when developing. I already know that there are many available tools that could make my life easier, but I was up for a challenge big time.

For me to make a nodemon like CLI tool for Go, I needed to figure out two things really:

A watcher for file system events
How to kill and start a child process over again
For the watcher I was hopping around watcher and fsnotify. I saw that watcher already had a recursive watching capability, so I wanted to use that but I couldn't end up setting it up perhaps for the limited documentation for my use case. I ended up settling on fsnotify for its rich documentation. I also came across inotify Linux API, but I was too scared to deal with C :) (hopefully someday it won't be so).
I needed to know how I could execute a child process, coming from a node background I know it's as easy as using a child_process.spawn or child_process.fork for spinning up a new child process. Eventually, I read through the standard library documentation of Go and stumbled upon the os/exec package and I already knew that's what I wanted. I further Googled it up for more documentation and came across this gobyexample page and this zetcode page which provided me with solid insights on using this API.

The Implementation

Naming the nodemon clone

We all know that coding isn't that hard; it's naming things that takes the most time. Initially, my lazy self was thinking of naming it gomon, but was brought to enlightenment as I googled the name up and a package already existed with the same name, so I thought hard, and I was inspired by how Evan You the creator of Vue and Vite, on how he came up with the name Vite. Well, Vite stands for quick in French, and it struck me. I didn't waste a single minute and just converted the word watch to French and what I got was montre and I was like, it sounds okayish, so I rolled along with it.

The Code

Here is the repo if this article is already too long :3

Since nodemon used a nodemon.json file for extended config, I also thought that montre should have that too. Obviously, it will be called montre.json duh!

I wanted to have custom errors and wanted those errors and normal logs to have a bit of flair, so I pulled up a bash cheat sheet and found how to have colorful logs by prefixing certain sequences of characters (idk the term for these characters). \ I went with these custom errors:

var (
  ErrMainFileNotFound    error = getErr("specified main file not found")
  ErrNoMainFile          error = getErr("no main file specified in config or command")
  ErrEmptyAllowList      error = getErr("cannot have empty allow list")
  ErrReadingConfigFile   error = getErr("error reading config file")
  ErrParsingConfigFile   error = getErr("error while parsing config file")
  ErrStartChildProcess   error = getErr("error while starting program")
  ErrRestartChildProcess error = getErr("error while restarting program")
  ErrKillChildProcess    error = getErr("error while killing program")
  ErrMainFileIsDir       error = getErr("main file should'nt be a directory"
  ErrSettingPWD          error = getErr("error while setting present working directory")
  ErrWatcherSetup        error = getErr("error while setting up watcher")
  ErrWalkingFS           error = getErr("error while walking file system")
)

// getErr() is a function that is in the repo; I haven't included it here

and these color sequences for flavorful logs.

var (
    ResetColor    string = "\033[0m"
    GreenColor    string = "\033[0;32m"
    YellowColor   string = "\033[0;33m"
    BoldRedColor  string = "\033[1;31m"
    BoldCyanColor string = "\033[1;36m"
)

Here is the montre struct that I used:

type Montre struct {
    args    []string
    pwd     string
    config  Config
    child   *exec.Cmd
    watcher *fsnotify.Watcher
    blocker chan struct{}
}

I didn't really touch much on args and left it for future use cases.\
pwd represents the present working directory, which is used for walking the root directory (default behavior).\
config represents this structure:
```
type Config struct {
  MainFile   string   `json:"mainFile"`
  WatchExts  []string `json:"watchExtensions"`
  IgnoreDirs []string `json:"ignoreDirs"`
}
```
where all these fields are config fields that can be provided in montre.json config file and read by montre. Montre performs a file based watch rather that folder based watch (I was initially thinking of this approach), which enables it to have file based filtering of watch items.
child holds reference to the child process that montre keeps restarting on change.
watcher holds reference to fsnotify's watcher instance, and
blocker as the name suggests, it's for blocking the main context of the executing program. We need this because things such as accepting input from stdin for restarting or registering listening events are all goroutines as the main context can't be blocked, which is where Go's concurrency super powers kick in.

The StartWatching function starts the go routines for registering listening events, accepting input from the stdin and walking through all the files according to a given set of config or an extended set of config.

func (m *Montre) StartWatching() {
    w, err := fsnotify.NewWatcher()
    if err != nil {
        log.Fatalln(ErrWatcherSetup)
    }
    m.watcher = w

    defer m.watcher.Close()

    go m.initListenEvents()
    go m.acceptCommands()

    walkErr := filepath.WalkDir(m.pwd, func(path string, file fs.DirEntry, err error) error {
        if err != nil {
            return nil
        }

        if file.IsDir() && slices.Contains(m.config.IgnoreDirs, file.Name()) {
            return filepath.SkipDir
        }

        if !slices.Contains(m.config.WatchExts, filepath.Ext(path)) {
            return nil
        }

        addErr := m.watcher.Add(path)
        if addErr != nil {
            return nil
        }

        return nil
    })

    if walkErr != nil {
        log.Fatalln(ErrWalkingFS)
    }

    fmt.Println(MONTRE_LOG + YellowLog("watching extension(s): "+strings.Join(m.config.WatchExts, ",")))
    fmt.Println(MONTRE_LOG + YellowLog("ignoring folder(s): "+strings.Join(m.config.IgnoreDirs, ", ")))
    fmt.Println(MONTRE_LOG + YellowLog("to restart watcher enter ") + GreenLog("`rs`"))
    fmt.Println(MONTRE_LOG + YellowLog("to quit watching enter ") + RedLog("`q`"))
    fmt.Println(MONTRE_LOG + GreenLog("starting `go run main.go`"))

    m.reload()
    <-m.blocker
}

and here is how montre reloads or restarts the child process. Here m.child.Process.Kill() is where I was facing the dreadful nil pointer dereference issue as the child process didn't exist due to some issues regarding clean exit of the process, as I was calling the m.child.Wait() on it, which was apparently throwing an error as the process was being killed while being awaited upon to release resources and exit on its own.

I was also at first trying to use the m.child.StdoutPipe() method for grabbing the stdout of the process and then using the os.ReadAll() method for reading and then calling os.Stdout.Write(), but I don't know why I was facing issues with it, so I switched it to the version of the code below where the main process's stdout and stderr are the same for the child process stdout and stderr, kind of like piping the stdio of the child to the parent.

func (m *Montre) quitChildProcess(errStr error) {
    if m.child != nil {
        err := m.child.Process.Kill()
        if err != nil {
            log.Fatalln(RedLog(errStr.Error() + " " + err.Error()))
        }
    }
}

func (m *Montre) reload() {
    m.quitChildProcess(ErrRestartChildProcess)

    cmd := exec.Command("go", "run", m.config.MainFile)
    m.child = cmd

    cmd.Stdout = os.Stdout
    cmd.Stderr = os.Stderr

    if err := m.child.Start(); err != nil {
        log.Fatalln(ErrStartChildProcess)
    }

    fmt.Println(MONTRE_LOG + GreenLog("waiting for further changes"))
}

The listening events are setup in the following manner, where we are listening to events of any kind and then reloading the server. On error, we are logging the error to the console. The events case could be further expaned upon by for example filtering the process restart on specific events or some other functionality.

func (m *Montre) initListenEvents() {
    for {
        select {
        case err, ok := <-m.watcher.Errors:
            if !ok {
                log.Fatalln(RedLog(err.Error()))
                return
            }
        case _, ok := <-m.watcher.Events:
            if !ok {
                return
            }
            m.reload()
        }
    }
}

I know that I briefly mentioned the implementation details, so do follow my repo if interested.

Conclusion

I learned a lot from building this small CLI tool, and since I am new to Go, I was stuck for hours trying to debug goroutine related issues and nil pointer dereferences. I would really like to hear some feedback on this article, the code or any ways that the code could be improved upon, as this is my first time writing a Go article.

#Go #Nodemon #Montre #Goroutine #Concurrency

Nodemon clone for Go (Montre)

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