flask application with watchdog observer

Question

I am seeking for an example for flask-based web application with watchdog observer. More specifically, I want to use the watchdog observer to detect any changes in pre-defined directories and update a web application based on the changes. I can find many examples for each of them, i.e. flask-based web applications and watchdog observer examples.

But, I don't know how to integrate two examples and run them smoothly. Can anyone provide a simple example?

Also, I wonder if I can run the watchdog observer with Celery worker?

Thanks

EDIT: I used a celery worker to run the watchdog observer to watch a directory and its subdirectories as follows:

@celery.task(bind=True)
def _watcher(self):
   observer = Observer()
   handler = MyHandler()
   observer.schedule(handler, '.')
   observer.start()
   try:
      while True:
         if not handler.event_q.empty():
            event, ts = handler.event_q.get()
            self.update_state(state='PROGRESS', meta={'src_path': event.src_path, 'event_type': event.event_type})
            time.sleep(1)
  except KeyboardInterrupt:
      observer.stop()
  observer.join()
  return {'src_path': 'srcpath', 'event_type': 'eventtype'}

Then, from the front-end side, every 1 second, it called GET function to update any changes, if there are. This is a bit hacky.

What I eventually want to achieve is that 1) keep watching a directory and its subdirectories, 2) if there are any changes, update a database according to the changes and 3) update front-end side based on the changes.

So far, I could update a database based the changes in the filesystem using watchdog (MyHandler class in the above code). But, I am still seeking for a better solution to observe the changes within a flask framework and to update the changes in the front-end side.

score 11 · Answer 1 · answered May 30 '18 at 13:25

I ended up using multithreading.

from flask import Flask, render_template, jsonify
from watchdog.observers import Observer
from watchdog.events import FileSystemEventHandler

from queue import Queue
import time
import threading

class MyHandler(FileSystemEventHandler):
    def __init__(self, pattern=None):
        self.pattern = pattern or (".xml", ".tiff", ".jpg")
        self.event_q = Queue()
        self.dummyThread = None

    def on_any_event(self, event):
        if not event.is_directory and event.src_path.endswith(self.pattern):
        self.event_q.put((event, time.time()))

    def start(self):
        self.dummyThread = threading.Thread(target=self._process)
        self.dummyThread.daemon = True
        self.dummyThread.start()

    def _process(self):
        while True:
            time.sleep(1)

app = Flask(__name__)
handler = MyHandler()
handler.start()

eventlist_flag = 0
eventlist = []

def run_watcher():
    global eventlist_flag, eventlist

    observer = Observer()
    observer.schedule(handler, '.')
    observer.start()
    try:
        while True:
            if eventlist_flag == 0:
                eventlist_flag = 1
                while not handler.event_q.empty():
                    event, ts = handler.event_q.get()
                    eventlist.append(event)
                eventlist_flag = 0
            time.sleep(1)
    except KeyboardInterrupt:
        observer.stop()
    observer.join()

@app.route('/watcher/status', methods=['POST'])
def watchernow():
    global eventlist_flag, eventlist
    if eventlist_flag == 0 and len(eventlist) > 0:
        eventlist_flag = 2
        for e in eventlist:
            print(e)
            eventlist = []
    eventlist_flag = 0
    return jsonify({})

@app.route('/')
def index():
    return render_template('index.html')

if __name__ == '__main__':
    watcher_thread = threading.Thread(target=run_watcher)
    watcher_thread.start()
    app.run(debug=True)
    watcher_thread.join()

2022 here, this code does not seem to pick up any watchdog events... :( — svenema, Apr 15 '22 at 09:46

score 6 · Answer 2 · answered May 30 '18 at 00:32

6

Flask (Werkzeug) already integrate watchdog for development server when enable debug mode. The only thing you need to do is install watchdog:

$ pip install watchdog

More info:

answered May 30 '18 at 00:32

Grey Li

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i hope i learned about this earlier before i'd have had implemented something alike by hand, thanks that's helpful. – Abr001am May 30 '18 at 00:50
Thank you for the reply. I mean that I want to watch filesystem in production server such that if there are any changes in the filesystem (i.e. a specific directory and its subdirectories), update a database according to the changes, and update front-end using the changes. – Sungsoo Ha May 30 '18 at 11:42

flask application with watchdog observer

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