當我通過Gunicorn和Flask連接到Cassandra時，它爲什麼會懸掛？

Question

我有一個連接到Cassandra的Flask應用程序。當我在Gunicorn下運行這個應用程序並將Gunicorn作爲高腳本命令python manage.py gunicorn時，它會掛起。但是當我在命令行上運行這個相同的應用程序gunicorn manage:app，它會成功。爲什麼？

Answer 1

說明

Gunicorn分叉工人處理傳入的請求。 如果Cassandra會話（連接池）在之前創建工作者分支（例如，在使用應用程序工廠模式創建應用程序期間），則工作人員在使用Cassandra時會遇到問題。 DataStax建議每個工作人員獲得自己的會話，因此您需要推遲會話創建，直到之後的。 當您將Gunicorn和Flask作爲自定義應用程序捆綁在一起時，這是一個問題，但可能在命令行上，Gunicorn可以在創建Flask應用程序之前明顯初始化並分叉。

例

要看到兩種行爲，手動改變不良=假壞=真。

from cassandra.cluster import Cluster 
from flask import Flask 
from flask_script import Command, Manager 
from gunicorn.app.base import BaseApplication 
from gunicorn.six import iteritems 


class CassandraClient: 
    def __init__(self, bad=False): 
     self.session = None 
     self.cluster = None 
     if bad: 
      self.connect() 

    def connect(self): 
     self.cluster = Cluster(['127.0.0.1']) 
     self.session = self.cluster.connect('keyspace') 

    def execute(self): 
     if not self.session: 
      self.connect() 
     query = ''' 
      select now() 
      from system.local 
     ''' 
     return self.session.execute(query)[0] 


class GunicornApp(BaseApplication): 
    ''' 
    Bundle Gunicorn and Flask together, so that we can call it as a 
    flask-script command. 

    http://docs.gunicorn.org/en/stable/custom.html 
    ''' 

    def __init__(self, app, options=None): 
     self.options = options or {} 
     self.application = app 
     super(GunicornApp, self).__init__() 

    def load_config(self): 
     config = dict(
      [(key, value) for key, value in iteritems(self.options) 
      if key in self.cfg.settings and value is not None]) 
     for key, value in iteritems(config): 
      self.cfg.set(key.lower(), value) 

    def load(self): 
     return self.application 


class GunicornCommand(Command): 
    ''' 
    Modeled off of flask_script.Server 
    ''' 
    def __init__(self, app, options): 
     self.app = app 
     self.options = options 

    def __call__(self, *args, **kwargs): 
     GunicornApp(self.app, self.options).run() 


app = Flask(__name__) 
app.cassandra = CassandraClient() 
@app.route('/') 
def hello(): 
    return str(app.cassandra.execute()) 


if __name__ == '__main__': 
    manager = Manager(app) 
    gunicorn_options = { 
     'bind': '127.0.0.1', 
     'port': 8000, 
     'workers': 4 
    } 
    manager.add_command("gunicorn", GunicornCommand(app, gunicorn_options)) 
    manager.run() 

版本

Flask==0.12.1 
Flask-Script==2.0.5 
gunicorn==19.7.1 
cassandra-driver==3.8.1 

參考

• http://docs.gunicorn.org/en/stable/custom.html
• https://datastax.github.io/python-driver/faq.html
• How to use Flask-Script and Gunicorn
• https://groups.google.com/a/lists.datastax.com/forum/#!topic/python-driver-user/XuSjjWVnE9Y