LockError using flask-zodb

Question

I have a problem using flask-zodb in a simple Flask app deployed with mod_wsgi. Every time I try to use the DB for example setting defaults:

from flaskext.zodb import zodb, List

db = ZODB(app)
app.config.from_pyfile('settings.py') # here I have defined ZODB_STORAGE = "/home/username/webapps/myapp/htdocs/Data.fs"

@app.before_request
def set_db_defaults():
   if 'entries' not in db:
       db['entries'] = List()

or in a view like:

@app.route('/add', methods=['POST'])
def add_entry():
   db['entries'].append(request.form)
   flash('New entry was successfully posted')
   return redirect(url_for('show_entries'))

I got the following error:

[Sat May 19 16:52:30 2012] [error] [client 127.0.0.1]
self._lock_file = LockFile(file_name + '.lock')
[Sat May 19 16:52:30 2012] [error] [client 127.0.0.1]   File
"/home/userame/.virtualenvs/myapp/lib/python2.7/site-packages/zc/lockfile/__init__.py",
line 76, in __init__
[Sat May 19 16:52:30 2012] [error] [client 127.0.0.1]     _lock_file(fp)
[Sat May 19 16:52:30 2012] [error] [client 127.0.0.1]   File
"/home/username/.virtualenvs/myapp/lib/python2.7/site-packages/zc/lockfile/__init__.py",
line 59, in _lock_file
[Sat May 19 16:52:30 2012] [error] [client 127.0.0.1]     raise
LockError("Couldn't lock %r" % file.name)
[Sat May 19 16:52:30 2012] [error] [client 127.0.0.1] LockError:
Couldn't lock '/home/username/webapps/myapp/htdocs/Data.fs.lock'

The application works fine in the development enviroment and I do not think there's a premission problem since I tried to set:

ZODB_STORAGE = "/tmp"

And the same exception is raised. I can't understand why this happens and how to avoid this kind of error. Any ideas?

score 5 · Accepted Answer · edited May 23 '17 at 11:44

You are trying to open the ZODB from multiple processes; presumably in development you only use one WSGI process. This is basically a dupe of zc.lockfile.LockError in ZODB.

Your options are:

Limit your production environment to only one process.
Use ZEO, see the referenced question; the zodburi equivalent is zeo://localhost:9100.
Use RelStorage. You can use the ZConfig URI scheme to connect up, provided the RelStorage egg is available. This is documented in the RelStorage documentation under the old repoze.zodbconn name.

LockError using flask-zodb

1 Answers1