使用python检查数据库连接是否繁忙

Question

I want to create a Database class which can create cursors on demand. It must be possible to use the cursors in parallel (two or more cursor can coexist) and, since we can only have one cursor per connection, the Database class must handle multiple connections.

For performance reasons we want to reuse connections as much as possible and avoid creating a new connection every time a cursor is created: whenever a request is made the class will try to find, among the opened connections, the first non-busy connection and use it.

A connection is still busy as long as the cursor has not been consumed.

Here is an example of such class:

class Database:
    ...
    def get_cursos(self,query):
        selected_connection = None

        # Find usable connection
        for con in self.connections:
            if con.is_busy() == False: # <--- This is not PEP 249
                selected_connection = con
                break

        # If all connections are busy, create a new one
        if (selected_connection is None):
            selected_connection = self._new_connection()
            self.connections.append(selected_connection)


         # Return cursor on query
         cur = selected_connection.cursor()
         cur.execute(query)
         return cur

However looking at the PEP 249 standard I cannot find any way to check whether a connection is actually being used or not.

Some implementations such as MySQL Connector offer ways to check whether a connection has still unread content (see here ), however as far as I know those are not part of PEP 249.

Is there a way I can achieve what described before for any PEP 249 compliant python database API ?

Answer 1

Perhaps you could use the status of the cursor to tell you if a cursor is being used. Let's say you had the following cursor:

new_cursor = new_connection.cursor()
cursor.execute(new_query)

and you wanted to see if that connection was available for another cursor to use. You might be able to do something like:

if (new_cursor.rowcount == -1):
    another_new_cursor = new_connection.cursor()
    ...

Of course, all this really tells you is that the cursor hasn't executed anything yet since the last time it was closed. It could point to a cursor that is done (and therefore a connection that has been closed) or it could point to a cursor that has just been created or attached to a connection. Another option is to use a try/catch loop, something along the lines of:

try:
    another_new_cursor = new_connection.cursor()
except ConnectionError?: //not actually sure which error would go here but you get the idea.
    print("this connection is busy.")

Of course, you probably don't want to be spammed with printed messages but you can do whatever you want in that except block, sleep for 5 seconds, wait for some other variable to be passed, wait for user input, etc. If you are restricted to PEP 249, you are going to have to do a lot of things from scratch. Is there a reason you can't use external libraries?

EDIT: If you are willing to move outside of PEP 249, here is something that might work, but it may not be suitable for your purposes. If you make use of the mysql python library, you can take advantage of the is_connected method.

new_connection = mysql.connector.connect(host='myhost',
                         database='myDB',
                         user='me',
                         password='myPassword')

...stuff happens...

if (new_connection.is_connected()):
    pass
else:
    another_new_cursor = new_connection.cursor()
    ...