Description

Implementation of a persistent queue in Python. I looked around and couldn't find anything that fit my needs, so I made my own. Example usage:

from persistent_queue import PersistentQueue

queue = PersistentQueue('queue')

# Add stuff
queue.push(1)
queue.push(2)
queue.push(3)
queue.push(['a', 'b', 'c'])

data = queue.peek()  # 1
data = queue.peek(4)  # [1, 2, 3, 'a']
size = len(queue)  # 6

queue.push('foobar')

data = queue.pop()  # 1

queue.delete(2)
data = queue.pop()  # 3

queue.clear()

Objects that are added to the queue must be pickle-able. A file is saved to the file system based on the name given to the queue. The same name must be given if you want the data to persist.

I created this with the following workflow in mind:

data = queue.peek(5)

success = upload_data_somewhere(data)

if success:
    queue.delete(5)
    queue.flush()  # Remove extra space

By default, pickle is used to serialize objects. This can be changed depending on your needs by setting the dumps and loads options (see Parameters). dill and msgpack have been tested (see tests as an example).

When items are popped or deleted, the data isn't actually deleted. Instead a pointer is moved to the place in the file with valid data. As a result, the file will continue to grow even if items are removed. persistent_queue.flush() reclaims this space. You must call flush as you see fit!

Parameters

A persistent queue takes the following parameters:

• filename (required): The name of the file that will keep the data.
• path (optional, default='.'): The directory to put the file.
• dumps (optional, default=pickle.dumps): The method used to convert a Python object into bytes.
• loads (optional, default=pickle.loads): The method used to convert bytes into a Python object.
• flush_limit (optional, default=1048576): When the amount of empty space in the file is greater than flush_limit, the file will be flushed. This balances file I/O and storage space.

Install

pip install python-persistent-queue