QueueLink

The QueueLink library simplifies several queue patterns including linking queues together with one-to-many or many-to-one relationships. "Adapters" support reading and writing text-based files.

Documentation: https://queuelink.readthedocs.io/en/latest/

Use

A QueueLink is a one-way process that connects queues together. When two or more queues are linked, a sub-process is started to read from the "source" queue and write into the "destination" queue.

Circular references are not allowed.

Users create each queue from the Queue or Multiprocessing libraries. Those queues can then be added to a QueueLink instance as either the source or destination.

With standard queues

from queue import Queue
from queuelink import QueueLink

# Source and destination queues
source_q = Queue()
dest_q = Queue()

# Create the QueueLink
queue_link = QueueLink(name="my link")

# Connect queues to the QueueLink
source_id = queue_link.read(q=source_q)
dest_id = queue_link.write(q=dest_q)

# Text to send
text_in = "a😂" * 10

# Add text to the source queue
source_q.put(text_in)

# Retrieve the text from the destination queue!
text_out = dest_q.get()
print(text_out)

With a process manager

from multiprocessing import Manager
from queuelink import QueueLink

# Create the multiprocessing.Manager
manager = Manager()

# Source and destination queues
source_q = manager.JoinableQueue()
dest_q = manager.JoinableQueue()

# Create the QueueLink
queue_link = QueueLink(name="my link")

# Connect queues to the QueueLink
source_id = queue_link.read(q=source_q)
dest_id = queue_link.write(q=dest_q)

# Text to send
text_in = "a😂" * 10

# Add text to the source queue
source_q.put(text_in)

# Retrieve the text from the destination queue!
text_out = dest_q.get()
print(text_out)

Methods

Primary methods

These methods are used most common use cases.

• register_queue(q: UNION_SUPPORTED_QUEUES, direction: str, start_method: str=None) -> client id: str
• stop

Secondary methods

These methods are less common.

• destructive_audit(direction: str)
• get_queue(queue_id: [str, int])
• is_alive
• is_drained
• is_empty(queue_id:str =None)
• unregister_queue(queue_id: str, direction: str, start_method: str=None)

Queue Compatibility

QueueLink is tested against multiple native Queue implementations. When a source or destination queue is thread-based, the link will be created as a Thread instance. When all involved queues are process-based, the link will also be a Process instance.

Note that in thread-based situations throughput might be limited by the Python GIL.

Two thread-based queues in different processes cannot be bridged directly. They would require an intermediate multiprocessing queue that can be accessed across processes.

Tested against the following queue implementations:

• SyncManager.Queue (multiprocessing.Manager)
• SyncManager.JoinableQueue (multiprocessing.Manager)
• multiprocessing.Queue
• multiprocessing.JoinableQueue
• multiprocessing.SimpleQueue
• queue.Queue
• queue.LifoQueue
• queue.PriorityQueue
• queue.SimpleQueue

Implementation

QueueLink creates a new thread or process for each source queue, regardless of the number of downstream queues. The linking thread/process gets each element of the source queue and iterates over and puts to the set of destination queues.

Multiprocessing

Start Method: QueueLink is tested against fork, forkserver, and spawn start methods. It defaults to the system preference, but can be overridden by passing the preferred start method name to the class "start_method" parameter.

Linking with other channels

QueueLink includes two "adapters" to link queues with inbound and outbound connections.

Inbound Connections

To quickly link a pipe or handle with a queue, use QueueHandleAdapterReader. The Reader Adapter is tested against Multiprocessing Connections and Subprocess pipes. It calls flush and readline to consume from handles, so it should work against any object implementing those methods, with readline returning a string or byte array. For Multiprocessing Connections, the adapter injects a no-op flush method and a custom readline method.

# Text to send
text_in = "a😂" * 10

# Destination queue
dest_q = multiprocessing.Queue()  # Process-based

# Subprocess, simple example sending some text to stdout
# from subprocess import Popen, PIPE
proc = Popen(['echo', '-n', text_in],  # -n prevents echo from adding a newline character
             stdout=PIPE,
             universal_newlines=True,
             close_fds=True)

# Connect the reader
# from queuelink import QueueHandleAdapterReader
read_adapter = QueueHandleAdapterReader(queue=dest_q,
                                        handle=proc.stdout)

# Get the text from the queue
text_out = dest_q.get()
print(text_out)

Other Notes

Tuning link_timeout

Under heavily loaded conditions the "publisher" process/thread can thrash when trying to retrieve records from the source queue. Tuning link_timeout higher (default 0.1 seconds) can improve responsiveness. Higher values might be less responsive to stop requests and throw warnings during shutdown.