priotizing a message on Google Pubsub - google-cloud-pubsub

Question

I have a pubsub topic with a number of pull subscriptions. I would like some mechanism where I can publish a message with a "priority" label that causes the message to jump as near to the front of the queue as possible.
I don't need any guaranteed ordering semantics, just a "best effort" prioritization mechanism.
Is anything like this possible with pubsub?

Answer 1

No such mechanism exists within Google Cloud Pub/Sub, no. Such a feature really only becomes relevant if your subscribers are not able to keep up with the rate of publishing and consequently, a backlog is building up. If subscribers are keeping up and processing and acking messages quickly, then the notion of "priority" messages isn't really necessary.
If a backlog is being built up and some messages needs to be processed with higher priority, then one approach is to create a "high-priority" topic and subscription. The subscribers subscribe to this subscription as well as the "normal" subscription and prioritize processing messages from the "high-priority" subscription whenever they arrive.

Answer 2

Providing an example implementation to #Kamal's answer in an attempt to provide more context to:
...prioritize processing messages from the "high-priority" subscription whenever they arrive
import logging
import threading
from google.cloud import pubsub
from google.cloud.pubsub_v1.types import FlowControl
logging.basicConfig(format="%(asctime)s %(message)s", level=logging.INFO)
c = threading.Condition()
n_priority_messages = 0
def priority_callback(message):
logging.info(f"PRIORITY received: {message.message_id}")
global n_priority_messages
c.acquire()
n_priority_messages += 1
c.release()
handle_message(message)
logging.info(f"PRIORITY handled: {message.message_id}")
c.acquire()
n_priority_messages -= 1
if n_priority_messages == 0:
c.notify_all()
c.release()
def batch_callback(message):
logging.info(f"BATCH received: {message.message_id}")
done = False
modify_count = 0
global n_priority_messages
while not done:
c.acquire()
priority_queue_is_empty = n_priority_messages == 0
c.release()
if priority_queue_is_empty:
handle_message(message)
logging.info(f"BATCH handled: {message.message_id}")
done = True
else:
message.modify_ack_deadline(15)
modify_count += 1
logging.info(
f"BATCH modifyed deadline: {message.message_id} - count: {modify_count}"
)
c.acquire()
c.wait(timeout=10)
c.release()
subscriber = pubsub.SubscriberClient()
subscriber.subscribe(
subscription=batch_subscription,
callback=batch_callback,
# adjust according to latency/throughput requirements
flow_control=FlowControl(max_messages=5)
)
pull_future = subscriber.subscribe(
subscription=priority_subscription,
callback=priority_callback,
# adjust according to latency/throughput requirements
flow_control=FlowControl(max_messages=2)
)
pull_future.result()
Example output when there is a backlog of priority and batch messages:
...
2021-07-29 10:25:00,115 PRIORITY received: 2786647736421842
2021-07-29 10:25:00,338 PRIORITY handled: 2786647736421841
2021-07-29 10:25:00,392 PRIORITY received: 2786647736421843
2021-07-29 10:25:02,899 BATCH modifyed deadline: 2786667941800415 - count: 2
2021-07-29 10:25:03,016 BATCH modifyed deadline: 2786667941800416 - count: 2
2021-07-29 10:25:03,016 BATCH modifyed deadline: 2786667941800417 - count: 2
2021-07-29 10:25:03,109 BATCH modifyed deadline: 2786667941800418 - count: 2
2021-07-29 10:25:03,109 BATCH modifyed deadline: 2786667941800419 - count: 2
2021-07-29 10:25:03,654 PRIORITY handled: 2786647736421842
2021-07-29 10:25:03,703 PRIORITY received: 2786647736421844
2021-07-29 10:25:03,906 PRIORITY handled: 2786647736421843
2021-07-29 10:25:03,948 PRIORITY received: 2786647736421845
2021-07-29 10:25:07,212 PRIORITY handled: 2786647736421844
2021-07-29 10:25:07,242 PRIORITY received: 2786647736421846
2021-07-29 10:25:07,459 PRIORITY handled: 2786647736421845
2021-07-29 10:25:07,503 PRIORITY received: 2786647736421847
2021-07-29 10:25:10,764 PRIORITY handled: 2786647736421846
2021-07-29 10:25:10,807 PRIORITY received: 2786647736421848
2021-07-29 10:25:11,004 PRIORITY handled: 2786647736421847
2021-07-29 10:25:11,061 PRIORITY received: 2786647736421849
2021-07-29 10:25:12,900 BATCH modifyed deadline: 2786667941800415 - count: 3
2021-07-29 10:25:13,016 BATCH modifyed deadline: 2786667941800416 - count: 3
2021-07-29 10:25:13,017 BATCH modifyed deadline: 2786667941800417 - count: 3
2021-07-29 10:25:13,110 BATCH modifyed deadline: 2786667941800418 - count: 3
2021-07-29 10:25:13,110 BATCH modifyed deadline: 2786667941800419 - count: 3
2021-07-29 10:25:14,392 PRIORITY handled: 2786647736421848
2021-07-29 10:25:14,437 PRIORITY received: 2786647736421850
2021-07-29 10:25:14,558 PRIORITY handled: 2786647736421849
...