Queueing Strategies Explained #
This interactive study explores various queueing strategies used in handling HTTP requests and their implications for performance and user experience.
Key Takeaways:
- Queues are essential for smoothing request flow to servers, preventing dropped requests during high traffic.
- Different queue types offer trade-offs between minimizing request wait times, ensuring priority request processing, and optimizing overall efficiency.
- Active Queue Management (AQM) techniques like "random early detection" can help minimize dropped priority requests by selectively dropping low-priority requests before the queue is full.
Types of Queues:
- FIFO (First-In First-Out): Requests are processed in the order they are received. This is the most common queue type.
- LIFO (Last-In First-Out): The most recently added request is processed first. This can be advantageous for web requests where an outdated request is less valuable than a fresh one.
- Priority Queues: Requests are prioritized based on a designated scheme, with higher-priority requests moved to the front of the queue. This can be used to ensure critical actions like checkouts are processed rapidly.
- Priority Queues with RED: Combines priority queues with Active Queue Management (RED) to drop low-priority requests probabilistically based on queue fullness, improving the chance of processing high-priority requests even under heavy load.
"Queues help us smooth the flow of requests to the server. We can't control the rate at which requests arrive, but for the cost of some memory we can hold on to them until the server is ready." - Sam Rose
"It isn't a sure thing for priority with RED to always have the fewest dropped priority requests, it can sometimes have the same as the other queues depending on how you tapped. It will never have more dropped priority requests than the other queues, though." - Sam Rose
Queue Comparison:
- Wait Time: LIFO queues demonstrate good median wait times but potentially much higher wait times for requests at the tail end.
- Dropped Requests: Priority queues with RED consistently have the fewest dropped priority requests but the most dropped low-priority requests.
- Processed Timeouts: FIFO and LIFO queues tend to process more timed-out priority requests. This is because they're less efficient at prioritizing important requests.
Conclusion:
The choice of queue type depends on the specific application needs and acceptable trade-offs.
- For general HTTP requests, LIFO queues offer a suitable balance between minimizing wait times and ensuring timely processing of recent requests.
- Priority queues with AQM are a good choice for applications requiring the prioritization of specific requests, but they can lead to a higher rate of low-priority request drops.
Action Steps:
- Evaluate your application's specific needs: Consider the types of requests, performance expectations, acceptable wait times, and priority requirements.
- Experiment with different queueing strategies: Use simulation tools or real-world testing to compare the performance of FIFO, LIFO, priority queues, and RED.
- Implement the best queueing strategy for your application: Choose the queueing strategy that best balances your priorities and matches your application's requirements.
Further Learning:
The author recommends several resources for learning more about queueing:
- Two Weird Things
- Observing and Understanding Accept Queues in Linux
- Things We (Finally) Know About Network Queues
- SYN Packet Handling in the Wild
- TCP global synchronisation
About the Author:
Sam Rose is a software engineer with over 10 years of experience in backend development and SRE. He is passionate about creating interactive and educational content about distributed systems and event-driven applications.
Encore:
This blog is presented by Encore, a platform for building event-driven and distributed systems.