What is a Queue? Explain FIFO.

Queue and FIFO

javascript

1
FIFO ensures elements are processed in arrival order.
2
Enqueue adds an element at the rear.
3
Dequeue removes an element from the front.
4
A properly implemented queue supports enqueue and dequeue in O(1).
5
Queues are common in scheduling, messaging, buffering, and breadth-first search.

Difficulty: 4/10

Topics: FIFO, Producer-Consumer, Buffer Management

0-2 years experience

1
We are building a simple customer support ticketing system where tickets must be processed strictly in the order they are received. How would you structure the data to ensure the oldest ticket is always handled next, and what happens to the performance as the number of open tickets grows to thousands?
2
Imagine you're implementing an undo/redo feature, but your teammate suggested using a Queue. Why might a Queue be the wrong choice here, and what scenario in our application would actually require a Queue instead?

2-5 years experience

1
We have a multi-threaded background worker processing image uploads. We used a standard array-based queue to pass tasks to workers, but we're seeing race conditions where two workers occasionally grab the same image, or some images get skipped. How would you debug and resolve this?
2
In our notification service, we use an in-memory queue to send emails. If the email provider goes down for 10 minutes, our queue overflows and crashes the server. How would you redesign this queue mechanism to handle temporary downstream failures without losing user notifications?

5-8 years experience

1
We are designing a distributed order processing system where payment confirmation must happen before shipping. How do you guarantee strict FIFO ordering across multiple parallel consumer instances, especially when network retries and consumer crashes occur?
2
Our message queue is experiencing severe lag during peak hours because some heavy tasks block the queue, delaying quick, lightweight tasks. How would you design a priority or multi-queue system to prevent this head-of-line blocking?

8+ years experience

1
Our legacy monolithic system relies on synchronous HTTP calls between services, causing cascading failures. We want to migrate to an asynchronous, event-driven architecture. How would you evaluate the tradeoffs between a traditional message broker like RabbitMQ versus a log-based streaming platform like Kafka for our core transactional workflows?
2
We have multiple engineering teams consuming events from a central queue. One team's slow consumer is causing the queue to back up, impacting other teams' real-time features. How would you architect the event distribution system and establish governance to isolate workloads and prevent noisy neighbor issues?

❓What are the trade-offs of using a linked list versus a dynamic array to back your queue implementation?
❓How does a priority queue differ from a standard FIFO queue in terms of underlying data structures and time complexity?
❓How would you handle queue serialization if the payload size varies significantly?