Multiple Queues Vs Prioritized Queues For SaaS Background Workers was a dense discussion of queues, prioritization, trade offs, and outcomes.

This post is a much less dense discussion of the same topic with examples from airports.  Airports use a multiple queue system at Security, and a priority queue at Boarding.

Security Has Multiple Queues

Most airports in the US have 2 or 3 different queues to get through the security checkpoint: Clear, TSA Pre, and regular.  Agents help filter passengers into the different lines.  Each line represents different priorities and has a different number of agents conducting security screenings.  Once in a line, it operates as a FIFO (First in, first out) Queue.  There’s no additional sorting.

This is a human driven Multiple Queue system, and it makes sense:

1. The workload is highly variable.  There are peak times and slow times.  Times that favor high priority people, and times that favor regular people.  It is impractical to constantly shuffle the security checkpoint layout, so the system must accommodate all workloads.
2. You need to prevent resource starvation.  Ie – you need to keep the regular line moving no matter how many people show up at TSA Pre 
3. You want to minimize worker waste.  Ie – when the TSA Pre line is empty, the agent starts screening people from the regular security line.

Security checkpoints are slow and frustrating.  They are also well balanced to provide a simple, understandable, system that supports multiple priorities and minimizes agent idle time.

Boarding Gates Are Priority Queues

Boarding gates, where passengers wait to get on the airplane, are Priority Queues.  

The gates operate under different constraints from the security checkpoint:

1. Nearly all passengers are at the gate when boarding begins
2. There are a set number of passengers
3. All of the high priority passengers should board before any of the regular priority passengers board.  Unlike the security checkpoint, resource starvation is desirable.
4. The resources cannot be scaled.  There’s one plane, one door, and one person through at a time.

The queues take multiple forms.  They can be simple, like United’s

Or complex, like Southwest’s

The Priority Queues have a common structure.  They have self sorting guided by signs and instructions.  The ticket agent acts as a final filter, either accepting or rejecting people.  The ticket agent (the worker) always runs at full capacity, while the queue itself is extremely inefficient and keeps people waiting a long time.

Since the plane only has one entrance, a Priority Queue is the only way to ensure that the high priority passengers get on first.

Reminder – We’re Really Talking About Scaling

Airports are designed to scale.  They use Multiple Queues at the security checkpoint, because it fits the problem.  They use Prioritized Queues at the boarding gate because it fits the problem.

How should your Background Worker system be designed?

These are the considerations:

1. Resource Starvation aka job latency
2. Workload and priority variation
3. Worker waste
4. Scalability and configurability – aka how hard is it to add workers, or shift them around

If you get stuck, let me know and I’ll help you out in a future post!