fcfs

First-Come, First-Served (FCFS) Ordering Policy

Type: fcfs-ordering-policy

[!NOTE] This plugin is enabled by default when flow control is enabled. You do not need to explicitly declare it in your configuration.

The First-Come, First-Served (FCFS) ordering policy selects requests based on their arrival order at the Flow Control layer.

Why Choose This Policy?

• Simplicity and Intuitive Behavior: Requests are processed in the order they arrive, making the system behavior easy to understand and predict.
• No Special Inputs Required: Unlike policies that rely on deadlines or priorities, FCFS works without any additional request metadata or headers.
• Good for Homogeneous Workloads: When all requests have similar processing costs and importance, FCFS is often the fairest approach.

What It Does

The FCFS policy compares requests by their Logical Enqueue Time—the timestamp when the request first arrived at the controller.FlowController. The request with the earliest timestamp is rendered "less" than (i.e., higher priority than) requests that arrived later.

Inputs consumed

This policy inspects the following attributes of the request item:

• Logical Enqueue Time: Used as the arrival timestamp to determine order.

Behavior and Queue Pairing

The behavioral guarantees of this policy are critically dependent on the capabilities of the SafeQueue it is paired with in the configuration.

1. Strict FCFS (High Accuracy, Lower Throughput):
   • Paired with a CapabilityPriorityConfigurable queue (e.g., a heap-based queue).
   • Requests are strictly ordered by their logical enqueue time.
2. Approximate FCFS (High Throughput, Lower Accuracy):
   • Paired with a CapabilityFIFO queue (e.g., a list-based queue).
   • Requests are ordered by their physical arrival time at the specific queue shard.
   • This is the default behavior because it avoids the overhead of maintaining a min-heap across shards, yielding better performance for high-throughput workloads.

Configuration

This policy does not require any custom parameters.

orderingPolicyRef: fcfs-ordering-policy

Trade-offs

• Head-of-Line Blocking: A large request at the front of the queue can delay smaller, more urgent requests behind it.
• No Urgency Awareness: It does not consider deadlines or request priority; a request that has already timed out on the client side might still be processed if it was the oldest in the queue.

Related Documentation

• Ordering Overview
• Flow Control User Guide