Feature Overview

This document provides a high-level overview of the following core features:

  • HTTP/WebSocket-based communication
  • Flexible subscription models support HTTP Push, HTTP Pull, and WebSocket delivery methods
  • Robust data and delivery guarantees
  • High throughput and low resource utilization

HTTP/WebSocket Communication

Instead of inventing yet another wire protocol, Courier uses HTTP—along with WebSockets for full-duplex communication.

Additionally, Courier does not demand you use a particular serialization format. At the time of this writing it supports JSON with optional GZIP encoding, which is good enough for most use cases. Support for other “command and control” serializations are on the roadmap, though in any case we’re happy to deliver messages using whatever Content-Type you’d like!

Push, Pull, or Stream

Have your messages pushed to an HTTP URL, pulled from Courier at your leisure, or streamed over a persistent connection via the WebSocket protocol, it’s up to you!

Data and Delivery Guarantees

Courier is built on the back of replicated, highly-available systems that guarantee a consistent, correct view of data so long as a quorum exists. A response from Courier means your event was stored successfully and redundantly.

In addition to storage guarantees, Courier also offers delivery guarantees. Every Subscriber that exists at the point of publishing will receive the message. Courier also supplies an audit log of every message received by each Subscriber, along with how long it took to receive a reply, how many times sending was retried, etc. Every individual subscription may define its own policy, including:

  • Frequency and total number of retries
  • How large a message queue may become
  • How long the subscriber should remain offline before Courier gives up
  • How un-deliverable messages should be handled

And so forth. Policy options differ based on the type of subscription (push, pull, or stream).

Note

While Courier itself is not High-Availability (HA) yet, its supporting services are. Courier supplies end-to-end durable storage, meaning your messages won’t be lost if Courier itself crashes or otherwise disappears, making it ideal for volatile Cloud environments.

High Throughput; Low Utilization

Courier is an Event-driven system, meaning it can handle orders of magnitude more concurrent messages and subscribers than most other systems. It can effortlessly scale up to millions of messages per second on a single machine while maintaining a small memory footprint.

Note

Additionally, Courier’s supporting services scale horizontally—or near to it—meaning overall throughput can be continuously improved simply by adding more machines!