I like the idea of muticast.

I wonder how many times reliable UDP has been implemented? It's gotta be gazillions...

After years of maintaining and using an application suite that relies on multicast for internal communication, I would hesitate to use "reliable" and "multicast" in the same sentence. Multicast is great in theory, but comes with so many pitfalls and grievances in practice. Mostly due to unreliable handling of switches, routers, network adapters and TCP/IP stacks in operating systems.

Just to mention a few headaches I've been dealing with over the years: multicast sockets that joins the wrong network adapter interface (due to adapter priorities), losing multicast membership after resume from sleep/hibernate, switches/routers just dropping multicast membership after a while (especially when running in VMs and "enterprise" systems like SUSE Linux and Windows Server, all kinds of socket reuse problems, etc.

I don't even dare to think about how many hours I have wasted on issues listed above. I would never rely on multicast again when developing a new system.

But that said, the application suite, a mission control system for satellites, works great most of the time (typically on small, controlled subnets, using physical installations instead of VMs) and has served us well.

One of the greatest things about Aeron is just the fact it exists. If one goes e.g. to StackOverflow or a place with the same patronizing attitude of "experts", they will tell you no one needs UDP, even in the same DC on reliable network, especially no one needs multicast. Any sane person should use TCP with a loop for multiple destinations, they would say, and one should measure before optimizing, they would say. But they themselves probably never had a chance to measure. Yet Aeron guys, who are real expert in low-latency systems, just delivered an ultra-fast thing that is quite simple in design.

Aeron latency histograms vs TCP are quite nice in the same DC on enterprise-grade networking hardware. But it really makes sense to use if a single-digit or low-double digit microsecond latency improvement on P50 is worth the effort. Or if the long tail with TCP is a dealbreaker, as Aeron has much nicer P99+ regardless of how well optimized a TCP setup is. Also, if one can leverage multicast that's nice, but not only clouds have it disabled, and Aeron works fine with unicast to N.

However, there are gotchas with threading and configuration overall. Cross-DC setup may surprise in a bad way if buffers are not configured to account for bandwidth-delay product. Any packet loss on high-latency network leads to a nasty NACK storm that is slow to recover under load. It's better to set the highest QoS and ensure the network is never dropping packets, e.g. calculate the real peak instant load vs hardware capacity. Relative latency savings cross-DC become less interesting the longer the distance, so there's nothing wrong with TCP there. Another note is that, e.g. ZMQ is slow not because of TCP but because of its internals, almost 2x slower for small packets than raw well-tuned TCP sockets, which are not that bad vs Aeron. Also, Aeron is not for sending big blobs around, the best is to use it with small payloads.

Aeron is designed with mechanical sympathy in mind by the guys who coined this term and have been evangelizing it for years, and it's visible. Lots to learn from the design & implementation (tons of resources on the web) even without using it in prod.

At least in terms of open-source, I often say that the Aeron code base is one of the best projects to study in terms of software quality (especially Java). The real-logic (now Adaptive) guys are a skilled and knowledgeable bunch.

It’s easy to focus on the reliable UDP protocol and the multicast support, but what’s important about Aeron is its system architecture. As noted elsewhere, it all combines together in “mechanical sympathy” and once you have that you can interconnect with high performance transports [1].

So you set up an Aeron Server on your machine. That handles all external network communication at the message layer (NAKs, etc). Every “Aeron Client” process communicates with that Server to stand up shared memory pipes. The messaging client solely deals with those pipes (cache efficient, etc). They subscribe to channels at host:port/topic but it is not direct network delivery to them (the client is transport agnostic, besides the subscription). The network service directs data to the clients shared memory queue.

Once they have that base networked-IPC setup, you can maximize performance with it using reliable UDP and fan out common data (eg marlet data) using multicast.

Then Aeron further adds Archiving (to persist) and Clustering (to scale late join / state replication) components. This stuff works well with kernel bypass :)

[1] It could work with RDMA as well, but the ticket regarding that was closed 8 years ago. Maybe if there was AI/GPU workload interest.