[price]

The Zulip server uses very little CPU, but the RAM is important. (I work on Zulip.)

Here's what the docs say, for reference (excerpt of https://zulip.readthedocs.io/en/latest/production/maintain-s... ):

> For an organization with 100+ users, it’s important to have more than 4GB of RAM on the system. Zulip will install on a system with 2GB of RAM, but with less than 3.5GB of RAM, it will run its queue processors multithreaded to conserve memory; this creates a significant performance bottleneck.

> chat.zulip.org, with thousands of user accounts and thousands of messages sent every week, has 8GB of RAM, 4 cores, and 80GB of disk. The CPUs are essentially always idle, but the 8GB of RAM is important.

As a practical matter, I think 4GB of RAM is not a lot to ask for a service that 100+ users are actually concurrently using all day. That's a small fraction of the RAM the clients are consuming; and you can get a suitable cloud machine from Digital Ocean (simpler pricing than AWS, so good for a quick price check) for $20 USD/mo.

On the implementation side, it turns out that a lot of moving parts go into a full-featured chat app. Here's a partial architecture diagram, plus detailed exposition: https://zulip.readthedocs.io/en/latest/overview/architecture... Database, plus caches, plus code for lots of features x running in a number of processes, adds up to a few gigs of memory.

[buboard]

Well my question is why they consume so much memory/CPU? What makes these services so different from earlier chat systems? Are these users continuously doing video chat with each other or keep sockets open? For simple text chat, 4GB of RAM seems absurdly high, considering how irc was able to handle thousands of users 3 decades ago.

[price]

IRC is essentially stateless. The server doesn't remember what messages people have sent; so it can't tell you what was said last night when you come online in the morning, or what was just said in some channel you weren't previously listening to. That work gets pushed out to clients, and to add-on services.

The IRC server also doesn't store images or any other kind of file people want to show each other. There are lots of good practical reasons to want to share images in a conversation (e.g., screenshots), plus of course silly GIFs. That work also gets pushed out to add-on services.

When people say here that Slack or Zulip etc. are a much better user experience than IRC, I think those two things -- message history, and images -- are major reasons for that.

Message history means a database that gets big, and images mean a lot of data too. There's a large working set of both of those that you want fast access to. That means providing adequate RAM.