Open-source alternatives to Slack

Slack is good software at a fair price. Pro is £5.75 per user a month on an annual plan, so a team of ten pays £690 a year. Business+ is £12, or £1,440 a year. The free plan keeps only 90 days of messages, which is usually what sends people looking.

Honestly, if your team likes Slack, £690 a year isn't much. Open-source chat makes sense when you've got a lot of people, or you want to keep your whole chat history without paying per head.

Zulip

Zulip's big idea is topics. Every message sits in a channel and under a named topic, so a conversation stays together rather than scrolling past. If you've ever come back from a week off to 400 unread messages, you'll see the appeal.

• Everything is free when self-hosted. The free edition includes all of Zulip's features.
• Apps for everything: Windows, Mac, Linux, iPhone and Android.
• Slack import. Message history, attachments, users, channels and custom emoji come across, though Slack only lets you export private channels and direct messages on its Business+ and Enterprise plans.
• 130+ native integrations.
• Calls go through someone else. Zulip adds a link to a provider such as Zoom or Jitsi rather than running calls itself.

It's light enough for a small team, too. The docs say a team of dozens can run on 1 CPU, 2 GB of memory and 2 GB of swap. It does want a dedicated machine, as the installer expects to be the only thing on it.

One catch, and it's a real one. Free self-hosted Zulip sends phone notifications through Zulip's service for up to 10 users, unless you qualify as a community. Past that, the Basic plan is $3.50 per user a month. So a bigger business isn't truly free if people want notifications on their phones.

Mattermost

Mattermost looks far more like Slack, so a team moving across should feel at home faster. But you need to read the small print.

• Team Edition is the open-source build, meant for fewer than 250 activated users, with no single sign-on. Mattermost's docs say it's not recommended for sensitive commercial workloads.
• Entry is the free mode of the commercial Enterprise Edition. It has more features, but it shows only the last 10,000 messages across all channels. Older ones stay in the database, but you can't view or search them. For a business, that's a nasty surprise waiting to happen.
• Paid editions are priced through sales, with no public price on the page.

There's a Slack migration route using Mattermost's own tools, but it's a technical job. Canvases, user groups and profile pictures don't come across, and apps and workflows have to be rebuilt.

For running it, the docs list 1 CPU and 2 GB of memory for up to 1,000 users, with PostgreSQL 15 or newer.

The UK angle

For most small firms, Slack being a US company is fine, and I wouldn't move for that reason alone.

It matters more if your chat is full of sensitive details, such as health information. Business+ and Enterprise can store messages, files and search data in London, but that's set up through Slack's sales team, and profile and membership data can still sit elsewhere. Self-hosting on a UK server keeps all of it in one place.

Either way, your chat history is yours to look after. With self-hosting that includes backups, and deciding how long you keep messages.

Honest drawbacks

• Fewer integrations. Check the tools you depend on before you move.
• Zulip's topics take some getting used to. People have to name a topic for each conversation, and not everyone will enjoy that.
• Phone notifications need thought. With Zulip it's the 10-user limit. Plan for it up front.
• Mattermost's free editions have limits, and Team Edition has no single sign-on.
• Someone has to run it, and chat going down gets noticed within about a minute.

Which one?

My pick is Mattermost Team Edition. It's the closest to Slack, so your team won't have to relearn how to chat. Run Team Edition rather than Entry, and stay under 250 users.

The main drawback is that there's no proper video calling in the free version. You get 1:1 audio calls and screen sharing, capped at 40 minutes, and group calls need a paid licence. So you'll want something like Jitsi Meet alongside it.

Zulip is the realistic alternative. All its features are free when self-hosted, and the licence is about as simple as it gets, but its topics are different from Slack and a team that won't take to them will drift back.

Need it built or customised?

If you'd like chat tied into your own systems, the team at Dev Partners can build that kind of integration.

Sources

• Slack UK pricing
• Slack data residency
• Zulip on GitHub
• Zulip plans and pricing
• Zulip features
• Zulip server requirements
• Zulip: import from Slack
• Zulip: starting a call
• Mattermost licence
• Mattermost editions and offerings
• Mattermost Calls deployment guide
• Mattermost requirements
• Mattermost: migrating from Slack
• Mattermost pricing