Two companies share one Slack channel. Whose database holds it?

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Two companies share one Slack channel. A fintech, and its bank. So whose database holds the messages?

Each company lives on its own shard. Easy answer: copy it to both. That doubles storage. At more than a billion messages a week. Worse: two writes, two databases. They can land in different orders. Now the two companies see two different histories. So Slack kept one copy. With the side that created the channel.

The channel row holds every message, reaction and pin. Once. Each company gets a small row of its own. With its own name, topic and purpose. And privacy became a flag. Public on one side, private on the other.

One copy means one history. Both companies see messages in the same order. And one channel can host up to 250 organisations.

But here's the catch. Same workspace no longer means allowed. So every check asks one question. Is this person a member of this channel?

The tenant decides who may see it. Not where it's stored. Store it once. Show it two ways.

Full breakdown in the lecture.