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Slack RTM (real-time messaging)¶
Slack RTM is Slack's real-time messaging stack — the WebSocket-based server-to-client push channel for messages, presence, typing indicators, unreads, and other live events. Each Slack client maintains a persistent RTM connection to the backend; the backend pushes events as they happen rather than requiring the client to poll.
Pre-Unified-Grid fan-out pathology¶
Under the original Enterprise Grid design (systems/enterprise-grid), org-wide data (cross-workspace channels, org-level permissions, etc.) had to be pushed to the client for every workspace on the Grid the client was connected to. For customers with "thousands of workspaces" on a single Grid (Source: sources/2024-08-26-slack-unified-grid-how-we-re-architected-slack-for-our-largest-customers), this was pathological — every org-level event fanned out across the full workspace set.
The rework¶
Before Unified Grid was viable, Slack "enhanced our real-time messaging (RTM) stack to remove the need to fan-out org-wide data to every workspace on the grid." This is one of the two architectural preconditions that made the later Unified Grid re-architecture feasible — the other being the Vitess migration's re-sharding of key tables off the workspace axis.
The post does not detail the mechanism of the RTM rework (subscription model, connection topology, event routing) — the load-bearing disclosure is that the fan-out was removed before Unified Grid could ship.
Seen in¶
- sources/2024-08-26-slack-unified-grid-how-we-re-architected-slack-for-our-largest-customers — canonical mention as one of the two infrastructure preconditions for Unified Grid; "thousands of workspaces" fan-out justification.