[antiguru]

5 of 14 in the interactive-runtime stack. Stacks on #38387, followed by #38389. Tracked by CPU-216. The design document is doc/developer/design/20261107_interactive_runtime.md in #40559.

This adds the multiplexer that fans a single command stream out to both runtimes and merges their responses into one, and the DataflowClass that decides which runtime renders a dataflow.

DataflowDescription gains a class: Maintained for indexes, materialized views, sinks, subscribes and copy-tos, OneShotRead for a dataflow that exists to answer one read. It is a class, not a destination, because the command history is shared by replicas with different runtime layouts, and a single-runtime replica renders every class. The adapter marks a peek dataflow OneShotRead where it already sets until one step past the as_of. A read at Timestamp::MAX has no finite until and stays maintained. The compute controller soft-asserts that a OneShotRead dataflow has the shape the interactive runtime can serve (transient exports, a single time, no subscribe, no copy-to), and rewrites a misfit to Maintained. The interactive runtime publishes nothing, so nothing else could import a durable export it rendered. compatible_with ignores the class, so a class change across a reconnect does not rebuild a dataflow a runtime already renders.

The multiplexer decides nothing. It forwards every command to both runtimes, except Peek and CancelPeek, which go to the interactive runtime because it serves every peek in a two-runtime process. Each runtime consumes its commands at its own rate, so the order of the two sends guarantees nothing. Each runtime reads the class: the runtime that renders it builds the dataflow, and the other records its exports as peers (#38391). Both runtimes seeing the same commands in the same order is necessary for an index's since to stay at or below the as_of of every dataflow importing it, but not sufficient: #38390 adds the peer hold that covers a runtime lagging behind the other. Each runtime reports frontiers only for the collections it renders, which are disjoint, so responses are forwarded verbatim.

Inert: nothing constructs a multiplexer. Independent of #38386 and #38387, it sits above them only to keep the stack linear.

Tests in multiplex/tests.rs cover routing, per-runtime command order, verbatim frontier forwarding, no lost message when both runtimes are ready, and either runtime ending the endpoint.

A reviewer should check the shape check and its fallback in Instance::create_dataflow.

Posted by Claude Code.

🤖 Generated with Claude Code

[linear-code]

CPU-215