Multi-stream: partition by key, run in parallel, merge in a fixed order

Partitioning by key lets each partition run alone, which is what makes the parallelism safe. Collecting the futures in partition order and reading them in that order is what makes the OUTPUT reproducible.

• Multi-stream: a record with a mutable component is a broken Map key
• Multi-stream: an unmodifiable view is not a snapshot of the feed
• Multi-stream: toMap's merge function runs only on a collision
• Multi-stream: nothing runs until the terminal operation
• Multi-stream: indexing the other side costs memory a sorted merge does not
• Multi-stream: UNORDERED is a promise you make, not one you receive

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Published 2026-09-20

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