Streaming ≠ faster compute
It changes delivery order. Remove sequential dependencies separately.
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Streaming boundary field guide
Parallelize first. Design the shell. Reveal along product seams.
Boundary test
| Test | Ask | Pass condition |
|---|---|---|
| Value | Is this region useful and understandable by itself? | The fallback and reveal form a coherent intermediate state. |
| Latency | Can its work begin independently? | Reads start in parallel; the boundary is not hiding a waterfall. |
| Geometry | Can the fallback reserve the final shape? | Reveal does not create surprising layout shift. |
| Failure | Can this region time out or fail locally? | The shell remains useful and the error has a local recovery path. |
Shell checklist
- Resolve redirects, authorization, not-found, and critical metadata before bytes commit.
- Include a minimal but meaningful layout and primary context.
- Keep slow optional services below independent boundaries.
- Reserve dimensions and preserve accessible reading order.
- Define abort deadlines, local errors, logs, and correlation IDs.
- Measure shell time, reveal time, LCP, CLS, interaction readiness, and server occupancy.
Fast corrections
Suspense ≠ every fetch
Boundaries follow useful visual and failure seams, not implementation count.
Fallback ≠ spinner
A good fallback preserves context, geometry, accessibility, and recovery.
200 ≠ all succeeded
After the shell commits, later region failures need in-document semantics and telemetry.