Phase 4 had been written and never run. The hardware is not coming, so the
cluster is simulated instead — four nodes across two fabrics, eight GPUs each,
built with the onboarding scripts from the previous commit.
`sim` is now an engine adapter, not just a standalone command. Its leader binds
a rendezvous port and does not serve until every worker has joined; its workers
serve nothing at all. That is the part stub processes could never exercise, and
it is why this path survived four phases untested: the command line was
checked, the sequence was not.
Measured on the cluster:
- a 2-node replica on each fabric, node01+node02 on ib-a and node03+node04 on
ib-b, four GPUs from each
- a request served through one of them
- only the leader listening — node02 had no port open at all, which is
ServesHTTP working end to end rather than in a unit test
- node02 killed; at t+60s it was declared lost, the ib-a replica failed and was
reaped; at t+80s a replacement was ready on ib-b, the fabric that could still
host it, while node01 sat idle rather than being paired across fabrics
The invariant held under the one condition that tests it: when a replica could
not be rebuilt where it was, it moved to a fabric that could hold all of it
rather than spanning two.
What this does not prove is written into the plan next to what it does. The
GPUs are an inventory, the fabric is a sysfs-shaped directory, and the engine
speaks no NCCL — so the launch sequence is now exercised while the launch
arguments for vLLM and SGLang are still read from documentation rather than
from a running engine.
docs/node-onboarding.md is the procedure, including how to run all of this on
machines that have neither GPUs nor InfiniBand.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
