Don't route to a restarted upstream until it serves, not just accepts

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Dal 5/11/2026.

Valutazione

Questa issue non è ancora stata valutata.

Descrizione

When a local Harper restarts, symphony marks a failover route up again about 1 s before Harper serves on it. Every connection in that window is dropped.

Observed (symphony 1.6.0, two-node Fabric stage cluster, Harper 5.3.5)

The route is 443, TLS-terminating, with a uds upstream (<instance>-sockets/0-9926.sock). The local container was stopped and then started again, with a new 443 connection every 0.25 s:

After docker start New 443 connections
0–3.1 s forwarded to the peer (localUp: false)
3.1 s the probe clears the mark (localUp: true, refusingSlots: 0)
3.3–4.1 s 4 of 4 dropped, 4–7 ms each, counted as route error stream
4.4 s onward served locally

The upstream accepted the connection and then closed it without a response. Every restart of a node with failover therefore loses about 1 s of new connections on each route, the same exposure #79 addressed for the stop side.

Cause (partly confirmed)

failover::probe (src/failover.rs) is a connect-and-close. Its own comment says it "proves the socket accepts, not that the application behind it is ready", and a successful probe clears the slot. Harper's socket accepts about 1 s before Harper serves on it.

Not yet known: what accepts during that window, and whether Harper binds its per-thread sockets before its HTTP handler is ready. The socket files' mtimes in that run were later than the first drops, which suggests an earlier bind or a rebind.

What to settle

  1. Where readiness belongs. If Harper binds or accepts on these sockets before it can serve, the root fix is in Harper: bind only when ready. symphony can't recover a TLS-terminated connection after its handshake, so a symphony-only fix can't be complete there. If Harper is correct and something else accepts, find it.
  2. What symphony should do either way. For example: require the probe to see the socket accepting continuously for a hold-down period before clearing a slot that was marked down, or have the probe wait for a protocol-level signal. Harper writes a .yaml beside each socket; check whether it carries a readiness signal.

Tests

  • symphony: a UDS upstream that accepts and immediately closes for its first N connections after coming back must not have new connections routed to it during that period, or they must be retried at a peer.
  • End to end on a two-node setup: restart one node's Harper under steady connection load, and no connection fails.

Found during Fabric stage failover testing. — Claude Opus 5.5, drafted with Kris

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