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0016. volatile is accepted and drops only in the pre-connect window

Status: Accepted · 2026-08-04 · #110 Governed by: 0000

TL;DR Real socket.io drops a volatile emit only when the transport cannot buffer, and otherwise delivers it like a normal emit. smocket's one buffer is the pre-connect tick (0004), so it drops volatile there and is a plain emit elsewhere.

Decision​

Measured against real socket.io 4.8.3 before deciding. When a socket is connected, volatile behaves exactly like emit: the message is delivered, and it composes with rooms and acknowledgements the same way a normal emit does. The only case where a real server drops a volatile message is when the transport is not ready to write it. Concretely, an emit sent before the connection opens is buffered and delivered once connected if it is normal, but discarded if it is volatile.

smocket has no transport and no send buffer, so that drop condition has no source here, with one exception: the single tick between a connection being requested and the socket being connected, where smocket does hold a message (0004). That window is the one place a volatile emit has an observable meaning to reproduce. So smocket drops a volatile emit sent during the pre-connect window and delivers a normal one, and outside that window volatile is exactly emit.

Because this reproduces the measured real behaviour rather than inventing one, it is conformance, not a divergence, so it is not an entry in differences.md. Dropping a message omits it without reordering the per-socket stream, so the FIFO invariant in 0010 is untouched.

Alternatives rejected​

  • Accept and ignore, so volatile always behaves as emit. This delivers a volatile emit in the pre-connect window, exactly where a real server drops it, so dual-run would no longer compare one behaviour. The measurement is what rules it out: the divergence is real, not hypothetical.
  • Do not expose volatile at all. Code that calls volatile would fail against smocket while running against real socket.io, breaking the substitution the library exists for.