# sim-lib-expr-tree-server
`sim-lib-expr-tree-server` is the authoritative server backend for live SIM
expression trees. It exports `site/sim-lib-expr-tree-server` as a loadable
placement site and implements both the standard `EvalSite` frame boundary and
the kernel `EvalFabric` realization boundary.
The server owns a bounded registry of opaque session ids and live
`TreeHandle`s. Every request runs through the caller's `Cx`, so the existing
`expr-tree/*` capability checks remain authoritative. The server also accepts
the operations produced by `ExpressionTreeSurfaceCodec` and the generic
`web-session/*` request maps used by server-backed surfaces. It does not add a
tree-specific transport or evaluator.
Each session provides:
- optimistic snapshot revisions and one-time bounded continuation tokens;
- explicit session, idle-tick, page, depth, watch, and queue limits;
- independent bounded watches with lifetime overflow evidence;
- cancellation for watches and whole sessions;
- structured remote errors that keep a connected client live;
- mandatory monotone logical ticks on changes; and
- optional injected `WallClock` observations used only as human evidence.
Wall time is never compared for freshness, optimistic concurrency, or idle
expiry. Source, policy, calculation, receipts, and live authority remain in the
server-owned expression-tree runtime. Browser and transport adapters carry
only bounded snapshots, Intents, operations, and change records.
`ExpressionTreeWebSurfaceFactory` is the product composition point for the
generic SIM browser shell. It gives each opaque browser id a fresh caller
context, negotiated `RemoteTransport`, codec registry, and reversible session,
while mapping the browser alias to exactly one authoritative tree resource.
The shell keeps ownership of HTTP lifecycle, session expiry, PWA policy, and
the generic Scene interpreter.
Run the focused conformance specimen with:
```sh
cargo test -p sim-lib-expr-tree-server
cargo run -p xtask -- check-recipes
```