1# @ajax/mesh-placer
2
3One placed program's state machine across the cluster (`mpl_placer`), sans-IO. The driver feeds it
4events (a frame batch completed here, a peer frame arrived, a placement request, the fleet's view, a
5tick) and carries out the acts it returns (send a frame, write the state into the view, show a composed
6frame, publish the follow batch, change the run state, answer a request).
7
8The fleet's leader coordinates every change among the up members: it pauses the others (PL_PAUSE),
9collects their drained state and time per part (PL_PAUSED), picks the state that continues (the old
10mode's authority's, or a survivor's copy when it is gone), sets the new mode everywhere (PL_SET with
11that state) and waits for PL_DONE; a change not done in 5 s is rolled back (PL_FAIL). It re-plans at once
12when a member of the mode drops, and 5 s after one comes back; with no leader, a mode that needs a lost
13member goes local. Frames go point to point: a runner sends whole frames, a split owner sends its parts
14and the state, followers render their parts on the owner's state and send them on, and every machine
15(mirrors included) composes and shows the same whole frame.
16
17## Ported from
18
19[alpha-rust-mesh](https://github.com/rsafier/alpha-rust-mesh) `crates/mesh-host/src/place.rs`:
20`Placer`, `Act`, `Dest`, `Stats`, `CHANGE_TIMEOUT`, `JOIN_DWELL`. Instants are monotonic microseconds.
21
22## Tests
23
24`tests/placer.c`: the Rust's `change_protocol_moves_state_across_three`,
25`split_auto_uses_measured_times`, `change_times_out_and_rolls_back`,
26`split_frames_assemble_on_every_machine` (six splits over 2..4 machines),
27`follower_keeps_the_newest_and_rejects_strays` and `member_loss_replans_and_return_rejoins`, with the
28Rust's message pump between placers.
29
30## Licence
31
32MIT. See the repository's LICENSE.