My biggest goal was "make sure that my bottleneck is serialization or syscalls for sending to the client." Those are both things I can parallelize really well, so I could (probably) scale my way out of them vertically in a pinch.

So I tried to pick an architecture that would make that true; I evaluated a ton of different options but eventually did some napkin math and decided that a 64-million uint64 array with a single mutex was probably ok[1].

To validate that I made a script that spins up ~600 bots, has 100 of them slam 1,000,000 moves through the server as fast as possible, and has the other 500 request lots of reads. This is NOT a perfect simulation of load, but it let me take profiles of my server under a reasonable amount of load and gave me a decent sense of my bottlenecks, whether changes were good for speed, etc.

I had a plan to move from a single RWMutex to a row-locking approach with 8,000 of them. I didn't want to do this because it's more complicated and I might mess it up. So instead I just measure the number of nanos that I hold my mutex for and send that to a loki instance. This was helpful during testing (at one point my read lock time went up 10x!) but more importantly gave me a plan for what to do if prod was slow - I can look at that metric and only tweak the mutex if it's actually a problem.

I also took some free wins like using protobufs instead of JSON for websockets. I was worried about connection overhead so I moved to GET polling behind Cloudflare's cache for global resources instead of pushing them over websockets.

And then I got comfortable with the fact that I might miss something! There are plenty more measurements I could have taken (if there was money on the line I would have measured some things like "number of TCP connections sending 0 moves this server can support" but I was lazy) but...some of the joy of projects like this is the firefighting :). So I was just ready for that.

Oh and finally I consulted with some very talented systems/performance engineer friends and ran some numbers by them as a sanity check.

It looks like this was way more work than I needed to do! I think I could comfortable 25x the current load and my server would be ok. But I learned a lot and this should all make the next project faster to make :)

[1] I originally did my math wrong and modeled the 100x100 snapshots I send to clients as 10,000 reads from main memory instead of 100 copies of 100 uint64s, which lead me down a very different path... I'm not used to thinking about this stuff!

(edit: I realized I didn't answer your question. If we receive a captured for a piece we're optimistically tracking that always takes precedence, since once a piece is captured it can't move anymore!)

    * clients send a token with each move
    * they either receive a cancel or accept for each token, depending on if the move is valid. If they receive an accept, it comes with the sequence number of the move (everything has a seqnum) and the ID of the piece they captured, if applicable
    * clients receive batches of captures and moves. if a move captured a piece, it's guaranteed that that capture shows up in the same batch as the move

    * Write down all impacted squares for the move (2 most of the time, 4 if you castle)
    * Write down its move token
    * If you moved a piece that is already tracked optimistically from a prior not-yet-acked-or-canceled move, we note that dependency as well

When we receive a new move, we compare it with our optimistic state. If the move occupies the same square as a piece that we've optimistically moved, we ask "is it possible that we inadvertently captured this piece?" That requires that the piece is of the opposite color and that we made a move that could have captured a piece (for example, if you moved a pawn up that is not a valid capturing move).

If there's a conflict - if you moved a piece to a square that is now occupied by a piece of the same color, for example - we back out your optimistically applied move. We then look for any moves that depended on it - moves that touch the same squares or share the same move token (because you optimistically moved a piece twice before receiving a response).

So concretely, imagine you have this state:

    _ _ _ _
    K B _ R

    _ _ B _
    _ R K _

There's some more bookkeeping here. For example, we also have to track the IDs of the pieces that you moved (if you move a bishop and then we receive another move for the same bishop, that move takes precedence).

Returning the captured piece ID from the server ack is essential, because we potentially simulate after-the-fact captures (you move a bishop to a square, a rook of the opposite color moves to that square, we decide you probably captured that rook and don't revert your move). We track that and when we receive our ack, compare that state with the ID of the piece we actually captured.

I think that's most of it? It was a real headache but very satisfying once I got it working.

My goal with the board architecture was "just be fast enough that I'm limited by serialization and syscalls for sending back to clients" and go made that really easy to do; I spend a few hundred nanos holding the write lock and ~15k nanos holding the read lock (but obviously I can do that from many readers at once) and that was enough for me.

I definitely have some qualms with it, but after this experience it's hard to imagine using something else for a backend with this shape.

If you did want to experiment with supporting cross-board captures, an alternate way to address that could be by rotating the board 180° every other row, so that white pieces have other white pieces behind their home rank.

(Oh and I still owe you an email. I haven't forgotten!)

get the most kills, make a cool shape

or take a piece veeeeery far away from home

1) fill corner board with pieces 2) cover the border (from inside) with pawns 3) cover the promotion square on the border with the king

king can't exit the board, pawns can't walk backward to leave space, filler doesn't allow these 2 to make way

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the real holy grail that will never be achieved I fear