Currenty i'm working on creating a chess app with nodejs & socket.io
now the running games information are stored in an array like this:
games[token] = {
'creator': socket,
'players': [],
'interval': null,
'timeout': timeout,
'FEN' : '',
'PGN' : ''
};
The question is : Is better to save games info to DB at the creation of games and change the values of fields move by move, or save every game after finish?
Which is better approach?
if you wait until the end of a game to save state then you run the risk of losing it if something like a server crash occurs. Think unhandled exception or something outside of your control like a container restart or something worse.
Persist every bit of data that you want recoverable as soon as possible. I could imagine an rpg in which it wasn't super important to always be able to recover the players exact position on a map. It seems you'd always want to be able to recover the state of your chess games.
If you want crash proof implementation cheapest way is to write every move in journal log. When game ends you save the state and discard journal. On every game start load the state and then check if there is anything in the journal, if yes just play back the events.
Journal can be in database, disk or some light weight DB like Redis.
Related
For context, i am making a multi-player game using nodejs and my db is postgres.
players play one by one, and i save everything in the db.
when first user plays, they can't play again, until the other player played too.
what i am doing now is having a boolean on each player in the db that says "ableToPlay" which is true, then turns to false if it's not the user's turn.
issue is when user spams the "play button" and my db is in a remote server, it takes time to update from true to false, making the user play multiple times then causes the app to crash.
I am using aws Microservices architecture so the server must be stateless.
is there any way i can save the game progress in a way where the progress is accessible to all my micro-services?
How do you check the turn? Is it something:
select turn from db
if turn == X then
//allow the turn
do all the logic
update the turn to Y
endif
So the "do all the logic" may be called several times as several requests will get turn=X.
This is a very common problem in programming, there are several approaches you could do.
Two key observations to address:
the same player should not do a turn twice in a row
while one player is making the turn, the other player must wait
Easiest way it to use a transaction in the DB while the turn is happening. For example, when player X making the turn:
start transaction
update turn=X where turn=Y (Y is the other player)
if update done (one record is updates)
do all the logic
commit the transaction
In that approach, update will wait for the previous one to finish, and the WHERE clause will make sure the same player won't do two or more turns in a row. And the transaction isolation will avoid running turn logic at the same time.
If you don't want to use the transaction, you could build a state machine, with states:
waitingForTurnX
makingTurnX
waitingForTurnY
makingTurnY
this would be a nice model to code and these transitions could be handled without transactions:
update state=makingTurnX where state=waitingForTurnX
This approach will also eliminate race condition, because in vast majority of databases, updates are atomic when it comes to a single record.
For example, let's say I have a random game in which I have 500 independent objects and 10 players.
Independent object is an object that moves in a specific direction per update regardless of what players do (there is no need for players to come into contact with these objects).
Now if a player is shooting (lets say) a bullet, it is easier because it belongs to a specific player therefore it's easier to avoid in game lag. Lets look at something simpler, though, for example a player try to update their position. The typical thing I would do on client & server side would be this :
client side : update the coords of the player + send a message to the server as socket X
server side : receives the message from socket X, updates the coords of the player on the server side +
sends a message with the coords of that same player to all other sockets
When you do the communication like this, everyone will receive the new coords of the player and there will be little to no lag. (It is also sufficient for objects like bullets, because they are created upon firing a player event)
How do you handle 500+ independent objects that move in random directions with random speed all across the map and update them for all players efficiently? (Be aware that their velocity and speed can be changed upon contact with a player). What I've tried so far:
1) Put all of the movement + collission logic on the server side &
notifying all clients with a setTimeout loop & io.emit -
Result : causes massive lag even when you have only 500+ objects and 4 connected players. All of the players receive the server's response way too slow
2) Put all of the movement + collission logic on the client side & notifying the server about every object' position-
Result : To be honest, couldn't encounter much lag, but I am not sure if this is the correct idea as every time an object moves, I am literally sending a message to the server from each client to update that same object (server is getting notified N[number of connected clients] amount of times about that same object). Handling this entirely on the client side is also a bad idea because when a player randomly switches tabs [goes inactive], no more javascript will be executed in that players' browser and this whole logic will break
I've also noticed that games like agar.io, slither.io, diep.io, etc, all of them do not really have hundreds of objects that move in various directions. In agar.io and slither you mainly have static objects (food) and players, in diep.io there are dynamical objects, but none of them move at very high speeds. How do people achieve this? Is there any smart way to achieve this with minimal lag?
Thanks in advance
Convert your user interactions to enumerated actions and forward those. Player A presses the left arrow which is interpreted by the client as "MOVE_LEFT" with possible additional attributes (how much, angle, whatever) as well as a timestamp indicating when this action took place from Player A's perspective.
The server receives this and validates it as a possible action and forwards it to all the clients.
Each client then interprets the action themselves and updates their own simulation with respect to Player A's action.
Don't send the entire game state to every client every tick, that's too bloated. The other side is to be able to handle late or missing actions. One way of doing that is rollback where you keep multiple sets of state and then keep the game simulation going until a missinterpretation (late/missing packet) is found. Revert to the "right" state and replay all the messages since in order to get state to correct. This is the idea behind GGPO.
I suggest also reading every article related to networking that Gaffer on Games goes into, especially What Every Programmer Needs To Know About Game Networking. They're very good articles.
I'm developing an multiplayer turn based game (e.g chess), should support a lot of players (that's the idea). My question is about a service i'm developing, it's the pairing system, the responsible of pairing 2 players to start a room and start playing.
So, this is the pairing service:
matchPlayers() {
if (this.players.length >= 2) {
let player1 = this.players.shift();
let player2 = this.players.shift();
if (player1 !== undefined && player2 !== undefined) {
player1.getSocket().emit('opponent_found');
player2.getSocket().emit('opponent_found');
return this.createMatchInDataBaseApiRequest(player1, player2)
.then(function (data) {
let room = new RoomClass(data.room_id, player1, player2);
player1.setRoom(room);
player2.setRoom(room);
return room;
});
}
}
return false;
}
At the entrypoint of the server, each new socket connection I push it to an array "PlayersPool" this array is for players waiting to get matched up.
Right now my approach is to pair users when there are available, (FIFO - first in first out).
The problems (and question) I see with this pairing system is:
This depends on new users, this gets executed each time a new user is connected, The flow is: A user connects, get's added to the pool, and check if there are users waiting for being paired, if yes a room is created and they can play, if not he gets added to the waiting pool; Until a new user connects and the code get's executed and so on...
What would happen if in some weird case (not sure if this could happen) 2 players gets added to the waiting pool at the same exact time, this service would find the pool empty and would not create a room: To solve this maybe having another service running always and checking the pool? what would be the best approach? Could this even happen? in which scenario?
Thanks for the help.
I'm guessing this particular code snippet is on the server? If so, assuming there is only one server, then there is no "race condition": node.js is single-threaded, as IceMetalPunk mentioned, so if you're running this function every time you add a player to this.players, you should be fine.
There are other reasons to be examining the player pool periodically, though: players you've added to the pool may have gotten disconnected (due to timeout or closing the browser), so you should remove them; you also might want to handle situations where players have been waiting a long time - after X seconds, should you be updating the player on progress, calculating an estimated wait time for them, perhaps spawning an AI player for them to interact with while they wait, etc.
You can run into a "race condition", it's explained here in this package which provides you a Locking mechanism.
https://www.npmjs.com/package/async-lock
That package will be useful, only if you run node.js in a single process, meaning you are not having multiple servers, or having node cluster running multiple processes.
In that case, you will have to implement a distributed locking mechanism which is one of the most complex things in distributed computing, but today you can use the npm package for Redlock algorithm, set 3 redis servers and go.
Too much overhead for a game without players.
Node.js is not single threaded, here is the explanation of one of the creators.
Morning Keynote- Everything You Need to Know About Node.js Event Loop - Bert Belder, IBM
https://www.youtube.com/watch?v=PNa9OMajw9w
Conclusion, keep it simple, run it in a single node process and use the "async-lock" package.
If your server grows to become a MMO, you will need to read about distributed computing:
How to do distributed locking:
https://martin.kleppmann.com/2016/02/08/how-to-do-distributed-locking.html
Book on data intensive apps
http://dataintensive.net/
My midlet uses two record stores. Currently, I create/open both record stores when the app starts and I leave them both open for the entire lifetime of the app.
If I open/close the record store after each operation (e.g., reading or writing) the delays are really bad in the emulator.
Similarly, if I close the recordstores when the app exits, there is another very long delay.
So is it OK for me to never close the record stores in my code (thereby, presuming the device will do this itself when the app exits). If not, what is the best practice I can employ without causing a noticeable delay for the user and without risking any data loss?
There is nothing in the docs regarding this, and nothing I could find on google.
As far as I remember, on some phones changes in DB are stored permanently only when DB is closed. While in most J2ME implementations changes are saved on each record change.
I would suggest keeping DB open for whole app session, if it significantly improves performance. It is worth handling DB close in destroyApp() of course.
You also can consider implementing 'auto save' feature - close and reopen DB if IO is inactive for some time.
Usually heavy DB access is performed in some actions only, not constantly. In this case you could wrap bunch of IO operations in a 'transaction' finishing it with DB close.
In other words, on most devices you can go with the first approach (keeping DB open) but on some devices (do not remember exactly, probably on Nokia S40 or S60) it can lead to data loss when the app will be terminated by VM (and you can't handle it since destroyApp is not guarantied to be called), without proper DB close. So in general case it would be right to wrap critical transactions with DB.close() statements
I'm creating a board game where 2 players can play, and others can be spectators (viewers)
so, when a spectator joins, he gets the current state of the game, and from then on, he only gets the move each player has made (to save data obviously).
My question is: when the spectator first get the state of the game from the server, how can I make sure it is actually synced? I don't really know when he will get the state, and it might be a fraction of a second before something has changed, and then the Delta he gets for every move made won't make sense.
Should I use some kind of an internal? what would you suggest to make sure everything is synced?
Assuming that your state is result of, and only of, user actions, you could store you state in a table like format with an auto-increment integer ID.
In the move event, you pass the new ID and the previous ID. If the receiver's max ID is less than the previous ID, you know to ask server for the missing actions.