Select available number from MySQL - mysql

Question

I'm about to set up a large server that assigns ports to specific objects. Every object is granted a port on creation and the ports can theoretically be assigned for years. If someone destroys their object, the port is released and is up for grabs. If I had a MySQL table containing all objects AND their ports, how would I retrieve a random free port within the range specified?
So:
Table has id and port. Query should select number where between <min> and <max> and not in table. Is anything like this possible in MySQL or would I have to get another table to jump in?
Edit
I currently have the following two tables:
CREATE TABLE `scjs_relays` (
`id` INT(100) UNSIGNED NOT NULL AUTO_INCREMENT,
`displayname` VARCHAR(50) NOT NULL COMMENT 'Name the relay shows up with at the overview',
`origin` VARCHAR(15) NOT NULL COMMENT 'IP the packets originate from',
`auth` BIT(1) NOT NULL DEFAULT b'',
`authurl` VARCHAR(255) NULL DEFAULT '0' COMMENT 'URL to retrieve auth token from',
`authtimeout` INT(10) UNSIGNED NULL DEFAULT '0' COMMENT 'Time before person has to be reauthed',
`steamid` BIT(1) NOT NULL DEFAULT b'',
`teamchat` BIT(1) NOT NULL DEFAULT b'',
`timestamp` BIT(1) NOT NULL DEFAULT b'',
`disconnect` BIT(1) NOT NULL DEFAULT b'',
`playerip` BIT(1) NOT NULL DEFAULT b'',
`node` INT(10) UNSIGNED NOT NULL DEFAULT '0',
PRIMARY KEY (`id`)
)
COMMENT='Chat relays'
COLLATE='utf8_general_ci'
ENGINE=InnoDB
;
CREATE TABLE `scjs_ports` (
`id` INT(10) UNSIGNED NOT NULL AUTO_INCREMENT,
`port` INT(10) UNSIGNED NOT NULL DEFAULT '0',
`node` INT(10) UNSIGNED NOT NULL DEFAULT '0',
`relayid` INT(100) UNSIGNED NULL DEFAULT NULL,
PRIMARY KEY (`id`)
)
COMMENT='Ports per server'
COLLATE='utf8_general_ci'
ENGINE=InnoDB
;
However, if I try to add a foreign key I get errors. the id in the scjs_users table is AUTO_INCREMENT because this is going to be an incremental ID, every server will have their own ID that never will be reused. The foreign key I currently have is the following:
ALTER TABLE `scjs_ports`
ADD CONSTRAINT `FK_relayid` FOREIGN KEY (`relayid`) REFERENCES `scjs_relays` (`id`) ON UPDATE CASCADE ON DELETE SET NULL;
Should that work fine? If I update the row in scjs_relay, id will never change, only the other columns, so is CASCADE the right option here? And if a row is deleted, it's port is obviously free'd up, so it should be set to NULL.

Answer 1

FOr your application you would probably wantt a single table with all your available ports in it and a reference to the object that has the port (or is null if no object is currently assigned to port)
So that might look like this:
id - autoincrement primary key
port - port number, has unique index
object_id - foreign key reference to object that has port, NULL-able, unique index if needed for reverse object to port lookups
You would need to think about allowing for consistent reads so when you are about to search for open row and assign it an object, you would need to do this within a transaction and with a lock upon first select so no other connection can touch that row while you are trying to update it.
That sequence might look like this:
START TRANSACTION;
SELECT #update_id:=id
FROM table
WHERE object_id IS NULL
ORDER BY port ASC
/*
Note you could order by RAND here if you truly want random
but query would not be optimized
*/
LIMIT 1
FOR UPDATE;
UPDATE table SET object_id = ?
WHERE id = #update_id;
COMMIT;
Here ? is the object reference for object you want to assign to the port. As you can see, this might lend itself well to be a stored procedure, where you pass in the object id as parameter.

Answer 2

I'd implement this by having a table with all the available ports for usage.
Then when issuing a new port I'd check availability by LEFT JOIN this table to the issued ports table like this:
SELECT table1.port from table1 LEFT JOIN table2 on table1.port = table2.port
WHERE table2 is null
LIMIT 1;