MySQL错误：检测到死锁，在尝试获取锁时；建议重启事务（Node.js环境）

One easy trick that can help with most deadlocks is sorting the operations in a specific order.

You get a deadlock when two transactions are trying to lock two locks at opposite orders, ie:

connection 1: locks key(1), locks key(2);
connection 2: locks key(2), locks key(1);
If both run at the same time, connection 1 will lock key(1), connection 2 will lock key(2) and each connection will wait for the other to release the key -> deadlock.

Now, if you changed your queries such that the connections would lock the keys at the same order, ie:

connection 1: locks key(1), locks key(2);
connection 2: locks key(1), locks key(2);
it will be impossible to get a deadlock.

So this is what I suggest:
//翻译：特别是删除语句时，确保你没有其他查询语句使用加锁，如果你必须那么删除操作，那建议查询条件按字段升序排序，然后查询出主键，最后还是按主键来删除
Make sure you have no other queries that lock access more than one key at a time except for the delete statement. if you do (and I suspect you do), order their WHERE in (k1,k2,..kn) in ascending order.
Fix your delete statement to work in ascending order:
Change

DELETE FROM onlineusers WHERE datetime <= now() - INTERVAL 900 SECOND
To

DELETE FROM onlineusers WHERE id IN (SELECT id FROM onlineusers
    WHERE datetime <= now() - INTERVAL 900 SECOND order by id) u;
//翻译：另一件重要的事，当发现死锁时，客户端最好自动尝试3次机会，如果都失败就放弃吧，真的是失败了。
Another thing to keep in mind is that mysql documentation suggest that in case of a deadlock the client should retry automatically. you can add this logic to your client code. (Say, 3 retries on this particular error before giving up).