database-locking

I am reading a manual about innodb transactions but still, there is lots of unclear stuff to me. For instance, I don't quite understand to the following behaviour: -- client 1 -- client 2 mysql> create table simple (col int) engine=innodb; mysql> insert into simple values(1); Query OK, 1 row affected (0.00 sec) mysql> insert into simple values(2); Query OK, 1 row affected (0.00 sec) mysql> select @@tx_isolation; +-----------------+ | @@tx_isolation | +-----------------+ | REPEATABLE-READ | +-----------------+ mysql> begin; Query OK, 0 rows affected (0.01 sec) mysql> begin; Query OK, 0 rows

How can I guarantee that I can search if a username exists in my database, then insert that username into the database as a new row without any intercept between the SELECT and INSERT statements? Almost as if I am locking on a row that doesn't exist. I want to lock on the non-existent row with the username "Foo" , so that I can now check if it exists in the database AND insert it into the database if it doesn't already exist without any interruption. I know that using LOCK IN SHARE MODE and FOR UPDATE exist but as far as I know, that only works on rows that already exist. I am not sure what to

I am doing a project in wicket How to solve the problem. I came across such a message: WicketMessage: Can't instantiate page using constructor public itucs.blg361.g03.HomePage() Root cause: java.lang.UnsupportedOperationException: [SQLITE_BUSY] The database file is locked (database is locked) at itucs.blg361.g03.CategoryEvents.CategoryEventCollection.getCategoryEvents(CategoryEventCollection.java:41) public List<CategoryEvent> getCategoryEvents() { List<CategoryEvent> categoryEvents = new LinkedList<CategoryEvent>(); try { String query = "SELECT id, name, group_id" + " FROM event_category";