Transaction Propose

Hello everyone! In v4, transactions are removed to avoid keep datafile locked between commands. It was a hard decision, but was important to fix some v3 concurrency problems.

But transaction are awesome in some applications are really important. So I’m working on another way to implement “transaction-like” operation. My ideia is implement something like Entity Framework do: operate with objects in memory and, when call “SaveChanges”, save all changes in a single datafile operation (and if any error occurs, rollback everything).

So, here my propose:

using(var db = new LiteDatabase("..."))
{
    var customers = db.GetCollection<Customer>("customers");
    var orders = db.GetCollection<Order>("orders");
    
    var cust1 = new Customer { Name = "John" };
    var order1 = new Order { Date = DateTime.Now, Customer = cust1 };
    
    customers.Add(cust1);
    orders.Add(order1);
    
    // no datafile operation until here
    // all entities are in memory (as a List<Entity> Changes)
    // so, "customers.FindById(1) == null"
    
    db.SaveChanges(); 
    // now, in a single datafile operation:
    // - "cust1" will be serialized, inserted and setted "CustomerId"
    // - "order1" will be serialized (and will get reference from cust1), inserted and setted "OrderId"
    
    // "Updating"
    
    var cust2 = customers.FindById(2);
    
    cust2.Name = "Newton";
    
    customers.Set(cust2);
    
    // to avoid track all return objects (from Find), you will need use "Set" to be updated later
    
    db.SaveChanges();
    
    // also, will be avaiable
    // customers.Save(entity) - works as Upsert()
    // customers.Remove(entity) - works as Delete()
    
}

• This 4 methods will works with a change list (List<Entity>) that will be execute in order as included in this list when SaveChanges are called
• Current Insert(), Update(), Delete() method will work as excepted
• Until SaveChanges() all added/updated/deleted entities are not reflected in Find operations

Leave your comments, ideas and considerations 👍

[mbdavid]

Hi @masilver99, no. LiteDB has internal transactions (even in v4) but not expose to user to avoid keep opened. So, if engine has a fixed list of items to insert/update/delete, all operations can be done in a single transaction.

But, my “problem” now is that I read and understand more about WAL (Write Ahead Logging). All major databases implement this technique (SQL server, Postgres, Sqlite, MongoDB, …) and are great to work with transactions. WAL support multiple write threads and implement atomicity and durability So, I’m implementing WAL in LiteDB and BeginTrans() will be back in next major version.

[mbdavid]

Hi @ovule, problems with concurrency was not about using rollback journal, but way I implemented locks. Yes, using rollback journal needs lock file in write mode and lock readers too. I read about WAL long time ago and don’t was interested… until now. Reading in sqlite, WAL can be very useful to avoid writer lock reader… I will try this… thanks!