Pojo object:
public class Test {
#Id
#GeneratedValue(strategy = GenerationType.AUTO)
#Column(name="id")
private Integer id;
private String name;
#Column(columnDefinition="int default 100")
private int number;
.....getter and setter........
}
Backend Action:
#ResponseBody
#RequestMapping(value= "/common/index3")
public String index3(){
Session session = sessionFactory.getCurrentSession();
Test t = session.get(Test.class, 1);
t.setNumber(t.getNumber() - 1);
System.out.println("invoked");
session.update(t);
return "success";
}
Web page ajax trigger
$(document).ready(function(){
for(var i = 0; i < 3; i ++){
$.post("/common/index3");
}
}
issue
from client side I sent 3 times requests to backend action, and the action was invoked properly 3 times as well, however, in database the 'Number' column only was deducted 1 time, after invoked 3 times, column value become from 100 to 99, I don't know why it was only deducted 1 time, the expected value should be 97, I tried to flush & clear session to avoid the hibernate3 cache issue, anybody know how to solve this?
thank you.
From what I can see, your transactions aren't being committed to the database. You should use transactions while communicating with the DB.
After starting your session, start the transaction with
Transaction tx = session.beginTransaction()
and after completing your queries, commit them to the DB with a tx.commit().
Related
If I want to make some checks before inserting a row into the database, I know that I can run the following code:
public bool BookSeat(int userId, string seatNumber)
{
If (IsSeatStillAvailable(seatNumber))
{
var ticket = new Ticket(userId, seatNumber);
_dbContext.Tickets(ticket);
_dbContext.SaveChanges();
return true;
}
return false;
}
private bool IsSeatStillAvailable(seatNumber)
{
var seatTaken = _dbcontext.Tickets.Any(w => w.seatNumber == seatNumber);
return !seatTaken;
}
This will do a call to the database to see if the seat is taken and then do a second call to book the seat. BUT in that time it might have already been booked.
I know in simple examples, I can create an index, but my use case is much more complex.
Is there a way that I can generate the SQL statement on the insert so that it can produce an ATOMIC transaction?
To produce something like (excuse the clumsy SQL):
IF (SELECT TOP 1 Count(*) FROM Tickets = 0)
BEGIN
INSERT INTO Tickets (UserId, SeatNumber)
VALUES (#UserId, #SeatNumber);
RETURN true
END;
RETURN false
What you are looking for is concurrency handling and optimistic locking :
https://learn.microsoft.com/en-us/ef/core/saving/concurrency?tabs=data-annotations
I have to insert a row into the database but the problem is that the primary key is generated based on the total counts of rows.
E.g. if the db has 25601 rows, the ID of the newly inserted record would be CT25602.
I want to use transactions for primary key collisions.
Here is the code I wrote.
public void CreateContact(ContactViewModel input)
{
var transactionScopeOptions = new TransactionOptions
{
IsolationLevel = IsolationLevel.Serializable,
Timeout = TimeSpan.MaxValue
};
using (TransactionScope transaction = new TransactionScope(TransactionScopeOption.Required, transactionScopeOptions))
{
var contactNo = GenerateIdentity();
var contact = MapContactFields(new NavContact { No_ = contactNo }, input);
_db.Contacts.InsertOnSubmit(contact);
_db.SubmitChanges();
transaction.Complete();
}
}
This code gives me deadlocks if two persons are trying to insert a contact in a small timespan.
Any suggestions ? Thank you
Yes, the scenario you described is very likely to deadlock. I would recommend using a sequence instead. If not, then one solution is to acquire an exclusive app lock in the transaction, before scannig for the next identity. See sp_getapplock.
When SaveChanges() is called on the context, all insert/delete/update operations are executed in a single transaction. It is also possible to use DbContextTransaction for transactions. I am trying to simulate deadlock using both of these approaches. When I use DbContextTransaction, I get the deadlock exception right away but SaveChanges() alone does not throw any deadlock exceptions even after an hour. Am I doing something wrong?
Here is the code with DbContextTransaction. I try to update the first row and then the second row in the main thread. I also start another task which tries to update the second row first and then the first row.
while (true)
{
using (var context = new SchoolDBEntities())
{
using (System.Data.Entity.DbContextTransaction dbTran = context.Database.BeginTransaction())
{
Random r = new Random();
int r1 = r.Next();
int r2 = r.Next();
Student std1 = context.Students.First();
std1.StudentName = "test"+r1;
context.SaveChanges();
Student std2 = context.Students.Find(2);
std2.StudentName = "test"+r2;
context.SaveChanges();
dbTran.Commit();
}
}
}
But when I try it with just SaveChanges() it does not generate deadlock:
while (true)
{
using (var context = new SchoolDBEntities())
{
try
{
Random r = new Random();
int r1 = r.Next();
int r2 = r.Next();
Student std1 = context.Students.First();
std1.StudentName = "test" + r1;
Student std2 = context.Students.Find(2);
std2.StudentName = "test" + r2;
context.SaveChanges();
}
}
}
I am using SQL Profiler to trace the transactions. I even added more updates to the second approach just to make that transaction's duration equal to the DbContextTransaction case thinking it might be the reason but still no luck! When I look at the trace, I see that updates belonging to a particular transaction start only after the previous transaction is committed. What could be the reason?
Upon further investigation, I found out that regadless of the order of changes I have made in the context, the order in which SaveChanges() method always sends update queries to the SQL Server is based on the primary key of the table. In other words, even though I try to reverse the order of update request by first changing row 2 and then row 1, SaveChanges() first executes the update query for row 1 and then for row 2. That's why I don't get a deadlock by using just SaveChanges() method. It does not reverse the order of the queries.
I have a very strange error with dapper:
there is already an open DataReader associated with this Command
which must be closed first
But I don't use DataReader! I just call select query on my server application and take first result:
//How I run query:
public static T SelectVersion(IDbTransaction transaction = null)
{
return DbHelper.DataBase.Connection.Query<T>("SELECT * FROM [VersionLog] WHERE [Version] = (SELECT MAX([Version]) FROM [VersionLog])", null, transaction, commandTimeout: DbHelper.CommandTimeout).FirstOrDefault();
}
//And how I call this method:
public Response Upload(CommitRequest message) //It is calling on server from client
{
//Prepearing data from CommitRequest
using (var tr = DbHelper.DataBase.Connection.BeginTransaction(IsolationLevel.Serializable))
{
int v = SelectQueries<VersionLog>.SelectVersion(tr) != null ? SelectQueries<VersionLog>.SelectVersion(tr).Version : 0; //Call my query here
int newVersion = v + 1; //update version
//Saving changes from CommitRequest to db
//Updated version saving to base too, maybe it is problem?
return new Response
{
Message = String.Empty,
ServerBaseVersion = versionLog.Version,
};
}
}
}
And most sadly that this exception appearing in random time, I think what problem in concurrent access to server from two clients.
Please help.
This some times happens if the model and database schema are not matching and an exception is being raised inside Dapper.
If you really want to get into this, best way is to include dapper source in your project and debug.
Can I use a Criteria to execute a t-sql command to select the max value for a column in a table?
'select #cus_id = max(id) + 1 from customers'
Ta
Ollie
Use Projection:
session.CreateCriteria(typeof(Customer))
.SetProjection( Projections.Max("Id") )
. UniqueResult();
Max(id) + 1 is a very bad way to generate ids. If that's your goal, find another way to generate ids.
Edit: in answer to LnDCobra:
it's bad because it's hard to make sure that the max(id) you got is still the max(id) when you do the insert. If another process inserts a row, your insert will have the same id, and your insert will fail. (Or, conversely, the other process's insert will fail if your insert happened first.)
To prevent this, you have to prevent any other inserts/make your get and subsequent insert atomic, which generally means locking the table, which will hurt performance.
If you only lock against writes, the other process gets max(id), which is the same max(id) you got. You do your insert and release the lock, it inserts a duplicate id and fails. Or it tries to lock too, in which case it waits on you. If you lock against reads too, everybody waits on you. If it locks against writes also, then it doesn't insert the duplicate id, but it does wait on your read and your write.
(And it breaks encapsulation: you should let the rdbms figure out its ids, not the client programs that connect to it.)
Generally, this strategy will either:
* break
* require a bunch of "plumbing" code to make it work
* significantly reduce performance
* or all three
and it will be slower, less robust, and require more hard to maintain code than just using the RDBMS's built in sequences or generated autoincrement ids.
Best approach is to make additional Sequences table.
Where you can maintain sequence target and value.
public class Sequence : Entity
{
public virtual long? OwnerId { get; set; }
public virtual SequenceTarget SequenceTarget { get; set; }
public virtual bool IsLocked { get; set; }
public virtual long Value { get; set; }
public void GenerateNextValue()
{
Value++;
}
}
public class SequenceTarget : Entity
{
public virtual string Name { get; set; }
}
public long GetNewSequenceValueForZZZZ(long ZZZZId)
{
var target =
Session
.QueryOver<SequenceTarget>()
.Where(st => st.Name == "DocNumber")
.SingleOrDefault();
if (target == null)
{
throw new EntityNotFoundException(typeof(SequenceTarget));
}
return GetNewSequenceValue(ZZZZId, target);
}
protected long GetNewSequenceValue(long? ownerId, SequenceTarget target)
{
var seqQry =
Session
.QueryOver<Sequence>()
.Where(seq => seq.SequenceTarget == target);
if (ownerId.HasValue)
{
seqQry.Where(seq => seq.OwnerId == ownerId.Value);
}
var sequence = seqQry.SingleOrDefault();
if (sequence == null)
{
throw new EntityNotFoundException(typeof(Sequence));
}
// re-read sequence, if it was in session
Session.Refresh(sequence);
// update IsLocked field, so we acuire lock on record
// configure dynamic update , so only 1 field is being updated
sequence.IsLocked = !sequence.IsLocked;
Session.Update(sequence);
// force update to db
Session.Flush();
// now we gained block - re-read record.
Session.Refresh(sequence);
// generate new value
sequence.GenerateNextValue();
// set back dummy filed
sequence.IsLocked = !sequence.IsLocked;
// update sequence & force changes to DB
Session.Update(sequence);
Session.Flush();
return sequence.Value;
}
OwnerId - when you need to maintain different sequences for same entity, based on some kind of owner. For example you need to maintain numbering for document within contract, then OwnerId will be = contractId