I am using two windows as I want to implement concept of transaction.
Window1: begin;
Window1: update employee set salary = 45000 where ssn = '123456789';
Window2: begin;
Window2: select * from employee where ssn = '123456789';
Here, this command shows me previous data which is correct.
Window1: commit;
Window2: select * from employee where ssn = '123456789';
Here, I should get the updated salary of 45000. But my window 2 is showing previous data only. Where am I doing the mistake?
You expectations are incorrect, that's all. What transactions see from each other's work is determined by the so called transaction isolation levels. By default, mysql uses repeatable read isolation level, which means:
If the transaction isolation level is REPEATABLE READ (the default
level), all consistent reads within the same transaction read the
snapshot established by the first such read in that transaction. You
can get a fresher snapshot for your queries by committing the current
transaction and after that issuing new queries.
You can change the isolation level to read committed to enable the behaviour you expect:
With READ COMMITTED isolation level, each consistent read within a transaction sets and reads its own fresh snapshot.
Related
After my airflow upgraded to 2.0.0(and then 2.0.1) and scheduler expanded to 3 nodes, something weird happened：
dagruns were success but the task instances were not scheduled at all
task failed with a null hostname(https://github.com/apache/airflow/issues/13692)
Task is set "upstream_failed" while upstream tasks are success(https://github.com/apache/airflow/issues/13671)
These phenomena never happened when there was only one scheduler node.
And I found that after task instances of a new dagrun were created by a scheduler node, they were not found in another scheduler node's task_instance_scheduling_decisions function.
Then I checked the mysql configurations and found transaction isolation was set to be Repeatable read by default.
After I set transaction isolation to be read commited, everything seems to be good now. But I still wonder are there any side effects?
Yes, READ COMMITTED is different than REPEATABLE READ.
If you use REPEATABLE READ, then in this transaction:
START TRANSACTION;
SELECT * FROM mytable;
SELECT * FROM mytable; -- i.e. the same query
COMMIT;
You are guaranteed that both SELECTs return the same result (as long as they are not locking queries).
But in READ COMMITTED, if some other session has committed a change to data in between the two SELECTs, they can return different results.
In other words, REPEATABLE READ means your SELECT queries always return data that was committed at the moment your START TRANSACTION started. Whereas READ COMMITTED means your SELECT queries return data that was committed after your transaction started, up until the time each respective SELECT starts.
Both levels of transaction isolation have proper uses. But they do behave differently.
Greetings to everyone,
Recently I tried the scenario issued below and I want your opinion on this.
Using MySQL/INNODB, on a table t (a,b,c,d) values (10,20,30,40) we have these two transactions (A and B) executing in the order specified below:
1) A,B: set autocommit=0;
2)B: set transaction isolation level repeatable read;
3)B: select d from t;
4)A: update t set a=a+3;
5)A: commit;
6)B select * from t;
The values returned from (6) are (10,20,30,40) as if the selection of column d on (3) caused a local snapshot to be created on transaction B and since B has repeatable read it keeps consistent values. Is this true, or is there another explanation for this?
Thank you in advance for your help.
Yes, MySQL seems to automatically start a transaction in this case:
Suppose that you are running in the default REPEATABLE READ isolation
level. When you issue a consistent read (that is, an ordinary SELECT
statement), InnoDB gives your transaction a timepoint according to
which your query sees the database.
[…]
You can advance your timepoint
by committing your transaction and then doing another SELECT or START
TRANSACTION WITH CONSISTENT SNAPSHOT.
https://dev.mysql.com/doc/refman/5.7/en/innodb-consistent-read.html
Especially the last sentence sounds like the next SELECT will automatically create a new transaction. (And it also states what to do to do move on: commit or rollback, but I guess you already knew that.)
I want to test the transaction isolation issues : lost update. How can I do that in mysql console. I know the following one
SET SESSION TRANSACTION ISOLATION LEVEL READ UNCOMMITTED;
Using read uncommitted i can demo it does not allows lost update but allows dirty read. I want to demo the lost update how do I demo that practically in two different mysql consoles.
Suppose I do (note: the syntax below is probably not correct, but don't worry about it...it's just there to make a point)
Start Transaction
INSERT INTO table (id, data) VALUES (100,20), (100,30);
SELECT * FROM table WHERE id = 100;
End Transaction
Hence the goal of the select is to get ALL info from the table that just got inserted by the preceding insert and ONLY by the preceding INSERT...
Now suppose that during the execution, after the INSERT got executed, some other user also performs an INSERT with id = 100...
Will the SELECT statement in the next step of the transaction also get the row inserted by the executed INSERT by the other user or will it just get the two rows inserted by the preceding INSERT within the transaction?
Btw, I'm using MySQL so please tailor your answer to MySQL
This depends entirely on the Transaction Isolation that is used by the DB Connection.
According to MySQL 5.0 Certification Study Guide
Page 420 describes three transactional conditions handled by Isolation Levels
A dirty read is a read by one transaction of uncommitted changes made by another. Suppose the transaction T1 modifies a row. If transaction T2 reads the row and sees the modification neven though T1 has not committed it, that is a dirty read. One reason this is a problem is that if T1 rollbacks, the change is undone but T2 does not know that.
A non-repeatable read occurs when a transaction performs the same retrieval twice but gets a different result each time. Suppose that T1 reads some rows, and that T2 then changes some of those rows and commits the changes. If T1 sees the changes when it reads the rows again, it gets a different result; the initial read is non-repeatable. This is a problem because T1 does not get a consistent result from the same query.
A phantom is a row that appears where it was not visible before. Suppose that T1 and T2 begin, and T1 reads some rows. If T2 inserts a new and T1 sees that row when it reads again, the row is a phantom.
Page 421 describes the four(4) Transaction Isolation Levels:
READ-UNCOMMITTED : allows a transaction to see uncommitted changes made by other transactions. This isolation level allows dirty reads, non-repeatable reads, and phantoms to occur.
READ-COMMITTED : allows a transaction to see changes made by other transactions only if they've been committed. Uncommitted changes remains invisible. This isolation level allows non-repeatable reads, and phantoms to occur.
REPEATABLE READ (default) : ensure that is a transaction issues the same SELECT twice, it gets the same result both times, regardless of committed or uncommitted changesmade by other transactions. In other words, it gets a consistent result from different executions of the same query. In some database systems, REPEATABLE READ isolation level allows phantoms, such that if another transaction inserts new rows,in the inerbal between the SELECT statements, the second SELECT will see them. This is not true for InnoDB; phantoms do not occur for the REPEATABLE READ level.
SERIALIZABLE : completely isolates the effects of one transaction from others. It is similar to REPEATABLE READ with the additional restriction that rows selected by one transaction cannot be changed by another until the first transaction finishes.
Isolation level can be set for your DB Session globally, within your session, or for a specific transaction:
SET GLOBAL TRANSACTION ISOLATION LEVEL isolation_level;
SET SESSION TRANSACTION ISOLATION LEVEL isolation_level;
SET TRANSACTION ISOLATION LEVEL isolation_level;
where isolation_level is one of the following values:
'READ UNCOMMITTED'
'READ COMMITTED'
'REPEATABLE READ'
'SERIALIZABLE'
In my.cnf you can set the default as well:
[mysqld]
transaction-isolation = READ-COMMITTED
As other user is updating the same row, row level lock will be applied. So he is able to make change only after your transaction ends. So you will be seeing the result set that you inserted. Hope this helps.
Interfere is a fuzzy word when it comes to SQL database transactions. What rows a transaction can see is determined in part by its isolation level.
Hence the goal of the select is to get ALL info from the table that
just got inserted by the preceding insert and ONLY by the preceding
INSERT...
Preceding insert is a little fuzzy, too.
You probably ought to COMMIT the insert in question before you try to read it. Otherwise, under certain conditions not under your control, that transaction could be rolled back, and the row with id=100 might not actually exist.
Of course, after it's committed, other transactions are free to change the value of "id", of "value", or both. (If they have sufficient permissions, that is.)
The transaction will make it seem like that the statements in the transaction run without any interference from other transactions. Most DBMSs (including MySQL) maintain ACID properties for transactions. In your case, you are interested in the A for Atomic, which means that the DBMS will make it seem like all the statements in your transactions run atomically without interruption.
The only users that get effect is those that require access to the same rows in a table. Otherwise the user will not be affected.
However is is slightly more complicated as the row locking can be a read lock or a write lock.
Here is an explanation for the InnoDB storage engine.
For efficiency reasons, developers do not set transactions to totally isolated for each other.
Databases support multiples isolation levels namely Serializable, Repeatable reads, Read committed and Read uncommitted. They are list from the most strict to least strict.
I'm using the following format for commiting changes to my db using linq.
Begin Transaction (Scope Serialized, Required)
Check Business Rule 1...N
MyDataContext.SubmitChanges()
Save Changes Done In Previous Query To Log File
End Transaction Scope
But in the SQL Server profiler I see the following line in the Connection:Start.
set transaction isolation level read committed
I went through this (http://social.msdn.microsoft.com/Forums/en-US/adodotnetentityframework/thread/93a45026-0425-4d49-a4ac-1b882e90e6d5) and thought I had an answer;
Until I saw this (https://connect.microsoft.com/VisualStudio/feedback/details/565441/transactionscope-linq-to-sql?wa=wsignin1.0) on Microsoft Connect.
Can someone please tell me whether my code is actually executed under Serialized Isolation Level or whether it is infact just running under read committed?
It depends on how you created the transaction.
If you executed inline SQL to begin it (EG. BEGIN TRAN), L2S will not be aware of the transaction and will spin up a new nested one in READ COMMITTED.
However, if you used System.Transaction, or have a transaction set on your DataContext, SubmitChanges will participate in that transaction.
You can see these transaction starting and stopping in Profiler if you choose the TM: Begin Tran and TM: Commit Tran event classes.
Note: ADO.Net does not issue BEGIN TRAN nor does it issue SET TRANSACTION ISOLATION in batches, this is done at a lower level.
If you really want to confirm the behavior, create a trigger on a table that inserts the current isolation level into a logging table and check on it.
You can pick up your current isolation level by running:
SELECT CASE transaction_isolation_level
WHEN 0 THEN 'Unspecified'
WHEN 1 THEN 'Read Uncommitted'
WHEN 2 THEN 'Read Committed'
WHEN 3 THEN 'Repeatable Read'
WHEN 4 THEN 'Serializable'
WHEN 5 THEN 'Snapshot' END AS TRANSACTION_ISOLATION_LEVEL
FROM sys.dm_exec_sessions
where session_id = ##SPID
My guess is you created the DataContext, then used the TransactionScope. You have to open the connection inside the TransactionScope in order for it to enlist.
From http://entityframework.codeplex.com/workitem/1712
TransactionScope uses remote API calls rather than SQL commands to perform transactions in SQL Server. Those API calls are not included in the a standard trace in SQL Profiler.
You can include them by going to the "Event Selection" page, clicking on the "Show all events" checkbox and the selecting all the events from the Transactions category. This will allow you to see when such events as 'TM: Begin Tran Starting', 'SQLTransaction' and 'TM: Begin Tran Completed' actually occur.
You can also check the TransactionID column for the TSQL events in the "Event Selection" page to see the to which transaction each SQL batch being executed is associated.
Unfortunately I don't know of a direct way to observe the effective isolation level under which each command is being executed in SQL Profiler. But there is an indirect way...
When a connection is opened you will see an 'Audit Login' event in the trace. In many cases this event will contain the isolation level. Now, the 'Audit Login' happens before the actual isolation level is set, so the reported isolation level won't accurately reflect the isolation level of the transaction that is about to start. Here are some tips on how to interpret it:
When a connection opening actually hits a new connection it will always report the default transaction isolation level, e.g. you will see 'set transaction isolation level read uncommitted' (as I said, this is unrelated to the effective isolation level of your transaction as that one will be set at a later point)
After a connection has been opened and then returned to the connection pool (i.e. closed), subsequent connection openings will actually reuse that existing connection from the pool. In this case the 'Audit Login' will report the isolation level that was set when the connection got returned to the pool the last time. This can help you see the isolation level that was used, after the fact.
E.g. in your code snippet, the connection is open for a last time to roll back the transaction (because you have not marked the transaction as completed explicitly). In that 'Audit Login' event you should be able to see the isolation level that was effective when the connection was previously used to execute the query, represented by the line 'set transaction isolation level read uncommitted'.