Suppose we have 2 transactions as T1, T2 with T2 doing dirty read on data modified by T1 and committing before T1. Now suppose T1 fails and is rolled back. My question is- Since T2 is committed, changes made by T2 are transferred from shared buffer to original database or not(Since I have read that changes made by a transaction are made permanent to original db once transaction commits)? And if they are transferred to original database, then how will T1 rollback and get previous value of data item (which was read dirty by T2)? By its buffer or original database?
Unless you are specifically calling a dirty read by setting the isolation level, then this sort of problem simply can not happen. That is the whole idea of a transaction. T2 will be locked out of the row if T1 has updated it. If you are allowing dirty reads by setting set transaction isolation level read uncommitted then the handling of the data is up to you, usually by using a rowversion which your T2 checks hasnt changed before it commits.
Related
The specification for the Repeatable-Read isolation level defines that a transaction with this IL will prevent other transactions from updating any rows that this transaction has read until this transaction has completed. Thus, repeatable reads are guaranteed.
Consider the following order of operations for two concurrent transactions T1 and T2, both using repeatable read IL:
T1: Read row
T2: Read row
T1: Update row
T2: Update row
I think that the update in step 3 would violate the specification for the isolation level, since T2 would read a different value if it read the row again.
The converse can be said for the update in step 4.
So, what different options are available for RDBMSs in general resolve this conflict?
More specifically, how is this handled in SQL Server 2017+?
Will this result in a deadlock since neither transaction can complete its operations?
Or would one transaction be rolled back?
I've seen that Lost Updates are prevented in SQL Server. What does this mean for the resolution of this specific case?
I have perused the answers to these questions:
Repeatable read and lock compatibility table
Repeatable Read - am I understanding this right?
repeatable read and second lost updates issue
MySQL Repeatable Read isolation level and Lost Update phenomena
And although the last one asks a similar question but doesn't include any specific info about how RDBMSs which prevent lost updates for txs with this isolation level handle this case.
This might be a silly Question but if we use WITH (NOLOCK) option where does the sql server pulls the data from...Disk or Data pages.
begin transaction
Update table1
set col1 = 'abc'
In the above code I have not given Commit or rollback statement so data pages will get locked. But with NOLOCK option i can read the records that are not committed yet. select * from Table1 with (NOLOCK) seems to read the data from the table but Records go to the table only after the commit statement and in this case I have not issued any commit statement. So my question is that in this case sql server reads the modified data from DISK or Data pages.
As per my understanding sql follows below mentioned steps:
When a DML statements like INSERT UPDATE or DELETE are issued sql server first finds the records in data pages if not found then it loads the records in the data pages.
checks constraints rules.
Modification in the data pages are done.
Transaction entry is done in the Transaction log.
SQl server writes the data to the harddisk.
Issues checkpoint which is also recorded in the Transaction log.
You're missing some of the subtleties of SQL. Uncommitted changes can indeed go to disk, just not to the database data file (ndf/ndf), but even that isn't relevant for this question. WITH (NOLOCK) queries can read data from any in-progress transactions regardless of whether or not they have been committed. They can also skip data that has been deleted and not yet committed as well. This only makes a difference if the transactions are rolled back or if you care that you might get some results from a transaction without getting other results from the same transaction.
I have some questions about programming with a DBMS (no specific language needed, but I'm using Java; no specific DBMS in mind).
I open a transaction, select a row, then read a field, add 1 to the field, and update, then commit. What happens if another user runs in the same time a transaction on that field? Does it crash the transaction, or what?
Example: I'm a in a shop that has 1 kg of bread. Waiter1 has a client that needs 1 kg of bread. Waiter2 the same. If the program is:
select row "bread"
if quantity>=1 kg then quantity=quantity-1
update row
What happens if the two waiters run the transaction in the same time?
What are the best ways to implement multiuser, avoiding "collision"? Select and lock, transaction only, or what?
When to use optimistic lock, or pessimistic?
When to use lock, and when is it not needed?
Why are you handling this on the application side? Relational databases are built to handle situations like this. Just use an update statement:
UPDATE some_table
SET quantity = quantity - 1
WHERE item_name = 'bread' AND quantity >= 1
What you are looking for is Transaction Isolation. The official SQL standard would handle it like this:
If you don't lock specifically your database will generally lock either the row or even the table for you. Depending on your isolation level it will either wait or raise an error.
Serializable
The second transaction will wait for the first to complete before it can do anything.
Repeatable reads
As soon as the first transaction reads, the second will wait until the first one committed. Or the other way around, if somehow the second transaction starts reading before the first.
Read committed
If the first transaction writes before the second writes, the first will have to wait until the second has committed. Otherwise the second will have to wait until the first has committed.
Read uncommitted
Both can read without an issue, but the first to write will make the other write stall till the transaction has been committed.
If one of the transactions commits after the other reads, you could lose the data and end up with only 1 update.
I've read on Microsoft's site
http://msdn.microsoft.com/en-us/library/ms173763.aspx
that Sql Server doesn't request locks when reading data, except when a database is being recovered.
Does it mean that Sql Server using READ_COMMITTED_SNAPSHOT/SNAPSHOT ISOLATION doesn't use shared locks at all?
How is that possible?
For example, if there are 2 transactions.
First transaction T1 wants to update some row.
Second transaction T2 starts reading same row (this transaction is copying him to some output buffer, response buffer or whatever it's called in Sql Server).
At the same time transaction T1 starts updating that row (it created versioned row first).
Isn't there a possibility that transaction T2 will read uncommited data?
Remember, transaction T2 started copying that row before T1 made update, so there is no exclusive lock on that row.
Is this situation even possible and how can this be avoided without setting shared lock on that row during copying of it's data?
Beside logical locks there are also physical latches to protect the database structures (particularly, in this example, pages). Latches protect any changes (modification of bits), irrelevant of isolation level. So even if the T1 does not acquire locks, it still needs to acquire a shared latch on the pages it reads, otherwise it would be victim to low level concurrent modifications done to the very structures it reads. T2 can modify the page containing the rows it modifies only if it obtains a page exclusive latch. Thus T1 can only see the image of the row either before T2 modified it (and therefore the row is the one T1 wants) or after T2 is complete with the modifications done to the row (and now T1 has to lookup the previous row image in the version store).
The latching protocol must be honored by all isolation levels, including read uncommitted and versioned reads (ie. snapshot and friends).
Does it mean that Sql Server using READ_COMMITTED_SNAPSHOT/SNAPSHOT ISOLATION doesn't use shared locks at all? How is that possible?
It is possible because SQL Server is reading from a SNAPSHOT, which is not going to go through any change at all. It's already frozen at the state of the DB at the start of the current transaction, disregarding uncommitted transactions from other processes. This is done by SQL Server keeping a snapshot (row-versioned) copy of the record in tempdb for transactions to refer to, letting the current in-progress data/index page(s) get changed.
Isn't there a possibility that transaction T2 will read uncommited data? Remember, transaction T2 started copying that row before T1 made update, so there is no exclusive lock on that row.
The above narrative explains this already. But to illustrate (simplified):
Scenario 1:
T1: begin tran (implicit/explicit)
T1: read value (4)
T2: read value (4) -- *
T1: update value to (8)
* - This is the committed value at the time the T2 transaction started
Scenario 2:
T1: begin tran (implicit/explicit)
T1: read value (4)
T1: update value to (8)
version of the row with the value (4) is made
T2: read value (4) -- * from the versioned row
T1: commit
* - (4) is [still] the *committed* value at the time the T2 transaction started
i start with a simple question:
according to Dirty Read definition in
Wikipedia
and Msdn :
we have 2 concurrent transactions, T1 and T2
Dirty Reads Occur in ,when T1 is Updating a row and T2 is reading row that "is not Committed yet" by T1
but at Read Committed Level shared locks are released as soon as the data is read (not at the end of the transaction or even the end of the statement
then how Read Committed prevents Dirty Reads?
Bkaz as soon as the share lock released on updated row T2 can read the updated row and t1 can rollback the whole operation,,then we have a dirty read on the hand of t1
It prevents the dirty read because T1 has a lock on the row, so T2 can't read the "not yet committed" row that could be rollbacked later.
The problem Read Committed tries to resolve is:
T1 creates a transaction and writes something
T2 reads that something
T1 rollback the transaction
now T2 has a data that didn't really ever existed.
Depending on how the DB is structured, there are two "good" possibilities:
T1 creates a transaction and writes something
T2 waits for T1 to end the transaction
or
T2 reads a "snapshot" of how the DB was BEFORE T1 began the transaction (it's called Read committed using row versioning)
(the default on MSSQL is the first option)
Here for example there is a comparison of the various isolation levels: http://msdn.microsoft.com/en-us/library/ms345124(SQL.90).aspx (read under Isolation Levels Offered in SQL Server 2005)
When SQL Server executes a statement at the read committed isolation level, it acquires short lived share locks on a row by row basis. The duration of these share locks is just long enough to read and process each row; the server generally releases each lock before proceeding to the next row. Thus, if you run a simple select statement under read committed and check for locks (e.g., with sys.dm_tran_locks), you will typically see at most a single row lock at a time. The sole purpose of these locks is to ensure that the statement only reads and returns committed data. The locks work because updates always acquire an exclusive lock which blocks any readers trying to acquire a share lock.
Ripped from here