Due to security advantages through chaining of blocks and public key encryption, Is it possible to implement a transaction abort in blockchains?
No, you can't abort or revert a blockchain transaction when it's complete. When the transaction is mined, it's stored in the immutable block, meaning it'll never be changed.
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According to the reference documentation the READ ONLY transaction flag is useful other than allowing DEFERRABLE transactions?
SET SESSION CHARACTERISTICS AS TRANSACTION READ ONLY;
The DEFERRABLE transaction property has no effect unless the
transaction is also SERIALIZABLE and READ ONLY. When all three of
these properties are selected for a transaction, the transaction may
block when first acquiring its snapshot, after which it is able to run
without the normal overhead of a SERIALIZABLE transaction and without
any risk of contributing to or being canceled by a serialization
failure. This mode is well suited for long-running reports or backups.
Does the database engine runs other optimizations for read-only transactions?
To sum up the comments from Nick Barnes and Craig Ringer in the question comments:
The READ_ONLY flag does not necessarily provide any optimization
The main benefit of setting the READ_ONLY flag is to ensure that no tuple is going to be modified
Actually, it does. Let me just cite source code comment here:
/*
* Check if we have just become "RO-safe". If we have, immediately release
* all locks as they're not needed anymore. This also resets
* MySerializableXact, so that subsequent calls to this function can exit
* quickly.
*
* A transaction is flagged as RO_SAFE if all concurrent R/W transactions
* commit without having conflicts out to an earlier snapshot, thus
* ensuring that no conflicts are possible for this transaction.
*/
Let's say I need to perform two different kinds write operations on a datastore entity that might happen simultaneously, for example:
The client that holds a write-lock on the entry updates the entry's content
The client requests a refresh of the write-lock (updates the lock's expiration time-stamp)
As the content-update operation is only allowed if the client holds the current write-lock, I need to perform the lock-check and the content-write in a transaction (unless there is another way that I am missing?). Also, a lock-refresh must happen in a transaction because the client needs to first be confirmed as the current lock-holder.
The lock-refresh is a very quick operation.
The content-update operation can be quite complex. Think of it as the client sending the server a complicated update-script that the server executes on the content.
Given this, if there is a conflict between those two transactions (should they be executed simultaneously), I would much rather have the lock-refresh operation fail than the complex content-update.
Is there a way that I can "prioritize" the content-update transaction? I don't see anything in the docs and I would imagine that this is not a specific feature, but maybe there is some trick I can use?
For example, what happens if my content-update reads the entry, writes it back with a small modification (without committing the transaction), then performs the lengthy operation and finally writes the result and commits the transaction? Would the first write be applied immediately and cause a simultaneous lock-refresh transaction to fail? Or are all writes kept until the transaction is committed at the end?
Is there such a thing as keeping two transactions open? Or doing an intermediate commit in a transaction?
Clearly, I can just split my content-update into two transactions: The first one sets a "don't mess with this, please!"-flag and the second one (later) writes the changes and clears that flag.
But maybe there is some other trick to achieve this with fewer reads/writes/transactions?
Another thought I had was that there are 3 different "blocks" of data: The current lock-holder (LH), the lock expiration (EX), and the content that is being modified (CO). The lock-refresh operation needs to perform a read of LH and a write to EX in a transaction, while the content-update operation needs to perform a read of LH, a read of CO, and a write of CO in a transaction. Is there a way to break the data apart into three entities and somehow have the transactions span only the needed entities? Since LH is never modified by these two operations, this might help avoid the conflict in the first place?
The datastore uses optimistic concurrency control, which means that a (datastore primitive) transaction waits until it is committed, then succeeds only if someone else hasn't committed first. Typically, the app retries the failed transaction with fresh data. There is no way to modify this first-wins behavior.
It might help to know that datastore transactions are strongly consistent, so a client can first commit a lock refresh with a synchronous datastore call, and when that call returns, the client knows for sure whether it obtained or refreshed the lock. The client can then proceed with its update and lock clear. The case you describe where a lock refresh and an update might occur concurrently from the same client sounds avoidable.
I'm assuming you need the lock mechanism to prevent writes from other clients while the lock owner performs multiple datastore primitive transactions. If a client is actually only doing one update before it releases the lock and it can do so within seconds (well before the datastore RPC timeout), you might get by with just a primitive datastore transaction with optimistic concurrency control and retries. But a lock might be a good idea for simple serialization of, say, edits to a record in a user interface, where a user hits an "edit" button in a UI and you want that to guarantee that the user has some time to prepare and submit changes without the record being changed by someone else. (Whether that's the user experience you want is your decision. :) )
I've a question regarding distributed transactions. Let's assume I have 3 transaction programs:
Transaction A
begin
a=read(A)
b=read(B)
c=a+b
write(C,c)
commit
Transaction B
begin
a=read(A)
a=a+1
write(A,a)
commit
Transaction C
begin
c=read(C)
c=c*2
write(A,c)
commit
So there are 5 pairs of critical operations: C2-A5, A2-B4, B4-C4, B2-C4, A2-C4.
I should ensure integrity and confidentiality, do you have any idea of how to achieve it?
Thank you in advance!
What you have described in your post is a common situation in multi-user systems. Different sessions simultaneously start transactions using the same tables and indeed the same rows. There are two issues here:
What happens if Session C reads a record after Session A has updated it but before Session A has committed its trandsaction?
What happens if Session C updates the same record which Session A has updated but not committed?
(Your scenario only illustrates the first of these issues).
The answer to the first question is ioslation level. This is the definition of the visibility of uncommmitted transactions across sessions. The ANSI standard specifies four levels:
SERIALIZABLE: no changes from another session are ever visible.
REPEATABLE READ: phantom reads allowed, that is the same query executed twice may return different results.
READ COMMITTED: only changes which have been committed by another session are visible.
READ UNCOMMITTED: diryt readsallowed, that is uncommitted changes from one session are visible in another.
Different flavours or database implement these in different fashions, and not all databases support all of them. For instance, Oracle only supports READ COMMITTED and SERIALIZABLE, and it implements SERIALIZABLE as a snapsot (i.e. it is a read-only transaction). However, it uses multiversion concurrency control to prevent non-repeatable reads in READ COMMITTED transactions.
So, coming back to your question, the answer is: set the appropriate Isolation Level. What the appropriate level is depends on what levels your database supports, and what behaviour you wish to happen. Probably you want READ COMMITTED or SERIALIZABLE, that is you want your transactions to proceed on the basis of data values being consistent with the start of the transaction.
As to the other matter, the answer is simpler: transactions must issue locks on tables or preferably just the required rows, before they start to update them. This ensures that the transaction can proceed to change those values without causing a deadlock. This is called pessimistic locking. It is not possible in applications which use connection pooling (i.e. most web-based applications), and the situation there is much gnarlier.
How do you guys decide that you should be wrapping the sql in a transaction?
Please throw some light on this.
Cheers !!
A transaction should be used when you need a set of changes to be processed completely to consider the operation complete and valid. In other words, if only a portion executes successfully, will that result in incomplete or invalid data being stored in your database?
For example, if you have an insert followed by an update, what happens if the insert succeeds and the update fails? If that would result in incomplete data (in this case, an orphaned record), you should wrap the two statements in a transaction to get them to complete as a "set".
If you are executing two or more statements that you expect to be functionally atomic, you should wrap them in a transaction.
if your have more than a single data modifying statement to execute to complete a task, all should be within a transaction.
This way, if the first one is successful, but any of the following ones has an error, you can rollback (undo) everything as if nothing was ever done.
Whenever you wouldn't like it if part of the operation can complete and part of it doesn't.
Anytime you want to lock up your database and potentially crash your production application, anytime you want to litter your application with hidden scalability nightmares go ahead and create a transaction. Make it big, slow, and put a loop inside.
Seriously, none of the above answers acknowledge the trade-off and potential problems that come with heavy use of transactions. Be careful, and consider the risk/reward each time.
Ebay doesn't use them at all. I'm sure there are many others.
http://www.infoq.com/interviews/dan-pritchett-ebay-architecture
Whenever any operation falls under ACID(Atomicity,Consistency,Isolation,Durability) criteria you should use transactions
Read this article
When you want to use atomic or isolation property of database for a set of changes.
Atomicity: An atomic transaction is an indivisible and irreducible series of database operations such that either all occurs, or nothing occurs(according to wikipedia).
Isolation: isolation determines how transaction integrity is visible to other users and systems(according to wikipedia).
i have fired an 'update' query by mistake. But while processing i canceled the transaction. I want to know during that time of execution and cancellation has any data got manipulated.
What sort of query?
If it was a plain SELECT, then no damage will have been done.
If it was in a transaction, then the transaction should have been rolled back - and any damage should have been undone.
If the operation was not running in a transaction, the behaviour will be DBMS-specific. Most will treat statements as atomic - either it completes or it is as if the statement was never executed. Not all do things that way, though.
It would help if you specified which DBMS you are using - there can be differences in the answer depending on the nuances of the DBMS in question.
But while processing i cancelled the
transaction.
WP -
Basically, if it was a transaction, and you canceled it before it finished, then whatever had started would have been undone. What your database looks like now should be the same as it looked before the UPDATE.