Connection.setTransactionIsolation(int) warns:
Note: If this method is called during a transaction, the result is implementation-defined.
This bring up the question: how do you begin a transaction in JDBC? It's clear how to end a transaction, but not how to begin it.
If a Connection starts inside in a transaction, how are we supposed to invoke Connection.setTransactionIsolation(int) outside of a transaction to avoid implementation-specific behavior?
Answering my own question:
JDBC connections start out with auto-commit mode enabled, where each SQL statement is implicitly demarcated with a transaction.
Users who wish to execute multiple statements per transaction must turn auto-commit off.
Changing the auto-commit mode triggers a commit of the current transaction (if one is active).
Connection.setTransactionIsolation() may be invoked anytime if auto-commit is enabled.
If auto-commit is disabled, Connection.setTransactionIsolation() may only be invoked before or after a transaction. Invoking it in the middle of a transaction leads to undefined behavior.
See JDBC Tutorial by Oracle.
JDBC implicitly demarcates each query/update you perform on the connection with a transaction. You can customize this behavior by calling setAutoCommit(false) to turn off the auto-commit mode and call the commit()/rollback() to indicate the end of a transaction. Pesudo code
try
{
con.setAutoCommit(false);
//1 or more queries or updates
con.commit();
}
catch(Exception e)
{
con.rollback();
}
finally
{
con.close();
}
Now, there is a type in the method you have shown. It should be setTransactionIsolation(int level) and is not the api for transaction demarcation. It manages how/when the changes made by one operation become visible to other concurrent operations, the "I" in ACID (http://en.wikipedia.org/wiki/Isolation_(database_systems))
I suggest you read this you'll see
Therefore, the first call of
setAutoCommit(false) and each call of
commit() implicitly mark the start of
a transaction. Transactions can be
undone before they are committed by
calling
Edit:
Check the official documentation on JDBC Transactions
When a connection is created, it is in auto-commit mode. This means
that each individual SQL statement is treated as a transaction and is
automatically committed right after it is executed. (To be more
precise, the default is for a SQL statement to be committed when it is
completed, not when it is executed. A statement is completed when all
of its result sets and update counts have been retrieved. In almost
all cases, however, a statement is completed, and therefore committed,
right after it is executed.)
The way to allow two or more statements to be grouped into a
transaction is to disable the auto-commit mode. This is demonstrated
in the following code, where con is an active connection:
con.setAutoCommit(false);
Source: JDBC Transactions
Startingly, you can manually run a transaction, if you wish to leave your connection in "setAutoCommit(true)" mode but still want a transaction:
try (Statement statement = conn.createStatement()) {
statement.execute("BEGIN");
try {
// use statement ...
statement.execute("COMMIT");
}
catch (SQLException failure) {
statement.execute("ROLLBACK");
}
}
Actually, this page from the JDBC tutorial would be a better read.
You would get your connection, set your isolation level and then do your updates amd stuff and then either commit or rollback.
You can use these methods for transaction:
you must create the connection object like con
con.setAutoCommit(false);
your queries
if all is true con.commit();
else con.rollback();
Maybe this will answer your question:
You can only have one transaction per connection.
If autocommit is on (default), every select, update, delete will automatically start and commit (or rollback) a transaction.
If you set autocommit off, you start a "new" transaction (means commit or rollback won't happen automatically). After some statements, you can call commit or rollback, which finishes current transaction and automatically starts a new one.
You cannot have two transactions actively open on one JDBC connection on pure JDBC.
Using one connection for multiple transactions (reuse, pooling or chaining) some weird problems can lurk creating problems people have to live by since they usually cant identify the causes.
The following scenarios come to mind:
(Re-)Using a connection with an ongoing / uncommitted transaction
Flawed connection pool implementations
Higher isolation level implementations in some databases (especially the distributed SQL and the NoSQL one)
Point 1 is straight forward and understandable.
Point 2 basically leads to either point 1 or (and) point 3.
Point 3 is all about a system where a new transaction has begun before the first statement is issued. From a database perspective such a transaction might have started long before the 'first' real statement was issued. If the concurrency model is based on the snapshot idea where one reads only states/values that were valid at the point the transaction begins but no change that has changed later on, it is very important that on commit the full read set of the current transaction is also validated.
Since NoSQL and certain isolation levels like MS SQL-Server Snapshot often do not validate the read-set (in the right way), all bets are usually off to what to expect. While this is a problem always being present, it is way worse when one is dealing with transactions that start on the last commit or when the connection was pooled rather than the connection being actually used, it is usually important to make sure the transaction actually starts when it is expected to start. (Also very important if one uses a rollback-only read-only transaction).
I use the following rules when dealing with JDBC in JAVA:
Always rollback a JDBC connection before using it (scraps everyting and starts a new transaction), if the company uses plain JDBC in conjunction with any pooling mechanism
Use Hibernate for Transaction handling even if only using a session managed JDBC connection for plain SQL. Never had a problem with transactions till now.
Use BEGIN / COMMIT / ROLLBACK as SQL-Statements (like already mentioned). Most implementations will fail if you issue a BEGIN statement during an active transaction (test it for your database and remember the test database is not the production database and JDBC Driver and JDBC Server-side implementations can differ in behaviro than running a SQL console on the actual server).
Use 3 inside one's own wrapper for a JDBC connection instances. This way transaction handling is always correct (if no reflection is used and the connection pooling is not flawed).
3+4 I only use if response time is critical or if Hibernate is not available.
4 allows for using some more advanced performance (response time) improvement patterns for special cases
Related
Many articles and documents says that by setting Auto Commit off, You can start a transaction in JDBC. This Topic also ask same question but It doesn't answer to the question and just said:
Changing the auto-commit mode triggers a commit of the current transaction (if one is active).
Ok. but next?
for finding an answer, I searched and found this:
Autocommit transactions:
Each individual statement is a transaction.
Explicit transactions:
Each transaction is explicitly started with the
BEGIN TRANSACTION statement and explicitly ended with a COMMIT or
ROLLBACK statement.
Implicit transactions:
A new transaction is implicitly started when the prior transaction completes, but each transaction is explicitly
completed with a COMMIT or ROLLBACK statement.
And then I found this:
Committing Transactions
After the auto-commit mode is disabled, no SQL statements are committed until you call the method commit explicitly. All statements executed after the previous call to the method commit are included in the current transaction and committed together as a unit.
Therefore I conclude that after the auto-commit mode is disabled, we are in Implicit mode and we also know that for disabling auto-commit, a COMMIT statement has been run so after setting the Auto-commit off, we've started a new transaction.
Can we draw such a conclusion based on these cases? is it a right conclusion?
No, you cannot. In JDBC, auto-commit only governs when a transaction is to end. A driver is expected to start a transaction when it is needed. Specifically the JDBC 4.3 specification says in section 10.1 Transaction Boundaries and Auto-commit:
When to start a new transaction is a decision made implicitly by
either the JDBC driver or the underlying data source. Although some
data sources implement an explicit “begin transaction” statement,
there is no JDBC API to do so. Typically, a new transaction is started
when the current SQL statement requires one and there is no
transaction already in place. Whether or not a given SQL statement
requires a transaction is also specified by SQL:2003.
The Connection attribute auto-commit specifies when to end
transactions. Enabling auto-commit causes a transaction commit after
each individual SQL statement as soon as that statement is complete.
The point at which a statement is considered to be “complete” depends
on the type of SQL statement as well as what the application does
after executing it:
For Data Manipulation Language (DML) statements such as Insert, Update, Delete, and DDL statements, the statement is complete as soon
as it has finished executing.
For Select statements, the statement is complete when the associated result set is closed.
For CallableStatement objects or for statements that return multiple results, the statement is complete when all of the associated
result sets have been closed, and all update counts and output
parameters have been retrieved.
In other words, when you call connection.setAutoCommit(false) no transaction will be started. Only when a statement is executed (or another operation that requires a transaction), a transaction will be started if there is no active transaction.
Why do I need Transaction in Hibernate for read-only operations?
Does the following transaction put a lock in the DB?
Example code to fetch from DB:
Transaction tx = HibernateUtil.getCurrentSession().beginTransaction(); // why begin transaction?
//readonly operation here
tx.commit() // why tx.commit? I don't want to write anything
Can I use session.close() instead of tx.commit()?
Transactions for reading might look indeed strange and often people don't mark methods for transactions in this case. But JDBC will create transaction anyway, it's just it will be working in autocommit=true if different option wasn't set explicitly. But there are practical reasons to mark transactions read-only:
Impact on databases
Read-only flag may let DBMS optimize such transactions or those running in parallel.
Having a transaction that spans multiple SELECT statements guarantees proper Isolation for levels starting from Repeatable Read or Snapshot (e.g. see PostgreSQL's Repeatable Read). Otherwise 2 SELECT statements could see inconsistent picture if another transaction commits in parallel. This isn't relevant when using Read Committed.
Impact on ORM
ORM may cause unpredictable results if you don't begin/finish transactions explicitly. E.g. Hibernate will open transaction before the 1st statement, but it won't finish it. So connection will be returned to the Connection Pool with an unfinished transaction. What happens then? JDBC keeps silence, thus this is implementation specific: MySQL, PostgreSQL drivers roll back such transaction, Oracle commits it. Note that this can also be configured on Connection Pool level, e.g. C3P0 gives you such an option, rollback by default.
Spring sets the FlushMode=MANUAL in case of read-only transactions, which leads to other optimizations like no need for dirty checks. This could lead to huge performance gain depending on how many objects you loaded.
Impact on architecture & clean code
There is no guarantee that your method doesn't write into the database. If you mark method as #Transactional(readonly=true), you'll dictate whether it's actually possible to write into DB in scope of this transaction. If your architecture is cumbersome and some team members may choose to put modification query where it's not expected, this flag will point you to the problematic place.
All database statements are executed within the context of a physical transaction, even when we don’t explicitly declare transaction boundaries (e.g., BEGIN, COMMIT, ROLLBACK).
If you don't declare transaction boundaries explicitly, then each statement will have to be executed in a separate transaction (autocommit mode). This may even lead to opening and closing one connection per statement unless your environment can deal with connection-per-thread binding.
Declaring a service as #Transactional will give you one connection for the whole transaction duration, and all statements will use that single isolation connection. This is way better than not using explicit transactions in the first place.
On large applications, you may have many concurrent requests, and reducing database connection acquisition request rate will definitely improve your overall application performance.
JPA doesn't enforce transactions on read operations. Only writes end up throwing a TransactionRequiredException in case you forget to start a transactional context. Nevertheless, it's always better to declare transaction boundaries even for read-only transactions (in Spring #Transactional allows you to mark read-only transactions, which has a great performance benefit).
Transactions indeed put locks on the database — good database engines handle concurrent locks in a sensible way — and are useful with read-only use to ensure that no other transaction adds data that makes your view inconsistent. You always want a transaction (though sometimes it is reasonable to tune the isolation level, it's best not to do that to start out with); if you never write to the DB during your transaction, both committing and rolling back the transaction work out to be the same (and very cheap).
Now, if you're lucky and your queries against the DB are such that the ORM always maps them to single SQL queries, you can get away without explicit transactions, relying on the DB's built-in autocommit behavior, but ORMs are relatively complex systems so it isn't at all safe to rely on such behavior unless you go to a lot more work checking what the implementation actually does. Writing the explicit transaction boundaries in is far easier to get right (especially if you can do it with AOP or some similar ORM-driven technique; from Java 7 onwards try-with-resources could be used too I suppose).
It doesn't matter whether you only read or not - the database must still keep track of your resultset, because other database clients may want to write data that would change your resultset.
I have seen faulty programs to kill huge database systems, because they just read data, but never commit, forcing the transaction log to grow, because the DB can't release the transaction data before a COMMIT or ROLLBACK, even if the client did nothing for hours.
In Hibernate API, there is a property hibernate.connection.autocommit which can be set to true.
But in the API, they have mentioned that it is not recommended to set it like so:
Enables autocommit for JDBC pooled connections (it is not
recommended).
Why is it not recommended ?
What are the ill-effects of setting this property to true ?
All database statements are executed within the context of a physical transaction, even when we don’t explicitly declare transaction boundaries (BEGIN/COMMIT/ROLLBACK).
If you don't declare the transaction boundaries, then each statement will have to be executed in a separate transaction. This may even lead to opening and closing one connection per statement.
Declaring a service as #Transactional will give you one connection for the whole transaction duration, and all statements will use that single isolation connection. This is way better than not using explicit transactions in the first place. On large applications you may have many concurrent requests and reducing the database connection acquiring request rate is definitely improving your overall application performance.
So the rule of thumb is:
If you have read-only transactions that only execute one query, you can enable auto-commit for those.
If you have transactions containing more than one statement, you need to disable auto-commit, since you want all operations to execute in a single unit-of-work and you don't want to put extra pressure on your connection pool.
By default the autocommit value is false, therefore the transaction needs to be commited explicitly. This might be the reason why the changes not getting reflected in database, else can try flush to force the changes before commit.
When you close the session, then it will get commited in database implicitly [depends on the implementation].
When you have cascading transactions & needs to rollback for atomicity, you need to have control over transactions & in that case, autocommit should be false.
Either set autocommit as true or handle transactions explicitly.
Here is a good explanation on it.
Hibernate forum related to this.
Stackoverflow question on it.
My understanding is that if Hibernate autocommits, then a flush that fails part way through won't be rolled back. You'll have an incomplete/broken object graph.
If you want a connection with autocommit on for something, you can always unwrap a newly created Session to get the underlying JDBC Connection, setAutocommit(true) on it, do your work via JDBC APIs, setAutocommit(false), and close the session. I would not recommend doing this on a Session that's already done anything.
Do not use the session-per-operation antipattern: do not open and close a Session for every simple database call in a single thread. The same is true for database transactions. Database calls in an application are made using a planned sequence; they are grouped into atomic units of work. This also means that auto-commit after every single SQL statement is useless in an application as this mode is intended for ad-hoc SQL console work. Hibernate disables, or expects the application server to disable, auto-commit mode immediately. Database transactions are never optional. All communication with a database has to occur inside a transaction. Auto-commit behavior for reading data should be avoided, as many small transactions are unlikely to perform better than one clearly defined unit of work. The latter is also more maintainable and extensible.
find more information on this topic
Basically I have (ignoring exception handling, etc.):
connection.setAutoCommit(false);
Statement statement1 = connection.createStatement();
statement1.executeUpdate("...");
statement1.close();
Statement statement2 = connection.createStatement();
statement2.executeUpdate("...");
statement2.close();
connection.commit();
If I understand correctly it shouldn't have any impact because all it really does is free the resources for the GC. Especially with Derby: You should explicitly close Statements, ResultSets, and Connections when you no longer need them. Connections to Derby are resources external to an application, and the garbage collector will not close them automatically.
However will it cause any issues with the transaction? I don't believe the transaction relies on the Statement. Can anyone please confirm this?
Absolutely, you can close them, and you should.
Generally speaking, once a Statement is executed, the underlying datasource/database is responsible for ensuring successful execution. Any failures are expected to result in SQLExceptions being thrown in the Statement.executeXXX invocations. And any successful execution would result in the database tracking these updates in a temporary working area. Committing the transaction merely ensures that the updates caused by the statements are written to a durable store, from the temporary working area. This is often the case in most/all databases.
It is therefore safe to close a Statement object once you no longer need it, without encountering any side effects in the transaction.
Of course it is safe to close statements prior to committing transactions. You should have to read - Closing Statement object prior to committing from coderanch and JDBC Transactions tutorial.
Yup. Its a good practice and approach to close statement before committing the transactions.
We are trying to fix some issues on a testing harness, and having issues with a particular test, which basically tests a feature that creates an entity, does some processing and stores it in a database (Yes, the C in CRUD).
In the tearDown section of the fitnesse test, we execute a delete statement on that record. However, nothing is being deleted.
We suspect that this may be because the tearDown is executed before the SUT commits its transaction. So consequently, there's nothing to be deleted.
To try and fix this, we are making a pollable jdbc delete:
java.sql.Statement statement;
/*creates a statement*/
do{
recordsDeleted = statement.executeUpdate("delete...");
Thread.sleep(INTERVAL);
}while(recordsDeleted == 0);
So here comes the questions:
When is a jdbc transaction commited?
In the code above, will the updates be executed on the same transaction, or will a new transaction be created for each iteration of the do-while loop? (I'm inclined to think that they will be executed in the same transaction, since the java.sql.Connection holds the commit, rollback, etc methods).
Can you suggest another solution for this problem? I would think that this is quite common, but my teammates have not found any solution online, just the "poll until its deleted or timeout" suggestion.
I don't see anything wrong with your loop initially. You are calling statement.close() somewhere right? I assume the SUT is in another thread or is remote? Are you sure your delete criteria matches the input? Can you examine the database in another process to see if the create is making it to the database?
In terms of transactions, it depends on the database but typically there are no transactions by default. Typically auto-commit is enabled so each individual statement is executed and committed immediately. If you want to enable transactions then you need to disable auto-commit and then call databaseConnection.setSavePoint(). A transaction is committed when commit() is called. If the connection is closed (or rollback() called) then the transaction is rolled back.