Read Modifiers

GigaSpaces ReadModifiers class (see Javadoc) provides static methods and constants to decode read-type modifiers. The sets of modifiers are represented as integers with distinct bit positions representing different modifiers.

Four main types of modifiers can be used:

• REPEATABLE_READ - default modifier
• DIRTY_READ
• READ_COMMITTED
• EXCLUSIVE_READ_LOCK

These should be used for backward compatibility with older versions of GigaSpaces:

• MATCH_BY_ID
• THROW_PARTIAL_FAILURE

You can use bitwise or the | operator to unite different modifiers.

These modifiers can be set either at the proxy level - IJSpace.setReadModifiers(int), or at the operation level (e.g. using one of IJSpace read readIfExists readMultiple count methods with a modifiers parameter).

Spring - GigaSpaces ReadModifiers

The following table describes the mapping between the Spring TransactionDefinition Mapping to GigaSpaces ReadModifiers:

Spring TransactionDefinition	GigaSpaces ReadModifiers
ISOLATION_READ_UNCOMMITTED	DIRTY_READ
ISOLATION_READ_COMMITTED	READ_COMMITTED
ISOLATION_REPEATABLE_READ	REPEATABLE_READ

Repeatable Read

REPEATABLE_READ is the default modifier, defined by the JavaSpaces specification.

The REPEATABLE_READ isolation level allows a transaction to acquire read locks on an object it returns to an application, and write locks an object it write, updates, or deletes. By using the REPEATABLE_READ isolation level, space operations issued multiple times within the same transaction always yield the same result. A transaction using the REPEATABLE_READ isolation level can retrieve and manipulate the same object as many times as needed until it completes its task. However, no other transaction can write, update, or delete an object that can affect the result being accessed, until the isolating transaction releases its locks. That is, when the isolating transaction is either committed or rolled back.

Transactions using the REPEATABLE_READ isolation level wait until the object that is write-locked by other transactions are unlocked before they acquire their own locks. This prevents them from reading "dirty" data. In addition, because other transactions cannot update or delete an object that is locked by a transaction using the REPEATABLE_READ isolation level, non-repeatable read situations are avoided.

Dirty Read

The JavaSpaces specification defines the visibility of object for read operations as follows: A read operation performed under a null transaction can only access space objects that are not write-locked by non-null transactions. In other words, space objects that were written or taken by active transactions (transactions that have not been committed or rolled back) are not visible to the user performing a read operation.

Sometimes it is desirable for non-transactional read operations to have full visibility of the objects in the space. The DIRTY_READ modifier, once set, enables read/readIfExists/readMultiple/count operations under a null transaction to have this complete visibility.

Code Example

// write something under txn X and commit, making it publicly visible
space.write( something, txnX, Lease.FOREVER);
txnX.commit();

// update this something with a new one under a different txn Y
space.update( newSomething, txnY, Lease.FOREVER, IJSpace.NO_WAIT);

// all read operations (read, readIfExists, readMultiple, count) return the
// version of the object before txnY was committed (newSomething).
// operations can be performed with a new txn Z or a null txn
space.read( tmpl, null, ReadModifiers.DIRTY_READ);

// Note: using the same txn (txn Y) will return matches that are visible under the transaction

Read Committed

The READ_COMMITTED modifier enables a read-committed isolation level in read operations.

Read-committed is the isolation-level in which a read operation (under a transaction or a null transaction) can not see changes made by other transactions, until those transactions are committed. At this level of isolation, dirty-reads are not possible, but unrepeatable-reads and phantoms might occur.

Read-committed is the default isolation level in database systems. This isolation level means that the read operations return the space objects that are currently committed, regardless of the fact that these space objects might be updated (with a newer version) or taken under an uncommitted transaction. This is opposed to the default space isolation-level (derived from the JavaSpaces specification), which is repeatable-read.

The read-committed isolation level is useful for the local cache, local view, and GSIterators, which performs readMultiple and keep their current status by registering notify templates.

The READ_COMMITTED modifier is provided at the proxy level and the read API level. It is relevant for read, readIfExists, readMultiple, and count.

READ_COMMITTED and DIRTY_READ are mutually-exclusive. A space object under an (uncommitted) updating transaction or under a taking (unrolled) transaction returns the original (committed) value unless the operation is under the same transaction as the locking one.

If the read operation is under a transaction, there is no need to "enlist" the space object in the transaction (unless its already enlisted).

readIfExists has less space objects to wait for, unless these are new objects under an uncommitted transaction.

Locking and Blocking Rules

Operation A/Operation B	Update under transaction Y	Take under transaction Y	Read under transaction Y	Update, null transaction	Take, null transaction	Read, null transaction	Exclusive Read Lock	Dirty Read Transaction Y or null	Read Committed Transaction Y or null
Update under transaction X	Blocked	Blocked	Blocked	Blocked	Blocked	Blocked	Blocked	Allowed	Allowed
Take under transaction X	Blocked	Blocked	Blocked	Blocked	Blocked	Blocked	Blocked	Allowed	Allowed
Read under transaction X	Blocked	Blocked	Allowed	Blocked	Blocked	Allowed	Blocked	Allowed	Allowed
Update, null transaction	Allowed	Allowed	Allowed	Allowed	Allowed	Allowed	Allowed	Allowed	Allowed
Take, null transaction	Allowed	Allowed	Allowed	Allowed	Allowed	Allowed	Allowed	Allowed	Allowed
Read, null transaction	Allowed	Allowed	Allowed	Allowed	Allowed	Allowed	Allowed	Allowed	Allowed
Exclusive Read Lock	Blocked	Blocked	Blocked	Blocked	Blocked	Blocked	Blocked	Allowed	Allowed
Read Committed transaction X or null	Allowed	Allowed	Allowed	Allowed	Allowed	Allowed	Allowed	Allowed	Allowed
Dirty Read Transaction X or null	Allowed	Allowed	Allowed	Allowed	Allowed	Allowed	Allowed	Allowed	Allowed

Refer to the Locking and Blocking page for general GigaSpaces locking and blocking rules.

Code Example

The examples below assumes you are using IJSpace interface that is available via the GigaSpaces.getSpace(). If you are using the GigaSpaces interface and Spring automatic transaction demarcation, you will not need to specify the transaction object explicitly. Still, the blocking rules will be enforced.

GigaSpace space= ...
// write an object under txn X and commit, making it publicly visible
space.write( something, txnX, Lease.FOREVER);
txnX.commit();

// update this object with a new one under a different txn Y
space.update( newSomething, txnY, Lease.FOREVER, Space.NO_WAIT);

// all read operations (read, readIfExists, readMultiple, count) return the last publicly visible match.
// operations can be performed with a new txn Z or a null txn
space.read( tmpl, txnZ, ReadModifiers.READ_COMMITTED);

// Note: using the same txn (txn Y) will return matches that are visible under the transaction

Exclusive Read Lock

The Exclusive Read Lock is similar to select for update SQL for RDBMS, or update lock with ODBMS.

In the JavaSpaces specification, a read under a transaction does not allow other users to modify the Entry, but it does allow two readers or more to read the same Entry under different transactions. To allow a user to block other users from reading an object, a read using exclusive read lock mode with a transaction should be performed.

The following methods support exclusive read lock when used with a transaction:

• read()
• readIfExists()
• readByID()
• readMultiple()

The exclusive read lock is supported in a clustered environment when using the Jini Transaction Manager.

Code Example

IJSpace space = ...
space.setReadModifiers(ReadModifiers.EXCLUSIVE_READ_LOCK);
// this will allow all read operations with this proxy to use Exclusive Read Lock mode
Lock lock = new Lock();
lock.key = new Integer(1);
lock.data = "my data";
space.write(lock, null, Lease.FOREVER);
Transaction txn1 = getTX();
Lock lock_template1 = new Lock();
lock_template1.key = new Integer(1);
Lock lock1 = (Lock) space.read(lock_template1, txn1, 1000);
If (lock1!= null)
    System.out.println("Transaction " + txn1.id + " Got exclusive Read Lock on Entry:"
        + lock1.getId());

MATCH_BY_ID & THROW_PARTIAL_FAILURE

The matching behavior can be changed by adding one of these modifiers.

Setting MATCH_BY_ID changes the matching algorithm, such that once a SpaceID is set the other fields' values are ignored. The matching is only done according to the SpaceID value.

Setting THROW_PARTIAL_FAILURE is only effective when readMultiple() is called. When set and a matching can be performed only on part of the partitions instead of just returning the partial result a QueryMultiplePartialFailureException is thrown including the partial result and the source of the partial failure.