Castor JDO Long Transactions Support Introduction Bounded dirty checking Long transactions that do not depend on cache
Introduction
The usual Castor transactions are called here short
transactions: an object is read, modified and written within the
bounds of one transaction.
Long transactions consist of two Castor transactions:
an object is read in the first and written in the second.
Between them the object is sent "outwards" and modified there.
For example, the object may be send to a client application and
dispayed to a user, or it may be sent to a servlet engine and is
displayed on a web page. After that the modified object returns
back and is written to the database in the second transaction.
At this point the object is usually not the same physical instance
as one that was read in the first transaction.
The example code for writing the object in the second Castor
transaction follows:
// a customer go to a webpage to review her personal information.
// The servlet then call this server side function: getCustomerInfo
public CustomerInfo getCustomerInfo( Integer customNum ) {
// in most case, users simply review information and
// make no change. Even if they make changes, it often
// takes time for them to decide. We don't want to
// lock the database row, so commit right after we load.
db.begin();
CustomerInfo info = (CustomerInfo)
db.load( CustomerInfo.class, customNum );
// we also want to keep track of customers patterns
// well...it helps us provide better service.
info.setLastVisit( today );
db.commit();
return info;
}
// Three days passed, the indecisive customer finally agrees to
// marry Joe. She changes her last name in the webpage and
// clicked the "Submit" button on the webpage.
// The servlet then calls updateCustomerInfo to update the
// last name for the indecisive customer.
public void updateCustomerInfo( CustomerInfo info ) {
db.begin();
db.update(info);
db.commit();
}
Note, that it is natural to read the object in the first
transaction in the read-only mode.
Since the time interval between the first and the second
transaction is relatively big, it is desirable to perform dirty
checking, i.e. to check that the object has not been modified
in the database during the long transaction.
For that the object must hold a timestamp: it is set by
Castor during the first Castor transaction and is checked during
the second one.
In order to enable the dirty checking for long transactions,
the object should implement the interface
org.exolab.castor.jdo.TimeStampable
having two methods: long jdoGetTimeStamp() and
void jdoSetTimeStamp(long timeStamp)
Bounded dirty checking
The advantage of the bounded dirty checking is that it doesn't require
any changes to the database schema. It uses the Castor cache to store
object timestamps. The disadvantage of this algorithm is that it is
bounded by a lifetime of the cached copy of the object. After the
cached copy has been purged, db.update() causes ObjectModifiedException.
Thus, parameters of the cache define dirty checking capabilities.
The cache-type attribute is part of the
<class> element in the XML mapping. Consider the existing cache types:
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- | none - the bounded dirty checking is impossible
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- | count-limited - the count limit for the cache is a count
limit for the objects of this class that can participate in long
and short transactions simultaneously.
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- | time-limited - the time limit for the cache is a time limit
for the long transaction.
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- | unlimited - the bounded dirty checking gives correct results
while the cache exists, i.e. until the crash of the server.
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Long transactions that do not depend on cache
For long transactions (detached objects) it was required
that the entity has been kept in cache from being loaded until its update.
If the entity was expired from cache before the update an
ObjectModifiedException had been thrown. While this is no
problem if all entities of an application can be kept in
cache all the time, it is one for large scale applications
with millions of entities.
With release 1.3 we have changed the handling of timestamps.
While it is still possible to rely on cache only it is now
also possible to persist the timestamp together with the
other properties of the entity. Doing so will ensure that
the timestamp do not change even if the entity got expired
from cache from being loaded until it gets updated. If this
happens the entity gets reloaded during update which also
loads the previous timestamp. Having said that it still is
possible that an ObjectModifiedException is thrown when
another user has changed the same entity in the meantime.
See an example entity and its mapping below:
public class Entity implements TimeStampable {
private Integer _id;
private String _name;
private long _timeStamp;
public Integer getId() { return _id; }
public void setId(final Integer id) { _id = id; }
public String getName() { return _name; }
public void setName(final String name) { _name = name; }
public long getTimeStamp() { return _timeStamp; }
public void setTimeStamp(final long timeStamp) {
_timeStamp = timeStamp;
}
public long jdoGetTimeStamp() { return _timeStamp; }
public void jdoSetTimeStamp(final long timestamp) {
_timeStamp = timestamp;
}
} |
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<class name="Entity">
<cache-type type="time-limited" capacity="300"/>
<map-to table="entity"/>
<field name="id" type="integer" identity="true">
<sql name="id" type="integer"/>
</field>
<field name="name" type="string">
<sql name="name" type="char"/>
</field>
<field name="timeStamp" type="long">
<sql name="timestamp" type="numeric" />
</field>
</class> |
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