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title | weight |
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Transactions | 90 |
Transactions
Strolch Transactions play a central role in a Strolch agent. A transaction is
opened for a realm, and grants access to the model of the agent. Transactions
are implemented as a Java try-with-resources
by implementing
the AutoCloseable
interface. This makes it trivial to understand the scope of a transaction.
Transactions handle the following:
- Opening and closing database connections
- Releasing locks to strolch elements, if
tx.lock(StrolchRootElement)
ortx.lock(Locator)
was called - Performing Commands by executing them in the added order, and validating them first.
- Exception handling
- Auditing
- Updating observers
When a transaction is opened, it is by default read-only, i.e. does not perform
any commands when it is closed. Should the TX perform commands, then it is
important to call tx.commitOnClose()
, but only at the end of the work, so that
exception handling can properly work if something goes wrong.
StrolchTransaction
offers a myriad of methods:
- find element by its
Locator
- get methods for elements by type and id, or using a
StringParameter
orStringListParameter
references - methods to add, update or remove elements
- assert privilege access
- get a new element by its template
- check if an element exists by type and id
- get streams for elements
- add commands for execution
Transactions are opened by accessing the realm, but there are convenience methods depending on the use-case:
- In Services: by calling one of the
openTx()
-methods - In Commands: Transactions are already open, use method
tx()
to get instance. - REST API:
RestfulStrolchComponent.openTx()
{{% notice warning %}} Note: don't open a new TX inside a TX for the same realm! {{% /notice %}}
Important is to always open the transaction as a try-with-resource
block and to
define if the TX should commit, or not:
try (StrolchTransaction tx = openTx(...)) {
// read lock our object
Locator ferrariLoc = Resource.locatorFor("Car", "ferrari");
tx.lock(ferrariLoc);
// find a car by locator
Resource ferrari = tx.findElement(ferrariLoc);
// get a car by ID
Resource opel = tx.getResourceBy("Car", "opel", true);
// modify ball
opel.setName("Opel Corsa");
tx.update(opel);
// get by string reference
StringParameter ownerP = ferrari.getParameter("relations", "owner", true);
Resource owner = tx.getResourceBy(ownerP, true);
// get by string list reference
StringListParameter previousOwnersP = opel.getParameter("relations", "previousOwners", true);
List<Resource> previousOwners = tx.getResourcesBy(previousOwnersP, true);
// check resource exists
if (tx.hasResource("Car", "audi")) {
Resource audi = tx.getResourceBy("Car", "audi", true);
// assert has privilege to remove a car
tx.assertHasPrivilege(Operation.REMOVE, audi);
// remove the car
tx.remove(audi);
}
// iterate all cars
tx.streamResources("Car").forEach(car -> {
logger.info("Car: " + car.getId());
});
// commit if TX was changed
if (tx.needsCommit())
tx.commitOnClose();
}