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Transactions

Transactions are a core component of an Arez application they are not directly exposed by the Arez API. Transactions are created when actions are invoked, when computable values are computed and when observers are executed.

Transaction Mode

All work within an Arez system occurs within the scope of a transaction. Observable data can only be accessed within the scope of a transaction and observable data can only be updated if the transaction is a read-write transaction.

Transactions can be nested within one another but a read-write transaction can only be nested within another read-write transaction. A read-only transaction can NOT have a read-write transaction nested within it.

This restriction means that if a method is called in a read-only transaction then the invoker can assume that no Arez state has been mutated when the method returns. Typically read-only transactions are used by observers that want to reflect Arez state to non-Arez systems but do not want to mutate state. The most common example being an observer that reflects Arez state to a view as in Arez applications that use react4j. The restriction makes it easy to enforce this rule.

Tracking Transactions

Transaction can also be tracking or non-tracking. A tracking transaction detects accesses of observables and computable values within the scope of a transaction. The observables and computable values are recorded as dependencies of the running transaction. This makes it possible for Arez to monitor the dependencies and re-scheduled the observer that created the tracking transaction when the dependencies change.

Nesting Actions

Some transactions can contain nested transactions and some transactions can not contain nested transactions. In most cases it is easier to think about the element that initiated the transaction and what elements it can contain.

The simplified rules are:

• An action can invoke another action and access computable values but can never invoke an observer directly.
• A computable value can never invoke actions or observers but can access other computable values.
• An observer can access computable values and may be able to invoke actions depending on the configuration of the observer.

It should be noted that these rules are in addition to the rule that a read-write transaction can not be nested within a read-only transaction. Actions default to read-write transactions but may be made read-only. Observers default to read-only transactions but may be made read-write. Computable values create read-only transactions.