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Shared objects and collections

Shared objects and shared collections are specific objects and collections whose contents are shared between processes. Shared objects and shared collections are compatible with preemptive processes: they can be passed by reference as parameters to commands such as callWorker.

Shared objects and shared collections can be stored in standard object and collection type variables, but must be instantiated using specific commands:

note

Shared objects and collections can be set as properties of standard (not shared) objects or collections.

In order to modify a shared object/collection, the use...end structure must be called. Reading a shared object/collection value does not require use...end.

To know if an object or a collection is shared, use the objectIsShared command.

A unique, global catalog returned by the storage command is always available throughout the project, and can be used to store all shared objects and collections.

Using shared objects or collections

Once instantiated with the newSharedObject or newSharedCollection command, shared object/collection properties and elements can be modified or read from any process of the application.

Modification

Modifications can be applied to shared objects and shared collections:

  • adding or removing object properties,
  • adding or editing values (provided they are supported in shared objects), including other shared objects or collections (which creates a shared group, see below).

However, all modification instructions in a shared object or collection must be surrounded by the use...end keywords, otherwise an error is generated.

s_obj = newSharedObject("prop1","alpha")
use(s_obj)
s_obj.prop1 = "omega"
end

A shared object/collection can only be modified by one process at a time. use locks the shared object/collection from other threads, while end unlocks the shared object/collection (if the locking counter is at 0, see below). Trying to modify a shared object/collection without at least one use...end generates an error. When a process calls use...end on a shared object/collection that is already in use by another process, it is simply put on hold until the end unlocks it (no error is generated). Consequently, instructions within use...end structures should execute quickly and unlock the elements as soon as possible. Thus, it is strongly advised to avoid modifying a shared object or collection directly from the interface, e.g. through a web form.

Assigning shared objects/collections to properties or elements of other shared objects/collections is allowed and creates shared groups. A shared group is automatically created when a shared object/collection is set as property value or element of another shared object/collection. Shared groups allow nesting shared objects and collections but enforce additional rules:

  • Calling use on a shared object/collection belonging to a group locks properties/elements of all shared objects/collections of the group and increments its locking counter. Calling end decrements the locking counter of the group and when the counter is at 0, all the linked shared objects/collections are unlocked.
  • A shared object/collection can only belong to one shared group. An error is returned if you try to set an already grouped shared object/collection to a different group.
  • Grouped shared objects/collections cannot be ungrouped. Once included in a shared group, a shared object/collection is linked permanently to that group during the whole session. Even if all references of an object/collection are removed from the parent object/collection, they will remain linked.

Please refer to example 2 for an illustration of shared group rules.

info

Shared groups are managed through an internal property named locking identifier.

Read

Reading properties or elements of a shared object/collection is allowed without having to call the use...end structure, even if the shared object/collection is in use by another process.

However, it is necessary to read a shared object/collection within use...end when several values are linked together and must be read at once, for consistency reasons.

Duplication

Calling objectCopy with a shared object or with an object containing shared object(s) as properties is possible, but will return a standard (not shared) object including its contained objects (if any).

Storage

storage is a unique shared object, automatically available on each project. This shared object is returned by the storage command. You can use this object to reference all shared objects/collections defined during the session that you want to be available from any preemptive or standard processes.

Note that, unlike standard shared objects, the storage object does not create a shared group when shared objects/collections are added as its properties. This exception allows the storage object to be used without locking all connected shared objects or collections.

For more information, refer to the storage command description.

use...end

The formal syntax of the use...end structure is:

 use(Shared_object_or_Shared_collection)
statement(s)
end

The use...end structure defines a sequence of statements that will execute tasks on the Shared_object_or_Shared_collection parameter under the protection of an internal semaphore. Shared_object_or_Shared_collection can be any valid shared object or shared collection.

Shared objects and shared collections are designed to allow communication between processes, in particular, preemptive processes. They can be passed by reference as parameters from a process to another one. Surrounding modifications on shared objects or shared collections by the use...end keywords is mandatory to prevent concurrent access between processes.

  • Once the use line is successfully executed, all Shared_object_or_Shared_collection properties/elements are locked for all other process in write access until the corresponding end line is executed.
  • The statement(s) sequence can execute any modification on the Shared_object_or_Shared_collection properties/elements without risk of concurrent access.
  • If another shared object or collection is added as a property of the Shared_object_or_Shared_collection parameter, they become connected within the same shared group.
  • If another process tries to access one of the Shared_object_or_Shared_collection properties or connected properties while a use...end sequence is being executed, it is automatically put on hold and waits until the current sequence is terminated.
  • The end line unlocks the Shared_object_or_Shared_collection properties and all objects of the same group.
  • Several use...end structures can be nested in the QodlyScript code. In the case of a group, each use increments the locking counter of the group and each end decrements it; all properties/elements will be released only when the last end call sets the locking counter to 0.
note

If a function of the collection class modifies a shared collection, an internal use is automatically called for this shared collection while the function is executed.

Examples

The following examples highlight specific rules when handling shared groups:

 ob1 = newSharedObject
ob2 = newSharedObject
use(ob1)
ob1.a = ob2 //group 1 is created
end

ob3 = newSharedObject
ob4 = newSharedObject
use(ob3)
ob3.a = ob4 //group 2 is created
end

use(ob1) //use an object from group 1
ob1.b = ob4 //ERROR
//ob4 already belongs to another group
//assignment is not allowed
end

use(ob3)
ob3.a = null //remove any reference to ob4 from group 2
end

use(ob1) //use an object from group 1
ob1.b = ob4 //ERROR
//ob4 still belongs to group 2
//assignment is not allowed
end