Storage

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4D v20.1

Storage


Storage -> Function result

Parameter	Type	Description
Function result	Object	Catalog of shared objects and shared collections registered in Storage

Description

The Storage method returns the catalog of shared objects or shared collections that you have registered in the Storage object on the current machine or component.

The catalog returned by Storage is automatically created by 4D and is available to all processes of the database, including preemptive processes. There is one Storage catalog per machine and component: in a client/server application, there is one Storage shared object on the server, and one Storage shared object on each remote 4D application; if the database uses components, there is one Storage object per component.

Use the Storage catalog to reference any shared objects or shared collections that you want to be used from any preemptive or standard process. To register a shared object or a shared collection in the catalog, add its reference to the shared object returned by Storage.

Since the catalog returned by Storage is a shared object, it follows the rules described in the Shared objects and shared collections section, but with some specificities:

This object can only contain shared objects and shared collections. Trying to add other kinds of values (non-shared objects or collections, null, scalar values) will generate an error.
Adding a property to this object must be surrounded by the Use...End use structure, otherwise an error is returned. Reading an attribute outside of a Use...End use structure is, however, possible.
When surrounded by the Use...End use structure, first-level attributes of Storage are locked for other processes.
Unlike standard shared objects, the object returned by Storage will NOT share its locking identifier with shared objects or collections added as attributes (for more information, refer to the About the locking identifier (how shared groups work) section).

Example 1

A common practice could be initializing the Storage object in the :

 Use(Storage)

    Storage.counters:=New shared object("customers";0;"invoices";0)

 End use

Example 2

This example shows a standard way to set Storage values:

 Use(Storage)

    Storage.mydata:=New shared object

    Use(Storage.mydata)

       Storage.mydata.prop1:="Smith"

       Storage.mydata.prop2:=100

    End use

 End use

Example 3

Storage allows implementing a singleton with a lazy initialization, as shown in the following example.

Note: For more information about singleton patterns, you can refer to this Wikipedia page.

 C_LONGINT($0)

 C_LONGINT($counterValue)

 C_OBJECT(counter) //create a reference to counter for the process

 

 If(counter=Null) //if this reference is null, get if from Storage

    Use(Storage) //Use of Storage needed only once!

       If(Storage.counter=Null)

          Storage.counter:=New shared object("operation";0)

       End if

       counter:=Storage.counter //Get the reference of the counter shared object

    End use

 End if

 Use(counter) //directly use the shared object counter (no need to use Storage!)

    counter.operation:=counter.operation+1

    $counterValue:=counter.operation

 End use

 

 $0:=$counterValue