OpenEdge Development: Mobile Applications - Product ...

Coding AppServer services for OpenEdge Mobile
Coding AppServer services for OpenEdge Mobile
Several options and requirements apply especially to ABL written for an AppServer that
implements the services and resources of a Mobile Web application:
• Only a singleton class or procedure object can implement the interface to a Mobile
resource.
• The ABL routines that implement Mobile operations have specific coding
requirements.
Singleton classes and procedures as Mobile resources
The ABL to implement the interface to a single Mobile resource must be coded in a
single class or external procedure that can be executed as a singleton object. A
singleton object is a class or external procedure that once initially instantiated, the
same instance is shared by all consumers of the class or procedure no matter how
many times it is instantiated in a given AppServer session.
When on behalf of a Mobile App, a Mobile Web application executes any Mobile
operation of a Mobile resource on an AppServer agent, if the ABL class or external
procedure has not yet been instantiated as a singleton object, the agent instantiates it
and executes ABL routine that implements the operation. When the operation
completes, the object remains instantiated for access by other client requests. So,
when the Mobile Web application executes another operation of the same Mobile
resource, on the same AppServer agent, the same singleton object is then used to
execute the ABL routine for that operation, and so on. If another AppServer agent
executes the operation for the same Mobile resource, the same process repeats itself,
instantiating the singleton if it does not exist and remaining instantiated for all additional
calls to the same resource on that or another AppServer agent where the same object
is instantiated. Once all running AppServer agents have executed an operation for that
same Mobile resource, they all maintain their singleton objects as long as they continue
to run. Again, the process repeats for any additional agent that runs on the AppServer
and responds to a Mobile operation request.
ABL classes are inherently coded to be instantiated as singletons. However, external
procedures must meet a basic requirement to be instantiated as singletons, and that is
they cannot contain any DEFINE PARAMETER statements in the main block. They can
contain internal procedures and user-defined functions with their own parameters,
each of which can implement a Mobile operation exactly like a method of a class.
Note that the singleton coding requirement for external procedures applies only to an
external procedure that implements the interface for a Mobile resource and its
operations. Any additional procedures or classes that a singleton class or procedure
accesses can be implemented as any type of class or procedure that runs on the
AppServer.
For more information on singleton procedures instantiated in the context of an ABL
client, see the RUN statement in OpenEdge Development: ABL Reference. Although
from the ABL client viewpoint, the description of singleton procedure behavior and
coding requirements apply to OpenEdge Mobile as well.
OpenEdge ® Development: Mobile Applications 69