21. A software application development tool for assembling applications in which a check-in terminal communicates with two or more airline systems and services, the software application development tool stored on non-transitory computer readable media and comprising computer readable program code comprising: a common code base, wherein the common code base is incompatible with the two or more airline systems and services; data and messages used by the application described using a declarative data description language; a library of system components including: a) a terminal abstraction layer that allows interactions between the check-in terminal and the application to control the check-in terminal; and b) an airline systems and services abstraction layer that allows interactions between the two or more airline systems and services and the application, wherein the two or more airline systems and services comprise check-in systems; a graphical (GUI) tool to model the workflow of the application, the workflow including screens and services described declaratively by a declarative data description language; and an assembler for assembling the application using the graphical tool, declarative rules, and customizations of the system components selected from the library.
21. A software application development tool for assembling applications in which a check-in terminal communicates with two or more airline systems and services, the software application development tool stored on non-transitory computer readable media and comprising computer readable program code comprising: a common code base, wherein the common code base is incompatible with the two or more airline systems and services; data and messages used by the application described using a declarative data description language; a library of system components including: a) a terminal abstraction layer that allows interactions between the check-in terminal and the application to control the check-in terminal; and b) an airline systems and services abstraction layer that allows interactions between the two or more airline systems and services and the application, wherein the two or more airline systems and services comprise check-in systems; a graphical (GUI) tool to model the workflow of the application, the workflow including screens and services described declaratively by a declarative data description language; and an assembler for assembling the application using the graphical tool, declarative rules, and customizations of the system components selected from the library. 35. A software application development tool according to claim 21 , wherein the application is a check-in client.