Design and Implementation of TinyGALS: A Programming Model for ...

2.2.1 TinyGALS Components
Components are the most basic elements of a TinyGALS program. A TinyGALS compo-
nent C is a 3-tuple:
C = (VC,XC,IC), (1)
where VC is a set of internal variables, XC is a set of external variables, and IC is a set
of methods that constitute the interface of C. The internal variables carry the state of C
from one invocation of an interface method of C to another. The external variables can
be accessed by C through read and write operations. 2 The set of methods IC is further
divided into two disjoint sets: ACCEPT SC and USESC. The methods in ACCEPT SC can
be called by other components (these are the inputs of component C), while the methods
in USESC are those needed by C and may possibly belong to other components (these are
the outputs of component C). 3 Thus, a component is like an object in most object-oriented
programming languages, but with explicit definition of the external variables and methods
it uses. Syntactically, a component is defined in two parts – an interface definition and an
implementation.
Figure 2 shows a fragment of the code for the interface definition of the COUNTER
component shown in Figure 1, while Figure 3 shows a fragment of the code for its imple-
mentation. 4 In Figure 2, we see that the component has two ACCEPT S methods and one
USES method. In Figure 3, we see that the component accesses an internal variable of type
short named _counter. Using the tuple notation given in Equation 1, the COUNTER
component can be defined as C = (VC = { counter},XC = /0,IC = {init, f ire, f ireOut}).
2 External variables are implemented as TinyGUYS. See Section 2.4 for more information.
3 Using TinyOS conventions, ACCEPT SC is the set formed by the union of the TinyOS ACCEPTS and
HANDLES methods, and USESC is the set formed by the union of the TinyOS USES and SIGNALS methods.
4 The syntax is similar to that of TinyOS.
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