Figure 1

Analogies between Molecules and Web Services. (a)An oxygen atom has six electrons in its outer shell and needs twomore electrons to fill the shell. Meanwhile, a hydrogen atom has one electron in its outer shell that can be shared with the oxygen atom. Under the right condition, the oxygen atom shares one of its six electrons with each of the two hydrogen atoms. This form of bonding (called covalent bonding) is what holds a water molecule together. (b) The composite travel service needs to know whether a rental car is available within a given time period in a certain location. In addition, it also needs to know whether there is a room vacancy at the same proximity. These inquiries can be fulfilled by invoking operations provided by both the car rental and hotel services. (c) A gene-bearing portion of the DNA double helix is unraveled and information contained in one of the two single strands is transcribed into a messenger RNA (mRNA) through molecular recognition (c1). The mRNA is then detached from the DNA strand and serves as a template in the protein building process (c2). This process involves a transfer RNA (tRNA), which collects amino acids required to build the protein and carries them to the mRNA. Proteins are assembled on the mRNA one amino acid at a time. Links between adjacent amino acids called peptide bonds are established as the protein chain grows. (d) A travel process flow template can be first constructed specifying the logic order of sequential, concurrent (i.e., AND Split AND-S and AND Join AND-J) and alternative (i.e., OR Split OR-S and OR Join OR-J) service invocations (d1). Upon receiving a request, a process flow for a composite travel service is instantiated and grows (d2) based on this template.