Sequence diagramSequence diagrams represents dynamic behaviour of the system.A sequence diagram focuses on the time ordering of the messages that go back and forth of the systemBasically there are two elements involve in a sequence diagram,

1. Objects2. Messages

It consist of sequence of interactions among different objects over a period of time.The interactions between different objects in a sequence diagram is represented as messages.

There are three types of messages1. Synchronized messages2. Asynchronized messages(simple)3. Return messages

Sequence diagrams show a detailed flow for a specific use case or even just part of a specific use case. They are almost self explanatory; they show the calls between the different objects in their sequence and can show, at a detailed level, different calls to different objects.A sequence diagram has two dimensions: The vertical dimension shows the sequence of messages/calls in the time order that they occur; the horizontal dimension shows the object instances to which the messages are sent.

[diagram should come here]

In this example we describe the process of admitting a new patient. By-Stander or patient him/her self provides information to an admissions clerk,An appointment given to the patient,Patient participate to the particular clinic session,A patient record is created,Now the patient is forwarded to a doctor,A medical history is created by the doctor, Examination done , then the doctor order investigation to laboratory(including Biochemistry/Pathology/Radiology/etc..)Doctor may request patient to attend new clinic session(Patient receive Re-visit appointment)Now a laboratory report is generated and returned to the doctor.Then the doctor comes to a diagnosis and generate a drug prescription.Drug prescription directly goes to out door dispensary and pharmacist issue drug to the patient.

This pattern has the following advantages:

1. All information about a patient is recorded and documented even if there are re-examinations during that stay.

2. When a patient is admitted and discharged, the treatment history is added to the medical history and all treatments within the medical group will be kept for future reference.

3. If a patient discontinues treatment for any reason this information will be on record.

4. All the assignments of personnel or physical resources are explicitly described.

Role based access control can be used to protect this information. The access to patient treatment information can be controlled based on the role of the individuals.

Limitations of sequence diagrams:

1. Since there are snap shots of what happens in a given period of time,sequence diagram do not show a complete sequence-they are incomplete.

2. Number of sequence diagram required in a given design is not clearly known. It depends on the experience of the designer as to when to stop developing them.Theoretically number of sequence diagram can be drawn within a given design.

3. The way in which system designers use these diagrams(technical level) is different to the way system analysts use them(business process level).Inappropriate types of usage of these diagrams can be counter productive in practice.

(reference: Practical Object Oriented Design By Bhuvan Unhelkar,page 85)