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# Unit 6 Introduction to two-tier and three-tier architectures and the internet database environment
## Section 1: Client/Server architectures and database connection to web applications
Read Chapter 8 "Database Application Development":
* client/server architectures
* databases in a two-tier architecture
* three-tier architectures
* web application components
* databases in three-tier architectures
* key considerations in three-tier applications
### Client/server architectures
| presentation logic |
| processing logic (business rules) |
| storage logic |
> Application Partitioning: the process of assigning portions of application code to client or server
> partitions after it is written to achieve better performance and interoperability (ability of a component to function on different platforms).
> Fat client: A client PC that is responsible for processing presentation logic, extensive application and business rules logic and many DBMS functions.
Two tier architectures
|--------|
| Server | * storage logic
|--------|
| Client | * processing logic
| | * presentation logic
|--------|
|--------|
| Server | * storage logic
| | * processing logic
|--------|
| Client | * presentation logic
|--------|
n tier architecture
|--------|
| db | * storage logic
|--------|
| app | * processing logic
|--------|
| client | * presentation logic
|--------|
|--------|
| db | * storage logic (postgresql)
|--------|
| app | * processing logic (ruby on rails)
|--------|
| web | * processing logic (nginx)
|--------|
| client | * presentation logic (firefox)
|--------|
> database server: A computer that is responsible for database storage,
> access, and processing in a client/server environment.
* middleware: software that allows an application to interoperate with other
software without requiring the user to understand and code the low-level operations
necessary to achieve interoperability.
* API: set of routines that an application program uses to direct the performance
by the computers operating system.
* Open database connectivity (ODBC): An API that provides a common language for
application programs to access and process SQL databases independent of the particular DBMS that is accessed.
Connecting to a db:
* identify and register database driver
* open a connection to a database
* execute a query against the database
* process the results of the query
* repeat steps 3-4 as necessary
* close connection to the db
* Three tier architecture: client/server configuration that includes three layers; a client layer, and two server layers.
* common setup is to have an app server and a database server.
* thin client: an application where the client accessing the application primarily provides
the user interfaces and some application processing, usually with no or limited local data storage.
## Section 2: XML and Data Exchange on the Internet
Read Chapter 8: "Database Application Development"
* "Extensible Markup Language (XML)"
* XML: a text based markup language used to describe data structures hierarchically, using HTML-like tags.
* XML Schema Definition (XSD): language used for defining XML databases that has been recommended by the W3C.
XSD describes structure, data typing, and validation of a salesperson record.
Storing XML documents:
1. store xml in a relational db by shredding it: store each element in a relational table.
2. store XML as blob: not great for full text search unless the db supports xml datatypes.
3. store XML in db using database specific column datatypes.
4. Store xml using xml databases: this is non-relational database designed specifically for xml.
* XPath: expressions used to locate data in XML documents.
* XQuery: XML transformation language that allows application to query both relational and XML data.
* extensible stylesheet language transformation (XSLT): pretty up xml document display.
* Web services description language (WSDL): xml grammar to describe an public interface for an API.
* Simple Object Access Protocol (SOAP): xml communication protocol. Don't do this. just use json or protocol buffers.
* Service-oriented architecture (SOA): collection of services that communicate with each other in some manner, usually by passing data or coordinating a business activity.
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