Agrani Bank Ltd. — Senior Officer (IT) Written Test: 2017প্রশ্ন ও সমাধান

AJAX (Asynchronous JavaScript and XML) lets a web page update part of its content by communicating with the server in the background, without a full page reload.

sequenceDiagram
    participant U as User
    participant B as Browser (JS)
    participant X as XMLHttpRequest object
    participant S as Server
    participant D as Database

    U->>B: Triggers an action (e.g. click, keyup)
    B->>X: Creates/uses XMLHttpRequest
    X->>S: Sends async HTTP request
    S->>D: Query via JSP/PHP/Servlet/ASP.NET
    D-->>S: Data
    S-->>X: Response (JSON/XML)
    X-->>B: Invokes callback with data
    B-->>U: Updates HTML/CSS on page (no reload)

Step by step:

1. A user action in the UI triggers a JavaScript event handler.
2. The handler creates (or reuses) an XMLHttpRequest object (or fetch() in modern JS).
3. The browser sends an asynchronous HTTP request to the server — the rest of the page stays fully responsive while waiting.
4. The server processes the request (querying a database via PHP/JSP/Servlet/ASP.NET, etc.) and retrieves the needed data.
5. The server sends back the result, typically as JSON (or XML).
6. The XMLHttpRequest's callback function fires when the response arrives, and JavaScript uses it to update the relevant part of the page's HTML/CSS — without reloading the whole page.

Example: a live search box that shows matching results as you type, by sending each keystroke's partial query to the server and updating only the results <div>.

Exam takeaway: AJAX = async request via XMLHttpRequest/fetch leading to server processing and returning JSON/XML leading to a JS callback updating the DOM in place — the technique behind responsive, no-reload web UIs.

• Candidate Key: a minimal set of attributes that can uniquely identify every row in a table — a table may have several candidate keys, and exactly one is chosen as the Primary Key while the rest remain "alternate keys." Example: in Employee(EmpID, Email, NationalID), both {EmpID} and {Email} independently qualify as candidate keys.

• Foreign Key: a column (or set of columns) in one table that references the Primary Key of another table, creating a link between the two and enforcing referential integrity. Example: Orders.CustomerID referencing Customer.CustomerID — every order must belong to an existing customer.

• Horizontal Scaling ("scale out"): increasing capacity by adding more machines to a pool of resources (e.g., adding more database/web servers behind a load balancer), as opposed to Vertical Scaling ("scale up"), which increases capacity by adding more power (CPU/RAM/disk) to a single existing machine.

graph TD
    subgraph Vertical["Vertical Scaling - Scale Up"]
        S1[Single Server] --> S1B["Same server: add more CPU/RAM"]
    end
    subgraph Horizontal["Horizontal Scaling - Scale Out"]
        LB[Load Balancer] --> N1[Server 1]
        LB --> N2[Server 2]
        LB --> N3[Server 3]
    end

• Trade-off: vertical scaling is simpler (no application changes) but has a hardware ceiling and a single point of failure; horizontal scaling has no theoretical ceiling and improves fault tolerance, but requires the application to be designed for distributed operation (e.g., stateless services, data partitioning/sharding).

Exam takeaway: Candidate Key = any minimal unique identifier; Foreign Key = a link to another table's Primary Key; Horizontal scaling = add more machines (scale out), Vertical scaling = add more power to one machine (scale up).