Bangladesh Development Bank — Senior Officer (IT) Written Test: 2017প্রশ্ন ও সমাধান

ACID is the set of four guarantees a database transaction must satisfy to be considered reliable.

Property	Meaning
Atomicity	A transaction is "all or nothing" — either every operation in it completes, or none do (a failure rolls back all partial changes).
Consistency	A transaction moves the database from one valid state to another, never leaving it violating integrity constraints (e.g., foreign keys, uniqueness).
Isolation	Concurrently executing transactions do not interfere with each other — the result is the same as if transactions ran one after another (serially), even though they may run in parallel.
Durability	Once a transaction is committed, its changes survive permanently, even in the event of a power loss or crash (written to non-volatile storage).

graph TD
    T[Transaction] --> A["Atomicity: all-or-nothing"]
    T --> C["Consistency: valid state to valid state"]
    T --> I["Isolation: concurrent transactions do not interfere"]
    T --> D["Durability: committed changes are permanent"]

Classic example — bank transfer of Tk. 500 from Account A to Account B:

• Atomicity: both the debit from A and credit to B happen, or neither does (a crash mid-transfer cannot leave money "vanished").
• Consistency: total money in the system before and after the transfer stays the same.
• Isolation: if another transaction reads A's balance mid-transfer, it should not see a half-completed state.
• Durability: once the transfer is confirmed, it stays recorded even if the server crashes a second later.

Exam takeaway: ACID = Atomicity (all-or-nothing) + Consistency (valid states only) + Isolation (no interference between concurrent transactions) + Durability (permanent once committed) — the foundation of reliable transactional databases.

HTML Canvas (<canvas>) is an HTML5 element that provides a blank, resolution-dependent bitmap surface on which JavaScript can draw graphics, shapes, text, and images pixel-by-pixel at runtime, using the Canvas 2D (or WebGL) API.

<canvas id="c" width="150" height="150"></canvas>
<script>
  const ctx = document.getElementById("c").getContext("2d");
  ctx.beginPath();
  ctx.arc(75, 75, 50, 0, Math.PI * 2);   // draw a circle
  ctx.strokeStyle = "green";
  ctx.stroke();
</script>

Canvas vs SVG:

	Canvas	SVG
Type	Raster (pixel)	Vector (shape description)
Representation	No DOM per shape — one single <canvas> element	Each shape is its own DOM node (<circle>, <rect>, ...)
Scaling	Loses sharpness when zoomed/resized	Scales infinitely without quality loss
Performance	Better for many objects / frequent redraws (games, animations)	Better for fewer objects on a large area (icons, static diagrams)
Interactivity	Manual hit-testing (no per-shape events)	Native per-element event handlers (click, hover, etc.)
Editability after drawing	Must clear and redraw the whole scene to change anything	Modify the specific DOM node directly via CSS/JS

Exam takeaway: Canvas = imperative, raster, JS-drawn — ideal for games/animation with many redraws; SVG = declarative, vector, DOM-based — ideal for scalable, interactive static graphics like icons and charts.