What does this AdTech Foundations page explain?

It explains how programmatic advertising works from ad request to conversion, including auctions, delivery, measurement, common failures, and practical metrics.

Who should use this guide?

It is designed for learners who already know the basic AdTech terms and want a stronger working model for day-to-day troubleshooting and interview preparation.

How should I study this page?

Start with the request flow, then review failures and metrics, and finish with examples and interview questions to turn definitions into practical recall.

AdTech Foundations

A practical guide to how programmatic advertising actually works

This page is for learners who already know the basic terms and now want the full picture: request flow, auction logic, delivery, measurement, common failures, and the metrics people actually discuss in AdTech interviews and day-to-day work.

Open the flow Step-by-step guide Interview questions

What you will learn

The full request-to-conversion flow, where measurement happens, and how technical events connect to business outcomes.

What is inside

Flow explanations, troubleshooting patterns, metric tables, technical examples, short case studies, and interview-ready notes.

Best way to use it

Start with the flow, then move to failures and metrics, and finish with the examples and interview questions to turn theory into practical recall.

How an Ad Travels on the Internet

How an Ad Travels on the Internet (Beginner Friendly)

1. When You Open an App or Website

Imagine you open a news website or a mobile app. The app has an empty space where an ad needs to be shown. This is called an ad slot.

Now the app asks: "Which ad should I show to this user?"

This question starts the AdTech process. The request usually includes device type, location, time, app or website name, ad size, user data if available, and a user ID such as a cookie, device ID, RampID, or UID2.

This is called an Ad Request.

2. Forward Request Path

This is the forward path, where the request goes from the supply side to the demand side to find the best ad.

User → Publisher (Site/App) → Ad Server → SSP → Ad Exchange → DSP → Advertiser

Platform	What it does
Publisher	Owns the app or website.
Ad Server	Decides which demand source to ask.
SSP	Helps the publisher sell ad space.
Ad Exchange	Runs the auction marketplace.
DSP	Helps advertisers buy impressions.
Advertiser	Sets targeting and budget.

At the DSP, the advertiser decides: do I want this user, how much should I bid, and which creative should I show?

This decision happens in milliseconds, usually around 100-200 ms. This process is called RTB (Real Time Bidding).

3. Backward Response Path

Now the response comes back with the bid price, creative, ad URL, and tracking pixels.

DSP → Ad Exchange → SSP → Ad Server → Publisher → User sees Ad

The highest bidder wins the auction, and the ad is shown to the user.

4. After the Ad is Shown

Once the ad is shown, event tracking starts.

Event	What it means
Impression	The ad was shown.
Viewability	The ad was actually visible.
Click	The user clicked the ad.
Landing Page	The user visited the website.
Add to Cart	The user added a product.
Purchase	A conversion happened.
KPI	Performance is measured.
Optimization	The DSP improves the next bids.

So the full cycle is: Auction → Impression → Click → Conversion → KPI → Optimization → Next Auction. This loop keeps repeating and the system keeps learning.

5. Simple One-Line Summary

AdTech in one line:

The publisher tries to sell an impression, advertisers try to buy the right user, and an auction decides which ad the user sees.

Programmatic Advertising Flow

Interactive Diagram

Use the diagram and jump rail to follow the request path, response path, auction outcome, delivery, and measurement loop in one view.

Open App Ad Request Ad Server SSP Exchange DSPs DSP Rules No Bid Bid Response Response Auction Delivery Impression Click Conversion Optimize

Section 1

Step-by-step explanation

Sixteen steps from the first request to KPI review, written so you can understand what happens at each handoff in the chain.

User opens the site or app

A real user lands on a publisher page or app screen, which creates an ad slot that can now be sold.

Publisher creates the ad request

The page or app collects slot details, device context, location, time, identifiers, consent signals, and other fields needed to ask for an ad.

Site or app calls the ad server

The publisher property asks the ad server what demand source should handle this slot.

Ad server sends the opportunity to the SSP

If the impression is eligible for programmatic demand, the ad server forwards the request with placement rules, floor logic, and publisher priorities.

SSP forwards it to the ad exchange

The SSP packages the request with slot size, format, floor, site or app details, device data, IDs, geo, and consent information.

Ad exchange sends bid requests to DSPs

The exchange fans the same opportunity out to eligible buyers so multiple DSPs can evaluate it at once.

DSP checks advertiser rules

The DSP matches the request against targeting, budget, pacing, frequency, brand safety, predicted CTR, predicted CVR, and expected ROAS.

DSP may reject or no-bid

If the request does not fit targeting, quality rules, consent requirements, or budget controls, the DSP simply returns no bid.

DSP sends the bid response

If the impression is valuable, the DSP sends back a bid price, creative markup, adomain, trackers, and metadata needed for billing and validation.

Response path goes back through the chain

The response returns from DSP to exchange to SSP to ad server, and each hop can still reject it if the markup is invalid, late, or blocked.

Auction picks the winner

The ad server or exchange compares all valid bids and selects the winning bid based on price, deal priority, and publisher controls.

User sees the ad

The winning creative loads in the browser, app, or video player. If rendering succeeds, impression delivery is complete.

After delivery checks happen

Impression, viewability, and fraud checks start running once delivery conditions are met.

User clicks the ad

If the user engages, click tracking logs the click and sends the user to the landing page or app store.

User completes an action on the site

The advertiser site or app records actions such as signup, add to cart, purchase, install, or subscription through pixels, SDKs, or postbacks.

KPI review and optimization happen

The advertiser and DSP review CTR, CPA, ROAS, win rate, and quality signals so the next auction can perform better.

Section 2

Where things go wrong

A practical troubleshooting table for common delivery, auction, tracking, and CTV failures.

Step	Problem	Reason	Fix
SSP	Low bid requests	Floor price high	Reduce floor
DSP	No bid	Targeting mismatch	Expand targeting
DSP	No bid	Budget finished	Increase budget
Exchange	Lost auction	Bid too low	Increase bid
Creative	VAST error	Wrong format	Upload correct VAST
Impression	Not counted	Tracker missing	Fix tracker
Click	Click not tracked	Click tracker missing	Add click tracker
Conversion	Not tracked	MMP not integrated	Fix postback
Delivery	Under-delivery	Geo/device mismatch	Expand targeting
CTV	SSAI timeout	Ad too long	Fix ad duration

Section 3

Metrics at each step

This is the bridge between technical flow and business performance.

Step	Metric	Formula
Request	QPS	Requests per second
DSP	Bid Rate	Bids / Requests
Auction	Win Rate	Wins / Bids
Delivery	Fill Rate	Impressions / Requests
Click	CTR	Clicks / Impressions
Conversion	CVR	Conversions / Clicks
Revenue	eCPM	Revenue / Impressions * 1000
Performance	CPA	Spend / Conversions
Performance	ROAS	Revenue / Spend

Section 4

Real OpenRTB, VAST, and tracker examples

Technical samples you can read line by line while learning how programmatic delivery really works.

OpenRTB Bid Request Example

A simplified bid request showing video inventory, device context, app name, and floor.

JSON

{
  "imp": [
    {
      "id": "1",
      "video": {
        "mimes": ["video/mp4"],
        "minduration": 15,
        "maxduration": 30
      },
      "bidfloor": 5.0
    }
  ],
  "device": {
    "ua": "CTV",
    "geo": {
      "country": "IND"
    }
  },
  "app": {
    "name": "Samsung TV Plus"
  }
}

Bid Response Example

The DSP returns price, creative markup, and creative ID if it wants the impression.

JSON

{
  "seatbid": [
    {
      "bid": [
        {
          "price": 6.2,
          "adm": "<VAST XML>",
          "crid": "creative_123"
        }
      ]
    }
  ]
}

VAST Example

A compact VAST response with impression, click-through, and media file references.

XML

<VAST version="4.2">
  <Ad id="creative_123">
    <InLine>
      <AdSystem>Example Exchange</AdSystem>
      <AdTitle>CTV Mid-Roll Example</AdTitle>
      <Impression><![CDATA[https://tracker.com/imp?price=${AUCTION_PRICE}]]></Impression>
      <Creatives>
        <Creative>
          <Linear>
            <Duration>00:00:15</Duration>
            <VideoClicks>
              <ClickThrough><![CDATA[https://brand.com/landing-page]]></ClickThrough>
            </VideoClicks>
            <MediaFiles>
              <MediaFile delivery="progressive" type="video/mp4">
                <![CDATA[https://cdn.brand.com/spot.mp4]]>
              </MediaFile>
            </MediaFiles>
          </Linear>
        </Creative>
      </Creatives>
    </InLine>
  </Ad>
</VAST>

Tracker Examples

Tracking URLs tie delivery, click, and conversion events back to the winning impression.

TEXT

Impression Tracker
https://tracker.com/imp?price=${AUCTION_PRICE}

Click Tracker
https://tracker.com/click?id=123

Conversion Postback
https://tracker.com/postback?click_id=abc123&event=install

Section 5

Interview questions

Short answers for AdOps, Programmatic, SSP, DSP, exchange, and troubleshooting interviews.

Question	Short answer
Why DSP may not bid?	Targeting mismatch, budget finished, brand safety, consent, or low predicted value.
What is win notice?	The exchange tells the DSP that it won the auction and may include the clearing price macro.
What happens if VAST fails?	The video creative does not render correctly, so impression and downstream events may never count.
Where does frequency capping happen?	Usually in the DSP or ad server, depending on where the delivery decision is controlled.
What is bid shading?	A DSP reduces a first-price bid toward the expected clearing price to avoid overpaying.
Difference SSP vs Ad Server?	The SSP sells inventory to demand, while the ad server decides how the slot should be filled.
What is deal ID?	A private marketplace identifier that tells buyers and sellers they are transacting on a specific negotiated deal.
What is floor price?	The minimum CPM a publisher is willing to accept for an impression.
What is SSAI?	Server-side ad insertion stitches ads into the video stream before it reaches the device.
Why impression but no click?	The ad delivered, but the creative, audience, or placement did not drive engagement.

Numbers that matter

Metrics you should know cold

A compact reference for the metrics that come up most often in reporting, troubleshooting, and interviews.

Impression

One ad served to a user.

CTR

Clicks / impressions, shows engagement.

CPM

Cost per 1000 impressions.

CPC

Spend / clicks.

CPA

Spend / conversions.

ROAS

Revenue / ad spend.

Simple campaign math example

Budget $1,000

CPM $2

Impressions 500,000

CTR 2%

Clicks 10,000

Conversion rate 5%

Conversions 500

Revenue $2,500

ROI 150%

Quick scenarios

How this looks in real delivery work

Short examples that connect campaign goals, technical decisions, and performance outcomes.

App installs

Playables boosted CTR to 3.4% and CPI fell 18% after adding SKAN postbacks.

Brand awareness

PMP CTV mid-roll lifted viewability 10pts while keeping frequency <3.

Video performance

Lowering bitrate for mobile cut buffering and raised completion rate by 7%.