When managing multi-channel paid campaigns on Google Ads and Facebook Ads, consistent event tracking is essential for unified attribution. However, tracking platforms work in silos, and events like purchase, lead, or add_to_cart can fire with different payloads or timing.

🧰 Prerequisites

Component	Details
GTM Web + Server	Web (frontend) + Server container setup
Facebook Pixel + CAPI	Connected to Meta Events Manager
Google Ads	Conversion ID and Label available
OpenCart Website	E-commerce frontend
Consent Management	Optional but highly recommended

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🎯 Key Goals

• Receive frontend browser events in Server GTM
• Generate a shared event_id for deduplication
• Forward enriched payloads to both Google Ads CAPI & Meta CAPI
• Add hashed PII and marketing metadata

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⚙️ Step 1: Client-Side — Generate UUID & Push Event to GTM Server

In your OpenCart confirmation page (success.twig):

<script>
(function() {
  function uuidv4() {
    return 'xxxxxxxx-xxxx-4xxx-yxxx-xxxxxxxxxxxx'.replace(/[xy]/g, function(c) {
      var r = Math.random() * 16 | 0, v = c == 'x' ? r : (r & 0x3 | 0x8);
      return v.toString(16);
    });
  }

  const eventID = uuidv4();

  window.dataLayer = window.dataLayer || [];
  dataLayer.push({
    event: 'purchase',
    event_id: eventID,
    transaction_id: '{{ order_id }}',
    value: {{ total }},
    currency: '{{ currency }}',
    email: '{{ email | lower | sha256 }}', // Optional
    phone: '{{ phone | sha256 }}' // Optional
  });

  localStorage.setItem('event_id_purchase', eventID);
})();
</script>

✅ This ensures all platforms receive a consistent event_id.

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🧩 Step 2: GTM Web → GA4 & Server-Side GA4 Proxy

GA4 Purchase Tag in Web GTM:

• Event: purchase
• Parameters:
  • transaction_id, value, currency, event_id

Trigger: Custom Event = purchase

Also fire a Custom HTTP Request Tag to send data to Server GTM:

https://gtm.YOURDOMAIN.com/collect?event_id={{DLV - event_id}}&value={{DLV - value}}&currency={{DLV - currency}}&email={{DLV - email}}&phone={{DLV - phone}}&transaction_id={{DLV - transaction_id}}

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🛠️ Step 3: Server GTM — Event Router Setup

A. Create a Custom Client

In GTM Server container:

• Name: OpenCart Client
• Listen to collect endpoint
• Parse query string or request body
• Set event_name = purchase

Output event data to downstream tags.

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🔁 Step 4: Forward to Meta CAPI & Google Ads in Server GTM

A. Meta Conversions API Tag

In Server GTM:

• Event Name: Purchase
• Fields:
  • event_id: from request
  • event_time: {{Timestamp}}
  • user_data: hashed email, phone
  • custom_data: value, currency, transaction_id

Example Payload:

{
  "event_name": "Purchase",
  "event_time": {{Timestamp}},
  "event_id": {{event_id}},
  "user_data": {
    "em": {{hashed_email}},
    "ph": {{hashed_phone}}
  },
  "custom_data": {
    "value": {{value}},
    "currency": "{{currency}}",
    "transaction_id": "{{transaction_id}}"
  }
}

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B. Google Ads Enhanced Conversions Tag (Server-Side)

Use the native Google Ads Conversion Tag in Server GTM or send via HTTP Request.

Fields:

• Conversion ID: AW-XXXXXXX
• Conversion Label: XXXXXX
• Email, Phone, etc. (Hashed)
• Value, Currency, Transaction ID
• Event Time

Use deduplication via event_id in Enhanced Conversions field or match key.

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🔐 Step 5: Consent Filtering (Optional)

In both Web & Server GTM, use Consent Initialization triggers and enable Tag Consent Settings:

• Mark tags as requires marketing consent
• Fire only if consent is granted (via Cookiebot, OneTrust, etc.)

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🧪 Step 6: QA & Testing

Tool	Use Case
GTM Preview Mode	Ensure event_id & HTTP tag triggers
GA4 DebugView	Confirm deduped event in analytics
Meta Events Manager	Verify Purchase event & deduplication
Google Ads	Check Enhanced Conversions status
Server GTM Logs	Inspect payloads, headers, status codes

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📊 Use Cases

Insight	Platform	Use Case
Compare Meta vs Ads attribution	Meta & Google Ads	See which ad drove purchase
Audience building by platform	GA4 Audiences	Build audiences only from CAPI buyers
Event integrity reporting	Looker Studio	Confirm same event_id across channels

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🧠 Pro Tips

• Always log event_id + timestamp in order table for debugging
• Add retry logic or webhook fallback for failed server events
• Track client vs server events side-by-side in BigQuery for auditing

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