What Is an API? API Integration Explained with WhatsApp, SMS and e-Invoice Examples

An API (Application Programming Interface) is an interface that lets two pieces of software talk to each other using predefined rules. API integration means connecting your own software to another system through that interface: sending a customer an automatic WhatsApp message, sending an SMS appointment reminder or passing an invoice to Turkey's e-invoice (e-Fatura) system. Set up correctly, it cuts down repetitive manual work; set up badly, it can quietly lead to data loss or legal problems.

What is an API?

In MDN Web Docs' definition, an API is a set of features and rules within a piece of software that allows other software to interact with it. Unlike the interface a person uses (screens, buttons), an API is the interface through which software talks to software. MDN also describes it as a simple "contract" between an application and the other software that uses it.

An everyday comparison: in a restaurant, you don't walk into the kitchen. You order from the menu through a waiter, who passes the order to the kitchen and brings the food back. The menu sets what you can ask for and in what form. An API works the same way: it defines which request you can send to which address, in which format, and what kind of response you will get.

A website's frontend and backend also often talk through an API. We explained this relationship in our post Frontend vs. Backend.

What does API integration bring your business?

• Less manual re-entry. When a quote is approved, the details flow into accounting automatically; you don't type the same data twice.
• Speed. When an order, shipment or service status changes, the customer is informed immediately.
• Fewer errors. Typos made while copying by hand disappear.
• A record. You can track in the system which message was sent when and which invoice was passed on.

But every integration makes two systems dependent on each other. What your software will do when the other system changes, shuts down or slows down has to be designed from the start.

Example 1: WhatsApp Business Platform

There are two different ways to use WhatsApp for your business. The first is the WhatsApp Business app installed on a phone, where a person messages by hand. The second is the WhatsApp Business Platform (Cloud API), which software uses. The integration discussed in this post is the second one.

According to Meta's developer documentation, the Cloud API lets you send text, media and interactive messages programmatically. Incoming messages and delivery statuses reach your system through a webhook (an automatic notification the other system sends to your address when an event happens).

The basic rules to know when setting up the integration (as of September 2025):

• 24-hour customer service window: When a user messages you, a 24-hour window opens. Within this window, you can reply with free-form text.
• Outside the window, templates only: When the window is closed, you can only send template messages. Templates must be approved by Meta before they can be used.
• Template categories: Marketing, utility and authentication.
• Pricing: From 1 July 2025, Meta moved from conversation-based pricing to per-message pricing. The fee depends on the template category and the recipient's country code; free-form messages within an open window are free. Check Meta's page for current rates.

In practice, this means that if you want to send an automatic reminder such as "Your service appointment is tomorrow at 10:00," you need a pre-approved template for it, filled in with variable fields (name, date, time).

Example 2: SMS integration and İYS

SMS integration is usually done through an SMS provider's API: your software passes the recipient's number and the text to the provider, the provider delivers the message to the carriers and returns a delivery report.

The technical part is often easy; the real thing to watch is the legal framework. In Turkey, commercial electronic messages (SMS, email, calls) are governed by the Regulation on Commercial Communications and Commercial Electronic Messages (Ticari İletişim ve Ticari Elektronik İletiler Hakkında Yönetmelik). Under the Regulation:

• Individuals and legal entities that want to send commercial electronic messages register with İYS.
• For promotional and marketing messages, prior consent is obtained from the recipient, and commercial electronic messages cannot be sent to recipients who have no consent recorded in İYS.
• Notification messages, such as those about ongoing subscriptions, collections, payment reminders, purchases and deliveries, do not require prior consent; however, they cannot promote any goods or services.

For your software, this means your system must be able to tell a "notification" from a "promotional" message and check consent status for promotional messages. "Your service record has been completed" is a notification; adding "filter discounts this month" to the same message turns it into a promotion.

Example 3: e-invoice integration (GİB)

e-Fatura (e-invoice) means issuing an invoice electronically, in line with the standards set by Turkey's Revenue Administration (Gelir İdaresi Başkanlığı, GİB), and delivering it to the recipient. GİB defines three methods for using the e-invoice system:

If business software (a CRM, ordering or job tracking system) is going to work with e-Fatura, the connection is usually made through a private integrator's API. Alternatively, the software passes the invoice to an accounting program that can already send e-invoices. Which route suits you depends on your current accounting setup and invoice volume. The obligation to switch to e-Fatura and its scope are set by legislation and can change over time; check your own situation with your financial advisor.

Checklist for setting up an integration

1. Is there documentation? Is the other system's API documentation current and clear?
2. Is there a test environment? Can you try things out without messaging real customers or issuing real invoices?
3. Who holds the credentials? Are the API keys and accounts in the business's name, or in the developer's personal account?
4. What happens if something fails? If the other system doesn't respond, will the request be retried, will it be logged, and who will be notified?
5. Are records kept? Are the time, status and error details stored for every message and invoice sent?
6. Are the legal rules reflected in the design? Are things like İYS consent, privacy notices under Turkey's Personal Data Protection Law (PDPL, Law No. 6698) and template approvals part of the software's flow?
7. Are the costs known? Have per-message and per-transaction fees been added to the budget?

We explained how integrations affect the budget in our post What Determines the Cost of Custom Software? We covered payment and shipping connections on the e-commerce side in our post on e-commerce platforms.

Our experience

In corporate web projects, what people usually mean by "WhatsApp integration" is not actually an API integration but actions that start a phone call or a WhatsApp chat with one tap. On the Tuğra Karot site, for example, the phone and WhatsApp actions serve this purpose. This approach makes it easier for visitors to start a conversation with you, but it doesn't send automatic messages; if you need automatic notifications, you need a WhatsApp Business Platform integration. Telling the two apart is the first step to planning the budget correctly.

In custom software projects (CRM, dashboards, SaaS, APIs), we treat integrations as separate line items. For each connection, we plan error handling, logging and opening accounts in the business's name from the start.

Frequently asked questions

Conclusion

API integration lets your software exchange data in a rule-based, recorded way instead of by hand. As the WhatsApp, SMS and e-invoice examples show, the other side's rules (template approval, İYS, GİB methods) are as much a part of the design as the technical work. If you want to connect your existing systems or build new software with integrations from the start, take a look at our Software & Digital Solutions page, or contact us.