What are webhooks and how do they differ from traditional API calls in system integration?

Ever wonder how you get an instant email when someone logs into your account from a new device? Or how Slack updates a channel the moment a support ticket is created? That's often thanks to webhooks. But what exactly are webhooks, and how do they differ from traditional API calls in system integration? If you're new to these terms, don't worry—we'll explain them in a conversational, easy-to-understand way. In this article, we break down webhooks vs API calls with real-world examples, best practices, and tips on when to use each approach (and why this distinction matters even in technical interviews).

What is a Webhook?

A webhook is basically a way for one application to automatically send data to another application as soon as something happens. In formal terms, a webhook is a lightweight, event-driven communication that automatically sends data between applications via HTTP. In simpler words, you give one app a URL (the webhook endpoint) for another app, and whenever a specified event occurs, the first app makes an HTTP request to that URL to push the data. This means you don’t have to constantly check whether something new happened—the update is delivered to you the moment it’s available, almost like getting a text message alert for an event.

For example, imagine you run a newsletter service and a user unsubscribes. Instead of your CRM system querying the newsletter app every few minutes for updates, the newsletter app can send a webhook to your CRM as soon as the user clicks “unsubscribe.” The CRM receives that notification and updates the user’s status immediately. This one-way, event-triggered flow is why webhooks are often nicknamed “reverse APIs”—the server (source) calls the client (destination), flipping the usual request/response pattern.

Webhooks are great for real-time notifications and integrations. You’ll encounter webhooks in many services: GitHub can send a webhook to your CI/CD server when code is pushed (to trigger an automated build), Stripe or PayPal use webhooks to notify your app when a payment succeeds, and Slack provides incoming webhooks so external apps can send messages into a Slack channel. In each case, data flows one way—from the source to the destination—only when an event occurs. This makes webhooks efficient and timely for connecting systems without the need for the receiving app to ask “Is there anything new?” over and over.

What is an API Call?

API stands for Application Programming Interface. In simple terms, an API is like a contract that allows one software program to request information or actions from another. When you make an API call (also known as an API request), one application (the client) sends a request over the network to another application’s API (the server), asking for some data or to perform an operation. The server then processes that request and sends back a response with the data. If a webhook is a push, think of an API call as a pull – the client has to pull information by asking for it.

A helpful analogy: APIs are like the waiters of the internet—taking your order (request), relaying it to the kitchen (server), and bringing back exactly what you asked for. In other words, the client only gets data when it asks for it. You use API calls every day without realizing it: when a weather app fetches the current temperature when you open it, it’s calling a weather API; when you search for flights on a travel site, your search triggers API calls to airline databases; even clicking "refresh" in an email app makes an API call to check for new messages.

One key aspect of traditional API calls is that they are client-initiated and request-driven. The server will only respond when a request comes in. If you need continuous updates via pure API calls, the client must keep sending requests periodically to check for new data – this is known as polling. Polling works (and is simple to implement), but it can be inefficient and introduce a delay in getting updates. For instance, a news app that polls an API every 5 minutes might inform you of breaking news up to 5 minutes late. There are techniques like long-polling or streaming to make API calls more real-time, but fundamentally, with APIs the client is in charge of asking for data.

Key Differences Between Webhooks and API Calls

Both webhooks and API calls enable communication between systems, but they differ in how and when that communication happens. Here are the key differences at a glance:

Initiation (Push vs. Pull): Webhooks are event-driven – the server pushes data to the client when an event occurs. In contrast, API calls are request-driven – the client pulls data by making a request when it needs something. In short, webhooks are like the server calling you, whereas API calls are like you calling the server.
Timing and Frequency: Webhooks happen in real-time. As soon as the triggering event happens, a webhook delivers the data immediately. API calls happen on demand – the client decides when to ask. If continuous updates are required via API calls, the client might poll repeatedly (e.g. every few seconds or minutes), which can introduce delays between when an event happens and when the client finds out.
Data Flow Direction: Webhook communication is typically one-way. The server sends a notification and payload to the client’s webhook endpoint and usually doesn’t expect a detailed response (aside from a basic acknowledgment like an HTTP 200 OK). An API call involves a two-way interaction – the client sends a request, and the server sends back a response with data. This means APIs support a back-and-forth dialogue (multiple requests and responses), whereas a webhook is a fire-and-forget notification (one shot per event).
Efficiency: Webhooks tend to be more efficient for event-driven updates. They use no resources until an event actually occurs, which saves bandwidth and processing time. By eliminating the need for constant polling, webhooks reduce unnecessary traffic. On the other hand, frequent API polling can waste resources (imagine asking “Any new data?” every few seconds when most of the time the answer is no change). If an integration only uses API calls for updates, it needs to strike a balance (like polling at reasonable intervals or using caching) to avoid hammering the server.
Complexity & Setup: Setting up a webhook requires the consumer to have an endpoint that can receive incoming HTTP requests (and handle things like security verification and parsing the data). There’s a bit of upfront complexity in configuring and testing webhooks. API calls, by contrast, are often simpler to get started with – any script or app can call an API endpoint with a GET or POST request when needed. However, the client does need to handle things like API authentication, rate limiting, and error responses for robust integration. Webhooks shift some complexity to the server side (the server has to know when to send, and retry if a webhook fails), while API calls put more responsibility on the client to keep asking at the right times.

Both approaches can complement each other in a system. In fact, many modern systems provide both options: for example, an email service might offer webhooks for events (like email opened or bounced) and also traditional APIs for querying data (like fetching a list of sent emails on demand). Understanding the difference is important for choosing the right integration pattern in your system architecture. (For a deeper dive into polling techniques versus webhooks, see DesignGurus’ article on what is polling vs long-polling vs webhooks for more details.)

Use Cases and Real-World Examples

When should you use a webhook, and when is a normal API call better? It depends on the scenario and requirements. Here, we’ll illustrate some common use cases for each approach to make the differences more concrete.

When to Use Webhooks (Examples)

Webhooks are ideal for event-driven scenarios and real-time notifications, especially in system integration. Some real-world examples include:

Real-time notifications: If your application needs to react instantly to events, webhooks are the way to go. For example, a payment gateway like Stripe sends a webhook to your server the moment a payment succeeds or a charge fails. Your app can immediately update the order status or send a receipt without polling the payment API for updates.
Third-party app integrations: Many apps use webhooks to talk to each other without manual intervention. For instance, GitHub can be configured to send a webhook to your CI/CD service (like Jenkins or CircleCI) whenever code is pushed to a repository. This instantly triggers a build or deployment process. Similarly, a project management tool might send a webhook to a team chat app when a new task is created, so the team gets notified in Slack or Microsoft Teams right away.
Automated workflows: Webhooks shine in automation and system architecture patterns where one event in System A should immediately lead to an action in System B. For example, an e-commerce site can set up a webhook with its inventory system: when a product’s stock level drops to zero in the inventory database, a webhook notifies the online store to mark that item as “out of stock.” This kind of event-driven system integration keeps data consistent across services in real-time.

In all these cases, the webhook removes the need for the receiving system to continuously check the source system for changes. The updates are pushed as they happen, which is efficient and timely.

Check out polling vs long-polling vs webhooks

When to Use API Calls (Examples)

Traditional API calls are suited for on-demand requests and scenarios where you want control over when and what to fetch. They are also necessary when a service doesn’t support webhooks. Consider these examples:

On-demand data retrieval: If your app needs information only at specific times or in response to user actions, an API call is perfect. For example, a weather application calls a weather API when you open the app or ask for the forecast. Similarly, a maps app uses API calls to fetch route or place details when you search for something. The data is fetched as needed (pulled by the client), which is efficient for one-off queries.
Periodic polling (no webhook available): Not all systems provide webhooks. If a third-party service or legacy system only offers a traditional API, your integration might resort to periodic API calls to check for new data. For instance, an email client without push notifications might poll the server’s API every few minutes to see if new emails have arrived. While not real-time, a well-tuned polling interval (say every 60 seconds) can be acceptable for certain applications. (If possible, using long-polling or server-sent events can improve efficiency, but those are still client-initiated techniques.)
Aggregating data from multiple sources: In some cases, your application needs to compile data from several APIs. For example, a dashboard might call one API to get user info and another API to get that user’s activity history, then combine the results. Here, API calls give you fine-grained control to fetch and coordinate data on demand. Also, during mock interview practice for system design, you might design a solution where one service calls multiple microservice APIs to fulfill a request. In all such scenarios, the request/response model of API calls is appropriate.

The general rule of thumb: use webhooks for event-driven updates or notifications (especially if timeliness is crucial), and use API calls for querying data on demand or when you need the latest data at a specific moment. Often, robust systems will use a mix of both – webhooks for instant pushes of critical events, and APIs for on-demand fetching or deeper queries.

FAQs

Q1: What is a webhook in simple terms? A webhook is like a automatic alert that one application sends to another. In simple terms, it’s an HTTP callback triggered by an event. When something happens on the source app, it instantly sends a message (HTTP request with data) to a URL of the destination app. This way, the second app gets the info without asking for it.

Q2: How do webhooks differ from API calls? Webhooks and API calls are opposites in how they communicate. With a webhook, the server notifies the client (push model) when an event occurs. With a traditional API call, the client must ask the server (pull model) for data or updates. In short, a webhook delivers data automatically on events, whereas an API call retrieves data only when requested. This makes webhooks real-time and event-driven, while API calls are client-driven and on-demand.

Q3: When should I use a webhook vs an API call? Use a webhook when you need immediate, real-time updates or asynchronous notifications – for example, receiving payment confirmations, form submissions, or other events from an external system as they happen. Use an API call when you need data on demand or at scheduled intervals – for instance, fetching user data when a page loads, or querying a service that doesn’t support sending automatic updates. In many integration designs, you might use both: webhooks for instant events and API calls for on-the-spot data retrieval.

Q4: Are webhooks more efficient than polling? Yes, in most cases webhooks are more efficient than constant polling. Polling means your app keeps sending API requests (e.g., every few seconds) to ask “Any new data yet?”, which can waste resources and bandwidth if nothing has changed. Webhooks eliminate that by only sending data when there’s actually an update, reducing unnecessary traffic. This event-driven approach uses resources more sparingly and provides data with minimal delay. (For a detailed comparison of polling, long-polling, and webhooks, check out our DesignGurus Q&A on polling vs long-polling vs webhooks.)

Q5: Is a webhook a type of API? You can think of a webhook as a special kind of API endpoint. In fact, a webhook is sometimes called an “event-driven API” or reverse API. It’s essentially an API endpoint on your system that external services call (via HTTP) when an event happens. However, unlike a typical REST API that your app calls on demand, a webhook API is one your app listens to for incoming data. So while webhooks use APIs under the hood (they communicate via HTTP requests just like APIs), they operate in a reversed, event-based fashion compared to traditional API calls.

Conclusion

In summary, webhooks vs API calls comes down to how integration is achieved: webhooks push data to you when events happen, and API calls let you pull data when you need it. Webhooks offer real-time updates and can make your system integration more efficient by avoiding needless queries. API calls give you control to fetch exactly what you want, when you want, which is useful for on-demand operations and complex queries. Rather than seeing one as better than the other, recognize that both have their place in modern system architecture. The best solutions often use a combination of both webhooks and APIs to balance immediacy and control.

For junior developers and anyone preparing for system design interviews, understanding these patterns is invaluable. It’s a common technical interview tip to know the trade-offs between an event-driven approach (like webhooks) and a request-driven approach (like polling APIs). By mastering concepts like this, you’ll be better equipped to design efficient systems and answer interview questions with confidence.

If you’re eager to deepen your knowledge and practice these concepts, we encourage you to explore our Grokking the System Design Interview course at DesignGurus.io. It’s packed with system design interview prep, including real-world scenarios, mock interview practice, and insights into scalable system architecture. By continuing to learn and apply fundamentals like webhooks vs API calls, you’ll be well on your way to building robust integrations and acing your next coding interview. Good luck, and happy learning!

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