What Is a Webhook and Why Do You Need It?

You've probably already heard of webhook integrations but aren't sure what they are or whether you should use them. 

According to data from Google Trends, webhooks have become more popular over the last five years. But while they're clearly in demand – what exactly are they, and what do they mean for businesses that send emails?

What Is a Webhook?

A webhook is a programming interface that is driven by events instead of requests. Typically a software application will request data from another app or service via API whenever it needs. Webhooks, on the other hand, allow one program to send data to another as soon as a specific event occurs. Since the communication is initiated by the app sending the data rather than the one receiving it, webhooks are often called "reverse APIs".

Assume you want to be notified via your company’s messaging system whenever a specific product is mentioned in a blog or a specific hashtag is used. Instead of regularly querying the blogging platform for new posts that meet these criteria, the latter may send a notification only when this event occurs. The idea of a webhook is to allow the receiving app to sit back and have all the data it requires without sending repeated requests to another system.

Webhooks are a popular lightweight solution for real-time notifications and data updates without having to develop a full-scale API for interconnecting web services.

Using webhooks is extremely effective because everything is decoupled. In the image above, system B, which receives the notification via the chosen URL, is not only aware of the event on another system, but can also respond to it.

Why use a POST request? Because it allows you to send data in the request body. It is usually a simple JSON object, but it could also be an XML document. This will always be specified in the webhook documentation, so be sure to check it before getting started.

An added benefit is that webhooks can be public or private. The owner of a specific system account can register a webhook as private. It is not possible to track the account's activities in the wild.

How Webhooks Work: The Basics

To better understand what a webhook is, picture it as a lightweight messenger between applications. Instead of constantly polling another server for updates, your system exposes a specific URL. When something important happens inside the provider’s platform, it automatically sends fresh data to that URL in real time.

A webhook uses a simple idea: “We’ll let you know the moment something changes”. No extra code, no manual checks, and no delays.

The Webhook Flow, Step by Step

Here’s a clear breakdown of how webhook works:

1. An event happens
   An application registers something meaningful (an email is delivered, a payment succeeds, a form is submitted, etc.)
   
   
2. Trigger is pulled
   When the event happens, the provider prepares a small message containing details about the event.
   
   
3. Payload is created
   The event details are packaged into a JSON or XML object.
   
   
4. Data is sent to your app
   The webhook POST request goes to your configured callback URL.
   
   
5. Your system processes the data
   Your infrastructure receives the event and launches whatever workflow or processes you’ve defined.
   
   
6. A response is returned
   Your server acknowledges that the webhook was processed – typically with a simple 200 OK response code.

This flow allows systems to communicate smoothly and automatically without polling.

Webhook Payload Formats

While webhook payloads can use different structures, most modern platforms rely on:

• JSON – the most common format for webhook data: lightweight, flexible, and easy to parse in any programming language.
• XML – older but still widely used in enterprise systems.
• Form-encoded data – simple key-value pairs similar to a standard HTML form submission.

Regardless of the format, the payload includes everything your system needs to understand what happened and what action to take next.

Webhook vs. API: Key Differences

APIs and webhooks are often mentioned in similar contexts. While they both help you achieve your goals, they are not the same.

An application programming interface (API) is another tool that helps different apps communicate with one another. Although their use cases are very similar, you need to know the primary difference between APIs and webhooks.

It has to do with how they send and receive information.

Pushing vs. Pulling

The “pulling” process helps obtain data via an API. This is when your application sends a request to an API server to check for new data. In contrast, a webhook allows providers to send, or "push" data to your application only when an event actually occurs. APIs are used to periodically pull data from a server in order to stay current. With webhooks, however, the server can push this data to you via webhooks as soon as something happens.

APIs are like you calling an electronic store repeatedly to see if they have a new smartphone model you’d like. Webhooks are like asking the store to call you back when they have the smartphone in stock, which saves both time and resources.

Webhooks are less resource-consuming because they save you time by not having to keep constantly checking for new data.

So, why use APIs at all if webhooks are easier to set up, less resource-intensive, and faster than APIs?

APIs are still widely used for various reasons:

• Not all applications support webhook integrations.
• Webhooks can only send you event notifications; if you want to make changes based on new data or modified objectives, you'll need an API.
• A webhook payload might not have all the information you need.

APIs are still handy, and many apps support both APIs and webhooks.

When to Use Webhooks vs. APIs

Both webhooks and APIs help systems communicate, yet the purposes you can use them for differ. 

Use webhooks when:

• You need real-time updates automatically.
• A provider should notify your server about new events.
• You want to minimize infrastructure load and avoid constant polling.
  
  

Use APIs when:

• You need to control when exactly you receive the data with a request.
• Your system must retrieve large data sets or run complex operations.
• You’re building dashboards, syncing records, or managing resources.
  
  

A simple way to define webhook vs. API behavior:

• Webhook = “I’ll send the data to you when something happens”.
  
  
• API = “You ask me for the data and get it whenever you want”.

Most modern systems use both to build reliable workflows.

Real-World Examples of Webhooks

Webhooks appear in almost every online application today. Here are a few everyday examples based on the software we often interact with:

• Email services
  Customers receive webhook events for deliveries, opens, clicks, unsubscribes, and bounces. This is the most common way used by many email service providers, including UniOne, to facilitate detailed email analytics instantly.
  
  
• Payment platforms
  Users get notified when a transaction succeeds, fails, or needs manual review.
  
  
• E-commerce stores
  Trigger workflows when an order is created, canceled, or shipped.
  
  
• CRM platforms
  Automatically send data to your server when a contact is updated or a lead is created.
  
  
• Marketing automation
  Push new subscriber information instantly to your internal systems and launch an email automation process to run a welcome chain or promo campaign.
  
  

Whenever you need fast, automatic data exchange, webhooks deliver it.

Why Do You Need to Use Webhooks?

Assuming you're running a membership site. You must manually enter a customer's information into your membership management application every time they pay you through a payment gateway like Visa. As numbers increase, this task becomes even more tedious. But what if Visa and your membership management software could communicate? Can anyone who pays with a Visa card be automatically added as a member?

One way to accomplish this scenario is via a webhook. Assume Visa and your management software both support webhooks. The Visa integration can then be set up to automatically transfer a user's information over each time a payment occurs.

Here's an example of how this could work:

Webhooks are a fantastic technology that saves you time and effort. Due to their popularity, you'll be able to integrate most of the web apps you're already using.

Connecting your email marketing software to other applications via webhooks can open up a whole new world of possibilities:

• You connect a payment service to your email marketing software via a webhook so that when a payment bounces, the customer receives an email.
• Sync customer data across multiple applications. For example, if a user's email address changes, you can make sure the change is reflected in your CRM.
• You can use webhooks to send event data to external databases or data warehouses for further analysis. 

Consuming a Webhook

Giving the data provider an URL to deliver requests to is the first step in consuming a webhook. The most common method is to use a backend panel or an API. You'll need to create an URL in your app that can be accessed from the internet.

The majority of webhooks will send you data in one of the following formats: JSON (usually), XML (blech), or form data. These are simple to understand, and most web frameworks will take care of it.

UniOne’s Web API webhook methods only accept HTTPS POST requests in JSON format with a maximum size of 10 megabytes and return JSON responses. In case of an error, a suitable HTTP code is returned along with the JSON body containing the error description. API endpoint (aka base URL) must be submitted to the server that matches the user's registration server:

• eu1.unione.io — UniOne’s European server;
• us1.unione.io — US UniOne server.

Calling any method requires authentication with the user's or project's API key, using either of these two methods:

• The X-API-KEY HTTP header;
• The api_key JSON field in the request body.

You can find your API key on the account’s security or the project’s details page. 

Testing Webhooks

Before going live, it’s essential to test how your system receives and processes webhook requests. 

Dedicated webhook inspectors like Webhook.site or RequestBin allow you to:

• view the raw payload
• inspect headers
• replay events
• debug failing responses
• confirm your application processes data correctly

Testing webhooks will ensure your infrastructure handles events reliably and prevent unexpected production issues. 

Best Practices for Receiving Webhooks 

To make webhook integrations robust and secure, pay attention to the following matters. 

Delivery Guarantees and Idempotency

Providers often retry webhook deliveries if your server fails to respond.
Because the same event may arrive more than once, your code must be idempotent – it should handle repeated messages without creating duplicates, sending multiple notifications, or triggering the same workflow twice.

Response Time Limits

Webhook senders expect fast acknowledgment, usually within a few seconds. 

If your system needs more time to process data, return 200 OK first, then handle the event asynchronously in the background.

Throughput and Rate Management

High-volume applications may receive thousands of events per minute.

To mitigate load congestions, use:

• queueing systems;
• worker processes;
• rate limiting;
• horizontal scaling.

This protects your infrastructure from overload during traffic spikes.

Asynchronous Handling

Never block your webhook endpoint with heavy processing.
The recommended processing routine is:

1. Validate the message.
2. Store it.
3. Hand it off to a queue.
4. Let workers handle the slower tasks later.

This ensures stability and consistent performance.

Message Ordering

Webhooks may not always arrive in chronological order.
To prevent logic errors:

• use timestamps;
• track event IDs;
• enforce processing rules inside your system.

This is especially important for workflows involving status change.

Security and Signature Validation

Webhooks transmit sensitive data, so security must be your priority.
Always protect your webhook endpoints by:

• validating HMAC signatures;
• using HTTPS;
• rotating secrets;
• restricting IP ranges (if supported);
• requiring an authentication token in the URL.

Signature validation ensures that the webhook actually comes from the trusted sender, not an attacker.

When an App Doesn’t Have Webhook Integrations

Despite their popularity and importance, some applications still do not support webhooks. This can be mitigated with middleware apps such as Zapier, IFTTT, and Automate.io that can be used to regularly "pull" APIs and send updates via webhooks. You can use them to connect apps that don't have native webhook integrations and share data between them.

Conclusion

Webhooks are an important technology on the internet that is currently growing in popularity. They enable your applications to communicate with one another in real time.

While webhooks and APIs are very much alike, they serve different purposes and have different use cases. The best way to learn about webhooks is to use them for your needs. To get a feel for it, create a few webhook connections between your existing applications.

FAQ: Quick Answers to Common Webhook Questions

What are webhooks used for?

Webhooks are used to automatically transmit data from one application to another when a specific event occurs. They power real-time notifications, workflow automation, and seamless system-to-system communication.

What format does webhook data use?

Most webhook providers send data in JSON, though some support XML or form-encoded payloads. The exact format depends on the platform.

How is a webhook different from an API?

An API requires your system to request information, while a webhook automatically sends updates to your server when something happens. APIs pull data; webhooks push it.

Do I need an API key for webhooks?

Some platforms require an API key or secret token to authenticate webhook messages. Others use signature validation instead. Check the provider’s documentation for the exact answer, while also note that securing your endpoint is crucial anyway.