一、概述
Webhook是一个API概念，并且变得越来越流行。我们能用事件描述的事物越多，webhook的作用范围也就越大。Webhook作为一个轻量的事件处理应用，正变得越来越有用。
准确的说webhoo是一种web回调或者http的push API，是向APP或者其他应用提供实时信息的一种方式。Webhook在数据产生时立即发送数据，也就是你能实时收到数据。这一种不同于典型的API，需要用了实时性需要足够快的轮询。这无论是对生产还是对消费者都是高效的，唯一的缺点是初始建立困难。
Webhook有时也被称为反向API，因为他提供了API规则，你需要设计要使用的API。Webhook将向你的应用发起http请求，典型的是post请求，应用程序由请求驱动。
二、使用webhook
消费一个webhook是为webhook准备一个URL，用于webhook发送请求。这些通常由后台页面和或者API完成。这就意味你的应用要设置一个通过公网可以访问的URL。
多数webhook以两种数据格式发布数据：JSON或者XML，这需要解释。另一种数据格式是application/x-www-form-urlencoded or multipart/form-data。这两种方式都很容易解析，并且多数的Web应用架构都可以做这部分工作。
三、Webhook调试
调试webhook有时很复杂，因为webhook原则来说是异步的。你首先要解发他，然后等待，接着检查是否有响应。这是枯燥并且相当低效。幸运的是还有其他方法：
1、明白webhook能提供什么，使用如RequestBin之类的工具收集webhook的请求；
2、用cURL或者Postman来模拟请求；
3、用ngrok这样的工具测试你的代码；
4、用Runscope工具来查看整个流程。
四、webhook安全
因为webhook发送数据到应用上公开的URL，这就给其他人找到这个URL并且发送错误数据的机会。你可采用技术手段，防止这样的事情发生。最简单的方法是采用https（TLS connection）。除了使用https外，还可以采用以下的方法进一步提高安全性：
1、首先增加Token，这个大多数webhook都支持；
2、增加认证；
3、数据签名。
五、重要的问题
当作为webhook的消费者时有两件事需要铭记于心：
1、webhook通过请求发送数据到你的应用后，就不再关注这些数据。也就是说如果你的应用存在问题，数据会丢失。许多webhook会处理回应，如果程序出现错误会重传数据。如果你的应用处理这个请求并且依然返回一个错误，你的应用就会收到重复数据。
2、webhook会发出大量的请求，这样会造成你的应用阻塞。确保你的应用能处理这些请求。
Webhook vs API: What’s the Difference Between Them?
As you become familiar with SendGrid’s services, or even those of many Internet companies, you’ll see the term “API” used a lot. Increasingly, you may also see someone mention a “webhook.”
At SendGrid, we’ve consciously made a distinction between the two in our documentation and any time we write or speak about our seven services for simplifying email. Here’s how we break down the differences between webhooks and APIs.
What is an API?
API stands for Application Programming Interface, but what does that really mean? Rather than what it is, I find it easier to talk about what APIs enable. APIs can share data or functionality. For example, you might use a places API to look up restaurants by location or name, pulling out a lot of data about each place. Then you might combine that with a mapping API, using it as an interface for displaying your data.
Programmers who make a request to an API will then receive a response. For example, using our Web API to send an email, you’d pass the email contents with the request. If all goes well, you will receive a response declaring success.
What is a Webhook?
Sometimes people call webhooks reverse APIs, but perhaps more accurately a webhook lets you skip a step. With most APIs there’s a request followed by a response. No request is required for a webhook, it just sends the data when it’s available.
To use a webhook, you register a URL with the company providing the service. That URL is a place within your application that will accept the data and do something with it. In some cases, you can tell the provider the situations when you’d like to receive data. Whenever there’s something new, the webhook will send it to your URL.
“Call Me When He’s Warmed Up”
Baseball managers could really use webhooks. As the game progresses, they often want to change pitchers. To do this requires the new pitcher to first warm up in the bullpen, which is usually over 300 feet from the team’s dugout. If you watch baseball on television, you’ll often see the manager pick up a phone in the dugout. He’s making a call to the bullpen to check on the new pitcher.
- “Is he warmed up yet?”
- “Not yet”
Then he hangs up the phone. In a few minutes he’ll have to call again. Programmers would call this polling and it’s process-intensive for both sides. A webhook lets you say, “call me when he’s warmed up.”
Example Webhooks
SendGrid has two distinct webhooks related to each direction that email flows:
- Event Webhook provides data about the emails you send, such as bounces and when the recipient clicks a link.
- Inbound Parse Webhook allows your application to receive email as soon as a message comes in.
Some non-SendGrid webhooks:
- Twilio’s webhooks send text messages and voice calls to your application.
- DropBox just announced webhooks to provide real-time notifications about file changes in Dropbox.
- Metwit added webhooks to its weather API to provide notifications when specific types of weather occur in a particular location. It inspired my post on why every API needs webhooks.
Hopefully that gives you a few ideas about what a webhook is and how it is different from an API.
Now it’s time to start doing something with those webhooks. Here are four things you can do with webhooks, including testing them on your local machine or connecting them to other services.