Intercepting routes

Introduction

One of the features of Ktor is its routing mechanism, which allows developers to define the behavior of their application based on different HTTP requests and paths. However, sometimes developers need to perform some actions at a specific stage of the routing workflow. This is where intercepting routes comes into play. By intercepting a route, we can inject custom behavior or functionality into our application's request handling process.

What is the Pipeline in Ktor?

Before we intercept something, we need to understand what exactly we want to intercept. To do that, let's refresh our memory with some theory.

In Ktor, an ApplicationCall represents a single transaction between a client and the server. It encapsulates the incoming request and its corresponding response.

On the other hand, ApplicationCallPipeline is the sequence of steps, or phases, that an ApplicationCall goes through. It's a mechanism for organizing the processing of an ApplicationCall.

When an HTTP request comes into a Ktor server, it is wrapped into an ApplicationCall object. Then this ApplicationCall is passed through the ApplicationCallPipeline and its phases.

These phases of ApplicationCallPipeline are what we are going to intercept.

ApplicationCallPipeline has standart phases:

• Setup: This is the initial phase where the call and its attributes for processing are prepared. It's rarely used for intercepting, as it is mostly for internal Ktor usage.
• Monitoring: This phase is handy for tracing calls, useful for logging, metrics, error handling, and so on.
• Plugins: Most plugins and features should intercept this phase. They are installed and processed here.
  
  Previously, the name of this phase was "Features". It has now been deprecated.
• Call: Phase for processing a call and sending a response.
• Fallback: This phase is for handling unprocessed calls.

You can add your phases to the pipeline, but this is a theme for another topic. Now let's intercept something.

Intercept function

Let's consider a simple example of how to use the intercept function to add some additional functionality, like logging, to our routing.

Suppose we have such a routing:

get("/") {
    call.respondText("This is home page")
}

get("/first") {
    call.respondText("Hello!")
}

get("/second") {
    call.respondText("Hello again!")
}

get("/third") {
   call.respondText("I say hello!")
}

To implement logging, just add the intercept function inside routing:

routing {
    intercept(ApplicationCallPipeline.Monitoring) {
        val x = call.request
        call.application.environment.log.info("intercepting request to: ${x.host()}:${x.port()}${x.path()}")
    }

    get("/") {
        call.respondText("This is home page")
    }
    //...
}

In the code above, we use the intercept function to add logging functionality to the Monitoring phase of the ApplicationCallPipeline. This means that for every request, the application will log our message, and we get logs like these:

2023-07-25 01:00:18.095 [eventLoopGroupProxy-4-1] INFO  ktor.application - intercepting request to: 127.0.0.1:8080/

2023-07-25 01:00:51.468 [eventLoopGroupProxy-4-1] INFO  ktor.application - intercepting request to: 127.0.0.1:8080/first

2023-07-25 01:00:57.681 [eventLoopGroupProxy-4-1] INFO  ktor.application - intercepting request to: 127.0.0.1:8080/second

2023-07-25 01:01:03.447 [eventLoopGroupProxy-4-1] INFO  ktor.application - intercepting request to: 127.0.0.1:8080/third

Use cases

Intercepting routes in Ktor can be used in a variety of scenarios:

1. Logging: As shown in the example above, you can use intercepting to log details about requests and responses.

2. Authentication: You can use intercepting to check whether a user is authenticated before accessing a particular route. If they're not, you can redirect them to a login page.

3. Error Handling: You can use intercepting to catch exceptions that occur during the handling of a request and return a custom error response.

4. Rate Limiting: You can use intercepting to limit the number of requests a user or IP address can make within a certain timeframe.

Note that while some of these use cases can be handled by existing Ktor features or plugins, intercepting routes gives you more flexibility and control over your application's behavior.

Rate limiting is a technique used to limit the network traffic. It sets a limit on how many requests a client can make to the server in a specific timeframe. To consolidate the studied, let's create a simple rate limiting.

First, we need a simple rate limiter. For this example, we'll use a HashMap to store the IP addresses and a counter for each IP:

class RateLimiter(private val limit: Int, private val timeWindow: Long) {
    private val requests = HashMap<String, Int>()
    private val timestamps = HashMap<String, Long>()

    fun shouldLimit(ip: String): Boolean {
        val currentTime = System.currentTimeMillis()
        timestamps[ip]?.let {
            if (currentTime - it > timeWindow) {
                timestamps[ip] = currentTime
                requests[ip] = 0
            }
        } ?: run {
            timestamps[ip] = currentTime
        }

        val currentRequests = requests[ip]?.inc() ?: 1
        requests[ip] = currentRequests

        return currentRequests > limit
    }
}

Then, we can use this RateLimiter in the intercept function:

val rateLimiter = RateLimiter(3, 60 * 1000) // Limit to 3 requests per minute

intercept(ApplicationCallPipeline.Plugins) {
    val ip = call.request.origin.remoteHost
    if (rateLimiter.shouldLimit(ip)) {
        call.respond(HttpStatusCode.TooManyRequests, "Too many requests. Please slow down.")
        finish() // Finishes current pipeline execution
    }
}

In the code above, first, we create a RateLimiter that allows three requests per minute. Then, we use the intercept function to add a rate limiting step to the Plugins phase of the ApplicationCallPipeline. We get the IP address of the client from the call.request.origin.remoteHost property. If the RateLimiter determines that the IP has exceeded the limit, we respond with a "429 Too Many Requests" status code and a message, and we stop the processing of the call with finish().

This is a basic example and might not be suitable for a production. For a more robust solution, consider using a library that provides rate limiting features.

Conclusion

Intercepting routes in Ktor is a powerful tool that allows you to add custom behavior to your application's request handling process. By understanding the concept of pipelines and how to use the intercept function, you can effectively manage and control the flow of your application. Whether for logging, authentication, error handling, or rate limiting, intercepting routes can make your application more robust and adaptable to your specific needs.