FetchDSL is an easy to use way to make HTTP requests

View the Project on GitHub devslash-paul/fetchDSL

View the Javadoc for the API, and service

FetchDSL is a compact DSL that allows for fast, easy to maintain, HTTP request scripts.

The DSL runs using kotlin, ktor, and kotlinx-coroutines to provide highly parallel requests, without requiring many resources to run quickly.

Getting started

The runHttp context is the main entrypoint for the DSL. Each instance of runHttp could be considered similar to an incognito browser window. That is, within the runHttp block, cookies and state may be shared, and as soon as the block is over, any future blocks start from a fresh state.

Multiple runHttp blocks can be run at the same time in different threads. There is no global state.

Making a simple call

To get started with a simple call, open the runHttp context, and create a call

runHttp {

At it most simple, this will not output anything, or provide anything in the request body. This is analogous to the following silent, suppressed curl call.

curl -s --output /dev/null example.com

Making multiple calls

Multiple calls can be added to a single runHttp block. Cookies are shared with all calls in a runHttp block.

runHttp {

This is quite slow, encumbering, and not very useful.

One of the major advantages of fetchDSL is its ability to perform many thousands of requests efficiently. Even when there’s changes in each request. To take advantage of this, we have to define a DataSupplier.

Changing request method


Adding a body

Making multiple requests with Data Suppliers

The concept of data suppliers comes from the fact that most of the time when you require many requests to occur, there’s usually something a little different each time.

For instance, it may be that you want to download all the HTML pages given a sitemap. Each time you’re simply doing a GET call and saving the response, but the URL will change each time.

Data suppliers are the basis for providing the changed data to a request.

In the following example, we’ll be utilising the FileDataSupplier. This is a basic line-by-line supplier. Where each line indicates a new request, with the arguments for that request on the line.

For example, if we were trying to add many items to our shopping cart, we may have a file that contains each item.

...many more lines

Then, to create a POST request for each of those lines, you’d simply create the following

runHttp {
  call("https://www.coolshoppingcart.com/addItem") {
    type = HttpMethod.POST
    data = FileDataSupplier("inputfile.txt")
    body {

There’s a few new things here, so lets go through them.

First of all, a call takes a second parameter which is a block. In kotlin this can be specified outside of the parameters, so the convention is to place it after the closed parenthses. Within this, you may specify details about this specific call.

First of all, we’ll change the call from its default (a GET) to a POST by specfying the type.

Then, we set our data. The FileDataSupplier reads line by line from the input file.

Finally, we set the body. Note that the body, like the call receives a block as a parameter. There are many ways to set a body, depending on your requirements. We choose the simplest, which just sets the body to specific text content.

We also see "!1!". This is a replacement section and at the heart of fetchDSL. This allows you to specify something that should be replaced on a per call basis. The way this works, is that the data supplier is called at the start of each request, and returns a map of replacements. In the default case, the replacements are specified as !1!, !2! , !3! and so on. A one based index wrapped in !. The replacements themselves come from the line in the supplied file.

In a file such as

hello there
Bye now

And using a supplier such as FileDataSupplier. Then !1! would be replaced with hello on the first request. Any Bye on the second. !2! would be there on th first request, and now on the second.

FileDataSupplier allows you to split by more than just spaces. The optional second argument to FileDataSupplier is the splitting char.

Before Hooks

Sometimes you’ll want something to take place before your HTTP request gets sent. fetchDSL is bundled with a few that should help, but you’re encouraged to create your own as well.


The Skip hook is a bundled hook that can be utilised to figure out if a request should be avoided. This is useful in the case where you have a large set of requests to occur any no simple way to pre-screen which ones should be re-attempted. This is especially useful when you are dealing with a flaky endpoint that may require restarting your batch of requests.

val cart = getCurrentShoppingCart()

runHttp {
  call("https://www.coolshoppingcart.com/addItem") {
    before {
      +SkipIf { data -> card.contains("!1!".asReplaceableValue().get(data)) }

There’s quite a bit to unpack there. So lets start with the block statement. before indicates a series of things to occur before a request.

The evaluation of the block occurs in a builder-style pattern, so simply writing code within the block will execute it all before the first request is created. To have something that occurs every time it must be ‘added’ to the before set of actions. This occurs via the + symbol. You’ll see this in after blocks as well.

To be a valid + target you must implement BeforeHook. As of now, this interface is empty, but fetchDSL knows how to run the following specific children:


The Once hook is simply a hook that itself takes another hook. That inner hook will only ever be performed once. Due to an exclusive lock on the inner parts of the Once look, all other requests will block behind the Once block finishing.

Note: Currently the Once hook is running an experiment where it will accept custom declarations of BeforeHook. It will attempt to use reflection to find an appropriate set of parameters to provide to invoke the hook. This also means that the hooks provided to the Once can be suspended. If an appropriate method could not be found, an InvalidHookException is thrown.


This is a simple logging hook that logs to StdOut.

After Hooks


This simply outputs to StdOut. You are able to customise the output by providing an implementation to OutputFormat in the optional second argument.


Often, you’ll want to see the results of a call or store it in some way. In `f