Conditional Headers

The ConditionalHeaders feature avoids sending the body of content if it has not changed since the last request. This is achieved by using the following headers:

• The Last-Modified response header contains a resource modification time. For example, if the client request contains the If-Modified-Since value, Ktor will send a full response only if a resource has been modified after the given date. Note that for static files Ktor appends the Last-Modified header automatically after installing ConditionalHeaders.

• The Etag response header is an identifier for a specific resource version. For instance, if the client request contains the If-None-Match value, Ktor won’t send a full response in case this value matches the Etag. You can specify the Etag value when configuring ConditionalHeaders.

Install ConditionalHeaders

To install the ConditionalHeaders feature, pass it to the install function in the application initialization code. This can be the main function …

import io.ktor.features.*
// ...
fun Application.main() {
  install(ConditionalHeaders)
  // ...
}

… or a specified module:

import io.ktor.features.*
// ...
fun Application.module() {
    install(ConditionalHeaders)
    // ...
}

Configure Headers

To configure ConditionalHeaders, you need to call the version function inside the install block. This function provides access to a list of resource versions for a given OutgoingContent. You can specify the required versions by using the EntityTagVersion and LastModifiedVersion class objects.

The code snippet below shows how to add a specified Etag value for CSS:

install(ConditionalHeaders) {
    version { outgoingContent ->
        when (outgoingContent.contentType?.withoutParameters()) {
            ContentType.Text.CSS -> listOf(EntityTagVersion("123abc"))
            else -> emptyList()
        }
    }
}