Conditional headers

The ConditionalHeaders plugin (previously known as 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 plugin, pass it to the install function in the application initialization code. Depending on the way used to create a server, this can be the embeddedServer function call ...

            import io.ktor.features.*
            // ...
            fun main() {
                embeddedServer(Netty, port = 8080) {
                    install(ConditionalHeaders)
                    // ...
                }.start(wait = true)
            }
        

... 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 Etag and Last-Modified headers for CSS:

    install(ConditionalHeaders) {
        val file = File("src/main/kotlin/com/example/Application.kt")
        version { outgoingContent ->
            when (outgoingContent.contentType?.withoutParameters()) {
                ContentType.Text.CSS -> listOf(
                    EntityTagVersion(file.lastModified().hashCode().toString()),
                    LastModifiedVersion(Date(file.lastModified()))
                )
                else -> emptyList()
            }
        }
    }

You can find the full example here: conditional-headers.

Last modified: 11 May 2022