Storages

Ktor allows you to store session data on the server and pass only a session ID between the server and the client. In this case, you can choose where to keep the payload on the server.

Built-in storages

The following storage types are available out-of-the-box:

SessionStorageMemory enables storing a session's content in memory. This storage keeps data while the server is running and discards information once the server stops.
directorySessionStorage can be used to store a session's data in a file under the specified directory.

To create the required storage type, pass it as a second parameter to the cookie or header method. For example, you can store cookies in the server memory as follows:

install(Sessions) {
    cookie<SampleSession>("SAMPLE_SESSION", storage = SessionStorageMemory())
}

To store cookies in a file under the .sessions directory, create the directorySessionStorage in this way:

import java.io.File
// ...
install(Sessions) {
    cookie<SampleSession>("SAMPLE_SESSION", storage = directorySessionStorage(File(".sessions")))
}

Custom storage

Ktor provides the SessionStorage interface that allows you to implement a custom storage.

interface SessionStorage {
    suspend fun write(id: String, provider: suspend (ByteWriteChannel) -> Unit)
    suspend fun invalidate(id: String)
    suspend fun <R> read(id: String, consumer: suspend (ByteReadChannel) -> R): R
}

All three functions are suspending and use ByteWriteChannel and ByteReadChannel to read and write data from/to an asynchronous channel. The example below shows how to implement the SessionStorage interface to store session data in a ByteArray:

abstract class SimplifiedSessionStorage : SessionStorage {
    abstract suspend fun read(id: String): ByteArray?
    abstract suspend fun write(id: String, data: ByteArray?): Unit

    override suspend fun invalidate(id: String) {
        write(id, null)
    }

    override suspend fun <R> read(id: String, consumer: suspend (ByteReadChannel) -> R): R {
        val data = read(id) ?: throw NoSuchElementException("Session $id not found")
        return consumer(ByteReadChannel(data))
    }

    override suspend fun write(id: String, provider: suspend (ByteWriteChannel) -> Unit) {
        return provider(CoroutineScope(Dispatchers.IO).reader(coroutineContext, autoFlush = true) {
            write(id, channel.readAvailable())
        }.channel)
    }
}

suspend fun ByteReadChannel.readAvailable(): ByteArray {
    val data = ByteArrayOutputStream()
    val temp = ByteArray(1024)
    while (!isClosedForRead) {
        val read = readAvailable(temp)
        if (read <= 0) break
        data.write(temp, 0, read)
    }
    return data.toByteArray()
}

Last modified: 28 May 2021