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Kotlin I/O : Streams + TCP Client/Server + VFS for Multiplatform Kotlin 1.3

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All KOR libraries

Sublibraries (deprecated):

Use with gradle:

repositories {
  maven { url "" }

dependencies {
  compile "com.soywiz.korlibs.korio:korio:$korVersion"

This is a kotlin coroutine library that provides asynchronous non-blocking I/O and virtual filesystem operations for custom and extensible filesystems with an homogeneous API. This repository doesn't require any special library dependency and just requires Kotlin 1.1-M04 or greater.

This library is specially useful for webserver where asynchronous is the way to go. And completely asynchronous or single threaded targets like JavaScript or AS3, with kotlin-js or JTransc (Node.JS and Browser). So if you use korio you will be able to target several platforms without any problem.

It has a modern and useful API. And all works in Java 7, so it is compatible with Android.

Event Loop and async primitives

Korio provides an Event Loop that integrates with each supported platform seamlessly. So in JS the event loop will use setTimeout and setInterval, and will queue actions with it. In the case of Android it will use runOnUiThread and timer primitives, and in the JVM it would use en emulated Event Loop. You can even create your own event loop implementation and hook it.

Korio also provides some async primitives until they are officially available at a common place like kotlinx.coroutines, and will provide typealias + @Deprecated for the future migration when available.


Embedded Json, Xml and Yaml parsers. Can also write Json (with pretty print support) and Xml. Support Json to Object mapping with kotlin data classes support.


Korio provides AsyncStream and SyncStream classes with a simplified readable, writable and seekable API, for reading binary and text potentially huge data from files, network or whatever. AsyncStream is designed to be able to read from disk or network asynchronously. While SyncStream is designed to be able to read in-memory data faster while keeping the same API.

Both stream classes allow to read and write raw bytes, little and big endian primitive data, strings and structs while allowing optimized stream slicing and reading for a simple binary file handling.

Some stream methods:

read, write
setPosition, getPosition
setLength, getLength
getAvailable, sliceWithSize, sliceWithBounds, slice, readSlice
readStringz, readString, readExact, readBytes, readBytesExact
readU8, readU16_le, readU32_le, readS16_le, readS32_le, readS64_le, readF32_le, readF64_le, readU16_be, readU32_be, readS16_be, readS32_be, readS64_be, readF32_be, readF64_be, readAvailable
writeBytes, write8, write16_le, write32_le, write64_le, writeF32_le, writeF64_le, write16_be, write32_be, write64_be, writeF32_be, writeF64_be

AsyncClient + AsyncServer

Korio includes a TCP client (implementing AsyncStream) and a TCP server with a lazy asynchronous connection iterator for all supported platforms but browser javascript.


Korio includes a WebSocket client. It has two implementations: one simple and generic for targets supporting AsyncClient and other for browser javascript. So this is supported on all targets.

HttpClient + HttpServer

Korio includes a HttpClient client that uses available native implementations. UrlVfs uses HttpClient. Korio also includes a HttpServer server that uses available native implementations. That webserver support websockets. It is extensible and hooks with Korio's router, static VfsFile serving, cookies, sessions, oauth and so on.


Korio provides a router class for creating web applications that uses Korio's HttpServer so works everywhere. It supports per class and per method injections, route annotations, injection of params, getters, posts, the request itself. Also support websocket routes.

Static File Serving

Korio allows to serve VfsFiles directly using HttpServer supporting mime types, Last-Modified, ETag and Range. There is in the works a refactor in the system that internally uses MappedByteBuffer with zero copy overhead to transfer from local files into sockets transparently using Korio async streams and VfsFile at the JVM level, and equivalent methods in other targets.

Cookies and Sessions

Korio supports cookies and sessions as extensions for its HttpServer.

Korte integration

Korio has a tight integration with Korte, a hybrid template engine compatible with both twig and liquid (jekyll) in an asynchronous fashion supporting asynchronous lazy loading of data (executing suspend getters). Prepared to support chunked serving to reduce memory requirements per request.


Korio includes an oauth client + Google and Facebook oauth implementation for logins.



Korio includes basic support for connecting and querying to cassandra.


Korio includes basic support for connecting and querying to elasticsearch with its DSL.


Korio includes an extension with some database/cache clients. At this point, there is a Redis client implementation, but will provide more soon.


Korio includes a pure Kotlin DynamoDB asynchronous simple and fast implementation using Korio's HttpClient implementations, which leverages: jvm, android, js (browser and nodejs) + pure asynchronous http clients from vertx when including korio-vertx.


Korio provides an asynchronous Virtual File System extensible engine. There is a Vfs class and a Vfs.Proxy class that provides you a base for your VFS. But when using it you are using a VfsFile class that represents a node (file or folder) inside a Vfs.

As an example, in a suspend block, you can do the following:

val zip = ResourcesVfs()[""].openAsZip() // Non blocking opening zip file
for (file in zip.listRecursively()) { // Lazy non blocking recursive file listing


In order to increase security, Vfs engine provides a JailVfs that allows you to sandbox VFS operations inside a specific folder. So you can do the following:

val jail = LocalVfs(File("/path/to/sandbox/folder")).jail()
jail["../../../etc/passwd"].readString() // this won't work


Korio includes a MountableVfs that allows you to mount other filesystems like this:

val resources = ResourcesVfs
val root = MountableVfs {
	mount("/zip", resources[""].openAsZip())
	mount("/iso", resources["isotest.iso"].openAsIso())
assertEquals("ZIP!", root["/zip/hello/world.txt"].readString())
assertEquals("ISO!", root["/iso/hello/world.txt"].readString())

(root.vfs as Mountable).unmount("/zip")

Memory Vfs

Korio includes an in-memory vfs to create volatile vfs:

val mem = MemoryVfs(mapOf(
    "hello/secret.txt" to "SECRET!".openAsync(Charsets.UTF_8),
    "hello/world/test.txt" to "HELLO WORLD!".openAsync(Charsets.UTF_8)


val mem = MemoryVfsMix(
    "hello/secret.txt" to "SECRET!",
    "hello/world/test.txt" to "HELLO WORLD!"


Korio includes an open base NodeVfs to support node based vfs like in-memory vfs.

S3Vfs (Amazon S3 Vfs)

In the korio-ext-amazon-s3 submodule, there is an Amazon S3 Client without external dependencies that is implemented as a VFS for convenience.


Korio includes a PathInfo utility integrated with VfsFile in order to obtain path information (folder, basename, extension...)


Korio includes a mechanism to execute commands inside a VfsFile. This allows you to execute asynchronous commands in your local file system, but also allows to implement RPC mechanisms that will work seamlessly. You can implement here ssh or ftp commands as an example.

Included Vfs

There are several filesystems included and you can find examples of their usage in the test folder:

LocalVfs, UrlVfs, ZipVfs, IsoVfs, ResourcesVfs, JailVfs, MountableVfs, MemoryVfs, S3Vfs

For Vfs implementations:

Vfs, Vfs.Proxy, Vfs.Decorate, NodeVfs, MergedVfs


To understand which kind of operations can be performed, this is the VfsFile API:

class VfsFile {
	val vfs: Vfs
	val path: String
	val basename: String
	val pathInfo: PathInfo
	// Accessing parent
	val parent: VfsFile
	// Accessing descendants, and ascendants using relative paths
	operator fun get(path: String): VfsFile
	// Opening file as AsyncStream
	suspend fun open(mode: VfsOpenMode): AsyncStream
	// Convenience methods for fully reading/writing files
	suspend fun read(): ByteArray
	suspend fun readString(charset: Charset = Charsets.UTF_8): String
	suspend fun readChunk(offset: Long, size: Int): ByteArray
	suspend fun readAsSyncStream(): SyncStream
	suspend fun write(data: ByteArray): Unit
	suspend fun writeString(data: String, charset: Charset = Charsets.UTF_8): Unit
	suspend fun writeChunk(data: ByteArray, offset: Long, resize: Boolean = false): Unit
	// Stat + convenience methods
	suspend fun stat(): VfsStat
	suspend fun size(): Long
	suspend fun exists(): Boolean
	// Modification operations
	suspend fun delete(): Boolean
	suspend fun mkdir(): Boolean
	suspend fun mkdirs(): Boolean
	suspend fun renameTo(path: String): Boolean
	suspend fun setSize(size: Long): Unit
	// Directory listing
	suspend fun list(): AsyncSequence<VfsFile>
	suspend fun listRecursive(): AsyncSequence<VfsFile>
	// Executing in this folder/filesystem. You can implement RPC here or ftp/sftp commands. Implemented in default korio for LocalVfs.
	suspend fun exec(cmdAndArgs: List<String>, handler: VfsProcessHandler = VfsProcessHandler()): Int
	// Convenience execution methods
	suspend fun execToString(cmdAndArgs: List<String>, charset: Charset = Charsets.UTF_8): String
	suspend fun execToString(vararg cmdAndArgs: String, charset: Charset = Charsets.UTF_8): String
	suspend fun passthru(cmdAndArgs: List<String>, charset: Charset = Charsets.UTF_8): Int
	suspend fun passthru(vararg cmdAndArgs: String, charset: Charset = Charsets.UTF_8): Int
	// File watching
	suspend fun watch(handler: (VfsFileEvent) -> Unit): Closeable =, handler)
	// Jail this file so generated VfsFile can't access ancestors
	fun jail(): VfsFile = JailVfs(this)

data class VfsStat {
	val file: VfsFile
	val exists: Boolean
	val isDirectory: Boolean
	val size: Long
	val device: Long
	val inode: Long
	val mode: Int
	val owner: String
	val group: String
	val createTime: Long
	val modifiedTime: Long
	val lastAccessTime: Long
	val extraInfo: Any?

class PathInfo(val fullpath: String) {
	val folder: String
	val basename: String
	val pathWithoutExtension: String
	val basenameWithoutExtension: String
	val extension: String

You can create custom virtual file systems and combine them (for S3, for Windows Registry, for FTP/SFTP, an ISO file...) or whatever you need.

Also, since you are using a single interface (VfsFile), you can create generic code that will work for files, for network, for redis... You can use a MemoryVfs for testing while using a real folder in your code without having to mock code.


Korio supports JVM, Android, Browser and Node.JS out of the box at this point. But it is extensible so you can create your own targets or benefit from new ones when available.


  • JVM uses NIO and common runtime tools
  • Android uses threads when required and simplifies and unifies resources/assets loading/listing
  • Node.JS uses all asynchronous methods available and supports full korio
  • Browser allows reading "embedded" resource lists supported by jtransc + reading urls chunks/streaming with buffering in same domain or with CORs. Do not support raw client/server sockets.


Korio: Kotlin cORoutines I/O : Virtual File System + Async/Sync Streams + Async TCP Client/Server + WebSockets for Multiplatform Kotlin 1.3




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