Suave is a simple web development F# library providing a lightweight web server and a set of combinators to manipulate route flow and task composition.

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type ErrorHandler = Exception -> String -> HttpContext -> HttpContext

and HttpRuntime =
  { protocol          : Protocol
    errorHandler      : ErrorHandler
    mimeTypesMap      : MimeTypesMap
    homeDirectory     : string
    compressionFolder : string
    logger            : Log.Logger
    sessionProvider   : ISessionProvider }

and HttpContext =
  { request   : HttpRequest
    runtime   : HttpRuntime
    userState : Map<string, obj>
    response  : HttpResult }

and ISessionProvider =
  abstract member Generate : TimeSpan * HttpContext -> string
  abstract member Validate : string * HttpContext -> bool
  abstract member Session<'a>  : string -> SessionStore<'a>

Default-supported HTTP Verbs

See RFC 2616.

These applicatives match on HTTP verbs.

let GET     (x : HttpRequest) = ``method`` "GET" x
let POST    (x : HttpRequest) = ``method`` "POST" x
let DELETE  (x : HttpRequest) = ``method`` "DELETE" x
let PUT     (x : HttpRequest) = ``method`` "PUT" x
let HEAD    (x : HttpRequest) = ``method`` "HEAD" x
let CONNECT (x : HttpRequest) = ``method`` "CONNECT" x
let PATCH   (x : HttpRequest) = ``method`` "PATCH" x
let TRACE   (x : HttpRequest) = ``method`` "TRACE" x
let OPTIONS (x : HttpRequest) = ``method`` "OPTIONS" x

Server configuration

The first argument to startWebServer is a configuration record with the following signature. (See below for tips on customizing this.)

/// The core configuration of suave. See also Suave.Web.defaultConfig which
/// you can use to bootstrap the configuration:
/// <code>{ defaultConfig with bindings = [ ... ] }</code>
type SuaveConfig =
  { /// The bindings for the web server to launch with
    bindings                : HttpBinding list

    /// A server-key to use for cryptographic operations. When generated it
    /// should be completely random; you can share this key between load-balanced
    /// servers if you want to have them cryptographically verify similarly.
    serverKey              : byte []

    /// An error handler to use for handling exceptions that are
    /// are thrown from the web parts
    errorHandler           : ErrorHandler

    /// Timeout to wait for the socket bind to finish
    listenTimeout          : TimeSpan

    /// A cancellation token for the web server. Signalling this token
    /// means that the web server shuts down
    cancellationToken      : Threading.CancellationToken

    /// buffer size for socket operations
    bufferSize             : int

    /// Buffer manager auto grow
    autoGrow               : bool

    /// max number of concurrent socket operations
    maxOps                 : int

    /// MIME types
    mimeTypesMap          : MimeTypesMap

    /// Home or root directory
    homeFolder             : string option

    /// Folder for temporary compressed files
    compressedFilesFolder  : string option

    /// Suave's logger. You can override the default instance if you wish to
    /// ship your logs, e.g. using
    /// Also, this logger will be configured by default for Suave unless you
    /// explicitly use `Suave.Logging.Global.initialise` before starting the
    /// web server (the first time – the second time, the static will already
    /// have been initialised).
    logger                : Logger

    /// Pluggable TCP async sockets implementation. You can choose betwee libuv
    /// and CLR's Async Socket Event Args. Currently defaults to the managed-only
    /// implementation.
    tcpServerFactory      : TcpServerFactory

    /// The cookie serialiser to use for converting the data you save in cookies
    /// from your application into a byte array.
    cookieSerialiser      : CookieSerialiser

    /// A TLS provider implementation.
    tlsProvider           : TlsProvider

    /// Make this true, if you want Suave not to display its server header in
    /// every response. Defaults to false.
    hideHeader            : bool

    /// Maximun upload size in bytes
    maxContentLength      : int }

With Protocol , HttpBinding and MimeType defined like follows:

type ITlsProvider =
  abstract member Wrap  : Connection -> SocketOp<Connection>

/// Gets the supported protocols, HTTP and HTTPS with a certificate
type Protocol =
  /// The HTTP protocol is the core protocol
  | HTTP
  /// The HTTP protocol tunneled in a TLS tunnel
  | HTTPS of ITlsProvider

/// A HTTP binding is a protocol is the product of HTTP or HTTP, a DNS or IP binding
/// and a port number
type HttpBinding =
  /// The scheme in use
  { scheme : Protocol
    /// The host or IP address to bind to. This will be interpreted by the operating system
    ip     : IPAddress
    /// The port for the binding
    port   : uint16 }

type MimeType =
  /// The name of the mime type, i.e "text/plain"
  { name         : string
    /// If the server will compress the file when clients ask for gzip or
    /// deflate in the `Accept-Encoding` header
    compression  : bool }


A request life-cycle begins with the HttpProcessor that takes an HttpRequest and the request as bytes and starts parsing it. It returns an HttpRequest option that, if Some, gets run against the WebParts passed.

The WebPart

A web part is a thing that acts on a HttpContext, the web part could fail by returning None or succeed and produce a new HttpContext. Each web part can execute asynchronously, and it’s not until it is evaluated that the async is evaluated. It will be evaluated on the same fibre (asynchronous execution context) that is consuming from the browser’s TCP socket.

type SuaveTask<'a> = Async<'a option>
type WebPart = HttpContext -> SuaveTask<HttpContext>
// hence: WebPart = HttpContext -> Async<HttpContext option>

The ErrorHandler

An error handler takes the exception, a programmer-provided message, a request (that failed) and returns a web part for the handling of the error.

/// An error handler takes the exception, a programmer-provided message, a
/// request (that failed) and returns
/// an asynchronous workflow for the handling of the error.
type ErrorHandler = Exception -> String -> WebPart

Changing the Default Configuration

defaultConfig (defined in Suave.Web) has sane defaults, and for many users, these will be fine. However, since SuaveConfig is a record type, it is easy to swap out one or more of the default settings, tweaking the configuration to your precise needs. While we will discuss the default values below, you can review the defaults at the bottom of Web.fs.

If you’re looking to get started quickly, you can jump straight to sections detailing how to change the IP addres or port, specify a home directory, and set the server’s cryptography key. For those whose descriptions are prefixed with “(advanced)”, there is the potential to degrade Suave’s performance; you should take great care when changing these from their default values.

Changing the server’s IP address or port

The default binding for Suave is It is rather simple to change that, though.

let myCfg =
  { defaultConfig with
      bindings = [ HttpBinding.createSimple HTTP "" 8082 ]

As bindings is a list, you can also configure Suave to listen on any combination of IP addresses and ports.

Changing the server’s cryptography key

Suave encrypts state stored in sessions, and the key used for that is the serverKey configuration setting. This key is required to be a specific length (256 bits as of this writing), so there is a ServerKey module that helps ensure that the key is the proper length. The importance of this key, how to generate one, and how to plug it into your Suave config can be found under the Server Keys heading on that page.

While the examples all demonstrate using a base64-encoded string, if you have 256 bits already in a byte array that you want to use as a server key, you can use ServerKey.validate instead of ServerKey.fromBase64; it will ensure that the key is the proper length.

Changing the error handler

Suave’s default error handler logs the error, then returns an HTTP 500 response. For local requests, it returns the error and stack trace in the body of the response; for others, it returns “Internal Server Error”. The best way to customize it would be to start with defaultErrorHandler near the top of Web.fs, and tweak it to your liking. Then…

let myCfg =
  { defaultConfig with
      errorHandler = myErrorHandler

Changing the listen timeout

(advanced) This is the TimeSpan Suave will wait, on startup, for its request to bind to a specific TCP port to be successful. The default value is for listenTimeout is 2 seconds.

Changing the cancellation token

As with any asynchronous process, Suave can be controlled by the cancellation token used to start the process. By default, it uses the default cancellation token; however, giving cancellationToken a specific value here will allow for scenarios such as stopping and restarting Suave programmatically.

Changing the buffer size

(advanced) The bufferSize that is used for socket operations (low-level communications). Its default value is 8192 bytes (8KB).

Changing whether the buffer can automatically grow

(advanced) This boolean specifies whether the buffer manager is allowed to grow itself; autoGrow defaults to true.

Changing the maximum concurrent operations

(advanced) This is the maximum number of concurrent socket operations that Suave will attempt to serve. The default value for maxOps is 100.

Changing the MIME type map

The Writers module has a default MIME type map, and that is the default map in the configuration. Suave will not serve a file for which it cannot determine a MIME type, so if you are serving files that are not current in the MIME type map, you will need to add this type.

The above paragraph uses the word “map” several times, but it’s technically a mapping function; to modify it, you need a function of your own. As of this writing, .iso files are not in the default mapping. Here’s how we could add it.

let myMimeTypesMap ext =
  match Writers.defaultMimeTypesMap ext with
  | Some mime -> mime
  | _ ->
      match ext with
      | ".iso" -> createMimeType "application/octet-stream" false
      | _ -> None

// and then

let myCfg =
  { defaultConfig with
      mimeTypesMap = myMimeTypesMap

Changing the home folder

Suave does not have a default home folder. If you want to serve files from a folder, just specify it as a Some string. If an absolute path is not given, it is interpreted from the current working directory when Suave was started. For example, if we follow the .NET Core convention of putting our publicly-available files under wwwroot, this example sets it as the home folder.

let myCfg =
  { defaultConfig with
      homeFolder = Some "./wwwroot"

Changing the compressed files folder

Suave writes temporary files to disk when performing compression; the default directory where these files are placed is the current working directory when Suave was started. However, if you want these files to go somewhere specific, you can specify them the exact same way we did above for the home folder; just set the compressedFilesFolder field instead. (Suave deletes these files once they are served, so their lifetime is usually less than a second.)

Changing logging options

A good overview can be found on the logging page.

Changing the default TCP server

Suave’s default TCP server uses the .NET CLR’s socket implementation to bind and listen. Suave also comes with a TCP server implementation based on LibUV; to use it…

open Suave.LibUv

let myCfg =
  { defaultConfig with
      tcpServerFactory = new LibUvServerFactory()

By default, Suave uses a binary formatter to serialize a Map<string, obj> into a string that is encrypted and used as a cookie payload. On the “State and Sessions” page, there is an example of swapping out the default cookie serializer for one based on JSON.NET.

Changing the default TLS provider

(advanced) The TLS provider supports encrypted communications (HTTPS). The default is an instance of the DefaultTlsProvider class; customizing it would require implementing the TlsProvider interface, and providing an instance of that class in your configuration’s tlsProvider field.

Changing whether the header is shown

By default, Suave adds a Server header to each response, identifying itself as the software that handled the request. If this behavior is not desired, you can set hideHeader to true.

Changing the maximum file upload size

The maxContentLength field controls the maximum allowed upload size, in bytes; its default is 10000000 (10 MiB).