HTTP Model

Akka HTTP model contains a deeply structured, fully immutable, case-class based model of all the major HTTP data structures, like HTTP requests, responses and common headers. It lives in the akka-http-core module and forms the basis for most of Akka HTTP’s APIs.


Since akka-http-core provides the central HTTP data structures you will find the following import in quite a few places around the code base (and probably your own code as well):

import akka.http.scaladsl.model._

This brings all of the most relevant types in scope, mainly:

  • HttpRequest and HttpResponse, the central message model
  • headers, the package containing all the predefined HTTP header models and supporting types
  • Supporting types like Uri, HttpMethods, MediaTypes, StatusCodes, etc.

A common pattern is that the model of a certain entity is represented by an immutable type (class or trait), while the actual instances of the entity defined by the HTTP spec live in an accompanying object carrying the name of the type plus a trailing plural ‘s’.

For example:

  • Defined HttpMethod instances live in the HttpMethods object.
  • Defined HttpCharset instances live in the HttpCharsets object.
  • Defined HttpEncoding instances live in the HttpEncodings object.
  • Defined HttpProtocol instances live in the HttpProtocols object.
  • Defined MediaType instances live in the MediaTypes object.
  • Defined StatusCode instances live in the StatusCodes object.


HttpRequest and HttpResponse are the basic case classes representing HTTP messages.

An HttpRequest consists of

  • a method (GET, POST, etc.)
  • a URI (see URI model for more information)
  • a seq of headers
  • an entity (body data)
  • a protocol

Here are some examples how to construct an HttpRequest:

import HttpMethods._

// construct a simple GET request to `homeUri`
val homeUri = Uri("/abc")
HttpRequest(GET, uri = homeUri)

// construct simple GET request to "/index" (implicit string to Uri conversion)
HttpRequest(GET, uri = "/index")

// construct simple POST request containing entity
val data = ByteString("abc")
HttpRequest(POST, uri = "/receive", entity = data)

// customize every detail of HTTP request
import HttpProtocols._
import MediaTypes._
import HttpCharsets._
val userData = ByteString("abc")
val authorization = headers.Authorization(BasicHttpCredentials("user", "pass"))
  uri = "/user",
  entity = HttpEntity(`text/plain` withCharset `UTF-8`, userData),
  headers = List(authorization),
  protocol = `HTTP/1.0`)

All parameters of HttpRequest.apply have default values set, so headers for example don’t need to be specified if there are none. Many of the parameters types (like HttpEntity and Uri) define implicit conversions for common use cases to simplify the creation of request and response instances.


An HttpResponse consists of

  • a status code
  • a seq of headers
  • an entity (body data)
  • a protocol

Here are some examples how to construct an HttpResponse:

import StatusCodes._

// simple OK response without data created using the integer status code

// 404 response created using the named StatusCode constant

// 404 response with a body explaining the error
HttpResponse(404, entity = "Unfortunately, the resource couldn't be found.")

// A redirecting response containing an extra header
val locationHeader = headers.Location("")
HttpResponse(Found, headers = List(locationHeader))

In addition to the simple HttpEntity constructors which create an entity from a fixed String or ByteString as shown here the Akka HTTP model defines a number of subclasses of HttpEntity which allow body data to be specified as a stream of bytes.


An HttpEntity carries the data bytes of a message together with its Content-Type and, if known, its Content-Length. In Akka HTTP there are five different kinds of entities which model the various ways that message content can be received or sent:

The simplest entity, which is used when all the entity are already available in memory. It wraps a plain ByteString and represents a standard, unchunked entity with a known Content-Length.
The general, unchunked HTTP/1.1 message entity. It has a known length and presents its data as a Source[ByteString] which can be only materialized once. It is an error if the provided source doesn’t produce exactly as many bytes as specified. The distinction of Strict and Default is an API-only one. One the wire, both kinds of entities look the same.
The model for HTTP/1.1 chunked content (i.e. sent with Transfer-Encoding: chunked). The content length is unknown and the individual chunks are presented as a Source[HttpEntity.ChunkStreamPart]. A ChunkStreamPart is either a non-empty Chunk or a LastChunk containing optional trailer headers. The stream consists of zero or more Chunked parts and can be terminated by an optional LastChunk part.
An unchunked entity of unknown length that is implicitly delimited by closing the connection (Connection: close). The content data are presented as a Source[ByteString]. Since the connection must be closed after sending an entity of this type it can only be used on the server-side for sending a response. Also, the main purpose of CloseDelimited entities is compatibility with HTTP/1.0 peers, which do not support chunked transfer encoding. If you are building a new application and are not constrained by legacy requirements you shouldn’t rely on CloseDelimited entities, since implicit terminate-by-connection-close is not a robust way of signaling response end, especially in the presence of proxies. Additionally this type of entity prevents connection reuse which can seriously degrade performance. Use HttpEntity.Chunked instead!
A streaming entity of unspecified length for use in a Multipart.BodyPart.

Entity types Strict, Default, and Chunked are a subtype of HttpEntity.Regular which allows to use them for requests and responses. In contrast, HttpEntity.CloseDelimited can only be used for responses.

Streaming entity types (i.e. all but Strict) cannot be shared or serialized. To create a strict, shareable copy of an entity or message use HttpEntity.toStrict or HttpMessage.toStrict which returns a Future of the object with the body data collected into a ByteString.

The HttpEntity companion object contains several helper constructors to create entities from common types easily.

You can pattern match over the subtypes of HttpEntity if you want to provide special handling for each of the subtypes. However, in many cases a recipient of an HttpEntity doesn’t care about of which subtype an entity is (and how data is transported exactly on the HTTP layer). Therefore, the general method HttpEntity.dataBytes is provided which returns a Source[ByteString, Any] that allows access to the data of an entity regardless of its concrete subtype.

When to use which subtype?
  • Use Strict if the amount of data is “small” and already available in memory (e.g. as a String or ByteString)
  • Use Default if the data is generated by a streaming data source and the size of the data is known
  • Use Chunked for an entity of unknown length
  • Use CloseDelimited for a response as a legacy alternative to Chunked if the client doesn’t support chunked transfer encoding. Otherwise use Chunked!
  • In a Multipart.Bodypart use IndefiniteLength for content of unknown length.

When you receive a non-strict message from a connection then additional data are only read from the network when you request them by consuming the entity data stream. This means that, if you don’t consume the entity stream then the connection will effectively be stalled. In particular no subsequent message (request or response) will be read from the connection as the entity of the current message “blocks” the stream. Therefore you must make sure that you always consume the entity data, even in the case that you are not actually interested in it!

Limiting message entity length

All message entities that Akka HTTP reads from the network automatically get a length verification check attached to them. This check makes sure that the total entity size is less than or equal to the configured max-content-length [1], which is an important defense against certain Denial-of-Service attacks. However, a single global limit for all requests (or responses) is often too inflexible for applications that need to allow large limits for some requests (or responses) but want to clamp down on all messages not belonging into that group.

In order to give you maximum flexibility in defining entity size limits according to your needs the HttpEntity features a withSizeLimit method, which lets you adjust the globally configured maximum size for this particular entity, be it to increase or decrease any previously set value. This means that your application will receive all requests (or responses) from the HTTP layer, even the ones whose Content-Length exceeds the configured limit (because you might want to increase the limit yourself). Only when the actual data stream Source contained in the entity is materialized will the boundary checks be actually applied. In case the length verification fails the respective stream will be terminated with an EntityStreamSizeException either directly at materialization time (if the Content-Length is known) or whenever more data bytes than allowed have been read.

When called on Strict entities the withSizeLimit method will return the entity itself if the length is within the bound, otherwise a Default entity with a single element data stream. This allows for potential refinement of the entity size limit at a later point (before materialization of the data stream).

By default all message entities produced by the HTTP layer automatically carry the limit that is defined in the application’s max-content-length config setting. If the entity is transformed in a way that changes the content-length and then another limit is applied then this new limit will be evaluated against the new content-length. If the entity is transformed in a way that changes the content-length and no new limit is applied then the previous limit will be applied against the previous content-length. Generally this behavior should be in line with your expectations.

[1] akka.http.parsing.max-content-length (applying to server- as well as client-side), akka.http.server.parsing.max-content-length (server-side only), akka.http.client.parsing.max-content-length (client-side only) or (only host-connection-pools)

Special processing for HEAD requests

RFC 7230 defines very clear rules for the entity length of HTTP messages.

Especially this rule requires special treatment in Akka HTTP:

Any response to a HEAD request and any response with a 1xx (Informational), 204 (No Content), or 304 (Not Modified) status code is always terminated by the first empty line after the header fields, regardless of the header fields present in the message, and thus cannot contain a message body.

Responses to HEAD requests introduce the complexity that Content-Length or Transfer-Encoding headers can be present but the entity is empty. This is modeled by allowing HttpEntity.Default and HttpEntity.Chunked to be used for HEAD responses with an empty data stream.

Also, when a HEAD response has an HttpEntity.CloseDelimited entity the Akka HTTP implementation will not close the connection after the response has been sent. This allows the sending of HEAD responses without Content-Length header across persistent HTTP connections.

Header Model

Akka HTTP contains a rich model of the most common HTTP headers. Parsing and rendering is done automatically so that applications don’t need to care for the actual syntax of headers. Headers not modelled explicitly are represented as a RawHeader (which is essentially a String/String name/value pair).

See these examples of how to deal with headers:

import akka.http.scaladsl.model.headers._

// create a ``Location`` header
val loc = Location("")

// create an ``Authorization`` header with HTTP Basic authentication data
val auth = Authorization(BasicHttpCredentials("joe", "josepp"))

// custom type
case class User(name: String, pass: String)

// a method that extracts basic HTTP credentials from a request
def credentialsOfRequest(req: HttpRequest): Option[User] =
  for {
    Authorization(BasicHttpCredentials(user, pass)) <- req.header[Authorization]
  } yield User(user, pass)

HTTP Headers

When the Akka HTTP server receives an HTTP request it tries to parse all its headers into their respective model classes. Independently of whether this succeeds or not, the HTTP layer will always pass on all received headers to the application. Unknown headers as well as ones with invalid syntax (according to the header parser) will be made available as RawHeader instances. For the ones exhibiting parsing errors a warning message is logged depending on the value of the illegal-header-warnings config setting.

Some headers have special status in HTTP and are therefore treated differently from “regular” headers:

The Content-Type of an HTTP message is modeled as the contentType field of the HttpEntity. The Content-Type header therefore doesn’t appear in the headers sequence of a message. Also, a Content-Type header instance that is explicitly added to the headers of a request or response will not be rendered onto the wire and trigger a warning being logged instead!
Messages with Transfer-Encoding: chunked are represented via the HttpEntity.Chunked entity. As such chunked messages that do not have another deeper nested transfer encoding will not have a Transfer-Encoding header in their headers sequence. Similarly, a Transfer-Encoding header instance that is explicitly added to the headers of a request or response will not be rendered onto the wire and trigger a warning being logged instead!
The content length of a message is modelled via its HttpEntity. As such no Content-Length header will ever be part of a message’s header sequence. Similarly, a Content-Length header instance that is explicitly added to the headers of a request or response will not be rendered onto the wire and trigger a warning being logged instead!
A Server header is usually added automatically to any response and its value can be configured via the akka.http.server.server-header setting. Additionally an application can override the configured header with a custom one by adding it to the response’s header sequence.
A User-Agent header is usually added automatically to any request and its value can be configured via the akka.http.client.user-agent-header setting. Additionally an application can override the configured header with a custom one by adding it to the request’s header sequence.
The Date response header is added automatically but can be overridden by supplying it manually.
On the server-side Akka HTTP watches for explicitly added Connection: close response headers and as such honors the potential wish of the application to close the connection after the respective response has been sent out. The actual logic for determining whether to close the connection is quite involved. It takes into account the request’s method, protocol and potential Connection header as well as the response’s protocol, entity and potential Connection header. See this test for a full table of what happens when.
HTTP Strict Transport Security (HSTS) is a web security policy mechanism which is communicated by the Strict-Transport-Security header. The most important security vulnerability that HSTS can fix is SSL-stripping man-in-the-middle attacks. The SSL-stripping attack works by transparently converting a secure HTTPS connection into a plain HTTP connection. The user can see that the connection is insecure, but crucially there is no way of knowing whether the connection should be secure. HSTS addresses this problem by informing the browser that connections to the site should always use TLS/SSL. See also RFC 6797.

Custom Headers

Sometimes you may need to model a custom header type which is not part of HTTP and still be able to use it as convenient as is possible with the built-in types.

Because of the number of ways one may interact with headers (i.e. try to match a CustomHeader against a RawHeader or the other way around etc), a helper trait for custom Header types and their companions classes are provided by Akka HTTP. Thanks to extending ModeledCustomHeader instead of the plain CustomHeader such header can be matched

final class ApiTokenHeader(token: String) extends ModeledCustomHeader[ApiTokenHeader] {
  override def renderInRequests = false
  override def renderInResponses = false
  override val companion = ApiTokenHeader
  override def value: String = token
object ApiTokenHeader extends ModeledCustomHeaderCompanion[ApiTokenHeader] {
  override val name = "apiKey"
  override def parse(value: String) = Try(new ApiTokenHeader(value))

Which allows the this CustomHeader to be used in the following scenarios:

val ApiTokenHeader(t1) = ApiTokenHeader("token")
t1 should ===("token")

val RawHeader(k2, v2) = ApiTokenHeader("token")
k2 should ===("apiKey")
v2 should ===("token")

// will match, header keys are case insensitive
val ApiTokenHeader(v3) = RawHeader("APIKEY", "token")
v3 should ===("token")

intercept[MatchError] {
  // won't match, different header name
  val ApiTokenHeader(v4) = DifferentHeader("token")

intercept[MatchError] {
  // won't match, different header name
  val RawHeader("something", v5) = DifferentHeader("token")

intercept[MatchError] {
  // won't match, different header name
  val ApiTokenHeader(v6) = RawHeader("different", "token")

Including usage within the header directives like in the following headerValuePF example:

def extractFromCustomHeader = headerValuePF {
  case t @ ApiTokenHeader(token) ⇒ s"extracted> $t"
  case raw: RawHeader            ⇒ s"raw> $raw"

val routes = extractFromCustomHeader { s ⇒

Get().withHeaders(RawHeader("apiKey", "TheKey")) ~> routes ~> check {
  status should ===(StatusCodes.OK)
  responseAs[String] should ===("extracted> apiKey: TheKey")

Get().withHeaders(RawHeader("somethingElse", "TheKey")) ~> routes ~> check {
  status should ===(StatusCodes.OK)
  responseAs[String] should ===("raw> somethingElse: TheKey")

Get().withHeaders(ApiTokenHeader("TheKey")) ~> routes ~> check {
  status should ===(StatusCodes.OK)
  responseAs[String] should ===("extracted> apiKey: TheKey")

One can also directly extend CustomHeader which requires less boilerplate, however that has the downside of matching against RawHeader instances not working out-of-the-box, thus limiting its usefulness in the routing layer of Akka HTTP. For only rendering such header however it would be enough.


When defining custom headers, prefer to extend ModeledCustomHeader instead of CustomHeader directly as it will automatically make your header abide all the expected pattern matching semantics one is accustomed to when using built-in types (such as matching a custom header against a RawHeader as is often the case in routing layers of Akka HTTP applications).

Parsing / Rendering

Parsing and rendering of HTTP data structures is heavily optimized and for most types there’s currently no public API provided to parse (or render to) Strings or byte arrays.


Various parsing and rendering settings are available to tweak in the configuration under akka.http.client[.parsing], akka.http.server[.parsing] and[.client.parsing], with defaults for all of these being defined in the akka.http.parsing configuration section.

For example, if you want to change a parsing setting for all components, you can set the akka.http.parsing.illegal-header-warnings = off value. However this setting can be still overridden by the more specific sections, like for example akka.http.server.parsing.illegal-header-warnings = on.

In this case both client and host-connection-pool APIs will see the setting off, however the server will see on.

In the case of settings, they default to settings set in akka.http.client, and can override them if needed. This is useful, since both client and host-connection-pool APIs, such as the Client API Http().outgoingConnection or the Host Connection Pool APIs Http().singleRequest or Http().superPool, usually need the same settings, however the server most likely has a very different set of settings.

Registering Custom Media Types

Akka HTTP predefines most commonly encountered media types and emits them in their well-typed form while parsing http messages. Sometimes you may want to define a custom media type and inform the parser infrastructure about how to handle these custom media types, e.g. that application/custom is to be treated as NonBinary with WithFixedCharset. To achieve this you need to register the custom media type in the server’s settings by configuring ParserSettings like this:

// similarily in Java: `akka.http.javadsl.settings.[...]` import akka.http.scaladsl.settings.ParserSettings import akka.http.scaladsl.settings.ServerSettings // define custom media type: val utf8 = HttpCharsets.`UTF-8` val `application/custom`: WithFixedCharset = MediaType.customWithFixedCharset("application", "custom", utf8) // add custom media type to parser settings: val parserSettings = ParserSettings(system).withCustomMediaTypes(`application/custom`) val serverSettings = ServerSettings(system).withParserSettings(parserSettings) val routes = extractRequest { r ⇒ complete(r.entity.contentType.toString + " = " + r.entity.contentType.getClass) } val binding = Http().bindAndHandle(routes, host, port, settings = serverSettings)

You may also want to read about MediaType Registration trees, in order to register your vendor specific media types in the right style / place.