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.javadsl.model.*;
import akka.http.javadsl.model.headers.*;

import java.util.Optional;

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 are defined as static fields of the HttpMethods class.
  • Defined HttpCharset instances are defined as static fields of the HttpCharsets class.
  • Defined HttpEncoding instances are defined as static fields of the HttpEncodings class.
  • Defined HttpProtocol instances are defined as static fields of the HttpProtocols class.
  • Defined MediaType instances are defined as static fields of the MediaTypes class.
  • Defined StatusCode instances are defined as static fields of the StatusCodes class.


HttpRequest and HttpResponse are the basic immutable 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:

// construct a simple GET request to `homeUri`
Uri homeUri = Uri.create("/home");
HttpRequest request1 = HttpRequest.create().withUri(homeUri);

// construct simple GET request to "/index" using helper methods
HttpRequest request2 = HttpRequest.GET("/index");

// construct simple POST request containing entity
ByteString data = ByteString.fromString("abc");
HttpRequest postRequest1 = HttpRequest.POST("/receive").withEntity(data);

// customize every detail of HTTP request
//import HttpProtocols.*
//import MediaTypes.*
Authorization authorization = Authorization.basic("user", "pass");
HttpRequest complexRequest =
        .withEntity(HttpEntities.create(ContentTypes.TEXT_PLAIN_UTF8, "abc"))

In its basic form HttpRequest.create creates an empty default GET request without headers which can then be transformed using one of the withX methods, addHeader, or addHeaders. Each of those will create a new immutable instance, so instances can be shared freely. There exist some overloads for HttpRequest.create that simplify creating requests for common cases. Also, to aid readability, there are predefined alternatives for create named after HTTP methods to create a request with a given method and URI directly.

Synthetic Headers

In some cases it may be necessary to deviate from fully RFC-Compliant behavior. For instance, Amazon S3 treats the + character in the path part of the URL as a space, even though the RFC specifies that this behavior should be limited exclusively to the query portion of the URI.

In order to work around these types of edge cases, Akka HTTP provides for the ability to provide extra, non-standard information to the request via synthetic headers. These headers are not passed to the client but are instead consumed by the request engine and used to override default behavior.

For instance, in order to provide a raw request uri, bypassing the default url normalization, you could do the following:

// imports akka.http.javadsl.model.headers.RawRequestURI


An HttpResponse consists of

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

Here are some examples how to construct an HttpResponse:

// simple OK response without data created using the integer status code
HttpResponse ok = HttpResponse.create().withStatus(200);

// 404 response created using the named StatusCode constant
HttpResponse notFound = HttpResponse.create().withStatus(StatusCodes.NOT_FOUND);

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

// A redirecting response containing an extra header
Location locationHeader = Location.create("");
HttpResponse redirectResponse =

In addition to the simple HttpEntities.create methods 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. All of these types can be created using the method on HttpEntites.


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 HttpEntityStrict and HttpEntityDefault 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<ChunkStreamPart, ?>. A ChunkStreamPart is either a non-empty chunk or the empty last chunk containing optional trailer headers. The stream consists of zero or more non-empty chunks parts and can be terminated by an optional last chunk.
An unchunked entity of unknown length that is implicitly delimited by closing the connection (Connection: close). Content data is 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 HttpEntityChunked instead!
A streaming entity of unspecified length for use in a Multipart.BodyPart.

Entity types HttpEntityStrict, HttpEntityDefault, and HttpEntityChunked are a subtype of RequestEntity which allows to use them for requests and responses. In contrast, HttpEntityCloseDelimited can only be used for responses.

Streaming entity types (i.e. all but HttpEntityStrict) 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 CompletionStage of the object with the body data collected into a ByteString.

The class HttpEntities contains static methods to create entities from common types easily.

You can use the isX methods of HttpEntity to find out of which subclass an entity is 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.getDataBytes() is provided which returns a Source<ByteString, ?> that allows access to the data of an entity regardless of its concrete subtype.

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

When you receive a non-strict message from a connection then additional data is only read from the network when you request it 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 HttpEntityDefault and HttpEntityChunked to be used for HEAD responses with an empty data stream.

Also, when a HEAD response has an HttpEntityCloseDelimited 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:

        // create a ``Location`` header
        Location locationHeader = Location.create("");

        // create an ``Authorization`` header with HTTP Basic authentication data
        Authorization authorization = Authorization.basic("user", "pass");

    // a method that extracts basic HTTP credentials from a request
	private Optional<BasicHttpCredentials> getCredentialsOfRequest(HttpRequest request) {
        Optional<Authorization> auth = request.getHeader(Authorization.class);
        if (auth.isPresent() && auth.get().credentials() instanceof BasicHttpCredentials)
            return Optional.of((BasicHttpCredentials) auth.get().credentials());
            return Optional.empty();

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 as a HttpEntityChunked entity. As such chunked messages that do not have another deeper nested transfer encoding will not have a Transfer-Encoding header in their headers list. 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.

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 convert a CustomHeader to a RawHeader or the other way around etc), a couple of helper classes for custom Header types are provided by Akka HTTP. Thanks to extending ModeledCustomHeader instead of the plain CustomHeader the following methods are at your disposal:

public static class ApiTokenHeader extends ModeledCustomHeader {

  ApiTokenHeader(String name, String value) {
    super(name, value);

  public boolean renderInResponses() {
    return false;

  public boolean renderInRequests() {
    return false;


static class ApiTokenHeaderFactory extends ModeledCustomHeaderFactory<ApiTokenHeader> {

  public String name() {
    return "apiKey";

  public ApiTokenHeader parse(String value) {
    return new ApiTokenHeader(name(), value);


Which allows the this modeled custom header to be used and created in the following ways:

final ApiTokenHeaderFactory apiTokenHeaderFactory = new ApiTokenHeaderFactory();
final ApiTokenHeader token = apiTokenHeaderFactory.create("token");
assertEquals("token", token.value());

final HttpHeader header = apiTokenHeaderFactory.create("token");
assertEquals("token", header.value());

final Optional<ApiTokenHeader> fromRaw = apiTokenHeaderFactory
  .from(RawHeader.create("apiKey", "token"));
assertTrue("Expected a header", fromRaw.isPresent());
assertEquals("apiKey", fromRaw.get().name());
assertEquals("token", fromRaw.get().value());

// will match, header keys are case insensitive
final Optional<ApiTokenHeader> fromRawUpper = apiTokenHeaderFactory
  .from(RawHeader.create("APIKEY", "token"));
assertTrue("Expected a header", fromRawUpper.isPresent());
assertEquals("apiKey", fromRawUpper.get().name());
assertEquals("token", fromRawUpper.get().value());

// won't match, different header name
final Optional<ApiTokenHeader> wrong = apiTokenHeaderFactory
  .from(RawHeader.create("different", "token"));

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

final ApiTokenHeaderFactory apiTokenHeaderFactory = new ApiTokenHeaderFactory();
final PartialFunction<HttpHeader, String> extractFromCustomHeader =
  new JavaPartialFunction<HttpHeader, String>() {

    public String apply(HttpHeader header, boolean isCheck) throws Exception {
      if (isCheck)
        return null;
      return apiTokenHeaderFactory.from(header)
        .map(apiTokenHeader -> "extracted> " + apiTokenHeader)
        .orElseGet(() -> "raw> " + header);

final Route route = headerValuePF(extractFromCustomHeader, this::complete);

  .run(HttpRequest.GET("/").addHeader(RawHeader.create("apiKey", "TheKey")))
  .assertEntity("extracted> apiKey: TheKey");

  .run(HttpRequest.GET("/").addHeader(RawHeader.create("somethingElse", "TheKey")))
  .assertEntity("raw> somethingElse: TheKey");

  .assertEntity("extracted> apiKey: TheKey");

One can also directly extend CustomHeader which requires less boilerplate, however that has the downside of having to deal with converting HttpHeader instances to your custom one. For only rendering such header however it would be enough.

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.get(sys).outgoingConnection or the Host Connection Pool APIs Http.get(sys).singleRequest or Http.get(sys).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:

// Define custom media type:
final MediaType.WithFixedCharset applicationCustom =
  MediaTypes.customWithFixedCharset("application", "custom", // The new Media Type name
    HttpCharsets.UTF_8, // The charset used
    new HashMap<>(), // Empty parameters
    false); // No arbitrary subtypes are allowed

// Add custom media type to parser settings:
final ParserSettings parserSettings = ParserSettings.create(system)
final ServerSettings serverSettings = ServerSettings.create(system)

final Route route = extractRequest(req ->
  complete(req.entity().getContentType().toString() + " = "
    + req.entity().getContentType().getClass())

final CompletionStage<ServerBinding> binding = Http.get(system)
  .bindAndHandle(route.flow(system, materializer),
    ConnectHttp.toHost(host, 0),

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

Registering Custom Status Codes

Similarly to media types, Akka HTTP predefines well-known status codes, however sometimes you may need to use a custom one (or are forced to use an API which returns custom status codes). Similarily to the media types registration, you can register custom status codes by configuring ParserSettings like this:

// Define custom status code:
final StatusCode leetCode = StatusCodes.custom(777, // Our custom status code
  "LeetCode", // Our custom reason
  "Some reason", // Our custom default message
  true, // It should be considered a success response
  false);// Does not allow entities

// Add custom method to parser settings:
final ParserSettings parserSettings = ParserSettings.create(system)
final ServerSettings serverSettings = ServerSettings.create(system)

final ClientConnectionSettings clientConSettings = ClientConnectionSettings.create(system)
final ConnectionPoolSettings clientSettings = ConnectionPoolSettings.create(system)

final Route route = extractRequest(req ->

// Use serverSettings in server:
final CompletionStage<ServerBinding> binding = Http.get(system)
  .bindAndHandle(route.flow(system, materializer),
    ConnectHttp.toHost(host, 0),

final ServerBinding serverBinding = binding.toCompletableFuture().get();

final int port = serverBinding.localAddress().getPort();

// Use clientSettings in client:
final HttpResponse response = Http.get(system)
    .GET("http://" + host + ":" + port + "/"),

// Check we get the right code back
assertEquals(leetCode, response.status());

Registering Custom HTTP Method

Akka HTTP also allows you to define custome HTTP methods, other than the well-known methods predefined in Akka HTTP. To use a custom HTTP method, you need to define it first, and then add it to parser settings like below:

// define custom method type: HttpMethod BOLT = HttpMethods.createCustom("BOLT", false, true, Expected); // add custom method to parser settings: final ParserSettings parserSettings = ParserSettings.create(system).withCustomMethods(BOLT); final ServerSettings serverSettings = ServerSettings.create(system).withParserSettings(parserSettings); final Route routes = route( extractMethod( method -> complete( "This is a " + + " request.") ) ); final Flow<HttpRequest, HttpResponse, NotUsed> handler = routes.flow(system, materializer); final Http http = Http.get(system); final CompletionStage<ServerBinding> binding = http.bindAndHandle( handler, ConnectHttp.toHost(host, port), serverSettings, loggingAdapter, materializer); HttpRequest request = HttpRequest.create() .withUri("http://" + host + ":" + Integer.toString(port)) .withMethod(BOLT) .withProtocol(HTTP_1_1); CompletionStage<HttpResponse> response = http.singleRequest(request, materializer);
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