Server-Side HTTP/2 support in akka-http is currently available as a preview. This means it is ready to be evaluated, but the APIs and behavior are likely to change.
HTTP/2 can then be enabled through configuration:
akka.http.server.preview.enable-http2 = on
HTTP/2 is primarily used over a secure HTTPS connection which takes care of protocol negotiation and falling back to HTTP/1.1 over TLS when the client does not support HTTP/2. See the HTTPS section for how to set up HTTPS.
Note that currently only
newServerAt(...).bindSync support HTTP/2 but not
HTTP/2 over TLS needs Application-Layer Protocol Negotiation (ALPN) to negotiate whether both client and server support HTTP/2. The JVM provides ALPN support starting from JDK 8u252. Make sure to use at least that version.
While un-encrypted connections are allowed by HTTP/2, this is sometimes discouraged.
There are 2 ways to implement un-encrypted HTTP/2 connections: by using the HTTP Upgrade mechanism or by starting communication in HTTP/2 directly which requires the client to have Prior Knowledge of HTTP/2 support.
We support both approaches transparently on the same port. This feature is automatically enabled when HTTP/2 is enabled:
The advantage of switching from HTTP/1.1 to HTTP/2 using the HTTP Upgrade mechanism is that both HTTP/1.1 and HTTP/2 clients can connect to the server on the same port, without being aware beforehand which protocol the server supports.
The disadvantage is that relatively few clients support switching to HTTP/2 in this way. Additionally, HTTP/2 communication cannot start until the first request has been completely sent. This means if your first request may be large, it might be worth it to start with an empty OPTIONS request to switch to HTTP/2 before sending your first ‘real’ request, at the cost of a roundtrip.
The other option is to connect and start communicating in HTTP/2 immediately. The downside of this approach is the client must know beforehand that the server supports HTTP/2. For the reason this approach is known as h2c with Prior Knowledge of HTTP/2 support.
At this point you should be able to connect, but HTTP/2 may still not be available.
You’ll need a recent version of cURL compiled with HTTP/2 support (for OSX see this article). You can check whether your version supports HTTP2 with
curl --version, look for the nghttp2 extension and the HTTP2 feature:
curl 7.52.1 (x86_64-pc-linux-gnu) libcurl/7.52.1 OpenSSL/1.0.2l zlib/1.2.8 libidn2/0.16 libpsl/0.17.0 (+libidn2/0.16) libssh2/1.8.0 nghttp2/1.23.1 librtmp/2.3 Protocols: dict file ftp ftps gopher http https imap imaps ldap ldaps pop3 pop3s rtmp rtsp scp sftp smb smbs smtp smtps telnet tftp Features: AsynchDNS IDN IPv6 Largefile GSS-API Kerberos SPNEGO NTLM NTLM_WB SSL libz TLS-SRP HTTP2 UnixSockets HTTPS-proxy PSL
When you connect to your service you may now see something like:
$ curl -k -v https://localhost:8443 (...) * ALPN, offering h2 * ALPN, offering http/1.1 (...) * ALPN, server accepted to use h2 (...) > GET / HTTP/1.1 (...) < HTTP/2 200 (...)
If your curl output looks like above, you have successfully configured HTTP/2. However, on JDKs up to version 9, it is likely to look like this instead:
$ curl -k -v https://localhost:8443 (...) * ALPN, offering h2 * ALPN, offering http/1.1 (...) * ALPN, server did not agree to a protocol (...) > GET / HTTP/1.1 (...) < HTTP/1.1 200 OK (...)
curl declaring it is ready to speak
h2 (the shorthand name of HTTP/2), but could not determine whether the server is ready to, so it fell back to HTTP/1.1. To make this negotiation work you’ll have to configure ALPN as described below.