nghttp2/doc/sources/tutorial-server.rst

Tutorial: HTTP/2 server
=========================

In this tutorial, we are going to write single-threaded, event-based
HTTP/2 web server, which supports HTTPS only. It can handle
concurrent multiple requests, but only GET method is supported. The
complete source code, `libevent-server.c`_, is attached at the end of
this page.  It also resides in examples directory in the archive or
repository.

This simple server takes 3 arguments, a port number to listen to, a
path to SSL/TLS private key file and certificate file.  Its synopsis
is like this::

    $ libevent-server PORT /path/to/server.key /path/to/server.crt

We use libevent in this tutorial to handle networking I/O.  Please
note that nghttp2 itself does not depend on libevent.

First we do some setup routine for libevent and OpenSSL library in
function ``main()`` and ``run()``, which is not so relevant to nghttp2
library use. The one thing you should look at is setup NPN callback.
The NPN callback is used for the server to advertise the application
protocols the server supports to a client.  In this example program,
when creating ``SSL_CTX`` object, we stores the application protocol
name in the wire format of NPN in statically allocated buffer. This is
safe because we only create 1 ``SSL_CTX`` object in the entire program
life time::

    static unsigned char next_proto_list[256];
    static size_t next_proto_list_len;

    static int next_proto_cb(SSL *s, const unsigned char **data, unsigned int *len,
                             void *arg)
    {
      *data = next_proto_list;
      *len = next_proto_list_len;
      return SSL_TLSEXT_ERR_OK;
    }


    static SSL_CTX* create_ssl_ctx(const char *key_file, const char *cert_file)
    {
      SSL_CTX *ssl_ctx;
      ssl_ctx = SSL_CTX_new(SSLv23_server_method());

      ...

      next_proto_list[0] = NGHTTP2_PROTO_VERSION_ID_LEN;
      memcpy(&next_proto_list[1], NGHTTP2_PROTO_VERSION_ID,
             NGHTTP2_PROTO_VERSION_ID_LEN);
      next_proto_list_len = 1 + NGHTTP2_PROTO_VERSION_ID_LEN;

      SSL_CTX_set_next_protos_advertised_cb(ssl_ctx, next_proto_cb, NULL);
      return ssl_ctx;
    }

The wire format of NPN is a sequence of length prefixed string. The
exactly one byte is used to specify the length of each protocol
identifier.  In this tutorial, we advertise the HTTP/2 protocol the
nghttp2 library supports. The nghttp2 library exports its identifier
in :macro:`NGHTTP2_PROTO_VERSION_ID`. The ``next_proto_cb()`` function
is the server-side NPN callback. In OpenSSL implementation, we just
assign the pointer to the NPN buffers we filled earlier. The NPN
callback function is set to ``SSL_CTX`` object using
``SSL_CTX_set_next_protos_advertised_cb()``.

We use ``app_content`` structure to store the application-wide data::

    struct app_context {
      SSL_CTX *ssl_ctx;
      struct event_base *evbase;
    };

We use ``http2_session_data`` structure to store the session-level
(which corresponds to 1 HTTP/2 connection) data::

    typedef struct http2_session_data {
      struct http2_stream_data root;
      struct bufferevent *bev;
      app_context *app_ctx;
      nghttp2_session *session;
      char *client_addr;
      size_t handshake_leftlen;
    } http2_session_data;

We use ``http2_stream_data`` structure to store the stream-level
data::

    typedef struct http2_stream_data {
      struct http2_stream_data *prev, *next;
      char *request_path;
      int32_t stream_id;
      int fd;
    } http2_stream_data;

1 HTTP/2 session can have multiple streams.  We manage these multiple
streams by intrusive doubly linked list to add and remove the object
in O(1). The first element of this list is pointed by the
``root->next`` in ``http2_session_data``.  Initially, ``root->next``
is ``NULL``.  The ``handshake_leftlen`` member of
``http2_session_data`` is used to track the number of bytes remaining
when receiving first client connection preface
(:macro:`NGHTTP2_CLIENT_CONNECTION_PREFACE`), which is 24 bytes magic
byte string, from the client.  We use libevent's bufferevent structure
to perform network I/O. Notice that bufferevent object is in
``http2_session_data`` and not in ``http2_stream_data``. This is
because ``http2_stream_data`` is just a logical stream multiplexed
over the single connection managed by bufferevent in
``http2_session_data``.

We first create listener object to accept incoming connections.
We use libevent's ``struct evconnlistener`` for this purpose::

    static void start_listen(struct event_base *evbase, const char *service,
                             app_context *app_ctx)
    {
      int rv;
      struct addrinfo hints;
      struct addrinfo *res, *rp;

      memset(&hints, 0, sizeof(hints));
      hints.ai_family = AF_UNSPEC;
      hints.ai_socktype = SOCK_STREAM;
      hints.ai_flags = AI_PASSIVE;
    #ifdef AI_ADDRCONFIG
      hints.ai_flags |= AI_ADDRCONFIG;
    #endif // AI_ADDRCONFIG

      rv = getaddrinfo(NULL, service, &hints, &res);
      if(rv != 0) {
        errx(1, NULL);
      }
      for(rp = res; rp; rp = rp->ai_next) {
        struct evconnlistener *listener;
        listener = evconnlistener_new_bind(evbase, acceptcb, app_ctx,
                                           LEV_OPT_CLOSE_ON_FREE |
                                           LEV_OPT_REUSEABLE, -1,
                                           rp->ai_addr, rp->ai_addrlen);
        if(listener) {
          return;
        }
      }
      errx(1, "Could not start listener");
    }

We specify ``acceptcb`` callback which is called when a new connection
is accepted::

    static void acceptcb(struct evconnlistener *listener, int fd,
                         struct sockaddr *addr, int addrlen, void *arg)
    {
      app_context *app_ctx = (app_context*)arg;
      http2_session_data *session_data;

      session_data = create_http2_session_data(app_ctx, fd, addr, addrlen);
      bufferevent_setcb(session_data->bev, handshake_readcb, NULL, eventcb,
                        session_data);
    }

Here we create ``http2_session_data`` object. The bufferevent for this
connection is also initialized at this time. We specify 2 callbacks
for the bufferevent: ``handshake_readcb`` and ``eventcb``.

The ``eventcb()`` is invoked by libevent event loop when an event
(e.g., connection has been established, timeout, etc) happens on the
underlying network socket::

    static void eventcb(struct bufferevent *bev, short events, void *ptr)
    {
      http2_session_data *session_data = (http2_session_data*)ptr;
      if(events & BEV_EVENT_CONNECTED) {
        fprintf(stderr, "%s connected\n", session_data->client_addr);
        return;
      }
      if(events & BEV_EVENT_EOF) {
        fprintf(stderr, "%s EOF\n", session_data->client_addr);
      } else if(events & BEV_EVENT_ERROR) {
        fprintf(stderr, "%s network error\n", session_data->client_addr);
      } else if(events & BEV_EVENT_TIMEOUT) {
        fprintf(stderr, "%s timeout\n", session_data->client_addr);
      }
      delete_http2_session_data(session_data);
    }

For ``BEV_EVENT_EOF``, ``BEV_EVENT_ERROR`` and ``BEV_EVENT_TIMEOUT``
event, we just simply tear down the connection. The
``delete_http2_session_data()`` function destroys
``http2_session_data`` object and thus its bufferevent member. As a
result, the underlying connection is closed.  The
``BEV_EVENT_CONNECTED`` event is invoked when SSL/TLS handshake is
finished successfully.

The ``handshake_readcb()`` is a callback function to handle 24 bytes
magic byte string from a client, since nghttp2 library does not handle
it::

    static void handshake_readcb(struct bufferevent *bev, void *ptr)
    {
      http2_session_data *session_data = (http2_session_data*)ptr;
      uint8_t data[24];
      struct evbuffer *input = bufferevent_get_input(session_data->bev);
      int readlen = evbuffer_remove(input, data, session_data->handshake_leftlen);
      const char *conhead = NGHTTP2_CLIENT_CONNECTION_PREFACE;

      if(memcmp(conhead + NGHTTP2_CLIENT_CONNECTION_PREFACE_LEN
                - session_data->handshake_leftlen, data, readlen) != 0) {
        delete_http2_session_data(session_data);
        return;
      }
      session_data->handshake_leftlen -= readlen;
      if(session_data->handshake_leftlen == 0) {
        bufferevent_setcb(session_data->bev, readcb, writecb, eventcb, ptr);
        /* Process pending data in buffer since they are not notified
           further */
        initialize_nghttp2_session(session_data);
        if(send_server_connection_header(session_data) != 0) {
          delete_http2_session_data(session_data);
          return;
        }
        if(session_recv(session_data) != 0) {
          delete_http2_session_data(session_data);
          return;
        }
      }
    }

We check that the received byte string matches
:macro:`NGHTTP2_CLIENT_CONNECTION_PREFACE`.  When they match, the
connection state is ready for starting HTTP/2 communication. First
we change the callback functions for the bufferevent object. We use
same ``eventcb`` as before. But we specify new ``readcb`` and
``writecb`` function to handle HTTP/2 communication. We describe
these 2 functions later.

We initialize nghttp2 session object which is done in
``initialize_nghttp2_session()``::

    static void initialize_nghttp2_session(http2_session_data *session_data)
    {
      nghttp2_session_callbacks callbacks = {0};

      callbacks.send_callback = send_callback;
      callbacks.on_frame_recv_callback = on_frame_recv_callback;
      callbacks.on_stream_close_callback = on_stream_close_callback;
      callbacks.on_header_callback = on_header_callback;
      callbacks.on_begin_headers_callback = on_begin_headers_callback;
      nghttp2_session_server_new(&session_data->session, &callbacks, session_data);
    }

Since we are creating server, nghttp2 session object is created using
`nghttp2_session_server_new()` function. We registers 5 callbacks to
nghttp2 session object. We'll talk about these callbacks later.

After initialization of nghttp2 session object, we are going to send
server connection header in ``send_server_connection_header()``::

    static int send_server_connection_header(http2_session_data *session_data)
    {
      nghttp2_settings_entry iv[1] = {
        { NGHTTP2_SETTINGS_MAX_CONCURRENT_STREAMS, 100 }
      };
      int rv;

      rv = nghttp2_submit_settings(session_data->session, NGHTTP2_FLAG_NONE,
                                   iv, ARRLEN(iv));
      if(rv != 0) {
        warnx("Fatal error: %s", nghttp2_strerror(rv));
        return -1;
      }
      return 0;
    }

The server connection header is SETTINGS frame. We specify
SETTINGS_MAX_CONCURRENT_STREAMS to 100 in SETTINGS frame.  To queue
the SETTINGS frame for the transmission, we use
`nghttp2_submit_settings()`. Note that `nghttp2_submit_settings()`
function only queues the frame and not actually send it. All
``nghttp2_submit_*()`` family functions have this property. To
actually send the frame, `nghttp2_session_send()` is used, which is
described about later.

Since bufferevent may buffer more than first 24 bytes from the client,
we have to process them here since libevent won't invoke callback
functions for these pending data. To process received data, we call
``session_recv()`` function::

    static int session_recv(http2_session_data *session_data)
    {
      int rv;
      struct evbuffer *input = bufferevent_get_input(session_data->bev);
      size_t datalen = evbuffer_get_length(input);
      unsigned char *data = evbuffer_pullup(input, -1);
      rv = nghttp2_session_mem_recv(session_data->session, data, datalen);
      if(rv < 0) {
        warnx("Fatal error: %s", nghttp2_strerror(rv));
        return -1;
      }
      evbuffer_drain(input, rv);
      if(session_send(session_data) != 0) {
        return -1;
      }
      return 0;
    }

In this function, we feed all unprocessed, received data to nghttp2
session object using `nghttp2_session_mem_recv()` function. The
`nghttp2_session_mem_recv()` processes the received data and may
invoke nghttp2 callbacks and also queue outgoing frames. Since there
may be pending frames, we call ``session_send()`` function to send
those frames. The ``session_send()`` function is defined as follows::

    static int session_send(http2_session_data *session_data)
    {
      int rv;
      rv = nghttp2_session_send(session_data->session);
      if(rv != 0) {
        warnx("Fatal error: %s", nghttp2_strerror(rv));
        return -1;
      }
      return 0;
    }

The `nghttp2_session_send()` function serializes the frame into wire
format and call :member:`nghttp2_session_callbacks.send_callback` with
it. We set ``send_callback()`` function to
:member:`nghttp2_session_callbacks.send_callback` in
``initialize_nghttp2_session()`` function described earlier. It is
defined as follows::

    static ssize_t send_callback(nghttp2_session *session,
                                 const uint8_t *data, size_t length,
                                 int flags, void *user_data)
    {
      http2_session_data *session_data = (http2_session_data*)user_data;
      struct bufferevent *bev = session_data->bev;
      /* Avoid excessive buffering in server side. */
      if(evbuffer_get_length(bufferevent_get_output(session_data->bev)) >=
         OUTPUT_WOULDBLOCK_THRESHOLD) {
        return NGHTTP2_ERR_WOULDBLOCK;
      }
      bufferevent_write(bev, data, length);
      return length;
    }

Since we use bufferevent to abstract network I/O, we just write the
data to the bufferevent object. Note that `nghttp2_session_send()`
continues to write all frames queued so far. If we were writing the
data to the non-blocking socket directly using ``write()`` system call
in the :member:`nghttp2_session_callbacks.send_callback`, we will
surely get ``EAGAIN`` or ``EWOULDBLOCK`` since the socket has limited
send buffer. If that happens, we can return
:macro:`NGHTTP2_ERR_WOULDBLOCK` to signal the nghttp2 library to stop
sending further data. But writing to the bufferevent, we have to
regulate the amount data to be buffered by ourselves to avoid possible
huge memory consumption. To achieve this, we check the size of output
buffer and if it is more than or equal to
``OUTPUT_WOULDBLOCK_THRESHOLD`` bytes, we stop writing data and return
:macro:`NGHTTP2_ERR_WOULDBLOCK` to tell the library to stop calling
send_callback.

The next bufferevent callback is ``readcb()``, which is invoked when
data is available to read in the bufferevent input buffer::

    static void readcb(struct bufferevent *bev, void *ptr)
    {
      http2_session_data *session_data = (http2_session_data*)ptr;
      if(session_recv(session_data) != 0) {
        delete_http2_session_data(session_data);
        return;
      }
    }

In this function, we just call ``session_recv()`` to process incoming
data.

The third bufferevent callback is ``writecb()``, which is invoked when
all data written in the bufferevent output buffer have been sent::

    static void writecb(struct bufferevent *bev, void *ptr)
    {
      http2_session_data *session_data = (http2_session_data*)ptr;
      if(evbuffer_get_length(bufferevent_get_output(bev)) > 0) {
        return;
      }
      if(nghttp2_session_want_read(session_data->session) == 0 &&
         nghttp2_session_want_write(session_data->session) == 0) {
        delete_http2_session_data(session_data);
        return;
      }
      if(session_send(session_data) != 0) {
        delete_http2_session_data(session_data);
        return;
      }
    }

First we check whether we should drop connection or not. The nghttp2
session object keeps track of reception and transmission of GOAWAY
frame and other error conditions as well. Using these information,
nghttp2 session object will tell whether the connection should be
dropped or not. More specifically, both `nghttp2_session_want_read()`
and `nghttp2_session_want_write()` return 0, we have no business in
the connection. But since we are using bufferevent and its deferred
callback option, the bufferevent output buffer may contain the pending
data when the ``writecb()`` is called. To handle this situation, we
also check whether the output buffer is empty or not. If these
conditions are met, we drop connection.

Otherwise, we call ``session_send()`` to process pending output
data. Remember that in ``send_callback()``, we may not write all data
to bufferevent to avoid excessive buffering. We continue process
pending data when output buffer becomes empty.

We have already described about nghttp2 callback ``send_callback()``.
Let's describe remaining nghttp2 callbacks we setup in
``initialize_nghttp2_setup()`` function.

The ``on_begin_headers_callback()`` function is invoked when reception
of header block in HEADERS or PUSH_PROMISE frame is started::

    static int on_begin_headers_callback(nghttp2_session *session,
                                         const nghttp2_frame *frame,
                                         void *user_data)
    {
      http2_session_data *session_data = (http2_session_data*)user_data;
      http2_stream_data *stream_data;

      if(frame->hd.type != NGHTTP2_HEADERS ||
         frame->headers.cat != NGHTTP2_HCAT_REQUEST) {
        return 0;
      }
      stream_data = create_http2_stream_data(session_data, frame->hd.stream_id);
      nghttp2_session_set_stream_user_data(session, frame->hd.stream_id,
                                           stream_data);
      return 0;
    }

We only interested in HEADERS frame in this function. Since HEADERS
frame has several roles in HTTP/2 protocol, we check that it is a
request HEADERS, which opens new stream. If frame is request HEADERS,
then we create ``http2_stream_data`` object to store stream related
data. We associate created ``http2_stream_data`` object to the stream
in nghttp2 session object using `nghttp2_set_stream_user_data()` in
order to get the object without searching through doubly linked list.

In this example server, we want to serve files relative to the current
working directory the program was invoked. Each header name/value pair
is emitted via ``on_header_callback`` function, which is called after
``on_begin_headers_callback()``::

    static int on_header_callback(nghttp2_session *session,
                                  const nghttp2_frame *frame,
                                  const uint8_t *name, size_t namelen,
                                  const uint8_t *value, size_t valuelen,
                                  void *user_data)
    {
      http2_stream_data *stream_data;
      const char PATH[] = ":path";
      switch(frame->hd.type) {
      case NGHTTP2_HEADERS:
        if(frame->headers.cat != NGHTTP2_HCAT_REQUEST) {
          break;
        }
        stream_data = nghttp2_session_get_stream_user_data(session,
                                                           frame->hd.stream_id);
        if(!stream_data || stream_data->request_path) {
          break;
        }
        if(namelen == sizeof(PATH) - 1 && memcmp(PATH, name, namelen) == 0) {
          size_t j;
          for(j = 0; j < valuelen && value[j] != '?'; ++j);
          stream_data->request_path = percent_decode(value, j);
        }
        break;
      }
      return 0;
    }

We search ``:path`` header field in request headers and keep the
requested path in ``http2_stream_data`` object. In this example
program, we ignore ``:method`` header field and always treat the
request as GET request.

The ``on_frame_recv_callback()`` function is invoked when a frame is
fully received::

    static int on_frame_recv_callback(nghttp2_session *session,
                                      const nghttp2_frame *frame, void *user_data)
    {
      http2_session_data *session_data = (http2_session_data*)user_data;
      http2_stream_data *stream_data;
      switch(frame->hd.type) {
      case NGHTTP2_DATA:
      case NGHTTP2_HEADERS:
        /* Check that the client request has finished */
        if(frame->hd.flags & NGHTTP2_FLAG_END_STREAM) {
          stream_data = nghttp2_session_get_stream_user_data(session,
                                                             frame->hd.stream_id);
          /* For DATA and HEADERS frame, this callback may be called after
             on_stream_close_callback. Check that stream still alive. */
          if(!stream_data) {
            return 0;
          }
          return on_request_recv(session, session_data, stream_data);
        }
        break;
      default:
        break;
      }
      return 0;
    }

First we retrieve ``http2_stream_data`` object associated to the
stream in ``on_begin_headers_callback()``. It is done using
`nghttp2_session_get_stream_user_data()`. If the requested path cannot
be served for some reasons (e.g., file is not found), we send 404
response, which is done in ``error_reply()``.  Otherwise, we open
requested file and send its content. We send 1 header field
``:status`` as a response header.

Sending content of a file is done in ``send_response()`` function::

    static int send_response(nghttp2_session *session, int32_t stream_id,
                             nghttp2_nv *nva, size_t nvlen, int fd)
    {
      int rv;
      nghttp2_data_provider data_prd;
      data_prd.source.fd = fd;
      data_prd.read_callback = file_read_callback;

      rv = nghttp2_submit_response(session, stream_id, nva, nvlen, &data_prd);
      if(rv != 0) {
        warnx("Fatal error: %s", nghttp2_strerror(rv));
        return -1;
      }
      return 0;
    }

The nghttp2 library uses :type:`nghttp2_data_provider` structure to
send entity body to the remote peer. The ``source`` member of this
structure is a union and it can be either void pointer or int which is
intended to be used as file descriptor. In this example server, we use
file descriptor. We also set ``file_read_callback()`` callback
function to read content of the file::

    static ssize_t file_read_callback
    (nghttp2_session *session, int32_t stream_id,
     uint8_t *buf, size_t length, int *eof,
     nghttp2_data_source *source, void *user_data)
    {
      int fd = source->fd;
      ssize_t r;
      while((r = read(fd, buf, length)) == -1 && errno == EINTR);
      if(r == -1) {
        return NGHTTP2_ERR_TEMPORAL_CALLBACK_FAILURE;
      }
      if(r == 0) {
        *eof = 1;
      }
      return r;
    }

If error happens while reading file, we return
:macro:`NGHTTP2_ERR_TEMPORAL_CALLBACK_FAILURE`. This tells the library
to send RST_STREAM to the stream.  When all data is read, set 1 to
``*eof`` to tell the nghttp2 library that we have finished reading
file.

The `nghttp2_submit_response()` is used to send response to the remote
peer.

The ``on_stream_close_callback()`` function is invoked when the stream
is about to close::

    static int on_stream_close_callback(nghttp2_session *session,
                                        int32_t stream_id,
                                        nghttp2_error_code error_code,
                                        void *user_data)
    {
      http2_session_data *session_data = (http2_session_data*)user_data;
      http2_stream_data *stream_data;

      stream_data = nghttp2_session_get_stream_user_data(session, stream_id);
      if(!stream_data) {
        return 0;
      }
      remove_stream(session_data, stream_data);
      delete_http2_stream_data(stream_data);
      return 0;
    }

We destroy ``http2_stream_data`` object in this function since the
stream is about to close and we no longer use that object.