nghttp2/_sources/tutorial-server.txt

Tutorial: HTTP/2.0 server
=========================

In this tutorial, we are going to write single-threaded, event-based
HTTP/2.0 web server, which supports HTTPS. 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 iteself does not depends 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 array of length prefixed string. The exactly
one byte is used to specify the length of the protocol identifier.  In
this tutorial, we advertise the HTTP/2.0 protocol the nghttp2 library
supports. We export 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/2.0 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/2.0 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 24 bytes magic values 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 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 disconnected\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_HEADER;

      if(memcmp(conhead + NGHTTP2_CLIENT_CONNECTION_HEADER_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;
        }
      }
    }

Nothing special here, we just compare the magic byte string received
and expected one :macro:`NGHTTP2_CLIENT_CONNECTION_HEADER`.  When
whole magic byte string is received, the connection state is ready for
starting HTTP/2.0 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/2.0 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_request_recv_callback = on_request_recv_callback;
      callbacks.on_stream_close_callback = on_stream_close_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 4 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 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_callbacks.nghttp2_send_callback` with
it. We set ``send_callback()`` function as
: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 have 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 if 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_frame_recv_callback()`` function is invoked when a frame is
received from the remote peer::

    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;
      size_t i;
      const char PATH[] = ":path";
      switch(frame->hd.type) {
      case NGHTTP2_HEADERS:
        if(frame->headers.cat != NGHTTP2_HCAT_REQUEST) {
          break;
        }
        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);
        for(i = 0; i < frame->headers.nvlen; ++i) {
          nghttp2_nv *nv = &frame->headers.nva[i];
          if(nv->namelen == sizeof(PATH) - 1 &&
             memcmp(PATH, nv->name, nv->namelen) == 0) {
            size_t j;
            for(j = 0; j < nv->valuelen && nv->value[j] != '?'; ++j);
            stream_data->request_path = percent_decode(nv->value, j);
            break;
          }
        }
        break;
      default:
        break;
      }
      return 0;
    }

We only interested HEADERS frame in this function. Since HEADERS frame
has several roles in HTTP/2.0 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 server static file relative to the
current working directory the program was invoked. 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.

It is ok for the server to start sending response in this callback. In
this example, we defer it to ``on_request_recv_callback()`` function.

The ``on_request_recv_callback()`` function is invoked when all HTTP
request, including entity body, was received::

    static int on_request_recv_callback(nghttp2_session *session,
                                        int32_t stream_id, void *user_data)
    {
      int fd;
      http2_session_data *session_data = (http2_session_data*)user_data;
      http2_stream_data *stream_data;
      nghttp2_nv hdrs[] = {
        MAKE_NV(":status", "200")
      };
      char *rel_path;

      stream_data = (http2_stream_data*)nghttp2_session_get_stream_user_data
        (session, stream_id);
      if(!stream_data->request_path) {
        if(error_reply(session, stream_data) != 0) {
          return NGHTTP2_ERR_CALLBACK_FAILURE;
        }
        return 0;
      }
      fprintf(stderr, "%s GET %s\n", session_data->client_addr,
              stream_data->request_path);
      if(!check_path(stream_data->request_path)) {
        if(error_reply(session, stream_data) != 0) {
          return NGHTTP2_ERR_CALLBACK_FAILURE;
        }
        return 0;
      }
      for(rel_path = stream_data->request_path; *rel_path == '/'; ++rel_path);
      fd = open(rel_path, O_RDONLY);
      if(fd == -1) {
        if(error_reply(session, stream_data) != 0) {
          return NGHTTP2_ERR_CALLBACK_FAILURE;
        }
        return 0;
      }
      stream_data->fd = fd;

      if(send_response(session, stream_id, hdrs, ARRLEN(hdrs), fd) != 0) {
        close(fd);
        return NGHTTP2_ERR_CALLBACK_FAILURE;
      }
      return 0;
    }

First we retrieve ``http2_stream_data`` object associated to the
stream in ``on_frame_recv_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);
      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 to use that object.


libevent-server.c
-----------------

.. literalinclude:: ../examples/libevent-server.c