Tutorial: HTTP/2 client

In this tutorial, we are going to write a very primitive HTTP/2 client. The complete source code, libevent-client.c_, is attached at the end of this page. It also resides in the examples directory in the archive or repository.

This simple client takes a single HTTPS URI and retrieves the resource at the URI. The synopsis is:

.. code-block:: text

$ libevent-client HTTPS_URI

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

The client starts with some libevent and OpenSSL setup in the main() and run() functions. This setup isn't specific to nghttp2, but one thing you should look at is setup of ALPN. Client tells application protocols that it supports to server via ALPN::

static SSL_CTX *create_ssl_ctx(void) {
  SSL_CTX *ssl_ctx;
  ssl_ctx = SSL_CTX_new(SSLv23_client_method());
  if (!ssl_ctx) {
    errx(1, "Could not create SSL/TLS context: %s",
         ERR_error_string(ERR_get_error(), NULL));
  }
  SSL_CTX_set_options(ssl_ctx,
                      SSL_OP_ALL | SSL_OP_NO_SSLv2 | SSL_OP_NO_SSLv3 |
                          SSL_OP_NO_COMPRESSION |
                          SSL_OP_NO_SESSION_RESUMPTION_ON_RENEGOTIATION);

  SSL_CTX_set_alpn_protos(ssl_ctx, (const unsigned char *)"\x02h2", 3);

  return ssl_ctx;
}

Here we see SSL_CTX_get_alpn_protos() function call. We instructs OpenSSL to notify the server that we support h2, ALPN identifier for HTTP/2.

The example client defines a couple of structs:

We define and use a http2_session_data structure to store data related to the HTTP/2 session::

typedef struct {
  nghttp2_session *session;
  struct evdns_base *dnsbase;
  struct bufferevent *bev;
  http2_stream_data *stream_data;
} http2_session_data;

Since this program only handles one URI, it uses only one stream. We store the single stream's data in a http2_stream_data structure and the stream_data points to it. The http2_stream_data structure is defined as follows::

typedef struct {
  /* The NULL-terminated URI string to retrieve. */
  const char *uri;
  /* Parsed result of the |uri| */
  urlparse_url *u;
  /* The authority portion of the |uri|, not NULL-terminated */
  char *authority;
  /* The path portion of the |uri|, including query, not
     NULL-terminated */
  char *path;
  /* The length of the |authority| */
  size_t authoritylen;
  /* The length of the |path| */
  size_t pathlen;
  /* The stream ID of this stream */
  int32_t stream_id;
} http2_stream_data;

We create and initialize these structures in create_http2_session_data() and create_http2_stream_data() respectively.

initiate_connection() is called to start the connection to the remote server. It's defined as::

static void initiate_connection(struct event_base *evbase, SSL_CTX *ssl_ctx,
                                const char *host, uint16_t port,
                                http2_session_data *session_data) {
  int rv;
  struct bufferevent *bev;
  SSL *ssl;

  ssl = create_ssl(ssl_ctx);
  bev = bufferevent_openssl_socket_new(
      evbase, -1, ssl, BUFFEREVENT_SSL_CONNECTING,
      BEV_OPT_DEFER_CALLBACKS | BEV_OPT_CLOSE_ON_FREE);
  bufferevent_enable(bev, EV_READ | EV_WRITE);
  bufferevent_setcb(bev, readcb, writecb, eventcb, session_data);
  rv = bufferevent_socket_connect_hostname(bev, session_data->dnsbase,
                                           AF_UNSPEC, host, port);

  if (rv != 0) {
    errx(1, "Could not connect to the remote host %s", host);
  }
  session_data->bev = bev;
}

initiate_connection() creates a bufferevent for the connection and sets up three callbacks: readcb, writecb, and eventcb.

The eventcb() is invoked by the libevent event loop when an event (e.g. connection has been established, timeout, etc.) occurs 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) {
    int fd = bufferevent_getfd(bev);
    int val = 1;
    const unsigned char *alpn = NULL;
    unsigned int alpnlen = 0;
    SSL *ssl;

    fprintf(stderr, "Connected\n");

    ssl = bufferevent_openssl_get_ssl(session_data->bev);

    SSL_get0_alpn_selected(ssl, &alpn, &alpnlen);

    if (alpn == NULL || alpnlen != 2 || memcmp("h2", alpn, 2) != 0) {
      fprintf(stderr, "h2 is not negotiated\n");
      delete_http2_session_data(session_data);
      return;
    }

    setsockopt(fd, IPPROTO_TCP, TCP_NODELAY, (char *)&val, sizeof(val));
    initialize_nghttp2_session(session_data);
    send_client_connection_header(session_data);
    submit_request(session_data);
    if (session_send(session_data) != 0) {
      delete_http2_session_data(session_data);
    }
    return;
  }
  if (events & BEV_EVENT_EOF) {
    warnx("Disconnected from the remote host");
  } else if (events & BEV_EVENT_ERROR) {
    warnx("Network error");
  } else if (events & BEV_EVENT_TIMEOUT) {
    warnx("Timeout");
  }
  delete_http2_session_data(session_data);
}

Here we validate that HTTP/2 is negotiated, and if not, drop connection.

For BEV_EVENT_EOF, BEV_EVENT_ERROR, and BEV_EVENT_TIMEOUT events, we just simply tear down the connection.

The BEV_EVENT_CONNECTED event is invoked when the SSL/TLS handshake has completed successfully. After this we're ready to begin communicating via HTTP/2.

The initialize_nghttp2_session() function initializes the nghttp2 session object and several callbacks::

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

  nghttp2_session_callbacks_new(&callbacks);

  nghttp2_session_callbacks_set_send_callback2(callbacks, send_callback);

  nghttp2_session_callbacks_set_on_frame_recv_callback(callbacks,
                                                       on_frame_recv_callback);

  nghttp2_session_callbacks_set_on_data_chunk_recv_callback(
      callbacks, on_data_chunk_recv_callback);

  nghttp2_session_callbacks_set_on_stream_close_callback(
      callbacks, on_stream_close_callback);

  nghttp2_session_callbacks_set_on_header_callback(callbacks,
                                                   on_header_callback);

  nghttp2_session_callbacks_set_on_begin_headers_callback(
      callbacks, on_begin_headers_callback);

  nghttp2_session_client_new(&session_data->session, callbacks, session_data);

  nghttp2_session_callbacks_del(callbacks);
}

Since we are creating a client, we use nghttp2_session_client_new() to initialize the nghttp2 session object. The callbacks setup are explained later.

The delete_http2_session_data() function destroys session_data and frees its bufferevent, so the underlying connection is closed. It also calls nghttp2_session_del() to delete the nghttp2 session object.

A HTTP/2 connection begins by sending the client connection preface, which is a 24 byte magic byte string (:macro:NGHTTP2_CLIENT_MAGIC), followed by a SETTINGS frame. The 24 byte magic string is sent automatically by nghttp2. We send the SETTINGS frame in send_client_connection_header()::

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

  /* client 24 bytes magic string will be sent by nghttp2 library */
  rv = nghttp2_submit_settings(session_data->session, NGHTTP2_FLAG_NONE, iv,
                               ARRLEN(iv));
  if (rv != 0) {
    errx(1, "Could not submit SETTINGS: %s", nghttp2_strerror(rv));
  }
}

Here we specify SETTINGS_MAX_CONCURRENT_STREAMS as 100. This is not needed for this tiny example program, it just demonstrates use of the SETTINGS frame. To queue the SETTINGS frame for transmission, we call nghttp2_submit_settings(). Note that nghttp2_submit_settings() only queues the frame for transmission, and doesn't actually send it. All nghttp2_submit_*() family functions have this property. To actually send the frame, nghttp2_session_send() has to be called, which is described (and called) later.

After the transmission of the client connection header, we enqueue the HTTP request in the submit_request() function::

static void submit_request(http2_session_data *session_data) {
  int32_t stream_id;
  http2_stream_data *stream_data = session_data->stream_data;
  const char *uri = stream_data->uri;
  const urlparse_url *u = stream_data->u;
  nghttp2_nv hdrs[] = {
      MAKE_NV2(":method", "GET"),
      MAKE_NV(":scheme", &uri[u->field_data[URLPARSE_SCHEMA].off],
              u->field_data[URLPARSE_SCHEMA].len),
      MAKE_NV(":authority", stream_data->authority, stream_data->authoritylen),
      MAKE_NV(":path", stream_data->path, stream_data->pathlen)};
  fprintf(stderr, "Request headers:\n");
  print_headers(stderr, hdrs, ARRLEN(hdrs));
  stream_id = nghttp2_submit_request2(session_data->session, NULL, hdrs,
                                      ARRLEN(hdrs), NULL, stream_data);
  if (stream_id < 0) {
    errx(1, "Could not submit HTTP request: %s", nghttp2_strerror(stream_id));
  }

  stream_data->stream_id = stream_id;
}

We build the HTTP request header fields in hdrs, which is an array of :type:nghttp2_nv. There are four header fields to be sent: :method, :scheme, :authority, and :path. To queue the HTTP request, we call nghttp2_submit_request2(). The stream_data is passed via the stream_user_data parameter, which is helpfully later passed back to callback functions.

nghttp2_submit_request2() returns the newly assigned stream ID for the request.

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

static void readcb(struct bufferevent *bev, void *ptr) {
  http2_session_data *session_data = (http2_session_data *)ptr;
  nghttp2_ssize readlen;
  struct evbuffer *input = bufferevent_get_input(bev);
  size_t datalen = evbuffer_get_length(input);
  unsigned char *data = evbuffer_pullup(input, -1);

  readlen = nghttp2_session_mem_recv2(session_data->session, data, datalen);
  if (readlen < 0) {
    warnx("Fatal error: %s", nghttp2_strerror((int)readlen));
    delete_http2_session_data(session_data);
    return;
  }
  if (evbuffer_drain(input, (size_t)readlen) != 0) {
    warnx("Fatal error: evbuffer_drain failed");
    delete_http2_session_data(session_data);
    return;
  }
  if (session_send(session_data) != 0) {
    delete_http2_session_data(session_data);
    return;
  }
}

In this function we feed all unprocessed, received data to the nghttp2 session object using the nghttp2_session_mem_recv2() function. nghttp2_session_mem_recv2() processes the received data and may invoke nghttp2 callbacks and queue frames for transmission. Since there may be pending frames for transmission, we call immediately session_send() to send them. session_send() 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 pending frames into wire format and calls the send_callback() function to send them. send_callback() has type :type:nghttp2_send_callback2 and is defined as::

static nghttp2_ssize send_callback(nghttp2_session *session _U_,
                                   const uint8_t *data, size_t length,
                                   int flags _U_, void *user_data) {
  http2_session_data *session_data = (http2_session_data *)user_data;
  struct bufferevent *bev = session_data->bev;
  bufferevent_write(bev, data, length);
  return (nghttp2_ssize)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 the write() system call, we'd soon receive an EAGAIN or EWOULDBLOCK error, since sockets have a limited send buffer. If that happens, it's possible to return :macro:NGHTTP2_ERR_WOULDBLOCK to signal the nghttp2 library to stop sending further data. When writing to a bufferevent, you should regulate the amount of data written, to avoid possible huge memory consumption. In this example client however we don't implement a limit. To see how to regulate the amount of buffered data, see the send_callback() in the server tutorial.

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

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

As described earlier, we just write off all data in send_callback(), so there is no data to write in this function. All we have to do is check if the connection should be dropped or not. The nghttp2 session object keeps track of reception and transmission of GOAWAY frames and other error conditions. Using this information, the nghttp2 session object can state whether the connection should be dropped or not. More specifically, when both nghttp2_session_want_read() and nghttp2_session_want_write() return 0, the connection is no-longer required and can be closed. Since we're using bufferevent and its deferred callback option, the bufferevent output buffer may still contain pending data when the writecb() is called. To handle this situation, we also check whether the output buffer is empty or not. If all of these conditions are met, then we drop the connection.

Now let's look at the remaining nghttp2 callbacks setup in the initialize_nghttp2_setup() function.

A server responds to the request by first sending a HEADERS frame. The HEADERS frame consists of response header name/value pairs, and the on_header_callback() is called for each name/value pair::

static int on_header_callback(nghttp2_session *session _U_,
                              const nghttp2_frame *frame, const uint8_t *name,
                              size_t namelen, const uint8_t *value,
                              size_t valuelen, uint8_t flags _U_,
                              void *user_data) {
  http2_session_data *session_data = (http2_session_data *)user_data;
  switch (frame->hd.type) {
  case NGHTTP2_HEADERS:
    if (frame->headers.cat == NGHTTP2_HCAT_RESPONSE &&
        session_data->stream_data->stream_id == frame->hd.stream_id) {
      /* Print response headers for the initiated request. */
      print_header(stderr, name, namelen, value, valuelen);
      break;
    }
  }
  return 0;
}

In this tutorial, we just print the name/value pairs on stderr.

After the HEADERS frame has been fully received (and thus all response header name/value pairs have been received), the on_frame_recv_callback() function is called::

static int on_frame_recv_callback(nghttp2_session *session _U_,
                                  const nghttp2_frame *frame, void *user_data) {
  http2_session_data *session_data = (http2_session_data *)user_data;
  switch (frame->hd.type) {
  case NGHTTP2_HEADERS:
    if (frame->headers.cat == NGHTTP2_HCAT_RESPONSE &&
        session_data->stream_data->stream_id == frame->hd.stream_id) {
      fprintf(stderr, "All headers received\n");
    }
    break;
  }
  return 0;
}

on_frame_recv_callback() is called for other frame types too.

In this tutorial, we are just interested in the HTTP response HEADERS frame. We check the frame type and its category (it should be :macro:NGHTTP2_HCAT_RESPONSE for HTTP response HEADERS). We also check its stream ID.

Next, zero or more DATA frames can be received. The on_data_chunk_recv_callback() function is invoked when a chunk of data is received from the remote peer::

static int on_data_chunk_recv_callback(nghttp2_session *session _U_,
                                       uint8_t flags _U_, int32_t stream_id,
                                       const uint8_t *data, size_t len,
                                       void *user_data) {
  http2_session_data *session_data = (http2_session_data *)user_data;
  if (session_data->stream_data->stream_id == stream_id) {
    fwrite(data, len, 1, stdout);
  }
  return 0;
}

In our case, a chunk of data is HTTP response body. After checking the stream ID, we just write the received data to stdout. Note the output in the terminal may be corrupted if the response body contains some binary data.

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;
  int rv;

  if (session_data->stream_data->stream_id == stream_id) {
    fprintf(stderr, "Stream %d closed with error_code=%d\n", stream_id,
            error_code);
    rv = nghttp2_session_terminate_session(session, NGHTTP2_NO_ERROR);
    if (rv != 0) {
      return NGHTTP2_ERR_CALLBACK_FAILURE;
    }
  }
  return 0;
}

If the stream ID matches the one we initiated, it means that its stream is going to be closed. Since we have finished receiving resource we wanted (or the stream was reset by RST_STREAM from the remote peer), we call nghttp2_session_terminate_session() to commence closure of the HTTP/2 session gracefully. If you have some data associated for the stream to be closed, you may delete it here.