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Merge branch 'bagder-server-tutorial'

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Tatsuhiro Tsujikawa 2014-08-01 20:55:45 +09:00
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doc/sources/tutorial-server.rst
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@ -3,29 +3,28 @@ 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
concurrent multiple requests, but only the 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::
This simple server takes 3 arguments, a port number to listen to, a path to
your SSL/TLS private key file and a path to your 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::
First we create a setup routine for libevent and OpenSSL in the functions
``main()`` and ``run()``. One thing in there you should look at, is the setup
of the NPN callback. The NPN callback is used for the server to advertise
which application protocols the server supports to a client. In this example
program, when creating ``SSL_CTX`` object, we store the application protocol
name in the wire format of NPN in a statically allocated buffer. This is safe
because we only create one ``SSL_CTX`` object in the program's entire life
time::
static unsigned char next_proto_list[256];
static size_t next_proto_list_len;
@ -54,25 +53,25 @@ life time::
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
The wire format of NPN is a sequence of length prefixed string. Exactly one
byte is used to specify the length of each protocol identifier. In this
tutorial, we advertise the specific HTTP/2 protocol version the current
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 the OpenSSL implementation, we just assign the
pointer to the NPN buffers we filled in earlier. The NPN callback function is
set to the ``SSL_CTX`` object using
``SSL_CTX_set_next_protos_advertised_cb()``.
We use ``app_content`` structure to store the application-wide data::
We use the ``app_content`` structure to store 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::
We use the ``http2_session_data`` structure to store session-level
(which corresponds to one HTTP/2 connection) data::
typedef struct http2_session_data {
struct http2_stream_data root;
@ -83,8 +82,7 @@ We use ``http2_session_data`` structure to store the session-level
size_t handshake_leftlen;
} http2_session_data;
We use ``http2_stream_data`` structure to store the stream-level
data::
We use the ``http2_stream_data`` structure to store stream-level data::
typedef struct http2_stream_data {
struct http2_stream_data *prev, *next;
@ -93,23 +91,20 @@ data::
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``.
A single HTTP/2 session can have multiple streams. We manage these multiple
streams with a doubly linked list. The first element of this list is pointed
to 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 the first client
connection preface (:macro:`NGHTTP2_CLIENT_CONNECTION_PREFACE`), which is a 24
bytes long magic string from the client. We use libevent's bufferevent
structure to perform network I/O. Note that the bufferevent object is kept 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::
We first create a 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)
@ -143,8 +138,8 @@ We use libevent's ``struct evconnlistener`` for this purpose::
errx(1, "Could not start listener");
}
We specify ``acceptcb`` callback which is called when a new connection
is accepted::
We specify the ``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)
@ -157,11 +152,11 @@ is accepted::
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``.
Here we create the ``http2_session_data`` object. The bufferevent for this
connection is also initialized at this time. We specify two callbacks for the
bufferevent: ``handshake_readcb`` and ``eventcb``.
The ``eventcb()`` is invoked by libevent event loop when an event
The ``eventcb()`` callback is invoked by the libevent event loop when an event
(e.g., connection has been established, timeout, etc) happens on the
underlying network socket::
@ -182,17 +177,15 @@ underlying network socket::
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.
For the ``BEV_EVENT_EOF``, ``BEV_EVENT_ERROR`` and ``BEV_EVENT_TIMEOUT``
events, we just simply tear down the connection. The
``delete_http2_session_data()`` function destroys the ``http2_session_data``
object and thus also 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::
``handshake_readcb()`` is a callback function to handle a 24 bytes magic byte
string coming from a client, since the nghttp2 library does not handle it::
static void handshake_readcb(struct bufferevent *bev, void *ptr)
{
@ -225,14 +218,13 @@ it::
}
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.
:macro:`NGHTTP2_CLIENT_CONNECTION_PREFACE`. When they match, the connection
state is ready to start the HTTP/2 communication. First we change the callback
functions for the bufferevent object. We use the same ``eventcb`` callback as
before, but we specify new ``readcb`` and ``writecb`` functions to handle the
HTTP/2 communication. These two functions are described later.
We initialize nghttp2 session object which is done in
We initialize a nghttp2 session object which is done in
``initialize_nghttp2_session()``::
static void initialize_nghttp2_session(http2_session_data *session_data)
@ -247,12 +239,12 @@ We initialize nghttp2 session object which is done in
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
Since we are creating a server, the nghttp2 session object is created using
`nghttp2_session_server_new()` function. We registers five callbacks for
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()``::
After initialization of the nghttp2 session object, we are going to send
a server connection header in ``send_server_connection_header()``::
static int send_server_connection_header(http2_session_data *session_data)
{
@ -270,18 +262,18 @@ server connection header in ``send_server_connection_header()``::
return 0;
}
The server connection header is SETTINGS frame. We specify
SETTINGS_MAX_CONCURRENT_STREAMS to 100 in SETTINGS frame. To queue
The server connection header is a SETTINGS frame. We specify
SETTINGS_MAX_CONCURRENT_STREAMS to 100 in the 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.
function only queues the frame and it does not actually send it. All
functions in the ``nghttp2_submit_*()`` family have this property. To
actually send the frame, `nghttp2_session_send()` should be used, as
described 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
Since bufferevent may buffer more than the first 24 bytes from the client, we
have to process them here since libevent won't invoke callback functions for
this pending data. To process the received data, we call the
``session_recv()`` function::
static int session_recv(http2_session_data *session_data)
@ -306,12 +298,12 @@ functions for these pending data. To process received data, we call
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::
In this function, we feed all unprocessed but already received data to the
nghttp2 session object using the `nghttp2_session_mem_recv()` function. The
`nghttp2_session_mem_recv()` function processes the data and may invoke the
nghttp2 callbacks and also queue outgoing frames. Since there may be pending
outgoing frames, we call ``session_send()`` function to send off those
frames. The ``session_send()`` function is defined as follows::
static int session_send(http2_session_data *session_data)
{
@ -325,8 +317,8 @@ those frames. The ``session_send()`` function is defined as follows::
}
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
format and calls :member:`nghttp2_session_callbacks.send_callback` with
it. We set the ``send_callback()`` function to
:member:`nghttp2_session_callbacks.send_callback` in
``initialize_nghttp2_session()`` function described earlier. It is
defined as follows::
@ -346,18 +338,17 @@ defined as follows::
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
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 a non-blocking socket
directly using ``write()`` system call in the
:member:`nghttp2_session_callbacks.send_callback`, we would surely get
``EAGAIN`` or ``EWOULDBLOCK`` back 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 when writing to
the bufferevent, we have to regulate the amount data to get buffered ourselves
to avoid using huge amounts of memory. To achieve this, we check the size of
the output buffer and if it reaches 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.
@ -377,8 +368,8 @@ data is available to read in the bufferevent input buffer::
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::
The third bufferevent callback is ``writecb()``, which is invoked when all
data in the bufferevent output buffer has been sent::
static void writecb(struct bufferevent *bev, void *ptr)
{
@ -397,29 +388,28 @@ all data written in the bufferevent output buffer have been sent::
}
}
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.
First we check whether we should drop the connection or not. The nghttp2
session object keeps track of reception and transmission of GOAWAY frames and
other error conditions as well. Using this information, the nghttp2 session
object will tell whether the connection should be dropped or not. More
specifically, if both `nghttp2_session_want_read()` and
`nghttp2_session_want_write()` return 0, we have no business left in the
connection. But since we are using bufferevent and its deferred callback
option, the bufferevent output buffer may contain pending data when the
``writecb()`` is called. To handle this, we check whether the output buffer is
empty or not. If all 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.
Otherwise, we call ``session_send()`` to process the pending output
data. Remember that in ``send_callback()``, we must not write all data to
bufferevent to avoid excessive buffering. We continue processing pending data
when the output buffer becomes empty.
We have already described about nghttp2 callback ``send_callback()``.
Let's describe remaining nghttp2 callbacks we setup in
We have already described the nghttp2 callback ``send_callback()``. Let's
learn about the 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::
The ``on_begin_headers_callback()`` function is invoked when the reception of
a header block in HEADERS or PUSH_PROMISE frame is started::
static int on_begin_headers_callback(nghttp2_session *session,
const nghttp2_frame *frame,
@ -438,17 +428,17 @@ of header block in HEADERS or PUSH_PROMISE frame is started::
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.
We are only interested in the HEADERS frame in this function. Since the
HEADERS frame has several roles in the HTTP/2 protocol, we check that it is a
request HEADERS, which opens new stream. If the frame is a request HEADERS, we
create a ``http2_stream_data`` object to store the stream related data. We
associate the created ``http2_stream_data`` object with the stream in the
nghttp2 session object using `nghttp2_set_stream_user_data()` to get the
object without searching through the 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
In this example server, we want to serve files relative to the current working
directory in which 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,
@ -479,10 +469,10 @@ is emitted via ``on_header_callback`` function, which is called after
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.
We search for the ``:path`` header field among the request headers and store
the requested path in the ``http2_stream_data`` object. In this example
program, we ignore ``:method`` header field and always treat the request as a
GET request.
The ``on_frame_recv_callback()`` function is invoked when a frame is
fully received::
@ -513,15 +503,15 @@ fully received::
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.
First we retrieve the ``http2_stream_data`` object associated with 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 reason (e.g., file is not found), we send a 404 response,
which is done in ``error_reply()``. Otherwise, we open the requested file and
send its content. We send the header field ``:status`` as a single response
header.
Sending content of a file is done in ``send_response()`` function::
Sending the content of the 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)
@ -539,12 +529,12 @@ Sending content of a file is done in ``send_response()`` function::
return 0;
}
The nghttp2 library uses :type:`nghttp2_data_provider` structure to
The nghttp2 library uses the :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::
the file descriptor. We also set the ``file_read_callback()`` callback
function to read the contents of the file::
static ssize_t file_read_callback
(nghttp2_session *session, int32_t stream_id,
@ -563,14 +553,14 @@ function to read content of the file::
return r;
}
If error happens while reading file, we return
If an error happens while reading the file, we return
:macro:`NGHTTP2_ERR_TEMPORAL_CALLBACK_FAILURE`. This tells the
library to send RST_STREAM to the stream. When all data are read, set
:macro:`NGHTTP2_DATA_FLAG_EOF` flag to ``*data_flags`` to tell the
nghttp2 library that we have finished reading file.
library to send RST_STREAM to the stream. When all data has been read, set
the :macro:`NGHTTP2_DATA_FLAG_EOF` flag to ``*data_flags`` to tell the
nghttp2 library that we have finished reading the file.
The `nghttp2_submit_response()` is used to send response to the remote
peer.
The `nghttp2_submit_response()` function is used to send the response to the
remote peer.
The ``on_stream_close_callback()`` function is invoked when the stream
is about to close::
@ -592,5 +582,5 @@ is about to close::
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.
We destroy the ``http2_stream_data`` object in this function since the stream
is about to close and we no longer use that object.