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Merge branch 'master' of https://github.com/skip2/nghttp2 into skip2-master

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Tatsuhiro Tsujikawa 2015-08-01 16:18:55 +09:00
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doc/sources/tutorial-server.rst
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@ -1,30 +1,31 @@
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 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.
In this tutorial, we are going to write a single-threaded, event-based
HTTP/2 web server, which supports HTTPS only. It can handle 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. The source also resides in the examples directory in the
archive or repository.
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::
This simple server takes 3 arguments: The port number to listen on,
the path to your SSL/TLS private key file, and the path to your
certificate file. The synopsis is::
$ 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 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::
The server 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 the NPN
callback. The NPN callback is used by the server to advertise which
application protocols the server supports to a client. In this
example program, when creating the ``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 lifetime::
static unsigned char next_proto_list[256];
static size_t next_proto_list_len;
@ -53,17 +54,20 @@ time::
return ssl_ctx;
}
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()``.
The wire format of NPN is a sequence of length prefixed strings, with
exactly one byte 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, which is
exported in the identifier :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 the ``app_content`` structure to store application-wide data::
Next, let's take a look at the main structures used by the example
application:
We use the ``app_context`` structure to store application-wide data::
struct app_context {
SSL_CTX *ssl_ctx;
@ -90,18 +94,21 @@ We use the ``http2_stream_data`` structure to store stream-level data::
int fd;
} http2_stream_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``. 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
A single HTTP/2 session can have multiple streams. To manage them, we
use 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``.
libevent's bufferevent structure is used to perform network I/O, with
the pointer to the bufferevent stored in the ``http2_session_data``
structure. 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 the bufferevent in
``http2_session_data``.
We first create a 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.
libevent's ``struct evconnlistener`` is used for this purpose::
static void start_listen(struct event_base *evbase, const char *service,
app_context *app_ctx) {
@ -135,7 +142,7 @@ libevent's ``struct evconnlistener`` for this purpose::
errx(1, "Could not start listener");
}
We specify the ``acceptcb`` callback which is called when a new connection is
We specify the ``acceptcb`` callback, which is called when a new connection is
accepted::
static void acceptcb(struct evconnlistener *listener _U_, int fd,
@ -148,13 +155,13 @@ accepted::
bufferevent_setcb(session_data->bev, readcb, writecb, eventcb, session_data);
}
Here we create the ``http2_session_data`` object. The bufferevent for
this connection is also initialized at this time. We specify three
callbacks for the bufferevent: ``readcb``, ``writecb`` and
Here we create the ``http2_session_data`` object. The connection's
bufferevent is initialized at the same time. We specify three
callbacks for the bufferevent: ``readcb``, ``writecb``, and
``eventcb``.
The ``eventcb()`` callback is invoked by the libevent event loop when an event
(e.g., connection has been established, timeout, etc) happens on the
(e.g. connection has been established, timeout, etc.) occurs on the
underlying network socket::
static void eventcb(struct bufferevent *bev _U_, short events, void *ptr) {
@ -181,17 +188,19 @@ underlying network socket::
delete_http2_session_data(session_data);
}
For the ``BEV_EVENT_EOF``, ``BEV_EVENT_ERROR`` and
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. Now we are ready to start the HTTP/2
communication.
``http2_session_data`` object and its associated bufferevent member.
As a result, the underlying connection is closed.
We initialize a nghttp2 session object which is done in
``initialize_nghttp2_session()``::
The
``BEV_EVENT_CONNECTED`` event is invoked when SSL/TLS handshake has
completed successfully. After this we are 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;
@ -217,13 +226,13 @@ We initialize a nghttp2 session object which is done in
nghttp2_session_callbacks_del(callbacks);
}
Since we are creating a server and uses options, the nghttp2 session
object is created using `nghttp2_session_server_new2()` function. We
registers five callbacks for nghttp2 session object. We'll talk about
these callbacks later.
Since we are creating a server, we use `nghttp2_session_server_new()`
to initialize the nghttp2 session object. We also setup 5 callbacks
for the nghttp2 session, these are explained later.
After initialization of the nghttp2 session object, we are going to send
a server connection header in ``send_server_connection_header()``::
The server now begins by sending the server connection preface, which
always consists of a SETTINGS frame.
``send_server_connection_header()`` configures and submits it::
static int send_server_connection_header(http2_session_data *session_data) {
nghttp2_settings_entry iv[1] = {
@ -239,11 +248,10 @@ a server connection header in ``send_server_connection_header()``::
return 0;
}
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 it does not actually send it. All
In the example SETTINGS frame we've set
SETTINGS_MAX_CONCURRENT_STREAMS to 100. `nghttp2_submit_settings()`
is used to queue the frame for transmission, but note it only queues
the frame for transmission, and doesn't 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.
@ -274,12 +282,14 @@ this pending data. To process the received data, we call the
return 0;
}
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::
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 both invoke the previously setup callbacks and also queue
outgoing frames. To send any pending outgoing frames, we immediately
call ``session_send()``.
The ``session_send()`` function is defined as follows::
static int session_send(http2_session_data *session_data) {
int rv;
@ -292,7 +302,7 @@ frames. The ``session_send()`` function is defined as follows::
}
The `nghttp2_session_send()` function serializes the frame into wire
format and calls ``send_callback()`` of type
format and calls the ``send_callback()``, which is of type
:type:`nghttp2_send_callback`. The ``send_callback()`` is defined as
follows::
@ -312,17 +322,16 @@ follows::
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 ``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.
data to a non-blocking socket directly using the ``write()`` system
call in the ``send_callback()``, 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. But here,
when writing to the bufferevent, we have to regulate the amount data
to 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`.
The next bufferevent callback is ``readcb()``, which is invoked when
data is available to read in the bufferevent input buffer::
@ -357,16 +366,18 @@ data in the bufferevent output buffer has been sent::
}
}
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.
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 can state whether the
connection should be dropped or not. More specifically, if both
`nghttp2_session_want_read()` and `nghttp2_session_want_write()`
return 0, the connection is no-longer required and can be closed.
Since we are using bufferevent and its deferred callback option, the
bufferevent output buffer may still contain pending data when the
``writecb()`` is called. To handle this, we check whether the output
buffer is empty or not. If all of these conditions are met, we drop
connection.
Otherwise, we call ``session_send()`` to process the pending output
data. Remember that in ``send_callback()``, we must not write all data to
@ -374,7 +385,7 @@ bufferevent to avoid excessive buffering. We continue processing pending data
when the output buffer becomes empty.
We have already described the nghttp2 callback ``send_callback()``. Let's
learn about the remaining nghttp2 callbacks we setup in
learn about the remaining nghttp2 callbacks setup in
``initialize_nghttp2_setup()`` function.
The ``on_begin_headers_callback()`` function is invoked when the reception of
@ -396,13 +407,15 @@ a header block in HEADERS or PUSH_PROMISE frame is started::
return 0;
}
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.
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()`. The
``http2_stream_data`` object can later be easily retrieved from the
stream, without searching through the doubly linked list.
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
@ -437,10 +450,10 @@ emitted via ``on_header_callback`` function, which is called after
return 0;
}
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.
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 the ``: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::
@ -470,15 +483,15 @@ fully received::
return 0;
}
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.
First we retrieve the ``http2_stream_data`` object associated with the
stream in ``on_begin_headers_callback()`` 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 using ``error_reply()``. Otherwise, we open
the requested file and send its content. We send the header field
``:status`` as a single response header.
Sending the content of the file is done in ``send_response()`` function::
Sending the file content is performed by the ``send_response()`` function::
static int send_response(nghttp2_session *session, int32_t stream_id,
nghttp2_nv *nva, size_t nvlen, int fd) {
@ -495,12 +508,13 @@ Sending the content of the file is done in ``send_response()`` function::
return 0;
}
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
the file descriptor. We also set the ``file_read_callback()`` callback
function to read the contents of the file::
nghttp2 uses the :type:`nghttp2_data_provider` structure to send the
entity body to the remote peer. The ``source`` member of this
structure is a union, which can be either a void pointer or an int
(which is intended to be used as file descriptor). In this example
server, we use it as a 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 _U_,
int32_t stream_id _U_, uint8_t *buf,
@ -520,11 +534,11 @@ function to read the contents of the file::
return r;
}
If an error happens while reading the file, we return
If an error occurs 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 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.
library to send RST_STREAM to the stream. When all data has been
read, the :macro:`NGHTTP2_DATA_FLAG_EOF` flag is set to signal nghttp2
that we have finished reading the file.
The `nghttp2_submit_response()` function is used to send the response to the
remote peer.
@ -546,5 +560,5 @@ is about to close::
return 0;
}
We destroy the ``http2_stream_data`` object in this function since the stream
is about to close and we no longer use that object.
Lastly, we destroy the ``http2_stream_data`` object in this function,
since the stream is about to close and we no longer need the object.