== Design Concept

Mongoose is a multi-protocol networking library that implements non-blocking,
asyncronous IO and provides event-based API. It has three basic data structures:

- link:#_struct_mg_mgr[`struct mg_mgr`] is an event manager
  that holds all active connections
- link:#_struct_mg_connection[`struct mg_connection`] describes a connection
- link:#_struct_mbuf[`struct mbuf`] describes data buffer
  (received or sent data)

Connections could be either *listening*, *outbound* or *inbound*.
Outbound connections are created by link:#_mg_connect[`mg_connect()`] call.
Listening connections are created by link:#_mg_bind[`mg_bind()`] call.
Inbound connections are those accepted by a listening connection.
Each connection is described by
link:#_struct_mg_connection[`struct mg_connection`] structure, which has
a number of fields like socket, event handler function, send/receive buffer,
flags, et cetera.

Mongoose usage pattern is to declare and initialize event manager, create
connections and create an event loop by calling
link:#_mg_mgr_poll[`mg_mgr_poll()`] in a loop.
link:#_mg_mgr_poll[`mg_mgr_poll()`] iterates over all sockets,
accepts new connections, sends and receives data, closes connections,
and calls event handler functions for the respective events.

=== Memory buffers

Each connection has send and receive buffer,
link:#_struct_mg_connection[`struct mg_connection::send_mbuf`]
and
link:#_struct_mg_connection[`struct mg_connection::recv_mbuf`] respectively.
When data arrives,
Mongoose appends received data to the `recv_mbuf` and
triggers `MG_EV_RECV` event. User may send data back by calling one of the
output functions, like link:#_mg_send[`mg_send()`] or
link:#_mg_printf[`mg_printf()`]. Output functions append data to the
`send_mbuf`. When Mongoose
successfully writes data to the socket, it discards data from
link:#_struct_mg_connection[`mg_connection::send_mbuf`] and
sends `MG_EV_SEND` event. When connection is closed, `MG_EV_CLOSE` event is sent.

image::mongoose/mbuf.png[width="600",align="center"]

=== Event handler function

Each connection has an event handler function associated with it. That
function must be implemented by user. Event handler is the key element of
the Mongoose application, since it defines application's behavior. This is how
an event handler function looks like:

[source,c]
----
static void ev_handler(struct mg_connection *nc, int ev, void *ev_data) {
  switch (ev) {
    /* Event handler code that defines behavior of the connection */
    ...
  }
}
----

struct mg_connection *nc::
  Connection that has received an event.

int ev::
  Event number, defined in `mongoose.h`. For example, when data arrives
  on inbound connection `ev` would be `MG_EV_RECV`.

void *ev_data::
  This pointer points to the event-specific data, and it has different
  meaning for different events. For example, for `MG_EV_RECV` event,
  `ev_data` is an `int *` pointer, pointing to the number of bytes received
  from the remote peer and saved into the receive IO buffer. Exact meaning
  of `ev_data` is described for each event. Protocol-specific events usually
  have `ev_data` pointing to structures that hold protocol-specific information.

NOTE: link:#_struct_mg_connection[`struct mg_connection`] has `void *user_data`
which is a placeholder for a application-specific data. Mongoose does not use
that pointer. Event handler can store any kind of information there.

=== Events

Mongoose accepts incoming connections, reads and writes data, and
calls specified event handler for each connection when appropriate. Typical
event sequence is this:

- For outbound connection: `MG_EV_CONNECT` -> (`MG_EV_RECV`, `MG_EV_SEND`, `MG_EV_POLL` ...) -> `MG_EV_CLOSE`
- For inbound connection: `MG_EV_ACCEPT` ->  (`MG_EV_RECV`, `MG_EV_SEND`, `MG_EV_POLL` ...) -> `MG_EV_CLOSE`


Below is a list
of core events triggered by Mongoose (note that each protocol triggers
protocol-specific events in addition to the core ones):

MG_EV_ACCEPT:: sent when new server connection is accepted by a
listening connection. `void *ev_data` is `union socket_address`
of the remote peer.
MG_EV_CONNECT:: sent when a new outbound connection created by
link:#_mg_connect[`mg_connect()`]
either failed or succeeded. `void *ev_data` is `int *success`.
If `success` is 0, then connection has been established,
otherwise it contains error code. See
link:#_mg_connect_opt[`mg_connect_opt()`] function for code example.

MG_EV_RECV:: New data is received and appended to the end of `recv_mbuf`.
`void *ev_data` is `int *num_received_bytes`. Typically, event handler
should check received data in `nc->recv_mbuf`,
discard processed data by calling link:#_mbuf_remove[`mbuf_remove()`],
set connection flags `nc->flags` if necessary
(see link:#_struct_mg_connection[`struct mg_connection`]), and write
data the remote peer by output functions like link:#_mg_send[`mg_send()`].

WARNING: Mongoose uses `realloc()` to expand receive buffer.
It is user's responsibility to discard processed
data from the beginning of receive buffer, note the `mbuf_remove()`
call in the example above.

MG_EV_SEND:: Mongoose has written data to the remote peer and discarded
written data from the
link:#_struct_mg_connection[`mg_connection::send_mbuf`]. `void *ev_data`
is `int *num_sent_bytes`.

NOTE: Mongoose output functions only append
data to the `mg_connection::send_mbuf`, they do not do any socket writes.
An actual IO is done by link:#_mg_mgr_poll[`mg_mgr_poll()`]. `MG_EV_SEND` event
is just a notification about an IO has been done.

MG_EV_POLL:: Sent to all connections on each invocation of
link:#_mg_mgr_poll[`mg_mgr_poll()`].
This event could be used to do any housekeeping, for example check whether
certain timeout has expired and close the connection, or send heartbeet
message, et cetera.

MG_EV_TIMER:: Sent to the connection if
link:#_mg_set_timer[`mg_set_timer()`] was called.

=== Connection flags

Each connection has a `flags` bit field. Some flags are set by Mongoose, for
example if a user creates an outbound UDP connection using `udp://1.2.3.4:5678`
address, Mongoose is going to set `MG_F_UDP` flag for that connection. Other flags
are meant to be set only by user event handler to tell Mongoose how to behave.
Below is a list of connection flags that are meant to be set by event handlers:

* `MG_F_FINISHED_SENDING_DATA` tells Mongoose that all data has been
  appended to the `send_mbuf`. As soon as Mongoose sends it to the
  socket, the connection will be closed.
* `MG_F_BUFFER_BUT_DONT_SEND` tells Mongoose to append data to the
  `send_mbuf` but hold on sending it, because the data will be modified
  later and then will be sent by clearing `MG_F_BUFFER_BUT_DONT_SEND` flag.
* `MG_F_CLOSE_IMMEDIATELY` tells Mongoose to close the connection
  immediately, usually after some error
* `MG_F_USER_1`, `MG_F_USER_2`, `MG_F_USER_3`, `MG_F_USER_4` could be
  used by a developer to store application-specific state

Flags below are set by Mongoose:

* `MG_F_SSL_HANDSHAKE_DONE` SSL only, set when SSL handshake is done.
* `MG_F_CONNECTING` set when connection is in connecting state after
  `mg_connect()` call but connect did not finish yet.
* `MG_F_LISTENING` set for all listening connections
* `MG_F_UDP` set if connection is UDP
* `MG_F_IS_WEBSOCKET` set if connection is a Websocket connection
* `MG_F_WEBSOCKET_NO_DEFRAG` should be set by a user if user wants to switch
  off automatic Websocket frame defragmentation