DESCRIPTION

RFC 7641 extends the CoAP protocol to be able to monitor the state of a resource over time.

This enables clients to "observe" resources with a defined query, i.e., to retrieve a representation of a resource and keep this representation updated by the server over a period of time.

The server has to flag a resource as "observable", and then the client has to request in a GET request that it wants to observe this resource by the use of the COAP_OPTION_OBSERVE Option with a value of COAP_OBSERVE_ESTABLISH. Optionally, the client can specify query options for the resource.

To remove the "observe" subscription, the client has to issue a GET request with the COAP_OPTION_OBSERVE Option with a value of COAP_OBSERVE_CANCEL. Alternatively, the server can remove a subscription by calling coap_delete_observer() or coap_delete_observers(), but this does not notify the client that the subscription has been removed.

The underlying library adds in and removes "subscribers" to the resource as appropriate in the server side logic.

Within the server application, it needs to determine that there is a change of state of the resource under observation, and then cause the CoAP library layer to initiate a "fake GET request" so that an observe GET response gets sent back to all the clients that are observing the resource. The appropriate GET handler within the server application is called to fill in the response packet with the appropriate information. This "fake GET request" is triggered by a call to coap_resource_notify_observers().

The call to coap_run_once() in the main server application i/o loop will do all the necessary processing of sending any outstanding "fake GET requests".

Whenever the server sends a copy of the state of the "observed" resource to the client, it will use the same token used by the client when the client requested the "observe". The client will receive this observe response in the handler defined by coap_register_response_handler(3). It is the responsibility of the client application to match the supplied token and update the appropriate internal information.

The coap_resource_set_get_observable() function enables or disables the observable status of the resource by the setting of mode. If mode is 1, then the resource is observable. If mode is 0, then the resource is no longer observable.

NOTE: It is not possible for the Unknown Resource, created by coap_resource_unknown_init(3), to be observable as the Uri-Path is not known when libcoap creates a "fake GET request". The Unknown Resource PUT handler must create a new resource and mark the resource as "observable" if a specific resource needs to be observable. The application must then manage the deleteion of the resource at the appropriate time.

NOTE: The type (confirmable or non-confirmable) of the triggered observe GET response is determined not by the initial GET request, but independently by the server as per RFC 7641 3.5. Transmission. This is controlled by the flags (one of COAP_RESOURCE_FLAGS_NOTIFY_NON or COAP_RESOURCE_FLAGS_NOTIFY_CON) used when creating the resource using coap_resource_init(3). Furthermore, the server must send at least one "observe" response as confirmable, when generally sending non-confirmable, every 24 hours. libcoap handles this by sending every fifth (COAP_OBS_MAX_NON) response as a confirmable response for detection that the client is still responding.

The coap_resource_notify_observers() function needs to be called whenever the server application determines that there has been a change to the state of resource, possibly only matching a specific query if query is not NULL.

The coap_resource_get_uri_path() function is used to obtain the UriPath of the resource definion.

The coap_find_observer() function is used to check whether the current GET request as determined from resource, session and token is currently being observed or not.

The coap_delete_observer() function deletes the specific observer associated with resource, session and has token.

The coap_delete_observers() function is used to delete all observers associated with the session that is a part of the context.

RETURN VALUES

The coap_resource_get_uri_path() function returns the uri_path or NULL if there was a failure.

The coap_find_observer() function returns the subscription or NULL if there was a failure.

The coap_resource_set_get_observable() function return 0 on failure, 1 on success.

The coap_delete_observer() function return 0 on failure, 1 on success.

EXAMPLES

Simple Time Server

#include <coap2/coap.h>

coap_resource_t *time_resource = NULL;

static int check_if_time_resource_has_changed(coap_resource_t *resource) {
  return 1;
}

/* specific GET "time" handler, called from hnd_get_generic() */

static void
hnd_get_time(coap_context_t *context, coap_resource_t *resource,
coap_session_t *session, coap_pdu_t *request, coap_string_t *token,
coap_string_t *query, coap_pdu_t *response) {

  unsigned char buf[40];
  size_t len;
  time_t now;

  /* ... Additional analysis code for resource, request pdu etc.  ... */

  /* After analysis, generate a suitable response */

  /* Note that token, if set, is already in the response pdu */

  now = time(NULL);

  if (query != NULL && coap_string_equal(query, coap_make_str_const("secs"))) {
    /* Output secs since Jan 1 1970 */
    len = snprintf((char *)buf, sizeof(buf), "%lu", now);
  }
  else {
    /* Output human-readable time */
    struct tm *tmp;
    tmp = gmtime(&now);
    if (!tmp) {
      /* If 'now' is not valid */
      response->code = COAP_RESPONSE_CODE(404);
      return;
    }
    len = strftime((char *)buf, sizeof(buf), "%b %d %H:%M:%S", tmp);
  }
  /*
   * Invoke coap_add_data_blocked_response() to do all the hard work.
   *
   * Define the format - COAP_MEDIATYPE_TEXT_PLAIN - to add in
   * Define how long this response is valid for (secs) - 1 - to add in.
   *
   * OBSERVE Option added internally if needed within the function
   * BLOCK2 Option added internally if output too large
   * ETAG Option added internally
   */
  coap_add_data_blocked_response(resource, session, request, response, token,
                                 COAP_MEDIATYPE_TEXT_PLAIN, 1,
                                 len,
                                 buf);

  /*
   * As resource->code has been updated in coap_add_data_blocked_response(),
   * the response pdu will be transmitted by the underlying library.
   */

}

/* Generic GET handler */

static void
hnd_get_generic(coap_context_t *ctx, coap_resource_t *resource,
coap_session_t *session, coap_pdu_t *request, coap_string_t *token,
coap_string_t *query, coap_pdu_t *response) {

  coap_str_const_t *uri_path = coap_resource_get_uri_path(resource);

  if (!uri_path) {
    /* Unexpected Failure */
    response->code = COAP_RESPONSE_CODE(400);
    return;
  }

  /* Is this the "time" resource" ? */
  if (coap_string_equal(uri_path, coap_make_str_const("time"))) {
    hnd_get_time(ctx, resource, session, request, token, query,
                 response);
    return;
  }

  /* Other resources code */

  /* Failure response */
  response->code = COAP_RESPONSE_CODE(400);
}

/* Initialize generic GET handler */

static void
init_resources(coap_context_t *ctx)
{

  coap_resource_t *r;

  /* Create a resource to return return or update time */
  r = coap_resource_init(coap_make_str_const("time"),
                         COAP_RESOURCE_FLAGS_NOTIFY_CON);

  /* We are using a generic GET handler here */
  coap_register_handler(r, COAP_REQUEST_GET, hnd_get_generic);

  coap_resource_set_get_observable(r, 1);

  coap_add_resource(ctx, r);
  time_resource = r;

}

int main(int argc, char *argv[]){

  coap_context_t *ctx = NULL;
  coap_endpoint_t *ep = NULL;
  coap_address_t addr;
  unsigned wait_ms;

  /* Create the libcoap context */
  ctx = coap_new_context(NULL);
  if (!ctx) {
    exit(1);
  }
  coap_address_init(&addr);
  addr.addr.sa.sa_family = AF_INET;
  addr.addr.sin.sin_port = ntohs(COAP_DEFAULT_PORT);
  ep = coap_new_endpoint(ctx, &addr, COAP_PROTO_UDP);

  /* Other Set up Code */

  init_resources(ctx);

  wait_ms = COAP_RESOURCE_CHECK_TIME * 1000;

  while (1) {
    int result = coap_run_once( ctx, wait_ms );
    if ( result < 0 ) {
      break;
    } else if ( result && (unsigned)result < wait_ms ) {
      /* decrement if there is a result wait time returned */
      wait_ms -= result;
    } else {
      /*
       * result == 0, or result >= wait_ms
       * (wait_ms could have decremented to a small value, below
       * the granularity of the timer in coap_run_once() and hence
       * result == 0)
       */
      time_t t_now = time(NULL);
      if (t_last != t_now) {
        /* Happens once per second */
        int i;
        t_last = t_now;
        if (time_resource) {
          coap_resource_notify_observers(time_resource, NULL);
        }
      }
      if (result) {
        /* result must have been >= wait_ms, so reset wait_ms */
        wait_ms = COAP_RESOURCE_CHECK_TIME * 1000;
      }
    }
  }
  exit(0);

}

Client Observe Request Setup

#include <coap2/coap.h>

/* Usually, requests are sent confirmable */

static unsigned char msgtype = COAP_MESSAGE_CON;

static unsigned int token = 0;

static coap_pdu_t *
coap_new_request(coap_context_t *context, coap_session_t *session, char request_code,
coap_optlist_t **options, unsigned char *data, size_t length, int observe) {

  coap_pdu_t *pdu;
  coap_optlist_t *opt;
  (void)context;

  /* Create the pdu with the appropriate options */
  pdu = coap_pdu_init(msgtype, request_code, coap_new_message_id(session),
                      coap_session_max_pdu_size(session));
  if (!pdu)
    return NULL;

  /*
   * Create uniqueness token for this request for handling unsolicited /
   * delayed responses
   */
  token++;
  if (!coap_add_token(pdu, sizeof(token), (unsigned char*)&token)) {
    coap_log(LOG_DEBUG, "cannot add token to request\n");
    goto error;
  }

  if (request_code == COAP_REQUEST_GET && observe) {
    /* Indicate that we want to observe this resource */
    if (!coap_insert_optlist(options,
                             coap_new_optlist(COAP_OPTION_OBSERVE,
                                         COAP_OBSERVE_ESTABLISH, NULL)))
      goto error;
  }

  /* ... Other code / options etc. ... */

  /* Add in all the options (after internal sorting) to the pdu */
  if (!coap_add_optlist_pdu(pdu, options))
    goto error;

  if (data && length) {
    /* Add in the specified data */
    if (!coap_add_data(pdu, length, data))
      goto error;
  }

  return pdu;

error:

  coap_delete_pdu(pdu);
  return NULL;

}

SEE ALSO

coap_attribute(3), coap_context(3), coap_handler(3), coap_pdu_setup(3) and coap_resource(3)

FURTHER INFORMATION

"RFC7252: The Constrained Application Protocol (CoAP)" "RFC7641: Observing Resources in the Constrained Application Protocol (CoAP)"

BUGS

Please report bugs on the mailing list for libcoap: libcoap-developers@lists.sourceforge.net

AUTHORS

The libcoap project <libcoap-developers@lists.sourceforge.net>