libcoap/man/coap_cache.txt.in

// -*- mode:doc; -*-
// vim: set syntax=asciidoc tw=0

coap_cache(3)
=============
:doctype: manpage
:man source:   coap_cache
:man version:  @PACKAGE_VERSION@
:man manual:   libcoap Manual

NAME
----
coap_cache,
coap_cache_derive_key,
coap_cache_derive_key_w_ignore,
coap_delete_cache_key,
coap_cache_ignore_options,
coap_new_cache_entry,
coap_delete_cache_entry,
coap_cache_get_by_key,
coap_cache_get_by_pdu,
coap_cache_get_pdu,
coap_cache_set_app_data,
coap_cache_get_app_data
- Work with CoAP cache functions

SYNOPSIS
--------
*#include <coap@LIBCOAP_API_VERSION@/coap.h>*

*coap_cache_key_t *coap_cache_derive_key(const coap_session_t *_session_,
const coap_pdu_t *_pdu_, coap_cache_session_based_t _session_based_);*

*coap_cache_key_t *coap_cache_derive_key_w_ignore(
const coap_session_t *_session_, const coap_pdu_t *_pdu_,
coap_cache_session_based_t _session_based_,
const uint16_t *_ignore_options_, size_t _ignore_count_);*

*void coap_delete_cache_key(coap_cache_key_t *_cache_key_);*

*int coap_cache_ignore_options(coap_context_t *_context_,
const uint16_t *_options_, size_t _count_);*

*coap_cache_entry_t *coap_new_cache_entry(coap_session_t *_session_,
const coap_pdu_t *_pdu_, coap_cache_record_pdu_t _record_pdu_,
coap_cache_session_based_t _session_based_, unsigned int _idle_timeout_);*

*void coap_delete_cache_entry(coap_context_t *_context_,
coap_cache_entry_t *_cache_entry_);*

*coap_cache_entry_t *coap_cache_get_by_key(coap_context_t *_context_,
const coap_cache_key_t *_cache_key_);*

*coap_cache_entry_t *coap_cache_get_by_pdu(coap_session_t *_session_,
const coap_pdu_t *_pdu_, coap_cache_session_based_t _session_based_);*

*const coap_pdu_t *coap_cache_get_pdu(const coap_cache_entry_t *_cache_entry_);*

*void coap_cache_set_app_data(coap_cache_entry_t *_cache_entry_, void *_data_,
coap_cache_app_data_free_callback_t _callback_);*

*void *coap_cache_get_app_data(const coap_cache_entry_t *_cache_entry_);*

For specific (D)TLS library support, link with
*-lcoap-@LIBCOAP_API_VERSION@-notls*, *-lcoap-@LIBCOAP_API_VERSION@-gnutls*,
*-lcoap-@LIBCOAP_API_VERSION@-openssl*, *-lcoap-@LIBCOAP_API_VERSION@-mbedtls*
or *-lcoap-@LIBCOAP_API_VERSION@-tinydtls*.   Otherwise, link with
*-lcoap-@LIBCOAP_API_VERSION@* to get the default (D)TLS library support.

DESCRIPTION
-----------

The CoAP Cache provides support for two opaque objects that can be used for
tracking requests and responses.

The first is the ability to derive a Cache Key from the cacheable parts of a
CoAP PDU as defined in
"https://rfc-editor.org/rfc/rfc7252#section-5.6[RFC7252 5.6. Caching]"
updated by
"https://rfc-editor.org/rfc/rfc7641#section-2[RFC7641 2. The Observe Option]"
and
"https://rfc-editor.org/rfc/rfc8132#section-2[RFC8132 2. Fetch Method]".

The Cache Key is a SHA256 digest if libcoap was built with TLS support,
otherwise it uses the internal coap_hash() function, using the information
abstracted from the PDU and (optionally) the CoAP session.

This Cache Key can then be used to match against incoming PDUs and then
appropriate action logic can take place.

There is support for excluding specific CoAP options from the Cache Key.
Examples could be to exclude CoAP BLOCK1 and BLOCK2 Options for the client or
server for ease of tracking a large PUT or GET response, but to not exclude
these CoAP options in a proxy where it makes sense to cache the individual
blocks.

The second is providing Cache Entries (which can be looked up by PDU and hence
by Cache Key) which hold additional information to make information tracking
simpler.  These Cache Entries are automatically deleted when a session closes
or a context is deleted. These Cache Entries are maintained on a hashed list
for speed of lookup.

The following enums are defined.

[source, c]
----
typedef enum coap_cache_session_based_t {
  COAP_CACHE_NOT_SESSION_BASED,
  COAP_CACHE_IS_SESSION_BASED
} coap_cache_session_based_t;

typedef enum coap_cache_record_pdu_t {
  COAP_CACHE_NOT_RECORD_PDU,
  COAP_CACHE_RECORD_PDU
} coap_cache_record_pdu_t;
----

FUNCTIONS
---------

*Function: coap_cache_derive_key()*

The *coap_cache_derive_key*() function abstracts all the non NoCacheKey CoAP
options, ignores the CoAP Observe option and includes a FETCH body from _pdu_.
If _session_based_ is COAP_CACHE_IS_SESSION_BASED, then _session_ pointer is
also included. CoAP options can be specifically ignored by the use of
*coap_cache_ignore_options*().  A digest is then built from all of the
information and returned. NULL is returned on error.

*Function: coap_cache_derive_key_w_ignore()*

The *coap_cache_derive_key_w_ignore*() function abstracts all the non
NoCacheKey CoAP options, ignores the CoAP Observe option and includes a FETCH
body from _pdu_. Further options to ignore are specified by the _ignore_count_
of _ignore_options_.  If _session_based_ is COAP_CACHE_IS_SESSION_BASED, then
_session_ pointer is also included. A digest is then built from all of the
information and returned. NULL is returned on error.

*Function: coap_delete_cache_key()*

The *coap_delete_cache_key*() function deletes the _cache_key_ that was
returned from a *coap_cache_derive_key*() or
*coap_cache_derive_key_w_ignore*() call.

*Function: coap_cache_ignore_options()*

The *coap_cache_ignore_options*() function is used to store in _context_ a
list of _count_ options held in _options_.  The specified _options_ will not
be included in the data used for the *coap_cache_derive_key*() function.

*Function: coap_new_cache_entry()*

The *coap_new_cache_entry*() function will create a new Cache Entry based on
the Cache Key derived from the _pdu_, _session_based_ and _session_. If
_record_pdu_ is COAP_CACHE_RECORD_PDU, then a copy of the _pdu_ is stored in
the Cache Entry for subsequent retrieval. The Cache Entry can also store
application specific data (*coap_cache_set_app_data*() and
*coap_cache_get_app_data*()).  _idle_timeout_ in seconds defines the length of
time not being used before it gets deleted.  If _idle_timeout_ is set to
0, then the Cache Entry will not get idle expired. The created Cache
Entry is returned, or NULL on error.

*Function: coap_delete_cache_entry()*

The *coap_delete_cache_entry*() function can be used to delete the Cache Entry
_cache_entry_ held within _context_.  This will remove the Cache Entry from
the hash lookup list and
free off any internally held data.  If the Cache Entry is session based, then
it will automatically get deleted when the session is freed off or when the
idle timeout expires.

*Function: coap_cache_get_by_key()*

The *coap_cache_get_by_key*() function will locate the Cache Entry held in the
_context_ environment that has Cache Key _cache_key_.  Returns NULL if the
Cache Key was not found.

*Function: coap_cache_get_by_pdu()*

The *coap_cache_get_by_pdu*() function will locate the Cache Entry held in the
_session_ environment that has a Cache Key derived from the _pdu_ and
whether _session_based_ or not. This function calls *coap_cache_derive_key*()
internally, and so normally *coap_cache_ignore_options*() would have
previously been called with COAP_OPTION_BLOCK1 or COAP_OPTION_BLOCK2 to
ignore the values held within these options.

*Function: coap_cache_get_pdu()*

The *coap_cache_get_pdu*() function returns the PDU that was stored with the
Cache Entry when it was created with *coap_new_cache_entry*() and _record_pdu_
was set to COAP_CACHE_RECORD_PDU.  If a PDU was not initially stored, NULL is
returned. +
*NOTE:* A copy of the returned PDU must be taken for use in sending a CoAP
packet using *coap_pdu_duplicate*().

*Function: coap_cache_set_app_data()*

The *coap_cache_set_app_data*() function is used to associate _data_ with the
_cache_entry_.  If _callback_ is not NULL, it points to a function to free off
_data_ when the _cache_entry_ is deleted.  If any data has been previously
stored in the _cache_entry_, the pointer to the old data will get overwritten,
but the old data will not get freed off.

The _callback_ handler function prototype is defined as:
[source, c]
----
typedef void (*coap_cache_app_data_free_callback_t)(void *data);
----
where _data_ is passed into the callback function whenever the Cache Entry is
deleted.

*Function: coap_cache_get_app_data()*

The *coap_cache_get_app_data*() function is used to get the previously stored
_data_ in the _cache_entry_.

RETURN VALUES
-------------
*coap_cache_derive_key*() and *coap_cache_derive_key_w_ignore*()
returns a newly created Cache Key or NULL if there is a creation failure.

*coap_cache_ignore_options*() returns 1 if success, 0 on failure.

*coap_new_cache_entry*(), *coap_cache_get_by_key*() and
*coap_cache_get_by_pdu*() return the Cache Entry or NULL if there
is a failure.

*coap_cache_get_pdu*() returns the PDU that is held within the Cache Entry or
NULL if there is no PDU available.

*coap_cache_get_app_data*() returns the application data value
previously set by the *coap_cache_set_app_data*() function or NULL.

EXAMPLES
--------
*PUT Handler supporting BLOCK1*

[source, c]
----
#include <coap@LIBCOAP_API_VERSION@/coap.h>

static coap_binary_t *example_data_ptr = NULL;
static int example_data_media_type = COAP_MEDIATYPE_TEXT_PLAIN;

static void
cache_free_app_data(void *data) {
  coap_binary_t *bdata = (coap_binary_t*)data;
  coap_delete_binary(bdata);
}

/*
 * Large Data PUT handler
 */

static void
hnd_put_example_data(coap_context_t *ctx,
        coap_resource_t *resource,
        coap_session_t *session,
        coap_pdu_t *request,
        coap_binary_t *token,
        coap_string_t *query,
        coap_pdu_t *response
) {
  size_t size;
  const uint8_t *data;
  coap_opt_iterator_t opt_iter;
  coap_opt_t *option;
  size_t offset;
  size_t total;
  coap_binary_t *data_so_far;

  /* Remove (void) definition if variable is used */
  (void)ctx;
  (void)token;
  (void)query;

  if (coap_get_data_large(request, &size, &data, &offset, &total) &&
    size != total) {
    /*
     * A part of the data has been received (COAP_BLOCK_SINGLE_BODY not set).
     * However, total unfortunately is only an indication, so it is not safe to
     * allocate a block based on total.  As per
     * https://rfc-editor.org/rfc/rfc7959#section-4
     *   o  In a request carrying a Block1 Option, to indicate the current
     *         estimate the client has of the total size of the resource
     *         representation, measured in bytes ("size indication").
     *
     * coap_cache_ignore_options() must have previously been called with at
     * least COAP_OPTION_BLOCK1 set as the option value will change per block.
     */
    coap_cache_entry_t *cache_entry = coap_cache_get_by_pdu(session,
                                                            request,
                                              COAP_CACHE_IS_SESSION_BASED);

    if (offset == 0) {
      if (!cache_entry) {
        /*
         * Set idle_timeout parameter to COAP_MAX_TRANSMIT_WAIT if you want
         * early removal on transmission failure. 0 means only delete when
         * the session is deleted as session_based is set here.
         */
        cache_entry = coap_new_cache_entry(session, request,
                                         COAP_CACHE_NOT_RECORD_PDU,
                                         COAP_CACHE_IS_SESSION_BASED, 0);
      }
      else {
        data_so_far = coap_cache_get_app_data(cache_entry);
        if (data_so_far) {
          coap_delete_binary(data_so_far);
          data_so_far = NULL;
        }
        coap_cache_set_app_data(cache_entry, NULL, NULL);
      }
    }
    if (!cache_entry) {
      if (offset == 0) {
        coap_log_warn("Unable to create a new cache entry\n");
      }
      else {
        coap_log_warn(
                 "No cache entry available for the non-first BLOCK\n");
      }
      coap_pdu_set_code(response, COAP_RESPONSE_CODE_INTERNAL_ERROR);
      return;
    }

    if (size) {
      /* Add in the new data to cache entry */
      data_so_far = coap_cache_get_app_data(cache_entry);
      data_so_far = coap_block_build_body(data_so_far, size, data,
                                          offset, total);
      /* Yes, data_so_far can be NULL if error */
      coap_cache_set_app_data(cache_entry, data_so_far, cache_free_app_data);
    }
    if (offset + size == total) {
      /* All the data is now in */
      data_so_far = coap_cache_get_app_data(cache_entry);
      coap_cache_set_app_data(cache_entry, NULL, NULL);
    }
    else {
      /* Give us the next block response */
      coap_pdu_set_code(response, COAP_RESPONSE_CODE_CONTINUE);
      return;
    }
  }
  else {
    /* single body of data received */
    data_so_far = coap_new_binary(size);
    if (data_so_far) {
      memcpy(data_so_far->s, data, size);
    }
  }

  if (example_data_ptr) {
    /* pre-existed response */
    coap_pdu_set_code(response, COAP_RESPONSE_CODE_CHANGED);
    coap_delete_binary(example_data_ptr);
  }
  else
    /* just generated response */
    coap_pdu_set_code(response, COAP_RESPONSE_CODE_CREATED);

  example_data_ptr = data_so_far;
  if ((option = coap_check_option(request, COAP_OPTION_CONTENT_FORMAT,
                                  &opt_iter)) != NULL) {
    example_data_media_type =
            coap_decode_var_bytes (coap_opt_value (option),
                                   coap_opt_length (option));
  }
  else {
    example_data_media_type = COAP_MEDIATYPE_TEXT_PLAIN;
  }

  coap_pdu_set_code(response, COAP_RESPONSE_CODE_CHANGED);
  coap_resource_notify_observers(resource, NULL);
}

int
main(int argc, char* argv[]) {
  coap_context_t *ctx = NULL;  /* Set up as normal */
  /* ... */
  uint16_t cache_ignore_options[] = { COAP_OPTION_BLOCK1,
                                      COAP_OPTION_BLOCK2 };

  /* Initialize libcoap library */
  coap_startup();

  /* Remove (void) definition if variable is used */
  (void)argc;
  (void)argv;

  /* ... */

  /** Define the options to ignore when setting up cache-keys */
  coap_cache_ignore_options(ctx, cache_ignore_options,
             sizeof(cache_ignore_options)/sizeof(cache_ignore_options[0]));

  /* ... */
  coap_cleanup();

}
----

SEE ALSO
--------
*coap_block*(3), *coap_init*(3), *coap_pdu_setup*(3), *coap_resource*(3)
and *coap_string*(3)

FURTHER INFORMATION
-------------------
See

"https://rfc-editor.org/rfc/rfc7252[RFC7252: The Constrained Application Protocol (CoAP)]"

"https://rfc-editor.org/rfc/rfc7959[RFC7959: Block-Wise Transfers in the Constrained Application Protocol (CoAP)]"

for further information.

BUGS
----
Please report bugs on the mailing list for libcoap:
libcoap-developers@lists.sourceforge.net or raise an issue on GitHub at
https://github.com/obgm/libcoap/issues

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