Unified Cache Interface

The unified cache interface (cache_req) is an integral component of the SSSD data flow, consumed by most SSSD responders including NSS, PAM, InfoPipe, autofs and ssh.

The cache req component implements a generic interface for SSSD requests. The interface is responsible for handling the logic for domain selection, cache lookups, and sending requests to the backend. cache_req implements a plugin architecture where a plugin exists for each object request type. The plugin dictates the behavior of cache request, which attributes should be used to lookup the object in the cache, etc. Each request follows similar logic through the cache request interface, making it easier to understand and debug.

graph LR app(Application) gnss(glibc-NSS) snss(sssd_nss) cache_req(cache_req) lcache(local cache) be(backend) style gnss fill:green,stroke:green,stroke-width:1px,fill-opacity:0.2 style snss fill:green,stroke:green,stroke-width:1px,fill-opacity:0.2 style cache_req fill:blue,stroke:blue,stroke-width:1px,fill-opacity:0.2 style lcache fill:yellow,stroke:yellow,stroke-width:1px,fill-opacity:0.2 style be fill:purple,stroke:purple,stroke-width:1px,fill-opacity:0.2 app --> gnss gnss --> snss snss --> cache_req cache_req --> lcache lcache <--> be be --> lcache

Initiating a cache request is done by calling the tevent request input function cache_req_send

struct tevent_req *cache_req_send(TALLOC_CTX *mem_ctx,
                                  struct tevent_context *ev,
                                  struct resp_ctx *rctx,
                                  struct sss_nc_ctx *ncache,
                                  int midpoint,
                                  enum cache_req_dom_type req_dom_type,
                                  const char *domain,
                                  struct cache_req_data *data);
midpoint

Handle background (out of band) object refreshes. It is used to enable the entry_cache_nowait_percentage configuration option in the NSS responder. Set this argument to 0 for non-NSS responder callers.

req_dom_type

Limit the request type lookups to only POSIX Domains (CACHE_REQ_POSIX_DOM), only CACHE_REQ_APPLICATION_DOM or any domain type (CACHE_REQ_ANY_DOM). POSIX domains are reachable by all services. Application domains are only reachable from the InfoPipe responder and the PAM responder. Only objects from POSIX domains are available to the operating system interfaces (NSS) and utilities.

domain

Request searching an explicit domain, NULL otherwise. When domain is NULL we delegate the logic of domain selection to cache req.

data

Must be a struct cache_req_data formatted data. Several helper functions exist to create this cache_req_data (See cache_req_data_* in cache_req.h) based on the type of input data.

Retrieving the output from the cache request is done with either of the two _recv functions:

errno_t cache_req_recv(TALLOC_CTX *mem_ctx,
                       struct tevent_req *req,
                       struct cache_req_result ***_results);

errno_t cache_req_single_domain_recv(TALLOC_CTX *mem_ctx,
                       struct tevent_req *req,
                       struct cache_req_result **_result);

You may want to only skip searching the local cache, or the data provider. The public functions in cache_req.h can be used to tune the behavior of the request lookups. See lookup_behavior

void
cache_req_data_set_bypass_cache(struct cache_req_data *data,
                                bool bypass_cache)

void
cache_req_data_set_bypass_dp(struct cache_req_data *data,
                             bool bypass_dp)

void
cache_req_data_set_requested_domains(struct cache_req_data *data,
                                     char **requested_domains)

A simple caller example (user initgroups request) may look as follows:

/* Prepare the input data */
data = cache_req_data_name(mem_ctx, CACHE_REQ_INITGROUPS, user_name);

/* Optional settings */
cache_req_data_set_bypass_cache(data, false);
cache_req_data_set_bypass_dp(data, true);
cache_req_data_set_requested_domains(data, requested_domains);

/* Initiate the request */
dpreq = cache_req_send(mem_ctx,
                       ev,
                       rctx,
                       rctx->ncache,
                       0,                  /* Disable midpoint refresh */
                       CACHE_REQ_ANY_DOM,  /* Don't limit domain type */
                       NULL,               /* No explicit domain */
                       data);

/* Set callback function */
tevent_req_set_callback(dpreq, sample_user_initgr_done, req);

And inside the callback function:

static void sample_user_initgr_done(struct tevent_req *subreq)
{
    struct cache_req_result *result;
    ...  /* Other vars */

    req = tevent_req_callback_data(subreq, struct tevent_req);
    state = tevent_req_data(req, struct sample_user_initgr_state);

    ret = cache_req_single_domain_recv(state, subreq, &result);
    talloc_zfree(subreq);
    if (ret != EOK) {
        tevent_req_error(req, ret);
        return;
    }

    /* Do something with result */

This section is used to describe the cache request interface details, and explain what happens behind the scenes.

Inside cache request code (cache_req_send, cache_req_create, and cache_req_process_input), setup and initialization of cache request plugins, input data and necessary cache request structures is performed.

Cache request lookup behavior is set here to one of the following 4 modes:

enum cache_req_behavior {
    CACHE_REQ_NORMAL,
    CACHE_REQ_CACHE_FIRST,
    CACHE_REQ_BYPASS_CACHE,
    CACHE_REQ_BYPASS_PROVIDER,
};
CACHE_REQ_NORMAL

Default lookup behavior, described in the SSSD Architecture document.

CACHE_REQ_CACHE_FIRST

SOn first iteration, the search will only check the cache and will not trigger any request to the data provider/backend. If the requested data is not found then a second lookup will search only in the data provider, not the cache. Set based on the sssd.conf cache_first option.

CACHE_REQ_BYPASS_CACHE

Always contact the data provider before searching the cache. Set with a call to cache_req_data_set_bypass_cache,

CACHE_REQ_BYPASS_DP

Always search in the local cache and do not perform a lookup to the data provider. Set with a call to cache_req_data_set_bypass_dp

Note

CACHE_REQ_BYPASS_CACHE does not bypass the cache entirely. Data returned from the data provider is always first added into the cache then a local cache lookup will return the refreshed data. It may help to think of bypass cache as dp_first instead.

All cache request plugins explicitly set the parse_name and ignore_default_domain structure members booleans.

const struct cache_req_plugin cache_req_user_by_name = {
    .name = "User by name",
    ...
    .parse_name = true,
    .ignore_default_domain = false,
    ...

Cache request plugins which accept a name as input will often set the parse_name boolean to true. The cache request logic will then assume an input name may contain a domain name which needs to be parsed. This domain name parsing also factors in the ignore_default_domain plugin boolean, determining if the cache request will also append the default domain when no domain component is found during parsing. When parse_name is false, the input type does not need to be parsed and cache request can assume the input as-is.

An input name may contain a name@UPN, instead of the typical name@domain. cache_req can automatically detect this when parsing an input name where no matching domain is found.

const struct cache_req_plugin cache_req_user_by_name = {
    .name = "User by name",
    ...
    .allow_switch_to_upn = true,
    .upn_equivalent = CACHE_REQ_USER_BY_UPN,
    ...

If the plugin sets allow_switch_to_upn then the plugin upn_equivalent is set with cache_req_set_plugin

As mentioned in How to use the cache_req API, the caller of cache_req_send() can specify a domain argument to tell cache request to perform a single-domain only search.

The function cache_req_data_set_requested_domains can be called, providing a list of domain names. This will cause cache_req to search for the object only in those requested domains.

If no domain argument is provided to cache_req_send, then a multi-domain search is executed. Here again we take cache_req plugin variables into consideration.

const struct cache_req_plugin cache_req_group_by_id = {
    ...
    .allow_missing_fqn = true,
    .get_next_domain_flags = SSS_GND_DESCEND,
    ...

If allow_missing_fqn is set to false, then this multi domain search iterating through domains will skip domains which require fully qualified names.

get_next_domain_flags sets the flags which are passed to get_next_domain() during iteration. At the time of this writing, cache request plugins set this to 0 or SSS_GND_DESCEND to determine behavior when iterating through multiple domains.

Domain Flags

Behavior

0

Skip subdomains in iteration

SSS_GND_DESCEND

Include subdomains in iteration

Before performing the object search inside the domain, the prepare_domain_data_fn is called, this is used by some plugins to alter lookup data per specific domain rules, such as case sensitivity, fully qualified format, etc.

The domain locator plugin exists to alleviate performance problems when SSSD must iterate over several domains (e.g. AD forest with multiple domains) for each unqualified name or by-ID lookups. The domain locator can help to find the correct domain to search early on in the lookup flow, instead of iterating through all domains.

The domain locator functionality is set currently only for CACHE_REQ_OBJECT_BY_ID, CACHE_REQ_GROUP_BY_ID, CACHE_REQ_USER_BY_ID plugins.

const struct cache_req_plugin cache_req_object_by_id = {
    ...
    .dp_get_domain_check_fn = cache_req_object_by_id_get_domain_check,
    .dp_get_domain_send_fn = cache_req_object_by_id_get_domain_send,
    .dp_get_domain_recv_fn = cache_req_common_get_acct_domain_recv,

A cache-only search is performed first to check if the object already exists in the cache. If not, then we execute the domain locator plugin dp_get_domain_check_fn function to check if the id exists in the negative cache. If it is not in the negative cache, then we call the dp_get_domain_send_fn

The send function sends an SBUS getAccountDomain request to the backend, if a domain is reported as containing an object, all domains except that one are marked with negative cache entries for that request(using the associated .ncache_add_fn plugin function.

The domain locator plugin code is only executed for unqualified requests with multiple domains or on the second pass of a CACHE_REQ_CACHE_FIRST lookup.

if (cr->plugin->dp_get_domain_send_fn != NULL
        && ((state->check_next && cr_domain->next != NULL)
            || ((state->cr->cache_behavior == CACHE_REQ_CACHE_FIRST)
                && !first_iteration))) {
    /* If the request is not qualified with a domain name AND
     * there are multiple domains to search OR if this is the second
     * pass during the "check-cache-first" schema, it makes sense
     * to try to run the domain-locator plugin
     */
    cache_req_domain_set_locate_flag(cr_domain, cr);
}
../_images/architecture-lookup.svg

In a multi-domain search, the below logic repeats for each domain until a result is found, or SSSD searched all available (or requested) domains. Let’s assume the normal cache behavior CACHE_REQ_NORMAL, most of the conditional logic in the search code flow is based on the cache behavior type.

  • First the search checks the negative cache using the plugin ncache_check_fn function. If the object exists in the negative cache, and the negative cache timeout has not been reached then cache_req will return ENOENT.

.ncache_check_fn = cache_req_user_by_name_ncache_check,
  • Next, search the cache to see if the object already exists in the local cache. Here, the plugin-defined lookup_fn is used to handle the different object types and SYSDB attributes to search. If the object is found and is not expired, it can be returned successfully.

  • If the object is expired, or not found then the plugin dp_send_fn and dp_recv_fn are used inside cache_req_search_dp to trigger a backend search. This backend search will update the cache, then cache_req_search_done() searches the cache again for the now-existing object.

Warning

If multiple objects are found when the plugin only_one_result is set to true then ERR_MULTIPLE_ENTRIES is returned.

Note

When results are found in a multiple domain search, if the plugin search_all_domains is true then cache_req continues to search all domains and merges the results.

A cache request contains an ID in cr, used for debugging and allows anyone viewing SSSD logs to follow along a certain request through the life of the cache request. This id is an unsigned integer which increments by 1 for each new request.

struct cache_req {
...

/* Debug information */
uint32_t reqid;

Due to the importance of cache_req in the SSSD data flow, cache_req will need to be extended going forward to handle additional SSSD use cases.

This may include adding new code paths into cache_req processing logic, see the domain locator feature addition in Use the domain-locator request to only search domains where the entry was found and Add plugin methods required for the domain-locator request

Plugins can be added to expand the functionality of cache_req, for example the commit add autofs map entries plugin adds the CACHE_REQ_AUTOFS_MAP_ENTRIES plugin.

When adding a new plugin you will need to decide which object-specific plugin operations need to be included.

/* Operations */
cache_req_is_well_known_result_fn is_well_known_fn;
cache_req_prepare_domain_data_fn prepare_domain_data_fn;
cache_req_create_debug_name_fn create_debug_name_fn;
cache_req_global_ncache_add_fn global_ncache_add_fn;
cache_req_ncache_check_fn ncache_check_fn;
cache_req_ncache_add_fn ncache_add_fn;
cache_req_ncache_filter_fn ncache_filter_fn;
cache_req_lookup_fn lookup_fn;
cache_req_dp_send_fn dp_send_fn;
cache_req_dp_recv_fn dp_recv_fn;
cache_req_dp_get_domain_check_fn dp_get_domain_check_fn;
cache_req_dp_get_domain_send_fn dp_get_domain_send_fn;
cache_req_dp_get_domain_recv_fn dp_get_domain_recv_fn;

At a minimum, you will need to add custom functions into src/responder/common/cache_req/plugins/cache_req_new_plugin.c for the below operations. The naming scheme below should also be used:

/* Search for object entries in local cache: lookup_fn */
cache_req_new_plugin_lookup()

/* Search for, and retrieve object in the backend: dp_send_fn, dp_recv_fn */
cache_req_new_plugin_dp_send()
cache_req_new_plugin_dp_recv()

/* Create debug name for object */
cache_req_new_plugin_create_debug_name()

Then you will need to consider if you need to do some domain preparation prior to lookups and define a prepare_domain_data_fn, or add negative cache custom functions in ncache_check_fn and ncache_add_fn if the object type supports negative cache entries.

Next you need to decide how your plugin will handle certain cases, these switches have been discussed earlier in this article, but you can see a basic description in src/responder/common/cache_req/cache_req_plugin.h

.name = "Group by name",
.attr_expiration = SYSDB_CACHE_EXPIRE,
.parse_name = true,
.ignore_default_domain = false,
.bypass_cache = false,
.only_one_result = true,
.search_all_domains = false,
.require_enumeration = false,
.allow_missing_fqn = false,
.allow_switch_to_upn = false,
.upn_equivalent = CACHE_REQ_SENTINEL,
.get_next_domain_flags = SSS_GND_DESCEND,