slapo-pcache

Sommaire

NAME

slapo-pcache - proxycache overlay to slapd

SYNOPSIS

/etc/ldap/slapd.conf

DESCRIPTION

The pcache overlay to slapd(8) allows caching of LDAP search requests (queries) in a local database. For an incoming query, the proxy cache determines its corresponding template. If the template was specified as cacheable using the proxytemplate directive and the request is contained in a cached request, it is answered from the proxy cache. Otherwise, the search is performed as usual and cacheable search results are saved in the cache for use in future queries.

A template is defined by a filter string and an index identifying a set of attributes. The template string for a query can be obtained by removing assertion values from the RFC 4515 representation of its search filter. A query belongs to a template if its template string and set of projected attributes correspond to a cacheable template. Examples of template strings are (mail=), (|(sn=)(cn=)), (&(sn=)(givenName=)).

The config directives that are specific to the proxycache overlay can be prefixed by proxycache-, to avoid conflicts with directives specific to the underlying database or to other stacked overlays. This may be particularly useful for those directives that refer to the backend used for local storage. The following cache specific directives can be used to configure the proxy cache:

overlay pcache
This directive adds the proxy cache overlay to the current backend. The proxy cache overlay may be used with any backend but is intended for use with the ldap, meta, and sql backends.
proxycache <database> <max_entries> <numattrsets> <entry_limit> <cc_period>
The directive enables proxy caching in the current backend and sets general cache parameters. A <database> backend will be used internally to maintain the cached entries. The chosen database will need to be configured as well, as shown below. Cache replacement is invoked when the cache size grows to <max_entries> entries and continues till the cache size drops below this size. <numattrsets> should be equal to the number of following proxyattrset directives. Queries are cached only if they correspond to a cacheable template (specified by the proxytemplate directive) and the number of entries returned is less than <entry_limit>. Consistency check is performed every <cc_period> duration (specified in secs). In each cycle queries with expired "time to live(TTL)" are removed. A sample cache configuration is:
proxycache bdb 10000 1 50 100
proxyattrset <index> <attrs...>
Used to associate a set of attributes <attrs..> with an <index>. Each attribute set is associated with an integer from 0 to <numattrsets>-1. These indices are used by the proxytemplate directive to define cacheable templates. A set of attributes cannot be empty. A set of attributes can contain the special attributes "*" (all user attributes), "+" (all operational attributes) or both; in the latter case, any other attribute is redundant and should be avoided for clarity. A set of attributes can contain "1.1" as the only attribute; in this case, only the presence of the entries is cached.
proxycachequeries <queries>
Specify the maximum number of queries to cache. The default is 10000.
proxycheckcacheability { TRUE | FALSE }
Check whether the results of a query being cached can actually be returned from the cache by the proxy DSA. When enabled, the entries being returned while caching the results of a query are checked to ensure consistency with the schema known to the proxy DSA. In case of failure, the query is not cached. By default, the check is off.
proxysavequeries { TRUE | FALSE }
Specify whether the cached queries should be saved across restarts of the caching proxy, to provide hot startup of the cache. Only non-expired queries are reloaded. The default is FALSE.

CAVEAT: of course, the configuration of the proxycache must not change across restarts; the pcache overlay does not perform any consistency checks in this sense. In detail, this option should be disabled unless the existing proxyattrset and proxytemplate directives are not changed neither in order nor in contents. If new sets and templates are added, or if other details of the pcache overlay configuration changed, this feature should not be affected.

proxytemplate <template_string> <attrset_index> <ttl> [<negttl> [<limitttl>]]
Specifies a cacheable template and "time to live" <ttl> of queries belonging to the template. An optional <negttl> can be used to specify that negative results (i.e., queries that returned zero entries) should also be cached for the specified amount of time. Negative results are not cached by default (<negttl> set to 0). An optional <limitttl> can be used to specify that results hitting a sizelimit should also be cached for the specified amount of time. Results hitting a sizelimit are not cached by default (<limitttl> set to 0).
response-callback { head | tail }
Specifies whether the response callback should be placed at the tail (the default) or at the head (actually, wherever the stacking sequence would make it appear) of the callback list. This affects how the overlay interacts with other overlays, since the proxycache overlay should be executed as early as possible (and thus configured as late as possible), to get a chance to return the cached results; however, if executed early at response, it would cache entries that may be later "massaged" by other databases and thus returned after massaging the first time, and before massaging when cached.
There are some constraints:

all values must be positive;

<entry_limit> must be less than or equal to <max_entries>;

<numattrsets> attribute sets SHOULD be defined by using the directive proxyattrset;

all attribute sets SHOULD be referenced by (at least) one proxytemplate directive;

The following adds a template with filter string (&(sn=)(givenName=)) and attributes mail, postaladdress, telephonenumber and a TTL of 1 hour.

 proxyattrset 0 mail postaladdress telephonenumber
 proxytemplate (&(sn=)(givenName=)) 0 3600

Directives for configuring the underlying database must also be given, as shown here:

 directory /var/tmp/cache
 cachesize 100

Any valid directives for the chosen database type may be used. Indexing should be used as appropriate for the queries being handled. In addition, an equality index on the queryid attribute should be configured, to assist in the removal of expired query data.

CAVEATS

Caching data is prone to inconsistencies because updates on the remote server will not be reflected in the response of the cache at least (and at most) for the duration of the proxytemplate TTL.

The remote server should expose the objectClass attribute because the underlying database that actually caches the entries may need it for optimal local processing of the queries.

The proxy server should contain all the schema information required for caching. Significantly, it needs the schema of attributes used in the query templates. If the objectClass attribute is used in a query template, it needs the definition of the objectClasses of the entries it is supposed to cache. It is the responsibility of the proxy administrator to keep the proxy schema lined up with that of the proxied server.

Another potential (and subtle) inconsistency may occur when data is retrieved with different identities and specific per-identity access control is enforced by the remote server. If data was retrieved with an identity that collected only partial results because of access rules enforcement on the remote server, other users with different access privileges on the remote server will get different results from the remote server and from the cache. If those users have higher access privileges on the remote server, they will get from the cache only a subset of the results they would get directly from the remote server; but if they have lower access privileges, they will get from the cache a superset of the results they would get directly from the remote server. Either occurrence may or may not be acceptable, based on the security policy of the cache and of the remote server. It is important to note that in this case the proxy is violating the security of the remote server by disclosing to an identity data that was collected by another identity. For this reason, it is suggested that, when using back-ldap, proxy caching be used in conjunction with the identity assertion feature of slapd-ldap(5) (see the idassert-bind and the idassert-authz statements), so that remote server interrogation occurs with a vanilla identity that has some relatively high search and read access privileges, and the "real" access control is delegated to the proxy's ACLs. Beware that since only the cached fraction of the real datum is available to the cache, it may not be possible to enforce the same access rules that are defined on the remote server. When security is a concern, cached proxy access must be carefully tailored.

FILES

/etc/ldap/slapd.conf
default slapd configuration file

SEE ALSO

slapd.conf(5), slapd-config(5), slapd-ldap(5), slapd-meta(5), slapd-sql(5), slapd(8).

AUTHOR

Originally implemented by Apurva Kumar as an extension to back-meta; turned into an overlay by Howard Chu.