16. The Proxy Cache Engine

LDAP servers typically hold one or more subtrees of a DIT. Replica (or shadow) servers hold shadow copies of entries held by one or more master servers. Changes are propagated from the master server to replica (slave) servers using LDAP Sync or slurpd(8). An LDAP cache is a special type of replica which holds entries corresponding to search filters instead of subtrees.

16.1. Overview

The proxy cache extension of slapd is designed to improve the responseiveness of the ldap and meta backends. It handles a search request (query) by first determining whether it is contained in any cached search filter. Contained requests are answered from the proxy cache's local database. Other requests are passed on to the underlying ldap or meta backend and processed as usual.

E.g. (shoesize>=9) is contained in (shoesize>=8) and (sn=Richardson) is contained in (sn=Richards*)

Correct matching rules and syntaxes are used while comparing assertions for query containment. To simplify the query containment problem, a list of cacheable "templates" (defined below) is specified at configuration time. A query is cached or answered only if it belongs to one of these templates. The entries corresponding to cached queries are stored in the proxy cache local database while its associated meta information (filter, scope, base, attributes) is stored in main memory.

A template is a prototype for generating LDAP search requests. Templates are described by a prototype search filter and a list of attributes which are required in queries generated from the template. The representation for prototype filter is similar to RFC 2254, except that the assertion values are missing. Examples of prototype filters are: (sn=),(&(sn=)(givenname=)) which are instantiated by search filters (sn=Doe) and (&(sn=Doe)(givenname=John)) respectively.

The cache replacement policy removes the least recently used (LRU) query and entries belonging to only that query. Queries are allowed a maximum time to live (TTL) in the cache thus providing weak consistency. A background task periodically checks the cache for expired queries and removes them.

The Proxy Cache paper (http://www.openldap.org/pub/kapurva/proxycaching.pdf) provides design and implementation details.

16.2. Proxy Cache Configuration

The cache configuration specific directives described below must appear after a overlay proxycache directive within a "database meta" or database ldap section of the server's slapd.conf(5) file.

16.2.1. Setting cache parameters

 proxyCache <DB> <maxentries> <nattrsets> <entrylimit> <period>

This directive enables proxy caching and sets general cache parameters. The <DB> parameter specifies which underlying database is to be used to hold cached entries. It should be set to bdb, hdb, or ldbm. The <maxentries> parameter specifies the total number of entries which may be held in the cache. The <nattrsets> parameter specifies the total number of attribute sets (as specified by the proxyAttrSet directive) that may be defined. The <entrylimit> parameter specifies the maximum number of entries in a cachable query. The <period> specifies the consistency check period (in seconds). In each period, queries with expired TTLs are removed.

16.2.2. Defining attribute sets

 proxyAttrset <index> <attrs...>

Used to associate a set of attributes to an index. Each attribute set is associated with an index number from 0 to <numattrsets>-1. These indices are used by the proxyTemplate directive to define cacheable templates.

16.2.3. Specifying cacheable templates

 proxyTemplate <prototype_string> <attrset_index> <TTL>

Specifies a cacheable template and the "time to live" (in sec) <TTL> for queries belonging to the template. A template is described by its prototype filter string and set of required attributes identified by <attrset_index>.

16.2.4. Example

An example slapd.conf(5) database section for a caching server which proxies for the "dc=example,dc=com" subtree held at server ldap.example.com.

        database        ldap
        suffix          "dc=example,dc=com"
        rootdn          "dc=example,dc=com"
        uri             ldap://ldap.example.com/dc=example%2cdc=com
        overlay proxycache
        proxycache    bdb 100000 1 1000 100
        proxyAttrset  0 mail postaladdress telephonenumber
        proxyTemplate (sn=) 0 3600
        proxyTemplate (&(sn=)(givenName=)) 0 3600
        proxyTemplate (&(departmentNumber=)(secretary=*)) 0 3600

        cachesize 20
        directory ./testrun/db.2.a
        index       objectClass eq
        index       cn,sn,uid,mail  pres,eq,sub Cacheable Queries

A LDAP search query is cacheable when its filter matches one of the templates as defined in the "proxyTemplate" statements and when it references only the attributes specified in the corresponding attribute set. In the example above the attribute set number 0 defines that only the attributes: mail postaladdress telephonenumber are cached for the following proxyTemplates. Examples:

        Filter: (&(sn=Richard*)(givenName=jack))
        Attrs: mail telephoneNumber

is cacheable, because it matches the template (&(sn=)(givenName=)) and its attributes are contained in proxyAttrset 0.

        Filter: (&(sn=Richard*)(telephoneNumber))
        Attrs: givenName

is not cacheable, because the filter does not match the template, nor is the attribute givenName stored in the cache

        Filter: (|(sn=Richard*)(givenName=jack))
        Attrs: mail telephoneNumber

is not cacheable, because the filter does not match the template ( logical OR "|" condition instead of logical AND "&" )