* Copyright (C) Internet Systems Consortium, Inc. ("ISC")
* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, you can obtain one at https://mozilla.org/MPL/2.0/.
* See the COPYRIGHT file distributed with this work for additional
* information regarding copyright ownership.
* The Additional Cache Object
* This module manages internal caching entries that correspond to
* the additional section data of a DNS DB node (an RRset header, more
* accurately). An additional cache entry is expected to be (somehow)
* attached to a particular RR in a particular DB node, and contains a set
* of information of an additional data for the DB node.
* An additional cache object is intended to be created as a per-view
* object, and manages all cache entries within the view.
* The intended usage of the additional caching is to provide a short cut
* to additional glue RRs of an NS RR. For each NS RR, it is often
* necessary to look for glue RRs to make a proper response. Once the
* glue RRs are known, the additional caching allows the client to
* associate the information to the original NS RR so that further
* expensive lookups can be avoided for the NS RR.
* Each additional cache entry contains information to identify a
* particular DB node and (optionally) an associated RRset. The
* information consists of its zone, database, the version of the
* database, database node, and RRset.
* A "negative" information can also be cached. For example, if a glue
* RR does not exist as an authoritative data in the same zone as that
* of the NS RR, this fact can be cached by specifying a NULL pointer
* for the database, version, and node. (See the description for
* dns_acache_getentry() below for more details.)
* Since each member stored in an additional cache entry holds a reference
* to a corresponding object, a stale cache entry may cause unnecessary
* memory consumption. For instance, when a zone is reloaded, additional
* cache entries that have a reference to the zone (and its DB and/or
* DB nodes) can delay the cleanup of the referred objects. In order to
* minimize such a bad effect, this module provides several cleanup
* The first one is a shutdown procedure called when the associated view
* is shut down. In this case, dns_acache_shutdown() will be called and
* all cache entries will be purged. This mechanism will help the
* situation when the configuration is reloaded or the main server is
* Per-DB cleanup mechanism is also provided. Each additional cache entry
* is associated with related DB, which is expected to have been
* registered when the DB was created by dns_acache_setdb(). If a
* particular DB is going to be destroyed, the primary holder of the DB,
* a typical example of which is a zone, will call dns_acache_putdb().
* Then this module will clean-up all cache entries associated with the
* DB. This mechanism is effective when a secondary zone DB is going to
* be stale after a zone transfer.
* Finally, this module supports for periodic clean-up of stale entries.
* Each cache entry has a timestamp field, which is updated every time
* the entry is referred. A periodically invoked cleaner checks the
* timestamp of each entry, and purge entries that have not been referred
* for a certain period. The cleaner interval can be specified by
* dns_acache_setcleaninginterval(). If the periodic clean-up is not
* enough, it is also possible to specify the upper limit of entries
* in terms of the memory consumption. If the maximum value is
* specified, the cleaner is invoked when the memory consumption reaches
* the high watermark inferred from the maximum value. In this case,
* the cleaner will use more aggressive algorithm to decide the "victim"
* entries. The maximum value can be specified by
* dns_acache_setcachesize().
* When a cache entry is going to be purged within this module, the
* callback function specified at the creation time will be called.
* The callback function is expected to release all internal resources
* related to the entry, which will typically be specific to DB
* implementation, and to call dns_acache_detachentry(). The callback
* mechanism is very important, since the holder of an additional cache
* entry may not be able to initiate the clean-up of the entry, due to
* the reference ordering. For example, as long as an additional cache
* entry has a reference to a DB object, the DB cannot be freed, in which
* a DB node may have a reference to the cache entry.
* The basic idea of this kind of short-cut for frequently used
* information is similar to the "pre-compiled answer" approach adopted
* in nsd by NLnet LABS with RIPE NCC. Our work here is an independent
* effort, but the success of nsd encouraged us to pursue this path.
* The design and implementation of the periodic memory management and
* the upper limitation of memory consumption was derived from the cache
* DB implementation of BIND9.
* There are two main locks in this module. One is for each entry, and
* the other is for the additional cache object.
* The callback function for a cache entry is called with holding the
* entry lock. Thus, it implicitly assumes the callback function does not
* call a function that can require the lock. Typically, the only
* function that can be called from the callback function safely is
* dns_acache_detachentry(). The breakage of this implicit assumption
* In a 32-bit architecture (such as i386), the following additional
* memory is required comparing to the case that disables this module.
* - 76 bytes for each additional cache entry
* - if the entry has a DNS name and associated RRset,
* * 44 bytes + size of the name (1-255 bytes)
* * 52 bytes x number_of_RRs
* - 28 bytes for each DB related to this module
* Using the additional cache also requires extra memory consumption in
* the DB implementation. In the current implementation for rbtdb, we
* - two additional pointers for each DB node (8 bytes for a 32-bit
* - for each RR associated to an RR in a DB node, we also need
* a pointer and management objects to support the additional cache
* function. These are allocated on-demand. The total size is
* 32 bytes for a 32-bit architecture.
* Since this module does not handle any low-level data directly,
* no security issue specific to this module is anticipated.
dns_acache_create(dns_acache_t **acachep, isc_mem_t *mctx,
isc_taskmgr_t *taskmgr, isc_timermgr_t *timermgr);
* Create a new DNS additional cache object.
* 'mctx' is a valid memory context
* 'taskmgr' is a valid task manager
* 'timermgr' is a valid timer or NULL. If NULL, no periodic cleaning of
* the cache will take place.
* 'acachep' is a valid pointer, and *acachep == NULL
* '*acachep' is attached to the newly created cache
dns_acache_attach(dns_acache_t *source, dns_acache_t **targetp);
* Attach *targetp to cache.
* 'acache' is a valid additional cache.
* 'targetp' points to a NULL dns_acache_t *.
* *targetp is attached to the 'source' additional cache.
dns_acache_detach(dns_acache_t **acachep);
* Detach *acachep from its cache.
* '*acachep' points to a valid additional cache.
* If '*acachep' is the last reference to the cache and the additional
* cache does not have an outstanding task, all resources used by the
dns_acache_setcleaninginterval(dns_acache_t *acache, unsigned int t);
* Set the periodic cleaning interval of an additional cache to 'interval'
dns_acache_setcachesize(dns_acache_t *acache, size_t size);
* Set the maximum additional cache size. 0 means unlimited.
dns_acache_setdb(dns_acache_t *acache, dns_db_t *db);
* Set 'db' in 'acache' when the db can be referred from acache, in order
* to provide a hint for resolving the back reference.
* 'acache' is a valid acache pointer.
* 'db' is a valid DNS DB pointer.
* 'acache' will have a reference to 'db'.
* ISC_R_EXISTS (which means the specified 'db' is already set)
dns_acache_putdb(dns_acache_t *acache, dns_db_t *db);
* Release 'db' from 'acache' if it has been set by dns_acache_setdb().
* 'acache' is a valid acache pointer.
* 'db' is a valid DNS DB pointer.
* 'acache' will release the reference to 'db'. Additionally, the content
* of each cache entry that is related to the 'db' will be released via
* ISC_R_NOTFOUND (which means the specified 'db' is not set in 'acache')
dns_acache_shutdown(dns_acache_t *acache);
* '*acache' is a valid additional cache.
dns_acache_createentry(dns_acache_t *acache, dns_db_t *origdb,
void (*callback)(dns_acacheentry_t *, void **),
void *cbarg, dns_acacheentry_t **entryp);
* Create an additional cache entry. A new entry is created and attached to
* the given additional cache object. A callback function is also associated
* with the created entry, which will be called when the cache entry is purged
* 'acache' is a valid additional cache.
* 'entryp' is a valid pointer, and *entryp == NULL
* 'origdb' is a valid DNS DB pointer.
* 'callback' and 'cbarg' can be NULL. In this case, however, the entry
* is meaningless (and will be cleaned-up in the next periodical
* '*entryp' will point to a new additional cache entry.
dns_acache_getentry(dns_acacheentry_t *entry, dns_zone_t **zonep,
dns_db_t **dbp, dns_dbversion_t **versionp,
dns_dbnode_t **nodep, dns_name_t *fname,
dns_message_t *msg, isc_stdtime_t now);
* Get content from a particular additional cache entry.
* 'entry' is a valid additional cache entry.
* 'zonep' is a NULL pointer or '*zonep' == NULL (this is the only
* 'dbp' is a valid pointer, and '*dbp' == NULL
* 'versionp' is a valid pointer, and '*versionp' == NULL
* 'nodep' is a valid pointer, and '*nodep' == NULL
* 'fname' is a valid DNS name.
* 'msg' is a valid DNS message.
* Several possible cases can happen according to the content.
* 1. For a positive cache entry,
* '*zonep' will point to the corresponding zone (if zonep is a valid
* '*dbp' will point to a DB for the zone,
* '*versionp' will point to its version, and
* '*nodep' will point to the corresponding DB node.
* 'fname' will have the DNS name of the DB node and contain a list of
* rdataset for the node (which can be an empty list).
* 2. For a negative cache entry that means no corresponding zone exists,
* '*zonep' == NULL (if zonep is a valid pointer)
* '*dbp', '*versionp', and '*nodep' will be NULL.
* 3. For a negative cache entry that means no corresponding DB node
* exists, '*zonep' will point to the corresponding zone (if zonep is a
* '*dbp' will point to a corresponding DB for zone,
* '*versionp' will point to its version.
* '*nodep' will be kept as NULL.
* 'fname' will not change.
* On failure, no new references will be created.
dns_acache_setentry(dns_acache_t *acache, dns_acacheentry_t *entry,
dns_zone_t *zone, dns_db_t *db, dns_dbversion_t *version,
dns_dbnode_t *node, dns_name_t *fname);
* Set content to a particular additional cache entry.
* 'acache' is a valid additional cache.
* 'entry' is a valid additional cache entry.
* All the others pointers are NULL or a valid pointer of the
dns_acache_cancelentry(dns_acacheentry_t *entry);
* Cancel the use of the cache entry 'entry'. This function is supposed to
* be called when the node that holds the entry finds the content is not
* correct any more. This function will try to release as much dependency as
* possible, and will be ready to be cleaned-up. The registered callback
* function will be canceled and will never called.
* 'entry' is a valid additional cache entry.
* true if the entry was active when canceled
dns_acache_attachentry(dns_acacheentry_t *source, dns_acacheentry_t **targetp);
* Attach *targetp to the cache entry 'source'.
* 'source' is a valid additional cache entry.
* 'targetp' points to a NULL dns_acacheentry_t *.
* *targetp is attached to 'source'.
dns_acache_detachentry(dns_acacheentry_t **entryp);
* Detach *entryp from its cache.
* '*entryp' points to a valid additional cache entry.
* If '*entryp' is the last reference to the entry,
* cache does not have an outstanding task, all resources used by the
* entry (including the entry object itself) will be freed.
dns_acache_countquerymiss(dns_acache_t *acache);
* Count up a missed acache query. XXXMLG need more docs.
#endif /* DNS_ACACHE_H */
