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Databases

BIND 9 DNS database allows named rdatasets to be stored and retrieved.
DNS databases are used to store two different categories of data:
authoritative zone data and non-authoritative cache data.  Unlike
previous versions of BIND which used a monolithic database, BIND 9 has
one database per zone or cache.  Certain database operations, for
example updates, have differing requirements and actions depending
upon whether the database contains zone data or cache data.


Database Semantics

A database instance either has zone semantics or cache semantics.  The
semantics are chosen when the database is created and cannot be
changed.  The differences between zone databases and cache databases
will be discussed further below.


Reference Safety

It is a general principle of the BIND 9 project, and of the database
API, that all references returned to the caller remain valid until the
caller discards the reference.

The database interface also mandates that the rdata in a retrieved
rdataset shall remain unaltered while any reference to the rdataset is
held.  Some other properties of the rdataset, e.g. its DNSSEC
validation status, may change.


Database Updates

A master zone is updated by a Dynamic Update message.  A slave zone is
updated by IXFR or AXFR.  AXFR provides the entire contents of the new
zone version, and replaces the entire contents of the database.  IXFR
and Dynamic Update, although completely different protocols, have the
same basic database requirements.  They are differential update
protocols, e.g. "add this record to the records at name 'foo'".  The
updates are also atomic, i.e. they must either succeed or fail.
Changes must not become visible to clients until the update has
committed.  In short, zone updates are transactional.  This
transaction occurs at a database level; the entire database goes from
one version to another.

Cache updates are done by the server in the ordinary course of
handling client requests.  Unlike zone databases, there's no need (and
indeed, no ability) to ensure that data in the cache is consistent.
For example, the cache may hold rdatasets from different versions of a
given zone.  A typical cache update involves looking at the existing
cache contents for the given name and type (if any), deciding if the
proposed replacement is better, and if so, doing the replacement.
Concurrent update attempts to the same node and rdataset type must
appear to have been executed in some order; there must be no merging
of data from multiple updates.  Caches are not globally versioned like
zones are.  There is no need to group changes to multiple rdatasets
into a cache transaction.


Database Concurrency and Locking

A principal goal of the BIND 9 project is multiprocessor scalabilty.
The amount of concurrency in database accesses is an important factor
in achieving scalability.  Consider a heavily used database, e.g. the
cache database serving some mail hubs, or ".com".  If access to these
databases is not parallalized, then adding another CPU will not help
the server's performance for the portion of the runtime spent in
database lookup.

Support for multiple concurrent readers certainly helps both cache
databases and zone databases.  Zones are typically read much more than
they are written, though less so than in prior years because dynamic
DNS support is now widely available.  Caches are frequently read and
frequently written; a non-scientific survey of caching statistics on a
few busy caching nameservers showed the ratio of cache hits to misses
was about 2 to 1.

As mentioned above, zone updates must be serialized, but cache updates
can often go in parallel.

A simple approach to these concurrency goals would be to have a single
read-write lock on the database.  This would allow for multiple
concurrent readers, and would provide the serialization of updates
that zone updates require.  This approach also has significant
limitations.  Readers cannot run while an update is running.  For a
short-lived transaction like a Dynamic Update, this may be acceptable,
but an IXFR can take a long time (even hours) to complete.  Preventing
read access for such a long time is unacceptable.  Another problem is
that it forces updates to be serialized, even for cache databases.
There are problems on the reader side of the lock too.  If the entire
database is protected by one lock, then any data retrieved from the
database must either be used while the lock is held, or it must be
copied, because the data in the database can change when the lock
isn't held.  Copying is expensive, and the server would like to be
able to hold a reference to database data for a long time.  The most
significant long-running reader problem is outbound AXFR, which could
potentially block updates for a long time (hours).

A finer-grained locking scheme, e.g. one lock per node, helps
parallelize cache updates, but doesn't help with the long-lived reader
or long-lived writer problems.  These problems are solved by zone
database versioning, described below.

The BIND 9 Database interface does not mandate any particular locking
scheme.  Database implementations are strongly encouraged to provide
as much concurrency as possible without violating the database
interface's rules.


Database Versioning

Versioning is not available in cache databases.

A zone database has a "current version" which is the version most
recently committed.  A database has a set of versions open for reading
(the "open versions").  This set is always non-empty, since the
current version is always open.  The openversion method opens a
read-only handle to the current version.  All retrievals using the
handle will see the database as it was at the time the version was
opened, regardless of subsequent changes to the database.  It is not
possible to open a specific version; only the current version may be
opened.  This helps limit the number of prior versions which must be
kept in the database.

Each zone update transaction is assigned a new version.  Only one such
"future version" may be open at any time.  It is the caller's
responsibility to serialize and handle the blocking and awakening of
multiple update requests.  The future version may be committed or
rolled back by the caller.  If the future version commits, its version
becomes the current version of the database.
