bind/doc/design/database

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 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'".  They
are transactional, and must either succeed or fail completely.
Changes must not become visible to clients until the transaction has
committed.  The differential nature of these updates requires
transaction serialization.

Cache updates are done by the server in the ordinary course of
handling client requests.  Unlike zone updates, cache updates do not
refer to the current contents of the cache, so concurrent writing to
the cache is possible.  The main requirement is that concurrent update
attempts to the same node and rdataset type must appear to have been
executed in some order.  In order to make DB versioning simpler, the DB
interface actually imposes a more restrictive set of requirements, namely
that access to a node is serialized and that database changes will become
visible in version order (more on this below).


Database Concurrency and Locking

A principle 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 written as
well as read; 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
often provide good opportunities for concurrency.

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 very 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 very 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.


Database Versioning

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