The Admin Guide still has not been updated with all of the relevant changes,
so here are some notes on new features in the 2.4 release... I believe all of
the manpages are up to date, so you can get specifics from them.
More complete cn=config functionality:
There is a new slapd-config(5) manpage for the cn=config backend.
the original design called for auto-renaming of config entries when you
insert or delete entries with ordered names, but that was not implemented in
2.3. It is now in 2.4. This means, e.g., if you have
olcDatabase={1}bdb,cn=config
olcSuffix: dc=example,dc=com
and you want to add a new subordinate, now you can:
ldapadd olcDatabase={1}bdb,cn=config
olcSuffix: dc=foo,dc=example,dc=com
this will insert a new BDB database in slot 1 and bump all following
databases down one, so the original BDB database will now be named
olcDatabase={2}bdb,cn=config
olcSuffix: dc=example,dc=com
In 2.3 you were only able to add new schema elements, not delete or
modify existing elements. In 2.4 you can modify schema at will. (Except for
the hardcoded system schema, of course.)
More sophisticated syncrepl configurations:
the original implementation of syncrepl in OpenLDAP 2.2 was intended to
support multiple consumers within the same database, but that feature never
worked and was removed from OpenLDAP 2.3. I.e., you could only configure a
single consumer in any database.
In 2.4 you can configure multiple consumers in a single database. The
configuration possibilities here are quite complex and numerous. You can
configure consumers over arbitrary subtrees of a database (disjoint or
overlapping). Any portion of the database may in turn be provided to other
consumers using the syncprov overlay. The syncprov overlay works with any
number of consumers over a single database or over arbitrarily many glued
databases.
As a consequence of the work to support multiple consumer contexts, the
syncrepl system now supports full N-way multimaster replication with
entry-level conflict resolution. There are some important constraints, of
course: In order to maintain consistent results across all servers, you must
maintain tightly synchronized clocks across all participating servers (e.g.,
you must use NTP on all servers). The entryCSNs used for replication now
record timestamps with microsecond resolution, instead of just seconds. The
delta-syncrepl code has not been updated to support multimaster usage yet,
that will come later in the 2.4 cycle.
On a related note, syncrepl was explicitly disabled on cn=config in 2.3.
It is now fully supported in 2.4; you can use syncrepl to replicate an entire
server configuration from one server to arbitrarily many other servers. It's
possible to clone an entire running slapd using just a small (less than 10
lines) seed configuration, or you can just replicate the schema subtrees,
etc. Tests 049 and 050 in the test suite provide working examples of these
capabilities.
In 2.3 you could configure syncrepl as a full push-mode replicator by
using it in conjunction with a back-ldap pointed at the target server. But
because the back-ldap database needs to have a suffix corresponding to the
target's suffix, you could only configure one instance per slapd.
In 2.4 you can define a database to be "hidden" which means that its
suffix is ignored when checking for name collisions, and the database will
never be used to answer requests received by the frontend. Using this hidden
database feature allows you to configure multiple databases with the same
suffix, allowing you to set up multiple back-ldap instances for pushing
replication of a single database to multiple targets. There may be other uses
for hidden databases as well (e.g., using a syncrepl consumer to maintain a
*local* mirror of a database on a separate filesystem).
More extensive TLS configuration control:
In 2.3, the TLS configuration in slapd was only used by the slapd
listeners. For outbound connections used by e.g. back-ldap or syncrepl their
TLS parameters came from the system's ldap.conf file.
In 2.4 all of these sessions inherit their settings from the main slapd
configuration but settings can be individually overridden on a
per-config-item basis. This is particularly helpful if you use
certificate-based authentication and need to use a different client
certificate for different destinations.
Various performance enhancements:
Too many to list. Some notable changes - ldapadd used to be a couple of
orders of magnitude slower than "slapadd -q". It's now at worst only about
half the speed of slapadd -q. A few weeks ago I did some comparisons of all
the 2.x OpenLDAP releases; the results are in the slides from my SCALE
presentation and you can find a copy here:
http://www.highlandsun.com/hyc/scale2007.pdf
That compared 2.0.27, 2.1.30, 2.2.30, 2.3.33, and HEAD (as of a couple
weeks ago). Toward the latter end of the "Cached Search Performance" chart it
gets hard to see the difference because the runtimes are so small, but the
new code is about 25% faster than 2.3, which was about 20% faster than 2.2,
which was about 100% faster than 2.1, which was about 100% faster than 2.0,
in that particular search scenario. That test basically searched a 1.3GB DB
of 380836 entries (all in the slapd entry cache) in under 1 second. i.e., on
a 2.4GHz CPU with DDR400 ECC/Registered RAM we can search over 500 thousand
entries per second. The search was on an unindexed attribute using a filter
that would not match any entry, forcing slapd to examine every entry in the
DB, testing the filter for a match.
Essentially the slapd entry cache in back-bdb/back-hdb is so efficient
the search processing time is almost invisible; the runtime is limited only
by the memory bandwidth of the machine. (The search data rate corresponds to
about 3.5GB/sec; the memory bandwidth on the machine is only about 4GB/sec
due to ECC and register latency.)
I think it goes without saying that no other Directory Server in the world is
this fast or this efficient. Couple that with the scalability, manageability,
flexibility, and just the sheer know-how behind this software, and nothing
else is even remotely comparable.