openldap/doc/guide/admin/replication.sdf

# $OpenLDAP$
# Copyright 1999-2003, The OpenLDAP Foundation, All Rights Reserved.
# COPYING RESTRICTIONS APPLY, see COPYRIGHT.
H1: Replication with slurpd

In certain configurations, a single {{slapd}}(8) instance may be
insufficient to handle the number of clients requiring
directory service via LDAP. It may become necessary to
run more than one slapd instance.  At many sites,
for instance, there are multiple slapd servers: one
master and one or more slaves.  {{TERM:DNS}} can be setup such that
a lookup of {{EX:ldap.example.com}} returns the {{TERM:IP}} addresses
of these servers, distributing the load among them (or
just the slaves). This master/slave arrangement provides
a simple and effective way to increase capacity, availability
and reliability.

{{slurpd}}(8) provides the capability for a master slapd to
propagate changes to slave slapd instances,
implementing the master/slave replication scheme
described above.  slurpd runs on the same host as the
master slapd instance.



H2: Overview

{{slurpd}}(8) provides replication services "in band". That is, it
uses the LDAP protocol to update a slave database from
the master. Perhaps the easiest way to illustrate this is
with an example. In this example, we trace the propagation
of an LDAP modify operation from its initiation by the LDAP
client to its distribution to the slave slapd instance.


{{B: Sample replication scenario:}}

^ The LDAP client submits an LDAP modify operation to
the slave slapd.

+ The slave slapd returns a referral to the LDAP
client referring the client to the master slapd.

+ The LDAP client submits the LDAP modify operation to
the master slapd.

+ The master slapd performs the modify operation,
writes out the change to its replication log file and returns
a success code to the client. 

+ The slurpd process notices that a new entry has
been appended to the replication log file, reads the
replication log entry, and sends the change to the slave
slapd via LDAP. 

+ The slave slapd performs the modify operation and
returns a success code to the slurpd process. 


Note: {{ldapmodify}}(1) and other tools distributed as part of
OpenLDAP Software do not support automatic referral chasing.



H2: Replication Logs

When slapd is configured to generate a replication logfile, it
writes out a file containing {{TERM:LDIF}} change records.  The
replication log gives the replication site(s), a timestamp, the DN
of the entry being modified, and a series of lines which specify
the changes to make. In the example below, Barbara ({{EX:uid=bjensen}})
has replaced the {{EX:description}} value.  The change is to be
propagated to the slapd instance running on {{EX:slave.example.net}}
Changes to various operational attributes, such as {{EX:modifiersName}}
and {{EX:modifyTimestamp}}, are included in the change record and
will be propagated to the slave slapd.

>	replica: slave.example.com:389
>	time: 809618633
>	dn: uid=bjensen,dc=example,dc=com
>	changetype: modify
>	replace: multiLineDescription
>	description: A dreamer...
>	-
>	replace: modifiersName
>	modifiersName: uid=bjensen,dc=example,dc=com
>	-
>	replace: modifyTimestamp
>	modifyTimestamp: 20000805073308Z
>	-

The modifications to {{EX:modifiersName}} and {{EX:modifyTimestamp}}
operational attributes were added by the master {{slapd}}.



H2: Command-Line Options

This section details commonly used {{slurpd}}(8) command-line options.

>	-d <level> | ?

This option sets the slurpd debug level to {{EX: <level>}}. When
level is a `?' character, the various debugging levels are printed
and slurpd exits, regardless of any other options you give it.
Current debugging levels (a subset of slapd's debugging levels) are

!block table; colaligns="RL"; align=Center; \
	title="Table 13.1: Debugging Levels"
Level	Description
4	heavy trace debugging
64	configuration file processing
65535	enable all debugging
!endblock

Debugging levels are additive. That is, if you want heavy trace
debugging and want to watch the config file being processed, you
would set level to the sum of those two levels (in this case, 68).

>	-f <filename>

This option specifies an alternate slapd configuration file.  Slurpd
does not have its own configuration file. Instead, all configuration
information is read from the slapd configuration file.

>	-r <filename>

This option specifies an alternate slapd replication log file.
Under normal circumstances, slurpd reads the name of the slapd
replication log file from the slapd configuration file. However,
you can override this with the -r flag, to cause slurpd to process
a different replication log file. See the {{SECT:Advanced slurpd
Operation}} section for a discussion of how you might use this
option.

>	-o

Operate in "one-shot" mode. Under normal circumstances, when slurpd
finishes processing a replication log, it remains active and
periodically checks to see if new entries have been added to the
replication log.  In one-shot mode, by comparison, slurpd processes
a replication log and exits immediately. If the -o option is given,
the replication log file must be explicitly specified with the -r
option.  See the {{SECT:One-shot mode and reject files}} section
for  a discussion of this mode.

>	-t <directory>

Specify an alternate directory for slurpd's temporary copies of
replication logs. The default location is {{F:/usr/tmp}}.


H2: Configuring slurpd and a slave slapd instance

To bring up a replica slapd instance, you must configure the master
and slave slapd instances for replication, then shut down the master
slapd so you can copy the database. Finally, you bring up the master
slapd instance, the slave slapd instance, and the slurpd instance.
These steps are detailed in the following sections. You can set up
as many slave slapd instances as you wish.


H3: Set up the master {{slapd}}

The following section assumes you have a properly working {{slapd}}(8)
instance. To configure your working {{slapd}}(8) server as a
replication master, you need to make the following changes to your
{{slapd.conf}}(5).

^ Add a {{EX:replica}} directive for each replica. The {{EX:binddn=}}
parameter should match the {{EX:updatedn}} option in the corresponding
slave slapd configuration file, and should name an entry with write
permission to the slave database (e.g., an entry listed as
{{EX:rootdn}}, or allowed access via {{EX:access}} directives in
the slave slapd configuration file).

+ Add a {{EX:replogfile}} directive, which tells slapd where to log
changes. This file will be read by slurpd.


H3: Set up the slave {{slapd}}

Install the slapd software on the host which is to be the slave
slapd server. The configuration of the slave server should be
identical to that of the master, with the following exceptions:

^ Do not include a {{EX:replica}} directive. While it is possible
to create "chains" of replicas, in most cases this is inappropriate.

+ Do not include a {{EX:replogfile}} directive.

+ Do include an {{EX:updatedn}} line. The DN given should match the
DN given in the {{EX:binddn=}} parameter of the corresponding
{{EX:replica=}} directive in the master slapd config file.

+ Make sure the DN given in the {{EX:updatedn}} directive has
permission to write the database (e.g., it is listed as {{EX:rootdn}}
or is allowed {{EX:access}} by one or more access directives).

+ Use the {{EX:updateref}} directive to define the URL the slave
should return if an update request is received.


H3: Shut down the master {{slapd}}

In order to ensure that the slave starts with an exact copy of the
master's data, you must shut down the master slapd. Do this by
sending the master slapd process an interrupt signal with {{EX:kill
-INT <pid>}}, where {{EX:<pid>}} is the process-id of the master
slapd process.

If you like, you may restart the master slapd in read-only mode
while you are replicating the database. During this time, the master
slapd will return an "unwilling to perform" error to clients that
attempt to modify data.


H3: Copy the master slapd's database to the slave

Copy the master's database(s) to the slave. For an {{TERM:BDB}} and
{{TERM:LDBM}} databases, you must copy all database files located
in the database {{EX:directory}} specified in {{slapd.conf}}(5).
In general, you should copy each file found in the database {{EX:
directory}} unless you know it is not used by {{slapd}}(8).

Note: This copy process assumes homogeneous servers with identically
configured OpenLDAP installations. Alternatively, you may use
{{slapcat}} to output the master's database in LDIF format and use
the LDIF with {{slapadd}} to populate the slave. Using LDIF avoids
any potential incompatibilities due to differing server architectures
or software configurations.  See the {{SECT:Database Creation and
Maintenance Tools}} chapter for details on these tools.


H3: Configure the master slapd for replication

To configure slapd to generate a replication logfile, you add a
"{{EX: replica}}" configuration option to the master slapd's config
file. For example, if we wish to propagate changes to the slapd
instance running on host {{EX:slave.example.com}}:

>	replica host=slave.example.com:389
>		binddn="cn=Replicator,dc=example,dc=com"
>		bindmethod=simple credentials=secret

In this example, changes will be sent to port 389 (the standard
LDAP port) on host slave.example.com. The slurpd process will bind
to the slave slapd as "{{EX:cn=Replicator,dc=example,dc=com}}" using
simple authentication with password "{{EX:secret}}".  Note that the
DN given by the {{EX:binddn=}} directive must exist in the slave
slapd's database (or be the rootdn specified in the slapd config
file) in order for the bind operation to succeed.  The DN should
also be listed as the {{EX:updatedn}} for the database in the slave's
slapd.conf(5).

Note: The use of strong authentication and transport security is
highly recommended.


H3: Restart the master slapd and start the slave slapd

Restart the master slapd process. To check that it is
generating replication logs, perform a modification of any
entry in the database, and check that data has been
written to the log file.


H3: Start slurpd

Start the slurpd process. Slurpd should immediately send
the test modification you made to the slave slapd. Watch
the slave slapd's logfile to be sure that the modification
was sent.

>	slurpd -f <masterslapdconfigfile>



H2: Advanced slurpd Operation

H3: Replication errors

When slurpd propagates a change to a slave slapd and receives an
error return code, it writes the reason for the error and the
replication record to a reject file. The reject file is located in
the same directory as the per-replica replication logfile, and has
the same name, but with the string "{{F:.rej}}" appended. For
example, for a replica running on host {{EX:slave.example.com}},
port 389, the reject file, if it exists, will be named

>	/usr/local/var/openldap/replog.slave.example.com:389.rej

A sample rejection log entry follows:

>	ERROR: No such attribute
>	replica: slave.example.com:389
>	time: 809618633
>	dn: uid=bjensen,dc=example,dc=com
>	changetype: modify
>	replace: description
>	description: A dreamer...
>	-
>	replace: modifiersName
>	modifiersName: uid=bjensen,dc=example,dc=com
>	-
>	replace: modifyTimestamp
>	modifyTimestamp: 20000805073308Z
>	-

Note that this is precisely the same format as the original replication
log entry, but with an {{EX:ERROR}} line prepended to the entry.



H3: One-shot mode and reject files

It is possible to use slurpd to process a rejection log with its
"one-shot mode." In normal operation, slurpd watches for more
replication records to be appended to the replication log file. In
one-shot mode, by contrast, slurpd processes a single log file and
exits. Slurpd ignores {{EX:ERROR}} lines at the beginning of
replication log entries, so it's not necessary to edit them out
before feeding it the rejection log.

To use one-shot mode, specify the name of the rejection log on the
command line as the argument to the -r flag, and specify one-shot
mode with the -o flag. For example, to process the rejection log
file {{F:/usr/local/var/openldap/replog.slave.example.com:389}} and
exit, use the command

>	slurpd -r /usr/tmp/replog.slave.example.com:389 -o

!if 0

H2: Replication to an X.500 DSA

In mixed environments where both {{TERM:X.500}} DSAs and slapd are
used, it may be desirable to replicate changes from a slapd directory
server to an X.500 {{TERM:DSA}}. This section discusses issues
involved with this method of replication, and describes the
currently-available facilities.

To propagate changes from a slapd directory server to an X.500 DSA,
slurpd runs on the master slapd host, and sends changes to an ldapd
which acts as a gateway to the X.500 DSA:

!import "replication.gif"; align="center"; \
	title="Replication from slapd to an X.500 DSA"
FT: Figure 10.1: Replication from slapd to an X.500 DSA

Note that the X.500 DSA must be a read-only copy. Since the replication
is one-way, updates from {{TERM:DAP}} clients connecting to the
X.500 DSA simply cannot be handled.

A problem arises where attribute names differ between the slapd
directory server and the X.500 DSA. At present, slapd and slurpd
do not support selective replication of attributes, nor do they
support translation of attribute names and values. For example,
slurpd will attempt to update the {{EX:modifiersName}} and
{{EX:modifyTimeStamp}} attributes on the slave it connects to.
However, the X.500 DSA may expect these attributes to be named
{{EX:lastModifiedBy}} and {{EX:lastModifiedTime}}.

A solution to this attribute naming problem is to have the LDAP/DAP
gateway to map {{EX:modifiersName}} to the Object Identifier
({{TERM:OID}}) for the {{EX:lastModifiedBy}} attribute and
{{EX:modifyTimeStamp}} to the OID for the {{EX:lastModifiedTime}}
attribute. Since attribute names are carried as OIDs over DAP, this
should perform the appropriate translation of attribute names.

!endif