11. Replication with slurpd

In certain configurations, a single slapd 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 the University of Michigan, for instance, there are four slapd servers, one master and three slaves. A DNS lookup of the name ldap.itd.umich.edu returns the IP addresses of those four servers, distributing the load among them. This master/slave arrangement provides a simple and effective way to increase capacity, availability and reliability.

Slurpd 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.

11.1 Overview

Slurpd 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.

Sample replication scenario:
Step 1: An LDAP client starts up and connects to a slave slapd.

Step 2: The LDAP client submits an LDAP modify operation to the slave slapd.

Step 3: The slave slapd returns a referral to the LDAP client, which causes the client to send the modify operation to the master slapd.

Step 4: The master slapd performs the modify operation, writes out the change to its replication log file and returns a success code to the client.

Step 5: 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.

Step 6: The slave slapd performs the modify operation and returns a success code to the slurpd process.

Note that if the LDAP client happened to connect to the master slapd to begin with, Step 3 is omitted, but the rest of the scenario remains the same.

11.2 Replication Logs

When slapd is configured to generate a replication logfile, it writes out a file in a format which is a variant of the LDIF format. 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 Jensen" has replaced a line of her multiLineDescription. The change is to be propagated to the slapd instance running on truelies.rs.itd.umich.edu. The lastModifiedBy and lastModified Time attributes are also propagated to the slave slapd.

replica: truelies.rs.itd.umich.edu:389
time: 809618633
dn: cn=Barbara Jensen, ou=People, o=University of Michigan, c=US
changetype: modify
delete: multiLineDescription
multiLineDescription: I enjoy sailing in my spare time
-
add: multiLineDescription
multiLineDescription: A dreamer...
-
delete: lastModifiedBy
-
add: lastModifiedBy
lastModifiedBy: cn=Barbara Jensen, ou=People, o=University of Michigan,
c=US
-
delete: lastModifiedTime
-
add: lastModifiedTime
lastModifiedTime: 950825073308Z
-

The modifications to lastModifiedBy and lastModifiedTime were initiated by the master slapd.

11.3 Command-Line Options

Slurpd supports the following command-line options.

-d <level> | ?

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

4 heavy trace debugging
64 configuration file processing
65535 enable all debugging

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 section 10.5, Advanced slurpd Operation, 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

-t <directory>

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

-k <filename>

When slurpd uses kerberos to authenticate to slave slapd instances, it needs to have an appropriate srvtab file for the remote slapd. This option allows you to specify an alternate filename containing kerberos keys for the remote slapd. The default filename is /etc/srvtab. You can also specify the srvtab file to use in the slapd configuration file's replica option. See the documentation on the srvtab directive in section 5.2.2, General Backend Options. A more complete discussion of using kerberos with slapd and slurpd may be found in Appendix D.

11.4 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.

11.4.1 Set up the master slapd

Follow the procedures in Section 4, Building and Installing slapd. Be sure that the slapd instance is working properly before proceeding. Be sure to do the following in the master slapd configuration file.
1. Add a replica directive for each replica. The binddn= parameter should match the 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 rootdn, or allowed access via access directives in the slave slapd configuration file).

2. Add a replogfile directive, which tells slapd where to log changes. This file will be read by slurpd.

11.4.2 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:
1. Do not include a replica directive. While it is possible to create "chains" of replicas, in most cases this is inappropriate.

2. Do not include a replogfile directive.

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

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

11.4.3 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 kill -TERM <pid>, where <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.

11.4.4 Copy the master slapd's database to the slave

Copy the master's database(s) to the slave. For an LDBM-based database, you must copy all index files as well as the "NEXTID" file. Index files will have a different suffix depending on the underlying database package used. The current possibilities are

dbb Berkeley DB B-tree backend
dbh Berkeley DB hash backend
gdbm GNU DBM backend
pag UNIX NBDM backend
dir UNIX NBDM backend

You should copy all files with such a suffix that are located in the index directory specified in your slapd config file.

11.4.5 Configure the master slapd for replication

To configure slapd to generate a replication logfile, you add a "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 truelies.rs.itd.umich.edu:

replica host=truelies.rs.itd.umich.edu:389 binddn="cn=Replicator, o=U of M, c=US" bindmethod=simple credentials=secret

In this example, changes will be sent to port 389 (the standard LDAP port) on host truelies. The slurpd process will bind to the slave slapd as "cn=Replicator, o=U of M, c=US" using simple authentication with password "secret". Note that the entry given by the 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.

11.4.6 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.

11.4.7 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>

11.5 Advanced slurpd Operation

11.5.1 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 with the per-replica replication logfile, and has the same name, but with the string ".rej" appended. For example, for a replica running on host truelies.rs.itd.umich.edu, port 389, the reject file, if it exists, will be named

/usr/tmp/truelies.rs.itd.umich.edu:389.

A sample rejection log entry follows:

ERROR: No such attribute
replica: truelies.rs.itd.umich.edu:389
time: 809618633
dn: cn=Barbara Jensen, ou=People, o=University of Michigan, c=US
changetype: modify
delete: multiLineDescription
multiLineDescription: I enjoy sailing in my spare time
-
add: multiLineDescription
multiLineDescription: A dreamer...
-
delete: lastModifiedBy
-
add: lastModifiedBy
lastModifiedBy: cn=Barbara Jensen, ou=People, o=University of Michigan,
c=US
-
delete: lastModifiedTime
-
add: lastModifiedTime
lastModifiedTime: 950825073308Z
-

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

11.5.2 Slurpd's 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 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 /usr/tmp/replog.truelies.rs.itd.umich.edu:389 and exit, use the command

slurpd -r /usr/tmp/truelies.rs.itd.umich.edu:389 -o

11.6 Replication from a slapd directory server to an X.500 DSA

In mixed environments where both X.500 DSAs and slapd are used, it may be desirable to replicate changes from a slapd directory server to an X.500 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:

Figure 6: 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 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 "modifiersName" and "modifyTimeStamp" attributes on the slave it connects to. However, the X.500 DSA may expect these attributes to be named "lastModifiedBy" and "lastModifiedTime".

A solution to this attribute naming problem is to have the ldapd read oidtables that map "modifiersName" to the objectID (OID) for the "lastModifiedBy" attribute and "modifyTimeStamp" to the OID for the "lastModifiedTime" attribute. Since attribute names are carried as OIDs over DAP, this should perform the appropriate translation of attribute names.

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