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[openldap] / doc / devel / replication-notes.txt

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-Written by Ganesan Rajagopal <rganesan@debian.org> and placed in the public
-domain.
-
-Replication in OpenLDAP
------------------------
-
-Please read "Section 10. Replication with slurpd" in the OpenLDAP guide for
-an overview and configuration of single-master replication. This document
-describes the internals of the replication mechanism.
-
-slapd/repl.c contains routines add_replica_info() and
-add_replica_suffix(). add_replica_info() adds a new host to the list of
-replicas for a backend. add_replica_info() returns a number for the
-replica. add_replica_suffix() must then be called with the replica number to
-add a suffix that is hosted on this replica. add_replica_info() and add_replica_suffix() do not lock the
-replog_mutex.
-
-Replicas are specified in the slapd.conf file. When slapd/config.c sees a
-"replica" line in slapd.conf, it calls add_replica_info() with the host
-specified in the "host=" directive and then calls add_replica_suffix() with
-the replica number and and the suffix specified in the "suffix="
-directive. 
-
-slapd writes out a replication log file containing LDIF change records for
-each configured replica for a suffix. The change records are generated for
-add, modify, delete and modrdn operations. A function called replog() is
-called at the end of the routines do_add (slapd/add.c),
-do_modify(slapd/modify.c), do_delete(slapd/delete.c) and
-do_modrdn(slapd/modrnd.c) to write out the change records.
-
-In master/slave replication, updates are not allowed on slave
-replicas. Therefore replog() is not called if the suffix is configured with
-a updatedn (which indicates that this is a slave replica), instead a
-referral is returned back to the client. If multi-master replication is
-enabled, replog() is always called whenever any of the above updates happen
-unless the dn which is making the change is the updatedn. When the dn making
-the change is the same as the updatedn, it is assumed that this entry is
-being replicated by a slurpd instance on another host. (Note: For this
-reason, the updatedn must not be a "regular" admin/user object in
-multi-master replication).
-
-The function replog() in slapd/repl.c generates the actual change
-records. Each change record is preceded by the list of replicas to which
-this change record needs to be replicated, the time when this change
-happened and the dn this change applies to. The pseudo code for replog() is
-follows
-
-1. Check that a replog exists.
-2. Lock the replog mutex.
-3. Open and lock the replog file. 
-4. Normalize the dn for the entry and write out a "replica:" entry for each
-   replica with a matching suffix. 
-5. Write out the the timestamp and the dn for the entry.
-6. Depending on the type of change, write out an appropriate changetype
-   record.
-7. Close the replication log
-8. Unlock the replog mutex
-
-slurpd has a file manager routine (function fm()) which watches for any
-change in the replication log. Whenever fm() detects a change in the
-replication log it locks the log, appends the records to slurpd's private
-copy of the replication log and truncates the log. See the slurpd/DESIGN
-file for a description of how slurpd works. 
-
-slapd can be configured to write out a replication log even if no replicas
-are configured. In this case the administrator has to truncate the
-replication log manually (under a lock!).