mirror of
https://git.openldap.org/openldap/openldap.git
synced 2024-12-27 03:20:22 +08:00
68 lines
3.2 KiB
Plaintext
68 lines
3.2 KiB
Plaintext
|
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!).
|