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