# $OpenLDAP: pkg/openldap-guide/admin/syncrepl.sdf,v 1.14.2.4 2008/02/11 23:24:08 kurt Exp $
# Copyright 2003-2008 The OpenLDAP Foundation, All Rights Reserved.
# COPYING RESTRICTIONS APPLY, see COPYRIGHT.

H1: LDAP Sync Replication

The LDAP Sync replication engine, syncrepl for short, is a consumer-side
replication engine that enables the consumer LDAP server to maintain
a shadow copy of a DIT fragment. A syncrepl engine resides at the
consumer-side as one of the {{slapd}} (8) threads. It creates and
maintains a consumer replica by connecting to the replication
provider to perform the initial DIT content load followed either
by periodic content polling or by timely updates upon content
changes.

Syncrepl uses the LDAP Content Synchronization (or LDAP Sync for
short) protocol as the replica synchronization protocol.  It provides
a stateful replication which supports both pull-based and push-based
synchronization and does not mandate the use of a history store.

Syncrepl keeps track of the status of the replication content by
maintaining and exchanging synchronization cookies. Because the
syncrepl consumer and provider maintain their content status, the
consumer can poll the provider content to perform incremental
synchronization by asking for the entries required to make the
consumer replica up-to-date with the provider content. Syncrepl
also enables convenient management of replicas by maintaining replica
status.  The consumer replica can be constructed from a consumer-side
or a provider-side backup at any synchronization status. Syncrepl
can automatically resynchronize the consumer replica up-to-date
with the current provider content.

Syncrepl supports both pull-based and push-based synchronization.
In its basic refreshOnly synchronization mode, the provider uses
pull-based synchronization where the consumer servers need not be
tracked and no history information is maintained.  The information
required for the provider to process periodic polling requests is
contained in the synchronization cookie of the request itself.  To
optimize the pull-based synchronization, syncrepl utilizes the
present phase of the LDAP Sync protocol as well as its delete phase,
instead of falling back on frequent full reloads. To further optimize
the pull-based synchronization, the provider can maintain a per-scope
session log as a history store. In its refreshAndPersist mode of
synchronization, the provider uses a push-based synchronization.
The provider keeps track of the consumer servers that have requested
a persistent search and sends them necessary updates as the provider
replication content gets modified.

With syncrepl, a consumer server can create a replica without
changing the provider's configurations and without restarting the
provider server, if the consumer server has appropriate access
privileges for the DIT fragment to be replicated. The consumer
server can stop the replication also without the need for provider-side
changes and restart.

Syncrepl supports both partial and sparse replications.  The shadow
DIT fragment is defined by a general search criteria consisting of
base, scope, filter, and attribute list.  The replica content is
also subject to the access privileges of the bind identity of the
syncrepl replication connection.


H2: The LDAP Content Synchronization Protocol

The LDAP Sync protocol allows a client to maintain a synchronized
copy of a DIT fragment. The LDAP Sync operation is defined as a set
of controls and other protocol elements which extend the LDAP search
operation. This section introduces the LDAP Content Sync protocol
only briefly. For more information, refer to the Internet Draft
{{The LDAP Content Synchronization Operation
<draft-zeilenga-ldup-sync-05.txt>}}.

The LDAP Sync protocol supports both polling and listening for
changes by defining two respective synchronization operations:
{{refreshOnly}} and {{refreshAndPersist}}.  Polling is implemented
by the {{refreshOnly}} operation.  The client copy is synchronized
to the server copy at the time of polling.  The server finishes the
search operation by returning {{SearchResultDone}} at the end of
the search operation as in the normal search.  The listening is
implemented by the {{refreshAndPersist}} operation.  Instead of
finishing the search after returning all entries currently matching
the search criteria, the synchronization search remains persistent
in the server. Subsequent updates to the synchronization content
in the server cause additional entry updates to be sent to the
client.

The {{refreshOnly}} operation and the refresh stage of the
{{refreshAndPersist}} operation can be performed with a present
phase or a delete phase.

In the present phase, the server sends the client the entries updated
within the search scope since the last synchronization. The server
sends all requested attributes, be it changed or not, of the updated
entries.  For each unchanged entry which remains in the scope, the
server sends a present message consisting only of the name of the
entry and the synchronization control representing state present.
The present message does not contain any attributes of the entry.
After the client receives all update and present entries, it can
reliably determine the new client copy by adding the entries added
to the server, by replacing the entries modified at the server, and
by deleting entries in the client copy which have not been updated
nor specified as being present at the server.

The transmission of the updated entries in the delete phase is the
same as in the present phase. The server sends all the requested
attributes of the entries updated within the search scope since the
last synchronization to the client. In the delete phase, however,
the server sends a delete message for each entry deleted from the
search scope, instead of sending present messages.  The delete
message consists only of the name of the entry and the synchronization
control representing state delete.  The new client copy can be
determined by adding, modifying, and removing entries according to
the synchronization control attached to the {{SearchResultEntry}}
message.

In the case that the LDAP Sync server maintains a history store and
can determine which entries are scoped out of the client copy since
the last synchronization time, the server can use the delete phase.
If the server does not maintain any history store, cannot determine
the scoped-out entries from the history store, or the history store
does not cover the outdated synchronization state of the client,
the server should use the present phase.  The use of the present
phase is much more efficient than a full content reload in terms
of the synchronization traffic.  To reduce the synchronization
traffic further, the LDAP Sync protocol also provides several
optimizations such as the transmission of the normalized {{EX:entryUUID}}s
and the transmission of multiple {{EX:entryUUIDs}} in a single
{{syncIdSet}} message.

At the end of the {{refreshOnly}} synchronization, the server sends
a synchronization cookie to the client as a state indicator of the
client copy after the synchronization is completed.  The client
will present the received cookie when it requests the next incremental
synchronization to the server.

When {{refreshAndPersist}} synchronization is used, the server sends
a synchronization cookie at the end of the refresh stage by sending
a Sync Info message with TRUE refreshDone.  It also sends a
synchronization cookie by attaching it to {{SearchResultEntry}}
generated in the persist stage of the synchronization search. During
the persist stage, the server can also send a Sync Info message
containing the synchronization cookie at any time the server wants
to update the client-side state indicator.  The server also updates
a synchronization indicator of the client at the end of the persist
stage.

In the LDAP Sync protocol, entries are uniquely identified by the
{{EX:entryUUID}} attribute value. It can function as a reliable
identifier of the entry. The DN of the entry, on the other hand,
can be changed over time and hence cannot be considered as the
reliable identifier.  The {{EX:entryUUID}} is attached to each
{{SearchResultEntry}} or {{SearchResultReference}} as a part of the
synchronization control.


H2: Syncrepl Details

The syncrepl engine utilizes both the {{refreshOnly}} and the
{{refreshAndPersist}} operations of the LDAP Sync protocol.  If a
syncrepl specification is included in a database definition, {{slapd}}
(8) launches a syncrepl engine as a {{slapd}} (8) thread and schedules
its execution. If the {{refreshOnly}} operation is specified, the
syncrepl engine will be rescheduled at the interval time after a
synchronization operation is completed.  If the {{refreshAndPersist}}
operation is specified, the engine will remain active and process
the persistent synchronization messages from the provider.

The syncrepl engine utilizes both the present phase and the delete
phase of the refresh synchronization. It is possible to configure
a per-scope session log in the provider server which stores the
{{EX:entryUUID}}s of a finite number of entries deleted from a
replication content.  Multiple replicas of single provider content
share the same per-scope session log. The syncrepl engine uses the
delete phase if the session log is present and the state of the
consumer server is recent enough that no session log entries are
truncated after the last synchronization of the client.  The syncrepl
engine uses the present phase if no session log is configured for
the replication content or if the consumer replica is too outdated
to be covered by the session log.  The current design of the session
log store is memory based, so the information contained in the
session log is not persistent over multiple provider invocations.
It is not currently supported to access the session log store by
using LDAP operations. It is also not currently supported to impose
access control to the session log.

As a further optimization, even in the case the synchronization
search is not associated with any session log, no entries will be
transmitted to the consumer server when there has been no update
in the replication context.

The syncrepl engine, which is a consumer-side replication engine,
can work with any backends. The LDAP Sync provider can be configured
as an overlay on any backend, but works best with the {{back-bdb}}
or {{back-hdb}} backend. The provider can not support refreshAndPersist
mode on {{back-ldbm}} due to limits in that backend's locking
architecture.

The LDAP Sync provider maintains a {{EX:contextCSN}} for each
database as the current synchronization state indicator of the
provider content.  It is the largest {{EX:entryCSN}} in the provider
context such that no transactions for an entry having smaller
{{EX:entryCSN}} value remains outstanding.  The {{EX:contextCSN}}
could not just be set to the largest issued {{EX:entryCSN}} because
{{EX:entryCSN}} is obtained before a transaction starts and
transactions are not committed in the issue order.

The provider stores the {{EX:contextCSN}} of a context in the
{{EX:contextCSN}} attribute of the context suffix entry. The attribute
is not written to the database after every update operation though;
instead it is maintained primarily in memory. At database start
time the provider reads the last saved {{EX:contextCSN}} into memory
and uses the in-memory copy exclusively thereafter. By default,
changes to the {{EX:contextCSN}} as a result of database updates
will not be written to the database until the server is cleanly
shut down. A checkpoint facility exists to cause the contextCSN to
be written out more frequently if desired.

Note that at startup time, if the provider is unable to read a
{{EX:contextCSN}} from the suffix entry, it will scan the entire
database to determine the value, and this scan may take quite a
long time on a large database. When a {{EX:contextCSN}} value is
read, the database will still be scanned for any {{EX:entryCSN}}
values greater than it, to make sure the {{EX:contextCSN}} value
truly reflects the greatest committed {{EX:entryCSN}} in the database.
On databases which support inequality indexing, setting an eq index
on the {{EX:entryCSN}} attribute and configuring {{contextCSN}}
checkpoints will greatly speed up this scanning step.

If no {{EX:contextCSN}} can be determined by reading and scanning
the database, a new value will be generated. Also, if scanning the
database yielded a greater {{EX:entryCSN}} than was previously
recorded in the suffix entry's {{EX:contextCSN}} attribute, a
checkpoint will be immediately written with the new value.

The consumer also stores its replica state, which is the provider's
{{EX:contextCSN}} received as a synchronization cookie, in the
{{EX:contextCSN}} attribute of the suffix entry.  The replica state
maintained by a consumer server is used as the synchronization state
indicator when it performs subsequent incremental synchronization
with the provider server. It is also used as a provider-side
synchronization state indicator when it functions as a secondary
provider server in a cascading replication configuration.  Since
the consumer and provider state information are maintained in the
same location within their respective databases, any consumer can
be promoted to a provider (and vice versa) without any special
actions.

Because a general search filter can be used in the syncrepl
specification, some entries in the context may be omitted from the
synchronization content.  The syncrepl engine creates a glue entry
to fill in the holes in the replica context if any part of the
replica content is subordinate to the holes. The glue entries will
not be returned in the search result unless {{ManageDsaIT}} control
is provided.

Also as a consequence of the search filter used in the syncrepl
specification, it is possible for a modification to remove an entry
from the replication scope even though the entry has not been deleted
on the provider. Logically the entry must be deleted on the consumer
but in {{refreshOnly}} mode the provider cannot detect and propagate
this change without the use of the session log.


H2: Configuring Syncrepl

Because syncrepl is a consumer-side replication engine, the syncrepl
specification is defined in {{slapd.conf}} (5) of the consumer
server, not in the provider server's configuration file.  The initial
loading of the replica content can be performed either by starting
the syncrepl engine with no synchronization cookie or by populating
the consumer replica by adding an {{TERM:LDIF}} file dumped as a
backup at the provider.

When loading from a backup, it is not required to perform the initial
loading from the up-to-date backup of the provider content. The
syncrepl engine will automatically synchronize the initial consumer
replica to the current provider content. As a result, it is not
required to stop the provider server in order to avoid the replica
inconsistency caused by the updates to the provider content during
the content backup and loading process.

When replicating a large scale directory, especially in a bandwidth
constrained environment, it is advised to load the consumer replica
from a backup instead of performing a full initial load using
syncrepl.


H3: Set up the provider slapd

The provider is implemented as an overlay, so the overlay itself
must first be configured in {{slapd.conf}} (5) before it can be
used. The provider has only two configuration directives, for setting
checkpoints on the {{EX:contextCSN}} and for configuring the session
log.  Because the LDAP Sync search is subject to access control,
proper access control privileges should be set up for the replicated
content.

The {{EX:contextCSN}} checkpoint is configured by the

>	syncprov-checkpoint <ops> <minutes>

directive. Checkpoints are only tested after successful write
operations.  If {{<ops>}} operations or more than {{<minutes>}}
time has passed since the last checkpoint, a new checkpoint is
performed.

The session log is configured by the

>	syncprov-sessionlog <size>

directive, where {{<size>}} is the maximum number of session log
entries the session log can record. When a session log is configured,
it is automatically used for all LDAP Sync searches within the
database.

Note that using the session log requires searching on the {{entryUUID}}
attribute. Setting an eq index on this attribute will greatly benefit
the performance of the session log on the provider.

A more complete example of the {{slapd.conf}} content is thus:

>	database bdb
>	suffix dc=Example,dc=com
>	rootdn dc=Example,dc=com
>	directory /var/ldap/db
>	index objectclass,entryCSN,entryUUID eq
>
>	overlay syncprov
>	syncprov-checkpoint 100 10
>	syncprov-sessionlog 100


H3: Set up the consumer slapd

The syncrepl replication is specified in the database section of
{{slapd.conf}} (5) for the replica context.  The syncrepl engine
is backend independent and the directive can be defined with any
database type.

>	database hdb
>	suffix dc=Example,dc=com
>	rootdn dc=Example,dc=com
>	directory /var/ldap/db
>	index objectclass,entryCSN,entryUUID eq
>
>	syncrepl rid=123
>		provider=ldap://provider.example.com:389
>		type=refreshOnly
>		interval=01:00:00:00
>		searchbase="dc=example,dc=com"
>		filter="(objectClass=organizationalPerson)"
>		scope=sub
>		attrs="cn,sn,ou,telephoneNumber,title,l"
>		schemachecking=off
>		bindmethod=simple
>		binddn="cn=syncuser,dc=example,dc=com"
>		credentials=secret

In this example, the consumer will connect to the provider slapd
at port 389 of {{FILE:ldap://provider.example.com}} to perform a
polling ({{refreshOnly}}) mode of synchronization once a day.  It
will bind as {{EX:cn=syncuser,dc=example,dc=com}} using simple
authentication with password "secret".  Note that the access control
privilege of {{EX:cn=syncuser,dc=example,dc=com}} should be set
appropriately in the provider to retrieve the desired replication
content. Also the search limits must be high enough on the provider
to allow the syncuser to retrieve a complete copy of the requested
content.  The consumer uses the rootdn to write to its database so
it always has full permissions to write all content.

The synchronization search in the above example will search for the
entries whose objectClass is organizationalPerson in the entire
subtree rooted at {{EX:dc=example,dc=com}}. The requested attributes
are {{EX:cn}}, {{EX:sn}}, {{EX:ou}}, {{EX:telephoneNumber}},
{{EX:title}}, and {{EX:l}}. The schema checking is turned off, so
that the consumer {{slapd}} (8) will not enforce entry schema
checking when it process updates from the provider {{slapd}} (8).

For more detailed information on the syncrepl directive, see the
{{SECT:syncrepl}} section of {{SECT:The slapd Configuration File}}
chapter of this admin guide.


H3: Start the provider and the consumer slapd

The provider {{slapd}} (8) is not required to be restarted.
{{contextCSN}} is automatically generated as needed: it might be
originally contained in the {{TERM:LDIF}} file, generated by
{{slapadd}} (8), generated upon changes in the context, or generated
when the first LDAP Sync search arrives at the provider.  If an
LDIF file is being loaded which did not previously contain the
{{contextCSN}}, the {{-w}} option should be used with {{slapadd}}
(8) to cause it to be generated. This will allow the server to
startup a little quicker the first time it runs.

When starting a consumer {{slapd}} (8), it is possible to provide
a synchronization cookie as the {{-c cookie}} command line option
in order to start the synchronization from a specific state.  The
cookie is a comma separated list of name=value pairs. Currently
supported syncrepl cookie fields are {{csn=<csn>}} and {{rid=<rid>}}.
{{<csn>}} represents the current synchronization state of the
consumer replica.  {{<rid>}} identifies a consumer replica locally
within the consumer server. It is used to relate the cookie to the
syncrepl definition in {{slapd.conf}} (5) which has the matching
replica identifier.  The {{<rid>}} must have no more than 3 decimal
digits.  The command line cookie overrides the synchronization
cookie stored in the consumer replica database.
