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'''本文的英文原文来自[http://www.openldap.org/doc/admin24/index.html OpenLDAP Software 2.4 Administrator's Guide]'''

由于OpenLDAP2.4版的复制部分产生了重大变化，所以优先翻译这部分, '''[http://www.openldap.org/doc/admin24/replication.html OpenLDAP Software 2.4 Administrator's Guide : Replication]'''

==复制==

为了提供一个有弹性的企业部署，复制目录是一个基础需求.

OpenLDAP有多种配置选项来建立一个可复制的目录. 在前一个版本里面, 复制被限定在一个主服务器和若干个从服务器的条件下来讨论。一个主服务器从其他客户端接受目录更新, 而一个从服务器则仅仅从一个（单个的）主服务器接受更新. 这个复制结构被僵化地定义并且任何典型的数据库只能完成一个单一角色,主或者从.

现在OpenLDAP支持一个更广泛的复制拓扑, 关于提供者和消费者的以下这些条件已经不推荐了: 一个提供者复制目录更新到消费者; 消费者从提供者接收复制更新. 不像僵化定义的主/从关系,提供者/消费者角色更加的流动化：一个接收复制更新的消费者可能传递给其它服务器的另一个消费者，所以一个消费者也可以同时成为一个提供者。而且，消费者不需要成为一个实际上的LDAP服务器;它也可以仅仅是一个LDAP客户端。

以下章节将描述复制技术和讨论各种可用的复制选项.

===复制技术===

====LDAP同步复制====

LDAP同步复制引擎, 简称syncrepl, 是一个消费方的复制引擎，能让消费者服务器维护一个抽取片断的影子副本. 一个syncrepl引擎以slapd的一个线程的方式驻留在消费者那里. 它通过连接一个复制提供者来建立和维护一个消费者复制，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 protocol (or LDAP Sync for short) as the replica synchronization protocol. LDAP Sync provides a stateful replication which supports both pull-based and push-based synchronization and does not mandate the use of a history store. In pull-based replication the consumer periodically polls the provider for updates. In push-based replication the consumer listens for updates that are sent by the provider in realtime. Since the protocol does not require a history store, the provider does not need to maintain any log of updates it has received. (Note that the syncrepl engine is extensible and additional replication protocols may be supported in the future.)

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 partial, sparse, and fractional 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.

=====LDAP内容同步协议=====

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 RFC4533.

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 consumer polls the provider using an LDAP Search request with an LDAP Sync control attached. The consumer copy is synchronized to the provider copy at the time of polling using the information returned in the search. The provider finishes the search operation by returning SearchResultDone at the end of the search operation as in the normal search. Listening is implemented by the refreshAndPersist operation. As the name implies, it begins with a search, like refreshOnly. Instead of finishing the search after returning all entries currently matching the search criteria, the synchronization search remains persistent in the provider. Subsequent updates to the synchronization content in the provider cause additional entry updates to be sent to the consumer.

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 provider sends the consumer the entries updated within the search scope since the last synchronization. The provider sends all requested attributes, be they changed or not, of the updated entries. For each unchanged entry which remains in the scope, the provider 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 consumer receives all update and present entries, it can reliably determine the new consumer copy by adding the entries added to the provider, by replacing the entries modified at the provider, and by deleting entries in the consumer copy which have not been updated nor specified as being present at the provider.

The transmission of the updated entries in the delete phase is the same as in the present phase. The provider sends all the requested attributes of the entries updated within the search scope since the last synchronization to the consumer. In the delete phase, however, the provider 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 consumer 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 provider maintains a history store and can determine which entries are scoped out of the consumer copy since the last synchronization time, the provider can use the delete phase. If the provider 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 consumer, the provider 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 entryUUIDs and the transmission of multiple entryUUIDs in a single syncIdSet message.

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

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

In the LDAP Sync protocol, entries are uniquely identified by the 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 entryUUID is attached to each SearchResultEntry or SearchResultReference as a part of the synchronization control.

=====Syncrepl细节=====

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 session log in the provider which stores the entryUUIDs of a finite number of entries deleted from a database. Multiple replicas share the same 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 LDAP Sync provider maintains a contextCSN for each database as the current synchronization state indicator of the provider content. It is the largest entryCSN in the provider context such that no transactions for an entry having smaller entryCSN value remains outstanding. The contextCSN could not just be set to the largest issued entryCSN because entryCSN is obtained before a transaction starts and transactions are not committed in the issue order.

The provider stores the contextCSN of a context in the 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 contextCSN into memory and uses the in-memory copy exclusively thereafter. By default, changes to the 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 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 contextCSN value is read, the database will still be scanned for any entryCSN values greater than it, to make sure the contextCSN value truly reflects the greatest committed entryCSN in the database. On databases which support inequality indexing, setting an eq index on the entryCSN attribute and configuring contextCSN checkpoints will greatly speed up this scanning step.

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

The consumer also stores its replica state, which is the provider's contextCSN received as a synchronization cookie, in the 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.

For configuration, please see the Syncrepl section.

===部署替代===

While the LDAP Sync specification only defines a narrow scope for replication, the OpenLDAP implementation is extremely flexible and supports a variety of operating modes to handle other scenarios not explicitly addressed in the spec.

====Delta-syncrepl复制====

:* LDAP同步复制的缺点:

LDAP同步复制是一个基于对象的复制机制. 当提供者的一个被复制对象中的任何属性值改变时, 每个消费者在复制过程中撷取并处理完整的变更对象, 包括所有改变和没改变的属性值. 这方法的一个好处是当多个变更发生在单一对象上时, 那些变更的精确顺序不需要保存; 只有最终状态是有意义的. 但是当使用模式(匹配的方式)在一次变更中处理很多对象时，这个方法可能有缺点。

例如, 假设你有一个数据库包含 100,000 对象，每个对象是 1 KB . 进一步, 假设你经常运行一个批处理工作来变更主服务器上的 100,000 对象的每一个对象中的一个两字节的属性值. 不算LDAP和TCP/IP协议的开销, 每次你运行这个工作每个消费者将传送并处理 1 GB 的数据，只是为了处理这个 200KB 的变更!

在类似这样的案例中，99.98% 被传送和处理的数据将是多余的, 因为它们代表那些未变更的值. 这是一个对宝贵的传输和处理带宽的浪费并且可能导致发展出不可接受的复制日志的积压. 虽然这个情形是一个极端, 但它有助于演示某些LDAP部署的一个非常真实的问题.

:* 看看Delta-syncrepl怎么处理:

Delta-syncrepl, 一个基于变更日志syncrepl变种, 被设计用来处理类似上面所说的情况. Delta-syncrepl通过在提供者一端维护一个可选择深度的变更日志来起作用. 复制消费者为它需要的变更检查这个变更日志，只要变更日志包含它需要的变更，消费者就从变更日志撷取这些变更并把它们应用到自己的数据库. 不过，一个复制（译者注：指变更日志里的变更）如果离上一次同步的状态太远(或消费者根本就是空的), 可以用常规的syncrepl把它（指消费者）恢复到最新的状态然后复制重新切换到delta-syncrepl模式.

关于配置请参考 Delta-syncrepl 章节.

====N-Way Multi-Master复制====

Multi-Master复制是一个使用Syncrepl复制数据到多个提供者（“主服务器”）目录服务器的复制技术.

=====对于Multi-Master replication有效的观点=====

:* 如果任何提供者失败了, 其他提供者将继续接受更新
:* 避免了单点失败
:* 提供者们可以在不同的物理位置例如跨越全球网络.
:* 好的自动容错/高可用性

=====对于Multi-Master replication无效的观点=====

(这些经常被声称是Multi-Master复制的优点但是那些说法是错误的):

:* 它不关负载均衡任何事
:* 提供者必须对所有其他的服务器进行写操作，这意味着分布在所有的服务器上的网络交通和写操作负载,和单一主服务器是一样的。
:* 多服务器的服务器利用率和负载在最好的情况下和单服务器一样; 最坏的情况下单服务器更优，因为在提供者和消费者之间使用不同的模式的时候索引可以做出不同的优化调整.

=====和Multi-Master replication抵触的观点=====

:* 打破了目录模式的数据一致性的保障
:* http://www.openldap.org/faq/data/cache/1240.html
:* 如果提供者的连接因为网络问题丢失了, 那么 "自动容错" 只会使问题复杂化
:* 通常, 一个特定的机器不能区分失去和一个节点的联系是因为该节点崩溃了还是因为网络连接失败了a
:* 如果一个网络是分割开的而多个客户端开始向每一个"主服务器"写操作，那么和解将是一个痛苦; 可能最好的办法是禁止那些被单一提供者分隔开的客户端的写操作

关于配置，请看下面的 N-Way Multi-Master 章节

====MirrorMode复制====

MirrorMode是一个混合配置，既提供单主服务器复制的所有一致性保障，也提供多主服务器模式的高可用性. 在 MirrorMode 两个提供者都被设置成从对方复制(就象一个多主服务器配置), 但是一个额外的前段被用来引导所有的写操作到仅仅到两台服务器中的其中一台. 第二个提供者将只在第一台服务器崩溃时进行写操作, 那时这个前端将切换路径引导所有的写操作到第二个提供者. 当一个崩溃的提供者被修复并且重启动后将自动从正在运行的提供者那里活得任何更新并重新同步.

=====MirrorMode的观点=====

:* 对于目录的写操作提供了一个高可用性 (HA) 方案(复制处理读操作)
:* 只有一个提供者是可操作的l, 写操作的安全是可接受的
:* 提供者节点从对方互相复制, 所以它们总是最新的并且可以随时准备好接管 (热备份)
:* Syncrepl也允许提供者节点在任何停机时间进行重新同步

=====和MirrorMode抵触的观点=====

:* MirrorMode 不能被称为多主机方案. 这是因为同一时间写操作不得不仅限于镜像节点中的一个
:* MirrorMode 可被称为Active-Active Hot-Standby（“双活热备份”,呃，这个翻译怎么样，传神不？）, 因此需要一个额外的服务器(代理模式的slapd)或设备(硬件负载平衡装置)来管理哪个提供者是当前激活的
:* 备份的管理稍微不同
:** 如果备份bdb本身并且定期备份事务日志文件，那么镜像对的相同数字需要用于收集日志文件直到下一次数据库备份发生
:** 为了确保所有数据库都是一致的, 当执行一个slapcat的时候每个数据库可能都不得不置于只读模式.
:* Delta-Syncrepl扔不支持

关于配置，请看下面的 MirrorMode 章节

====Syncrepl代理模式====

因为LDAP同步协议同时支持基于“拉”和“推”的复制, “推”模式 (refreshAndPersist) 在提供者开始"推"变更之前仍必须由消费者初始化. 在一些网络配置中, 特别是防火墙限制了连接的方向时, 一个提供者初始化的推模式是需要的.

这个模式可以被配置成LDAP Backend (Backends and slapd-ldap(8)). 不用在实际的消费者服务器上运行syncrepl引擎, 而是一个slapd-ldap代理设置在靠近（或搭配在）提供者的地方指向消费者, 而这个syncrepl引擎运行在这个代理服务器上.

关于配置, 请看 Syncrepl代理 章节.

=====替代Slurpd=====

旧的slurpd机制只操作主服务器初始化的推模式. Slurpd复制被Syncrepl复制取代了并且在OpenLDAP 2.4中被完全移除了.

slurpd守护进程是原来继承自UMich's LDAP的复制机制并且以推模式操作: 主服务器推变更到从服务器. 因为多种原因它被替换掉, 简短的说:

:* 它是不可靠的
:** 它对replog中的记录的次序极为敏感
:** 它可能很容易失去同步, 这时需要手工干预来从主目录重新同步从服务器数据库
:** 它对不可用的服务器不是非常宽容. 如果一个从服务器长时间停机, replog可能变得太大以至于slurpd无法处理
:* 它只工作在推模式
:* 它需要停止和重新启动主服务器来增加从服务器
:* 它只支持单一主服务器复制

Syncrepl没有那些弱点:

:* Syncrepl是自同步的; 你可以在任何状态启动一个消费者数据库，从完全空的到完全同步的，它将自动做正确的事来完成和维护同步
:** 它对变更发生的次序完全不敏感
:** 它保障消费者和提供者内容的合流,不用手工干预
:** 无论一个消费者多长时间没有联系提供者，它都能重新同步
:* Syncrepl能双向操作
:* 消费者能在不用碰提供者的情况下被加入
:* 支持多主服务器复制

===配置不同的复制类型===

====Syncrepl====

=====Syncrepl配置=====

因为syncrepl是一个消费方的复制引擎, syncrepl规范定义在 slapd.conf(5) 的消费者服务器, 而不是在提供者的服务器配置文件里. 复制内容的初始化装载可以有两种执行方式，以无同步cookie的方式启动一个syncrepl 引擎，或装载一个提供者服务器的全备份LDIF文件填充到消费者服务器.

当从一个备份装载的时候, 它不需要执行从提供者内容的最新备份初始化装载这个动作. syncrepl引擎将自动同步初始化的消费者复制当前的提供者内容. 结果是, 它不需要为了避免由于内容备份和装载过程中提供者服务器仍在更新而导致复制不一致的问题来停止提供者服务器.

当复制一个大规模的目录时, 特别是在一个带宽受限的环境, 建议从备份装载消费者而不是使用syncrepl执行一个完全的初始化装载.

=====设置提供者的slapd=====

提供者被实现为一个 overlay, 所以这个 overlay 本身在使用之前必须首先如 slapd.conf(5) 配置. 提供者只有两个配置指示, 在 contextCSN 上设定检查点和配置会话日志. 因为 LDAP 同步搜索受限于访问控制, 应为复制的内容设置正确的访问控制权限.

contextCSN检查点设置如下
<source lang="ini">
        syncprov-checkpoint <ops> <minutes>
</source>
检查点只在成功的写操作之后测试. 如果 <ops> 操作了或从上次检查点到现在超过了 <minutes> 时间, 将执行一个新的检查点.

会话日志设置如下
<source lang="ini">
        syncprov-sessionlog <size>
</source>
这里 <size> 是会话日志可以记录的条目的最大数量. 当一个会话日志被配置好, 它就自动用于所有对此数据库的 LDAP 同步搜索.

注意使用会话日志需要搜索 entryUUID 属性. 在这个属性上设一个 eq 索引将极有益于提供者服务器的会话日志的性能.

slapd.conf(5)中一个更复杂的例子内容如下:
<source lang="ini">
        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
</source>
=====设置消费者的slapd=====

在 slapd.conf(5) 的replica范围的数据库一节定义了syncrepl复制. syncrepl引擎是独立的后端并且可以使用任何数据库类型定义directive.
<source lang="ini">
        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
</source>
在这个例子中, 消费者将从ldap://provider.example.com的389端口连接到提供者 slapd(8) 来执行每天一次同步的拉操作(refreshOnly)模式. 它将以 cn=syncuser,dc=example,dc=com 绑定，以密码"secret"进行简单验证. 注意要在提供者服务器为cn=syncuser,dc=example,dc=com设置适当的访问控制权限以接收想要的复制内容. 另外提供者上的搜索限制必须足够高以允许同步用户接收请求内容完整的拷贝. 消费者使用 rootdn 写入它的数据库所以它总是有全部的权限来写所有的内容.

在上面的例子中同步搜索将在dc=example,dc=com的整个子树搜索 objectClass 是 organizationalPerson 的条目. 请求的属性是 cn, sn, ou, telephoneNumber, title, 和 l. schema 检查被关闭，这样当处理从提供者slapd(8)来的更新时消费者 slapd(8) 将不会强制对条目进行 schema 检查.

更多的详细信息参见 syncrepl 指示, 见本管理指南的slapd配置文件的syncrepl节.

=====启动提供者和消费者的slapd=====

提供者slapd(8)不需要重启. contextCSN将会根据需要自动生成: 它可能原来就包含在 LDIF 文件里, 由 slapadd (8) 生成, 在上下文中通过变更生成, 或当第一次 LDAP 同步搜索到达提供者时生成. 如果装载了一个之前不包含contextCSN的LDIF 文件, slapadd (8) 应使用 -w 选项来令它生成. 这将使服务器第一次运行时变得快一点.

当启动一个消费者 slapd(8) 时, 为了从一个特定的状态开始同步，它可能使用命令行参数 -c 即cookie选项，以提供一个同步cookie. cookie是一个逗号分隔的name=value对的列表. 目前支持的 syncrepl cookie 字段是 csn=<csn> 和 rid=<rid>. <csn>代表消费者复制的当前同步状态. <rid> 标识这个消费者服务器的一个本地消费者复制. 它用于把cookie关联到slapd.conf(5)中拥有匹配的复制标识的 syncrepl 定义. <rid>必须超过三位数. 命令行cookie会覆盖存储在消费者复制数据库中的同步cookie.

====Delta-syncrepl====

=====Delta-syncrepl提供者配置=====

设置 delta-syncrepl 需要同时改变主服务器和复制服务器的配置:
<source lang="ini">
     # 给予复制DN无限的读权限.  这个 ACL 需要和其他
     # ACL 声明合并, 并且/或者在数据库范围内移动
     # "by * break" 部分会执行随后的规则
     # 细节请看 slapd.access(5) .
     access to *
        by dn.base="cn=replicator,dc=symas,dc=com" read
        by * break

     # 设置模块路径
     modulepath /opt/symas/lib/openldap

     # 装载 hdb 后端
     moduleload back_hdb.la

     # 装载操作日志 overlay
     moduleload accesslog.la

     #装载 syncprov overlay
     moduleload syncprov.la

     # 操作日志数据库定义
     database hdb
     suffix cn=accesslog
     directory /db/accesslog
     rootdn cn=accesslog
     index default eq
     index entryCSN,objectClass,reqEnd,reqResult,reqStart

     overlay syncprov
     syncprov-nopresent TRUE
     syncprov-reloadhint TRUE

     # 让复制 DN 有无限的搜索权限
     limits dn.exact="cn=replicator,dc=symas,dc=com" time.soft=unlimited time.hard=unlimited size.soft=unlimited size.hard=unlimited

     # 主数据库定义
     database hdb
     suffix "dc=symas,dc=com"
     rootdn "cn=manager,dc=symas,dc=com"

     ## 任何期望的其他配置选项

     # syncprov 特别索引
     index entryCSN eq
     index entryUUID eq

     # 主数据库的syncrepl提供者
     overlay syncprov
     syncprov-checkpoint 1000 60

     # 主数据库的操作日志overlay定义
     overlay accesslog
     logdb cn=accesslog
     logops writes
     logsuccess TRUE
     # 每天扫描一次操作日志数据库, 并清除7天前的条目
     logpurge 07+00:00 01+00:00

     # 让复制DN有无限搜索权限
     limits dn.exact="cn=replicator,dc=symas,dc=com" time.soft=unlimited time.hard=unlimited size.soft=unlimited size.hard=unlimited
</source>
更多信息, 访问(slapo-accesslog(5) 和 slapd.conf(5))相关的 man 页

=====Delta-syncrepl消费者配置=====
<source lang="ini">
     # 复制数据库配置
     database hdb
     suffix "dc=symas,dc=com"
     rootdn "cn=manager,dc=symas,dc=com"

     ## 任何关于复制的其他配置, 例如你期望的索引
     ## 

     # syncrepl特有的索引
     index entryUUID eq

     # syncrepl参数
     syncrepl  rid=0
               provider=ldap://ldapmaster.symas.com:389
               bindmethod=simple
               binddn="cn=replicator,dc=symas,dc=com"
               credentials=secret
               searchbase="dc=symas,dc=com"
               logbase="cn=accesslog"
               logfilter="(&(objectClass=auditWriteObject)(reqResult=0))"
               schemachecking=on
               type=refreshAndPersist
               retry="60 +"
               syncdata=accesslog

     # 提交更新到主服务器
     updateref               ldap://ldapmaster.symas.com
</source>
以上配置假定你在你用于绑定到提供者的数据库中有一个复制者标识. 另外, 所有数据库 (主数据库, 复制数据库, 以及操作日志存储数据库) 也应该正确调整 DB_CONFIG 文件以满足你的需要.

====N-Way Multi-Master====

以下例子将使用三个主节点. Keeping in line with test050-syncrepl-multimaster of the OpenLDAP test suite, 我们将通过cn=config配置slapd(8)

这里设置配置数据库:
<source lang="ini">
     dn: cn=config
     objectClass: olcGlobal
     cn: config
     olcServerID: 1

     dn: olcDatabase={0}config,cn=config
     objectClass: olcDatabaseConfig
     olcDatabase: {0}config
     olcRootPW: secret
</source>
第二和第三服务器明显会有一个不同的 olcServerID:
<source lang="ini">
     dn: cn=config
     objectClass: olcGlobal
     cn: config
     olcServerID: 2

     dn: olcDatabase={0}config,cn=config
     objectClass: olcDatabaseConfig
     olcDatabase: {0}config
     olcRootPW: secret
</source>
这里设置 syncrepl 为提供者 (因为这些都是主服务器):
<source lang="ini">
     dn: cn=module,cn=config
     objectClass: olcModuleList
     cn: module
     olcModulePath: /usr/local/libexec/openldap
     olcModuleLoad: syncprov.la
</source>
现在我们设置第一个主节点 (使用你自己的确切的urls替换掉 $URI1, $URI2 和 $URI3 等.):
<source lang="ini">
     dn: cn=config
     changetype: modify
     replace: olcServerID
     olcServerID: 1 $URI1
     olcServerID: 2 $URI2
     olcServerID: 3 $URI3

     dn: olcOverlay=syncprov,olcDatabase={0}config,cn=config
     changetype: add
     objectClass: olcOverlayConfig
     objectClass: olcSyncProvConfig
     olcOverlay: syncprov

     dn: olcDatabase={0}config,cn=config
     changetype: modify
     add: olcSyncRepl
     olcSyncRepl: rid=001 provider=$URI1 binddn="cn=config" bindmethod=simple
       credentials=secret searchbase="cn=config" type=refreshAndPersist
       retry="5 5 300 5" timeout=1
     olcSyncRepl: rid=002 provider=$URI2 binddn="cn=config" bindmethod=simple
       credentials=secret searchbase="cn=config" type=refreshAndPersist
       retry="5 5 300 5" timeout=1
     olcSyncRepl: rid=003 provider=$URI3 binddn="cn=config" bindmethod=simple
       credentials=secret searchbase="cn=config" type=refreshAndPersist
       retry="5 5 300 5" timeout=1
     -
     add: olcMirrorMode
     olcMirrorMode: TRUE
</source>
现在启动主服务器和一个或多个消费者服务器, 也把上面的 LDIF 加入到第一个消费者, 第二个消费者等等. 然后它将复制cn=config. 你现在就在config数据库上拥有了多路多主机.

我们仍不得不复制实际的数据, 而不仅是 config, 所以添加下面这些到主服务器(所有激活的和配置好的消费者/主服务器将领取这个配置, 因为他们都是在同步的). 同样的, 以任何对你的安装可用的设置替换所有 ${} 变量:
<source lang="ini">
     dn: olcDatabase={1}$BACKEND,cn=config
     objectClass: olcDatabaseConfig
     objectClass: olc${BACKEND}Config
     olcDatabase: {1}$BACKEND
     olcSuffix: $BASEDN
     olcDbDirectory: ./db
     olcRootDN: $MANAGERDN
     olcRootPW: $PASSWD
     olcLimits: dn.exact="$MANAGERDN" time.soft=unlimited time.hard=unlimited size.soft=unlimited size.hard=unlimited
     olcSyncRepl: rid=004 provider=$URI1 binddn="$MANAGERDN" bindmethod=simple
       credentials=$PASSWD searchbase="$BASEDN" type=refreshOnly
       interval=00:00:00:10 retry="5 5 300 5" timeout=1
     olcSyncRepl: rid=005 provider=$URI2 binddn="$MANAGERDN" bindmethod=simple
       credentials=$PASSWD searchbase="$BASEDN" type=refreshOnly
       interval=00:00:00:10 retry="5 5 300 5" timeout=1
     olcSyncRepl: rid=006 provider=$URI3 binddn="$MANAGERDN" bindmethod=simple
       credentials=$PASSWD searchbase="$BASEDN" type=refreshOnly
       interval=00:00:00:10 retry="5 5 300 5" timeout=1
     olcMirrorMode: TRUE

     dn: olcOverlay=syncprov,olcDatabase={1}${BACKEND},cn=config
     changetype: add
     objectClass: olcOverlayConfig
     objectClass: olcSyncProvConfig
     olcOverlay: syncprov
</source>
注意: 你的所有服务器始终必须使用例如 NTP http://www.ntp.org/, 原子钟, 或一些其他可用的时间参照物紧紧同步.

注意: 如slapd-config(5)指出的, 在 olcSyncRepl 指示中定义的 URLs 是从它们那里复制的服务器的 URLs. 这些必须准确地匹配 slapd 监听(命令行参数选项的 -h )的URLs . 否则 slapd 可能尝试从它自身复制, 而导致循环.

====MirrorMode====

镜像模式配置实际上非常容易. 如果你已经配置了一个普通的 slapd syncrepl 提供者, 那么唯一的改变就是以下两个参数:
<source lang="ini">
       mirrormode  on
       serverID    1
</source>
注意: 你需要确保每个镜像的serverID是不同的并且把它作为一个全球配置选项.

=====Mirror Node配置=====

第一步是配置syncrepl提供者，就像 配置提供者slapd 一节写的那样.

注意: Delta-syncrepl还不支持镜像模式.

这里是从一个在refreshAndPersist模式下使用LDAP同步复制的例子中截取的片断:

镜像模式节点 1:
<source lang="ini">
       # 全球部分
       serverID    1
       # 数据库部分

       # syncrepl参数
       syncrepl      rid=001
                     provider=ldap://ldap-sid2.example.com
                     bindmethod=simple
                     binddn="cn=mirrormode,dc=example,dc=com"
                     credentials=mirrormode
                     searchbase="dc=example,dc=com"
                     schemachecking=on
                     type=refreshAndPersist
                     retry="60 +"

       mirrormode on
</source>
镜像模式节点 2:
<source lang="ini">
       # 全球部分
       serverID    2
       # 数据库部分

       # syncrepl参数
       syncrepl      rid=001
                     provider=ldap://ldap-sid1.example.com
                     bindmethod=simple
                     binddn="cn=mirrormode,dc=example,dc=com"
                     credentials=mirrormode
                     searchbase="dc=example,dc=com"
                     schemachecking=on
                     type=refreshAndPersist
                     retry="60 +"

       mirrormode on
</source>
它真的很简单; 每个镜像模式节点设置得完全一样, 除了 serverID 是唯一的, 并且每个消费者都被指向另一个服务器.

======容错配置======

这通常有两个选择; 1. 硬件代理/负载均衡 或 专用的代理软件, 2. 使用一个 Back-LDAP 代理作为一个 syncrepl 提供者

一个典型的企业例子可能是:

[http://www.openldap.org/doc/admin24/dual_dc.png http://www.openldap.org/doc/admin24/dual_dc.png]

图 X.Y: 在一个双数据中心配置中使用镜像模式

======标准消费者配置======

这和设置消费者slapd一节完全一样. 可以设置一个普通的复制模式, 也可以使用 delta-syncrepl 复制模式.

=====MirrorMode总结=====

现在你将有一个目录架构提供单主服务器复制的全部一致性保障, 同时也提供多主服务器复制的高可用性.

====Syncrepl代理====

Figure X.Y: Replacing slurpd

The following example is for a self-contained push-based replication solution:

        #######################################################################
        # Standard OpenLDAP Master/Provider
        #######################################################################

        include     /usr/local/etc/openldap/schema/core.schema
        include     /usr/local/etc/openldap/schema/cosine.schema
        include     /usr/local/etc/openldap/schema/nis.schema
        include     /usr/local/etc/openldap/schema/inetorgperson.schema

        include     /usr/local/etc/openldap/slapd.acl

        modulepath  /usr/local/libexec/openldap
        moduleload  back_hdb.la
        moduleload  syncprov.la
        moduleload  back_monitor.la
        moduleload  back_ldap.la

        pidfile     /usr/local/var/slapd.pid
        argsfile    /usr/local/var/slapd.args

        loglevel    sync stats

        database    hdb
        suffix      "dc=suretecsystems,dc=com"
        directory   /usr/local/var/openldap-data

        checkpoint      1024 5
        cachesize       10000
        idlcachesize    10000

        index       objectClass eq
        # rest of indexes
        index       default     sub

        rootdn          "cn=admin,dc=suretecsystems,dc=com"
        rootpw          testing

        # syncprov specific indexing
        index entryCSN eq
        index entryUUID eq

        # syncrepl Provider for primary db
        overlay syncprov
        syncprov-checkpoint 1000 60

        # Let the replica DN have limitless searches
        limits dn.exact="cn=replicator,dc=suretecsystems,dc=com" time.soft=unlimited time.hard=unlimited size.soft=unlimited size.hard=unlimited

        database    monitor

        database    config
        rootpw          testing

        ##############################################################################
        # Consumer Proxy that pulls in data via Syncrepl and pushes out via slapd-ldap
        ##############################################################################

        database        ldap
        # ignore conflicts with other databases, as we need to push out to same suffix
        hidden              on
        suffix          "dc=suretecsystems,dc=com"
        rootdn          "cn=slapd-ldap"
        uri             ldap://localhost:9012/

        lastmod         on

        # We don't need any access to this DSA
        restrict        all

        acl-bind        bindmethod=simple
                        binddn="cn=replicator,dc=suretecsystems,dc=com"
                        credentials=testing

        syncrepl        rid=001
                        provider=ldap://localhost:9011/
                        binddn="cn=replicator,dc=suretecsystems,dc=com"
                        bindmethod=simple
                        credentials=testing
                        searchbase="dc=suretecsystems,dc=com"
                        type=refreshAndPersist
                        retry="5 5 300 5"

        overlay         syncprov

A replica configuration for this type of setup could be:

        #######################################################################
        # Standard OpenLDAP Slave without Syncrepl
        #######################################################################

        include     /usr/local/etc/openldap/schema/core.schema
        include     /usr/local/etc/openldap/schema/cosine.schema
        include     /usr/local/etc/openldap/schema/nis.schema
        include     /usr/local/etc/openldap/schema/inetorgperson.schema

        include     /usr/local/etc/openldap/slapd.acl

        modulepath  /usr/local/libexec/openldap
        moduleload  back_hdb.la
        moduleload  syncprov.la
        moduleload  back_monitor.la
        moduleload  back_ldap.la

        pidfile     /usr/local/var/slapd.pid
        argsfile    /usr/local/var/slapd.args

        loglevel    sync stats

        database    hdb
        suffix      "dc=suretecsystems,dc=com"
        directory   /usr/local/var/openldap-slave/data

        checkpoint      1024 5
        cachesize       10000
        idlcachesize    10000

        index       objectClass eq
        # rest of indexes
        index       default     sub

        rootdn          "cn=admin,dc=suretecsystems,dc=com"
        rootpw          testing

        # Let the replica DN have limitless searches
        limits dn.exact="cn=replicator,dc=suretecsystems,dc=com" time.soft=unlimited time.hard=unlimited size.soft=unlimited size.hard=unlimited

        updatedn "cn=replicator,dc=suretecsystems,dc=com"

        # Refer updates to the master
        updateref   ldap://localhost:9011

        database    monitor

        database    config
        rootpw          testing

You can see we use the updatedn directive here and example ACLs (usr/local/etc/openldap/slapd.acl) for this could be:

        # Give the replica DN unlimited read access.  This ACL may need to be
        # merged with other ACL statements.

        access to *
             by dn.base="cn=replicator,dc=suretecsystems,dc=com" write
             by * break

        access to dn.base=""
                by * read

        access to dn.base="cn=Subschema"
                by * read

        access to dn.subtree="cn=Monitor"
            by dn.exact="uid=admin,dc=suretecsystems,dc=com" write
            by users read
            by * none

        access to *
                by self write
                by * read

In order to support more replicas, just add more database ldap sections and increment the syncrepl rid number accordingly.

Note: You must populate the Master and Slave directories with the same data, unlike when using normal Syncrepl

If you do not have access to modify the master directory configuration you can configure a standalone ldap proxy, which might look like:

Figure X.Y: Replacing slurpd with a standalone version

The following configuration is an example of a standalone LDAP Proxy:

        include     /usr/local/etc/openldap/schema/core.schema
        include     /usr/local/etc/openldap/schema/cosine.schema
        include     /usr/local/etc/openldap/schema/nis.schema
        include     /usr/local/etc/openldap/schema/inetorgperson.schema

        include     /usr/local/etc/openldap/slapd.acl

        modulepath  /usr/local/libexec/openldap
        moduleload  syncprov.la
        moduleload  back_ldap.la

        ##############################################################################
        # Consumer Proxy that pulls in data via Syncrepl and pushes out via slapd-ldap
        ##############################################################################

        database        ldap
        # ignore conflicts with other databases, as we need to push out to same suffix
        hidden              on
        suffix          "dc=suretecsystems,dc=com"
        rootdn          "cn=slapd-ldap"
        uri             ldap://localhost:9012/

        lastmod         on

        # We don't need any access to this DSA
        restrict        all

        acl-bind        bindmethod=simple
                        binddn="cn=replicator,dc=suretecsystems,dc=com"
                        credentials=testing

        syncrepl        rid=001
                        provider=ldap://localhost:9011/
                        binddn="cn=replicator,dc=suretecsystems,dc=com"
                        bindmethod=simple
                        credentials=testing
                        searchbase="dc=suretecsystems,dc=com"
                        type=refreshAndPersist
                        retry="5 5 300 5"

        overlay         syncprov

As you can see, you can let your imagination go wild using Syncrepl and slapd-ldap(8) tailoring your replication to fit your specific network topology.