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Chapter 6. Configuring cascading replication using the command line

In a cascading replication scenario, one server, a hub, acts both as a consumer and a supplier. The hub is a read-only replica that maintains a changelog. It receives updates from the supplier and supplies these updates to a consumer. Use cascading replication for balancing heavy traffic loads or to keep suppliers based locally in geographically-distributed environments.

6.1. Preparing the new hub server using the command line

To prepare the hub.example.com host, enable replication. This process:

  • Configures the role of this server in the replication topology
  • Defines the suffix that is replicated
  • Creates the replication manager account the supplier uses to connect to this host

Perform this procedure on the hub that you want to add to the replication topology.

Prerequisites

  • You installed the Directory Server instance.
  • The database for the dc=example,dc=com suffix exists.

Procedure

  • Enable replication for the dc=example,dc=com suffix:

    # dsconf -D "cn=Directory Manager" ldap://hub.example.com replication enable --suffix "dc=example,dc=com" --role "hub" --bind-dn "cn=replication manager,cn=config" --bind-passwd "password"

    This command configures the hub.example.com host as a hub for the dc=example,dc=com suffix. Additionally, the command creates the cn=replication manager,cn=config user with the specified password and allows this account to replicate changes for the suffix to this host.

Verification

  • Display the replication configuration:

    # dsconf -D "cn=Directory Manager" ldap://hub.example.com replication get --suffix "dc=example,dc=com"
    dn: cn=replica,cn=dc\3Dexample\2Cdc\3Dcom,cn=mapping tree,cn=config
    ...
    nsDS5ReplicaBindDN: cn=replication manager,cn=config
    nsDS5ReplicaRoot: dc=example,dc=com
    nsDS5ReplicaType: 2
    nsDS5ReplicaId: 65535
    ...

    These parameters indicate:

    • nsDS5ReplicaBindDN specifies the replication manager account.
    • nsDS5ReplicaRoot sets the suffix that is replicated.
    • nsDS5ReplicaType set to 2 defines that this host is a consumer, which is also valid for a hub.
    • nsDS5ReplicaId set to 65535 defines that this host is a hub. The dsconf utility automatically sets this value if you define the --role "hub" option.

6.2. Configuring the existing server as a supplier to the hub server using the command line

To prepare the existing server as a supplier, you need to:

  • Enable replication for the suffix.
  • Create a replication agreement to the hub.
  • Initialize the hub.

Perform this procedure on the existing supplier in the replication topology.

Prerequisites

  • You enabled replication for the dc=example,dc=com suffix on the hub to join.

Procedure

  1. Enable replication for the dc=example,dc=com suffix:

    # [command]`dsconf -D "cn=Directory Manager" ldap://supplier.example.com replication enable --suffix "dc=example,dc=com" --role "supplier" --replica-id 1

    This command configures the supplier.example.com host as a supplier for the dc=example,dc=com suffix, and sets the replica ID of this entry to 1.

    Important

    The replica ID must be a unique integer between 1 and 65534 for a suffix across all suppliers in the topology.

  2. Add the replication agreement and initialize the new server:

    # dsconf -D "cn=Directory Manager" ldap://supplier.example.com repl-agmt create --suffix "dc=example,dc=com" --host "hub.example.com" --port 389 --conn-protocol LDAP --bind-dn "cn=replication manager,cn=config" --bind-passwd "password" --bind-method SIMPLE --init example-agreement-supplier-to-hub

    This command creates a replication agreement named example-agreement-supplier-to-hub. The replication agreement defines settings, such as the hub’s host name, protocol, and authentication information that the supplier uses when connecting and replicating data to the hub.

    After the agreement was created, Directory Server initializes hub.example.com. Depending on the amount of data to replicate, initialization can be time-consuming.

Verification

  1. Display the replication configuration:

    # dsconf -D "cn=Directory Manager" ldap://supplier.example.com replication get --suffix "dc=example,dc=com"
    dn: cn=replica,cn=dc\3Dexample\2Cdc\3Dcom,cn=mapping tree,cn=config
    ...
    nsDS5ReplicaRoot: dc=example,dc=com
    nsDS5ReplicaType: 3
    ...

    These parameters indicate:

    • nsDS5ReplicaRoot sets the suffix that is replicated.
    • nsDS5ReplicaType set to 3 defines that this host is a supplier.
  2. Verify whether the initialization was successful:

    # dsconf -D "cn=Directory Manager" ldap://supplier.example.com repl-agmt init-status --suffix "dc=example,dc=com" example-agreement-supplier-to-hub
    Agreement successfully initialized.
  3. Display the replication status:

    # dsconf -D "cn=Directory Manager" ldap://supplier.example.com repl-agmt status --suffix "dc=example,dc=com" example-agreement-supplier-to-hub
    Status For Agreement: "example-agreement-supplier-to-hub" (hub.example.com:389)
    Replica Enabled: on
    Update In Progress: FALSE
    Last Update Start: 20210331105030Z
    Last Update End: 20210331105030Z
    Number Of Changes Sent: 0
    Number Of Changes Skipped: None
    Last Update Status: Error (0) Replica acquired successfully: Incremental update succeeded
    Last Init Start: 20210331105026Z
    Last Init End: 20210331105029Z
    Last Init Status: Error (0) Total update succeeded
    Reap Active: 0
    Replication Status: Not in Synchronization: supplier (Unknown) consumer (Unknown) State (green) Reason (error (0) replica acquired successfully: incremental update succeeded)
    Replication Lag Time: Unavailable

    Verify the Replication Status and Last Update Status fields.

Troubleshooting

  1. By default, the replication idle timeout for all agreements on a server is 1 hour. If the initialization of large databases fails due to timeouts, set the nsslapd-idletimeout parameter to a higher value. For example, to set the parameter to 7200 (2 hours), enter:

    # dsconf -D "cn=Directory Manager" ldap://supplier1.example.com config replace nsslapd-idletimeout=7200

    To set an unlimited period, set nsslapd-idletimeout to 0.

6.3. Preparing the new consumer of the hub using the command line

To prepare the consumer.example.com host, enable replication. This process:

  • Configures the role of this server in the replication topology
  • Defines the suffix that is replicated
  • Creates the replication manager account the hub uses to connect to this host

Perform this procedure on the consumer that you want to add to the replication topology.

Prerequisites

  • You installed the Directory Server instance.
  • The database for the dc=example,dc=com suffix exists.

Procedure

  • Enable replication for the dc=example,dc=com suffix:

    # dsconf -D "cn=Directory Manager" ldap://consumer.example.com replication enable --suffix "dc=example,dc=com" --role "consumer" --bind-dn "cn=replication manager,cn=config" --bind-passwd "password"

    This command configures the consumer.example.com host as a consumer for the dc=example,dc=com suffix. Additionally, the command creates the cn=replication manager,cn=config user with the specified password and allows this account to replicate changes for the suffix to this host.

Verification

  • Display the replication configuration:

    # dsconf -D "cn=Directory Manager" ldap://consumer.example.com replication get --suffix "dc=example,dc=com"
    dn: cn=replica,cn=dc\3Dexample\2Cdc\3Dcom,cn=mapping tree,cn=config
    ...
    nsDS5ReplicaBindDN: cn=replication manager,cn=config
    nsDS5ReplicaRoot: dc=example,dc=com
    nsDS5ReplicaType: 2
    ...

    These parameters indicate:

    • nsDS5ReplicaBindDN specifies the replication manager account.
    • nsDS5ReplicaRoot sets the suffix that is replicated.
    • nsDS5ReplicaType set to 2 defines that this host is a consumer.

6.4. Configuring the hub server as a supplier for the consumer using the command line

To prepare the hub, you need to:

  • Create a replication agreement to the consumer.
  • Initialize the consumer.

Perform this procedure on the hub in the replication topology.

Prerequisites

  • The hub is initialized, and replication from the supplier to the hub works.
  • You enabled replication for the dc=example,dc=com suffix on the hub.

Procedure

  • Add the replication agreement and initialize the consumer:

    # dsconf -D "cn=Directory Manager" ldap://hub.example.com repl-agmt create --suffix "dc=example,dc=com" --host "consumer.example.com" --port 389 --conn-protocol LDAP --bind-dn "cn=replication manager,cn=config" --bind-passwd "password" --bind-method SIMPLE --init example-agreement-hub-to-consumer

    This command creates a replication agreement named example-agreement-hub-to-consumer. The replication agreement defines settings, such as the consumer’s host name, protocol, and authentication information that the supplier uses when connecting and replicating data to this consumer.

    After the agreement was created, Directory Server initializes consumer.example.com. Depending on the amount of data to replicate, initialization can be time-consuming.

Verification

  1. Verify whether the initialization was successful:

    # dsconf -D "cn=Directory Manager" ldap://hub.example.com repl-agmt init-status --suffix "dc=example,dc=com" example-agreement-hub-to-consumer
    Agreement successfully initialized.
  2. Display the replication status:

    # dsconf -D "cn=Directory Manager" ldap://hub.example.com repl-agmt status --suffix "dc=example,dc=com" example-agreement-hub-to-consumer
    Status For Agreement: "example-agreement-hub-to-consumer" (consumer.example.com:389)
    Replica Enabled: on
    Update In Progress: FALSE
    Last Update Start: 20210331131534Z
    Last Update End: 20210331131534Z
    Number Of Changes Sent: 0
    Number Of Changes Skipped: None
    Last Update Status: Error (0) Replica acquired successfully: Incremental update succeeded
    Last Init Start: 20210331131530Z
    Last Init End: 20210331131533Z
    Last Init Status: Error (0) Total update succeeded
    Reap Active: 0
    Replication Status: Not in Synchronization: supplier (Unknown) consumer (Unknown) State (green) Reason (error (0) replica acquired successfully: incremental update succeeded)
    Replication Lag Time: Unavailable

    Verify the Replication Status and Last Update Status fields.

Troubleshooting

  1. By default, the replication idle timeout for all agreements on a server is 1 hour. If the initialization of large databases fails due to timeouts, set the nsslapd-idletimeout parameter to a higher value. For example, to set the parameter to 7200 (2 hours), enter:

    # dsconf -D "cn=Directory Manager" ldap://hub .example.com config replace nsslapd-idletimeout=7200

    To set an unlimited period, set nsslapd-idletimeout to 0.