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Chapter 3. Preparing for server loss with replication

Follow these guidelines to establish a replication topology that will allow you to respond to losing a server.

3.1. Guidelines for connecting IdM replicas in a topology
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Connect each replica to at least two other replicas
This ensures that information is replicated not just between the initial replica and the first server you installed, but between other replicas as well.
Connect a replica to a maximum of four other replicas (not a hard requirement)

A large number of replication agreements per server does not add significant benefits. A receiving replica can only be updated by one other replica at a time and meanwhile, the other replication agreements are idle. More than four replication agreements per replica typically means a waste of resources.

Note

This recommendation applies to both certificate replication and domain replication agreements.

There are two exceptions to the limit of four replication agreements per replica:

  • You want failover paths if certain replicas are not online or responding.
  • In larger deployments, you want additional direct links between specific nodes.

Configuring a high number of replication agreements can have a negative impact on overall performance: when multiple replication agreements in the topology are sending updates, certain replicas can experience a high contention on the changelog database file between incoming updates and the outgoing updates.

If you decide to use more replication agreements per replica, ensure that you do not experience replication issues and latency. However, note that large distances and high numbers of intermediate nodes can also cause latency problems.

Connect the replicas in a data center with each other
This ensures domain replication within the data center.
Connect each data center to at least two other data centers
This ensures domain replication between data centers.
Connect data centers using at least a pair of replication agreements
If data centers A and B have a replication agreement from A1 to B1, having a replication agreement from A2 to B2 ensures that if one of the servers is down, the replication can continue between the two data centers.

3.2. Replica topology examples
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You can create a reliable replica topology by using one of the following examples.

Figure 3.1. Replica topology with four data centers, each with four servers that are connected with replication agreements

A diagram showing 4 data centers - Geo 1 through 4. Each data center has four servers connected to each other with replication agreements. There are also replication agreements connecting two servers from Geo 1 to two servers in Geo 2. This pattern continues with two servers in Geo 2 connected to two servers in Geo 3 and two servers in Geo 3 connected to Geo 4. This connects each data center so each server is at most 3 hops away from another Geo.
View larger image
A diagram showing 4 data centers - Geo 1 through 4. Each data center has four servers connected to each other with replication agreements. There are also replication agreements connecting two servers from Geo 1 to two servers in Geo 2. This pattern continues with two servers in Geo 2 connected to two servers in Geo 3 and two servers in Geo 3 connected to Geo 4. This connects each data center so each server is at most 3 hops away from another Geo.



Figure 3.2. Replica topology with three data centers, each with a different number of servers that are all interconnected through replication agreements

A diagram showing 3 data centers: Geo 1 has 5 servers each connected to the other - Geo 2 has two servers connected to each other - Geo 3 has 3 servers connected in a triangle. There are 2 connections from each Geo connecting two of its servers to 2 servers in the next Geo.
View larger image
A diagram showing 3 data centers: Geo 1 has 5 servers each connected to the other - Geo 2 has two servers connected to each other - Geo 3 has 3 servers connected in a triangle. There are 2 connections from each Geo connecting two of its servers to 2 servers in the next Geo.

3.3. Protecting IdM CA data
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If your deployment contains the integrated IdM Certificate Authority (CA), install several CA replicas so you can create additional CA replicas if one is lost.

Procedure

  1. Configure three or more replicas to provide CA services.

    1. To install a new replica with CA services, run ipa-replica-install with the --setup-ca option. 

      [root@server ~]# ipa-replica-install --setup-ca
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    2. To install CA services on a preexisting replica, run ipa-ca-install. 

      [root@replica ~]# ipa-ca-install
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  2. Create CA replication agreements between your CA replicas. 

    [root@careplica1 ~]# ipa topologysegment-add
Suffix name: ca
Left node: ca-replica1.example.com
Right node: ca-replica2.example.com
Segment name [ca-replica1.example.com-to-ca-replica2.example.com]: new_segment
---------------------------
Added segment "new_segment"
---------------------------
  Segment name: new_segment
  Left node: ca-replica1.example.com
  Right node: ca-replica2.example.com
  Connectivity: both
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Warning

If only one server provides CA services and it is damaged, the entire environment will be lost. If you use the IdM CA, Red Hat strongly recommends having three or more replicas with CA services installed, with CA replication agreements between them.

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