An Oracle Data Guard implementation requires two DB systems, one containing the primary database and one containing the standby database. When you enable Oracle Data Guard for a virtual machine DB system database, a new DB system with the standby database is created and associated with the primary database. For a bare metal DB system, the DB system with the database that you want to use as the standby must already exist before you enable Oracle Data Guard.
Requirement details are as follows:
- Both DB systems must be in the same compartment.
- The DB systems must be the same shape type (for example, if the shape of the primary database is a virtual machine, then the shape of the standby database can be any other virtual machine shape).
- If your primary and standby databases are in different regions, then you must peer the virtual cloud networks (VCNs) for each database. See Remote VCN Peering (Across Regions).
- Configure the security list ingress and egress rules for the subnets of both DB systems in the Oracle Data Guard association to enable TCP traffic to move between the applicable ports. Ensure that the rules you create are stateful (the default).

Explanation
A fault domain is a grouping of hardware and infrastructure within an availability domain. Each availability domain contains three fault domains. Fault domains provide anti-affinity: they let you distribute your instances so that the instances are not on the same physical hardware within a single availability domain. A hardware failure or Compute hardware maintenance event that affects one fault domain does not affect instances in other fault domains. In addition, the physical hardware in a fault domain has independent and redundant power supplies, which prevents a failure in the power supply hardware within one fault domain from affecting other fault domains.
To control the placement of your compute instances, bare metal DB system instances, or virtual machine DB system instances, you can optionally specify the fault domain for a new instance or instance pool at launch time. If you don't specify the fault domain, the system selects one for you. Oracle Cloud Infrastructure makes a best-effort anti-affinity placement across different fault domains, while optimizing for available capacity in the availability domain. To change the fault domain for an instance, terminate it and launch a new instance in the preferred fault domain.
Use fault domains to do the following things:
Protect against unexpected hardware failures or power supply failures.
Protect against planned outages because of Compute hardware maintenance.

Explanation
a policy statement must specify the compartment for which access is being granted (or the tenancy). Where you create the policy determines who can update the policy. If you attach the policy to the compartment or its parent, you can simply specify the compartment name. If you attach the policy further up the hierarchy, you must specify the path. The format of the path is each compartment name (or OCID) in the path, separated by a colon:
<compartment_level_1>:<compartment_level_2>: . . . <compartment_level_n> For example, assume you have a three-level compartment hierarchy, shown here:

You want to create a policy to allow NetworkAdmins to manage VCNs in CompartmentC. If you want to attach this policy to CompartmentC or to its parent, CompartmentB, write this policy statement:
Allow group NewtworkAdmins to manage virtual-network-family in compartment CompartmentC However, if you want to attach this policy to CompartmentA (so that only administrators of CompartmentA can modify it), write this policy statement that specifies the path:
Allow group NewtworkAdmins to manage virtual-network-family in compartment CompartmentB:CompartmentC To attach this policy to the tenancy, write this policy statement that specifies the path from CompartmentA to CompartmentC:
Allow group NewtworkAdmins to manage virtual-network-family in compartment CompartmentA:CompartmentB:CompartmentC

You can think of a DRG as a virtual router that provides a path for private traffic (that is, traffic that uses private IPv4 addresses) between your VCN and networks outside the VCN's region.
You use a DRG when connecting your existing on-premises network to your virtual cloud network (VCN) with one (or both) of these:
IPSec VPN
Oracle Cloud Infrastructure FastConnect (Private Only)
You also use a DRG when peering a VCN with a VCN in a different region:
Remote VCN Peering (Across Regions)