How replication works

To enable replication between two HCP systems, you create a replication link between the two systems. A replication link is a secure, trusted relationship between two systems. The link determines what is replicated and how data is transmitted between the systems.

Link types

A replication link can be active/active or active/passive:

With an active/active link, the HCP tenants and namespaces and default-namespace directories being replicated are read-write on both systems, and data is replicated in both directions between the two systems. Active/active links are designed for use in environments where applications need seamless access to namespaces regardless of where those applications are located. All links in an erasure coding topology must be active/active links.

With an active/active link, HCP gives preference to synchronizing object metadata between the two systems, followed by data synchronization. This preferential treatment enables the fastest access to objects regardless of which system they were created on. If a full copy of the data for an object or a chunk for the object has not yet been replicated to the system targeted by a client request, HCP can read all the object data from a remote system. Additionally, synchronizing metadata first ensures that object names are reserved as quickly as possible.

Note: Until a full copy of the data for an object or a chunk for the object is replicated to a given system, the object is metadata-only on that system.

With an active/passive link, the HCP tenants and namespaces and default-namespace directories being replicated are read-write on one system, called the primary system, and read-only on the other system, called the replica. In this case, data is replicated in one direction only — from the primary system to the replica. Active/passive links are designed to support read-only content sharing and disaster recovery.

With an active/passive link, HCP replicates object data and metadata together, thereby ensuring that objects are complete on the replica and can be recovered should that become necessary. However, this approach means that the replica cannot service requests for an object until that object is fully replicated.

To a primary system, an active/passive link is an outbound link; that is, the system is sending data through the link. To a replica, an active/passive link is an inbound link; that is, the system is receiving data through the link. To both systems involved in an active/active link, the link functions as both an outbound link and an inbound link.

Link security

Replication relies on SSL to ensure the integrity and security of transmitted data. Before you can create a replication link, the two systems involved in the link must each have a valid SSL server certificate. Each system must also have the server certificate from the other system installed as a trusted replication server certificate.

Replication priority

Replication is asynchronous with object ingest. An HCP system can, therefore, develop a backlog of objects to be replicated.

You can choose to have the Replication service work on objects with the oldest changes first, regardless of which namespaces they’re in. Alternatively, you can have the service balance its processing time evenly across the namespaces being replicated. In this case, the service may replicate recent changes in some namespaces before older changes in others.

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