openstack-manuals/doc/arch-design/multi_site/section_architecture_multi_site.xml

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  xml:id="arch-design-architecture-multiple-site">
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    <title>Architecture</title>
    <para><xref linkend="multi-site_arch"/>
      illustrates a high level multi-site OpenStack
      architecture. Each site is an OpenStack cloud but it may be necessary
      to architect the sites on different versions. For example, if the
      second site is intended to be a replacement for the first site,
      they would be different. Another common design would be a private
      OpenStack cloud with a replicated site that would be used for high
      availability or disaster recovery. The most important design decision
      is configuring storage as a single shared pool or separate pools,
      depending on user and technical requirements.</para>
   <figure xml:id="multi-site_arch">
     <title>Multi-site OpenStack architecture</title>
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                fileref="../figures/Multi-Site_shared_keystone_horizon_swift1.png"/>
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    <section xml:id="openstack-services-architecture">
        <title>OpenStack services architecture</title>
        <para>The OpenStack Identity service, which is used by all other
            OpenStack components for authorization and the catalog of
            service endpoints, supports the concept of regions. A region
            is a logical construct used to group OpenStack services in
            close proximity to one another. The concept of
            regions is flexible; it may can contain OpenStack service
            endpoints located within a distinct geographic region or regions.
            It may be smaller in scope, where a region is a single rack
            within a data center, with multiple regions existing in adjacent
            racks in the same data center.</para>
        <para>The majority of OpenStack components are designed to run
          within the context of a single region. The OpenStack Compute
          service is designed to manage compute resources within a region,
          with support for subdivisions of compute resources by using
          availability zones and cells. The OpenStack Networking service
          can be used to manage network resources in the same broadcast
          domain or collection of switches that are linked. The OpenStack
          Block Storage service controls storage resources within a region
          with all storage resources residing on the same storage network.
          Like the OpenStack Compute service, the OpenStack Block Storage
          service also supports the availability zone construct which can
          be used to subdivide storage resources.</para>
        <para>The OpenStack dashboard, OpenStack Identity, and OpenStack
            Object Storage services are components that can each be deployed
            centrally in order to serve multiple regions.</para>
    </section>
    <section xml:id="arch-multi-storage">
        <title>Storage</title>
        <para>With multiple OpenStack regions, it is recommended to configure
          a single OpenStack Object Storage service endpoint to deliver
          shared file storage for all regions. The Object Storage service
          internally replicates files to multiple nodes which can be used
          by applications or workloads in multiple regions. This simplifies
          high availability failover and disaster recovery rollback.</para>
        <para>In order to scale the Object Storage service to meet the workload
            of multiple regions, multiple proxy workers are run and
            load-balanced, storage nodes are installed in each region, and the
            entire Object Storage Service can be fronted by an HTTP caching
            layer. This is done so client requests for objects can be served out
            of caches rather than directly from the storage modules themselves,
            reducing the actual load on the storage network. In addition to an
            HTTP caching layer, use a caching layer like Memcache to cache
            objects between the proxy and storage nodes.</para>
        <para>If the cloud is designed with a separate Object Storage
            Service endpoint made available in each region, applications are
            required to handle synchronization (if desired) and other management
            operations to ensure consistency across the nodes. For some
            applications, having multiple Object Storage Service endpoints
            located in the same region as the application may be desirable due
            to reduced latency, cross region bandwidth, and ease of
            deployment.</para>
          <note>
            <para>For the Block Storage service, the most important decisions
              are the selection of the storage technology, and whether
              a dedicated network is used to carry storage traffic
              from the storage service to the compute nodes.</para>
          </note>
    </section>
    <section xml:id="arch-networking-multiple">
        <title>Networking</title>
        <para>When connecting multiple regions together there are several design
            considerations. The overlay network technology choice determines how
            packets are transmitted between regions and how the logical network
            and addresses present to the application. If there are security or
            regulatory requirements, encryption should be implemented to secure
            the traffic between regions. For networking inside a region, the
            overlay network technology for tenant networks is equally important.
            The overlay technology and the network traffic of an application
            generates or receives can be either complementary or be at cross
            purpose. For example, using an overlay technology for an application
            that transmits a large amount of small packets could add excessive
            latency or overhead to each packet if not configured
            properly.</para>
    </section>
    <section xml:id="arch-dependencies-multiple">
        <title>Dependencies</title>
        <para>The architecture for a multi-site OpenStack installation
          is dependent on a number of factors. One major dependency to
          consider is storage. When designing the storage system, the
          storage mechanism needs to be determined. Once the storage
          type is determined, how it is accessed is critical. For example,
          we recommend that storage should use a dedicated network.
          Another concern is how the storage is configured to protect
          the data. For example, the Recovery Point Objective (RPO) and
          the Recovery Time Objective (RTO). How quickly recovery from
          a fault can be completed, determines how often the replication of
          data is required. Ensure that enough storage is allocated to
          support the data protection strategy.
      </para>
        <para>Networking decisions include the encapsulation mechanism that can
            be used for the tenant networks, how large the broadcast domains
            should be, and the contracted SLAs for the interconnects.</para>
    </section>
</section>