Scenario analysis doc - general issues for VNF HA

Scenario Analysis doc - general issues for VNF HA
JIRA:HA 15

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+2. Discussion for the General Issues for VNF HA schemes
+===========================================================
+
+This section is intended to talk about some general issues in the VNF HA schemes.
+In sections 1, the usecases of both stateful and stateless VNFs are discussed.
+While in this section, we would like to discuss some specific issues
+which are quite general for all the usecases proposed in the previous sections.
+
+1.1. VNF External Interfacece
+
+Regardless whether the VNF is stateful or stateless, all the VNFCs should act as
+a union from the perspective of the outside world. That means all the VNFCs should
+share a common interface where the outside modules (e.g., the other VNFs) can
+access the service from. There could be multiple solutions for this share of IP
+interface. However, all of this sharing and switching of IP address should be
+ignorant to the outside modules.
+
+There are several approaches for the VNFs to share the interfaces. A few of them
+are listed as follows and will be discussed in detail. 
+
+1) IP address of active/stand-by VM.
+
+2) Load balancers for active/active use cases
+
+Note that combinition of these two approaches is also feasible.
+
+For active/standby VNFCs, the HA manager will manage a common IP address
+to the active and standby VMs, so that they look as one instance from outside.
+(The HA manager may not be aware of this, I.e. the address may be configured
+and the active/standby state management is linked to the possession of the IP
+address, i.e. the active VNFC claims it as part of becoming active.) Only the
+active one possesses the IP address. And when failover happens, the standby
+is set to be active and can take possession of the IP address to continue traffic
+process.
+
+..[MT] In general I would rather say that the IP address is managed by the HA
+manager and not provided. But as a concrete use case "provide" works fine.
+So it depends how you want to use this text.
+..[fq] Agree, Thank you!
+
+For active/active VNFCs, LB(Load Balancer) could be used. In such scenario, there
+could be two cases for the deployment and usage of LB.
+
+Case 1: LB used before a cluster of VNFCs to distribute traffic flow.
+
+In such case, the LB is deployed in front of a cluster of multiple VNFCs. Such
+cluster can be managered by a seperate cluster manager, or can be managed just
+by the LB,  which is using heartbeat to monitor each VNFC. When one of VNFCs fails,
+the cluster manager should recover the failed one, and should also exclude the 
+failed VNFC from the cluster so that the LB will re-route the traffic to
+to the other VNFCs. In the case when the LB is acting as the cluster manager, it is
+the LB's responsibility to inform the VNFM to recover the failed VNFC if possible.
+
+
+Case 2: LB used before a cluster of VMs to distribute traffic flow.
+
+In this case, there exists a cluster manager(e.g. Pacemaker) to monitor and manage
+the VMs in the cluster. The LB sits in front of the VM cluster so as to distribute
+the traffic. When one of the VM fails, the cluster manager will ditect that and will
+be in charge of the recovery. The cluster manager will also exclude the failed VM
+out of the cluster, so that the LB won't re-route traffic to the failed one.
+ 
+In both two cases, the HA of the LB should also be considered.
+
+..[MT] I think this use case needs to show also how the LB learns about the new VNFC.
+Also we should distinguish VNFC and VM failures as VNFC failure wouldn't be detected
+in the NFVI e.g. LB, so we need a resolution, an applicability comment at least.
+..[fq] I think I have made a mistake here by saying the VNFC. Actually if the failure
+only happens in VNFC, the VNFC should reboot itself rather than have a new VNFC taking
+its place. So in this case, I think I should modify VNFC into VMs. And as you mentioned,
+the NFVI level can hardly detect VNFC level failure.
+
+..[MT] There could also be a combined case for the N+M redundancy, when there are N
+actives but also M standbys at the VNF level.
+..[fq] It could be. But I actually haven't see such a deployed case. So I am not sure
+if I can discribe the schemes correctly:)
+
+1.2. Intra-VNF Communication
+
+For stateful VNFs, data synchronization is necessary between the active and standby VMs.
+The HA manager is responsible for handling VNFC failover, and do the assignment of the
+active/standby states between the VNFCs of the VNF. Data synchronization can be handled
+either by the HA manager or by the VNFC itself.
+
+The state synchronization can happen as
+
+- direct communication between the active and the standby VNFCs
+
+- based on the information received from the HA manager on channel or messages using a common queue,
+
+..[MT] I don't understand the yellow inserted text
+..[fq] Neither do I, actually. I think it is added by some one else and I can't make
+out what it means as well:)
+
+- it could be through a shared storage assigned to the whole VNF
+
+- through in-memory database (checkpointing), when the database (checkpoint service) takes care of the data replication.