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The Archive Server can be scaled up both horizontally and vertically, but more manual actions are required.
Scale horizontally: Deploy a new Archive Server Virtual Machine. A load balancer or application gateway is used in front of the Archive and new VMs must be registered with the loadbalancer.
The scheduling of the harvester must be duplicated to the new VM. Since the harvester builds the index it takes some time before a new archive server is in sync. The load balancer should use sticky sessions to make sure consistent results are given all the time for the same user.
Scale vertically: Redeploy the Archive Server Virtual Machine with more resources.
Deploying multiple archive servers will avoid a single point of failure.
High available scenarios
On a high level the different scenarios for running Delft-FEWS in a highly available manner when the primary Azure region fails, from most high available to least available
Scenario | Description | Disaster recovery if primary regions fails | Manual interventions | Recover Time/Data Loss for Delft-FEWS components |
Dual MC in 2 Azure regions | 2 different databases in two regions, databases will be synchronized by Delft-FEWS. Use Geo redundant storage for file shares. | Automatically by Delft-FEWS. | Operator clients need to reconnect to secondary MC. This has been pre-configured. | No Recovery Time. (Hot Stand By) Data loss is minimal and can be restored by rerunning workflows. |
Single MC with Database in 2 Azure regions | Azure auto failover group Managed Instance Database with Azure Site Recovery for VMS. Use Geo redundant storage for file shares. | Use Azure Site Recovery to restore VMS in secondary region. Synchronized database will be made primary by Azure.
| Azure Site Recovery has to be enabled. | Recovery Time depends on the time it takes for Microsoft to enable ASR. Data loss is minimal and can be restored by rerunning workflows. |
Single MC with locally redundant Database | Single database with Delft-FEWS components | Database backups have to be restored to a new database in a secondary region. VM backups have to be restored to a secondary region | Database configurations on restored VMs have to be adjusted to new DB. Delft-FEWS configuration has to be adjusted to use new Database. | Recovery time is a few days. Data loss depends on the backup schedules. |
Disaster Recovery
This section will describe possible Disaster Recovery solutions for the different parts of a Virtual Machine based Azure system:
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