> For the complete documentation index, see [llms.txt](https://docs.tessell.com/tessell/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.tessell.com/tessell/database-engines/tessell-for-sqlserver/db-services/my-services/database-service-home-page/instances.md). # Instances The Instances tab displays all configured instances for your SQL Server database service in a table, with per-instance details and actions including switchover, DR drills, replica creation, and HA to SI conversion. *** ## What instance topologies does Tessell support for SQL Server? | Instance type | Description | | -------------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | **Single instance** | One instance in a given region and availability zone. | | **Multi-AZ (HA)** | Two instances — one primary and one synchronous replica — in the same region (same or different AZ). Both instances are always homogeneous (infrastructure, parameters, storage, IOPS). All planned operations run in rolling mode to minimise downtime. | | **Disaster Recovery (DR)** | One or more async replica instances added in the same or a different region. No automatic failover. DR-Drill scenario is supported (see below). Can be added to both single-instance and Multi-AZ services. | | **Read Replica (RR)** | An async replica opened in read-only mode. Use for read-only query offloading. | ### Which SQL Server editions support which topology? | Scenario | Supported editions (SQL Server 2016–2022) | Notes/restrictions | Not supported | | ---------------------------------- | ------------------------------------------- | -------------------------------------------------------------------------- | ------------- | | **SI → HA** | Enterprise, Standard, Developer, Evaluation | Standard edition: only one synchronous replica; no DR/RR at the same time. | Web, Express | | **SI with existing DR or RR → HA** | Enterprise, Developer, Evaluation | Standard edition with DR/RR is not supported. | Standard | *** ## What columns appear in the Instances table? | Column | Description | | ----------------------- | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | **Name** | Auto-generated name: `default-node-0` (primary), `default-node-1` (typically the HA replica configured for automatic failover), `-node-` (RR/DR, digit incrementing per node added). | | **Status** | Current instance state (see table below). | | **Role** | Primary, HA Replica (Automatic Failover), Read Replica (RR), or Disaster Recovery (DR). Shows the server name below the role. | | **Server** | Server where the instance is deployed. Click the server name to open its home page in the Servers app. | | **Region/Zone** | Cloud provider and region hosting the instance. | | **Parameter Profile** | Parameter profile applied at provisioning. | | **Compute** | Compute shape with vCPU and memory details below. | | **Created** | Date and time the instance was created. | | **Storage** | Storage size; click **See details** for storage provider, throughput, and IOPS. | | **Connection Endpoint** | Hostname; click **See details** for Service URL, Host, Port, and Username. | #### Instance status reference | Status | Description | Actions allowed | | -------------------------------------- | ----------------------------------------------------------------------------- | -------------------------------------------------------- | | **UP** | Instance is running and accepting connections. | Resize compute/storage, update parameter profile, delete | | **DOWN** | Instance is down. | Raise a Tessell support ticket to have it revived | | **STOPPED** | Instance is completely stopped. | Start the service | | **CREATING** | Provisioning is in progress, or a new instance is being added to the service. | No actions | | **DELETING** | Deletion in progress. | No actions | | **REBUILDING** | Compute or storage resize in progress. | No actions | | **REBUILD\_FAILED** | Resize was triggered and did not complete. | All actions | | **PATCHING** | A patch is being applied. | No actions | | **PATCH\_FAILED** | Patching failed. | All actions | | **UPDATING\_PARAMETER\_PROFILE** | Parameter profile update in progress. | No actions | | **PARAMETER\_PROFILE\_UPDATE\_FAILED** | Parameter profile update failed. | All actions | > **Note:** Using the ellipsis icon on individual instance rows, you can create, update, or delete private links for DR and RR nodes separately. Creation of private links for DR and RR nodes is currently supported for **AWS cloud** only — Azure support is a planned feature. *** ## How do I manually trigger a switchover to a replica? Switchover promotes a replica (HA or DR) to become the new primary. ### Steps to initiate a switchover 1. Click **Switch over** on the Instances tab. 2. In the **Switch over to an instance** window, select the target replica instance from the list of available replicas (shown with their names and statuses). > **Note:** Switching over to an instance may cause some downtime for the service. 3. Optionally add comments. If an HA replica is chosen as the target, the switchover is graceful (planned and synchronised with no data loss) and a switchback is allowed. 4. Select the **I understand the consequences** checkbox. 5. Click **Switch**. 6. Monitor progress by clicking **Submitting Request** below the service name. Once the switchover completes, the service returns to **Ready** status and Tessell metadata is updated. *** ## What is the difference between a DR Drill and an actual disaster recovery switchover? When switching over to a DR instance, Tessell offers two paths: | Mode | DR Drill option selected | Behaviour | | -------------------------------------------------- | ------------------------ | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | **DR Drill (Graceful Switchover)** | Yes | Attempts a graceful switchover: all transactions are safely committed, the current primary is demoted in a controlled manner, and the DR instance is promoted to primary with full data consistency. If graceful switchover isn't possible (e.g., connectivity issues, replication lag, or replication faults), the operation fails to avoid data inconsistency. Used for DR testing and compliance drills — switch back to the original primary is possible after testing. | | **Actual Disaster Recovery (Forceful Switchover)** | No | Assumes a real disaster. Tessell forcefully promotes the DR instance to primary; the original primary is marked unrecoverable and brought down. Tessell automatically triggers a rebuild of the original primary. Once the rebuild completes and replication is re-established, you may choose to switch back to the original primary as part of recovery workflows. | ### Feature comparison | Feature | DR Drill enabled | DR Drill disabled (Disaster mode) | | --------------------------- | ------------------------ | --------------------------------- | | Switchover type | Graceful | Forceful | | Data consistency check | Yes | Skipped | | Source primary status | Demoted cleanly | Brought down | | Automatic rebuild of source | No (optional switchback) | Yes (required for future use) | | Switchback possible | Yes (post-drill) | Yes (post-rebuild) | | Use case | Testing / compliance | Actual disaster / failover | *** ## How do I add a DR or Read Replica instance? You must open bi-directional network communication between the primary and the DR/RR subnets: * **Primary database port** (e.g., 1433 for SQL Server, or your specific DB port) * **Port 1599** * Applied at the **subnet level** (entire CIDR blocks), not VM level — NSG rules at the VM level are managed by Tessell. ### Steps to add a DR or Read Replica instance 1. Click **Create Replica** (top-right of the Instances tab) → select **Disaster Recovery** or **Read Only Replica**. 2. Provide the following details: * **Instance Name** — alphanumeric characters only. * **Region** — target region. * **Availability Zone** — target AZ. * **VPC** — target VPC. * **Private Subnet** — target private subnet. * **Advanced configuration** — (optional) specify IOPS and Throughput. A different compute shape for DR/RR is also supported. 3. Click **Add Instance**. Tessell automatically: 1. Submits the request to add the replica. 2. Creates the compute instance(s). 3. Prepares the primary database to accept the new replica. 4. Configures the replica with primary database information. 5. Sets up the Availability Machine for the new instance. As an alternative, use the **Code** button to view, copy, or download the API call for adding an instance in Shell, Python, Go, Java, JavaScript, or PowerShell. *** ## How do I add a High Availability (HA) replica? **Prerequisites:** The same bi-directional network communication and subnet-level NSG requirements as DR/RR (above) apply to the HA subnet as well. ### Steps to add an HA instance 1. Click **Create Replica** (top-right of the Instances tab) → select **High Availability**. 2. In the **Add High Availability Instance** window: * Specify a name in the **Instance Name** field. Region and VNet stay the same as the primary and cannot be changed. * Select an **Availability Zone** from the dropdown. * Select an available **Private Subnet** from the dropdown. * Review the HA properties inherited from the primary — Compute shape, vCPUs, Storage, Encryption Key, IOPS, Throughput. These cannot be changed for HA. 3. Click **Create**. Tessell automatically: 1. Runs pre-conversion validation on the SI service. 2. Takes a full database snapshot. 3. Provisions the secondary compute (creates the secondary VM). 4. Installs SQL Server on the secondary. 5. Configures the primary — enables an Always On Availability Group between the two instances. *** ## How do I convert an HA service back to Single Instance (SI)? HA to SI conversion removes the secondary replica from an existing HA configuration and returns the service to a single-instance deployment — useful for cost optimization when high availability is no longer required, or to simplify your database topology. Databases, configurations, and the service endpoint URL on the primary instance are all preserved, and the endpoint URL doesn't change, minimising disruption to applications. ### Steps to convert HA to SI 1. On the **Instances** tab, locate the secondary (HA) replica. 2. Click the ellipsis icon next to the secondary replica → **Delete Instance**. 3. In the confirmation dialog, review: * The synchronous secondary instance used for High Availability will be decommissioned. * Automatic failover will be disabled, increasing downtime risk if the primary fails. * Modifying actions on this service are temporarily disabled while deletion is in progress. 4. Select the confirmation checkbox and click **Delete**. Tessell automatically: 1. Removes the secondary replica from the Always On Availability Group. 2. Dismantles the Windows Server Failover Cluster. 3. Deletes the secondary compute instance. 4. Updates the service metadata to reflect the SI topology. 5. Preserves all primary instance configurations, server objects (logins, jobs, linked servers), and databases. > **Important considerations:** > > * **No HPC shape support:** HA to SI conversion is not supported for HPC shapes. > * **DR and RR preservation:** If the service has DR or RR configured, they remain intact after HA to SI conversion. > * **Service availability:** The conversion requires a brief maintenance window for cluster reconfiguration — plan it during low-traffic periods. > * **No rollback:** Once the secondary replica is removed, the conversion cannot be undone. To restore HA, add a new HA instance (SI to HA conversion). > * **No concurrent operations:** Don't perform other database operations (snapshot, backup, resize, patch) during the conversion.

Figure 5 — Instances tab

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