Auto-scaling Impala Virtual Warehouses

Your Impala Virtual Warehouse in Cloudera Data Warehouse (CDW) Private Cloud has an auto-scaler process that works with coordinators and executors to make resources available for queued queries. This ensures that workload demand is met without wasting cloud resources.

How autoscaling in Impala works

How coorninators and executors are used to autoscale resources is explained as follows:

  • Coordinator processes: Handle all incoming queries, creating execution plans and handing off the query to executor processes for execution. By default, there are two coordinator processes to enable high availability and fault tolerance. You can configure a single coordinator or an active-passive coordinator pair to resolve or mitigate query concurrency problems.. If one coordinator process fails, a backup coordinator process takes over so there is no single point of failure. By using only one coordinator process, you are likely to save on cloud resource consumption.
  • Executor processes: Processes that execute query fragments. Executor processes run on executor nodes, the unit of sizing for Virtual Warehouses. Each executor node runs one executor.
  • Executor groups: A group of executors that can execute queries. By default Impala Virtual Warehouses can run 3 large queries per executor group. Executors can handle more queries that are simpler and that do not utilize concurrency on the executor. When you enable legacy multithreading, the Virtual Warehouse can run 12 queries per executor group. For most read-only queries the default setting of 3 queries per executor group is sufficient. The size of executor groups is determined by the size you choose when you first create the Virtual Warehouse (XSMALL, SMALL, MEDIUM, or LARGE).

    It is recommended that you select the size of a Virtual Warehouse based on the number of nodes you need to run a typical query for your workloads based on your normal query size and complexity. For example, if you expect that 20 nodes are needed to run a typical query in your workloads, you can create a medium-sized Virtual Warehouse, which by default has 20 executor nodes. However, if you have a large number of queries that must be run in your workloads concurrently, you can set the minimum executor node count to 40 and maximum count to 80 by using the Nodes: Min: Max: setting. Then, each executor group for this Virtual Warehouse contains the 20 executors required to run an average query, but you have two executor groups to handle the volume of queries in your workloads. If the total number of executor nodes auto-scales up to 80, then there will be 4 executor groups containing 20 executor nodes each.

    The rationale behind selecting a size based on the normal size and complexity of your queries is that then the Virtual Warehouse can run all your normal queries in an acceptable time. Selecting size based on this criteria prevents you from "over-sizing" your Virtual Warehouse and as a result, unnecessarily incurring extra costs. On the other hand, keep in mind that under-sizing your Virtual Warehouse might result in queries spilling, which makes them run slower, in spite of the fact that they may eventually complete correctly.

  • Auto-scaler: A process that monitors Impala to determine when more or fewer resources are needed. When the auto-scaler detects an imbalance in resources, it sends a request to the Kubernetes framework to increase or decrease the number of executor groups in the Virtual Warehouse.
Impala uses memory-based auto-scaling to manage resources:

  • Coordinator(s): When users connect to Impala Virtual Warehouses using SQL applications such as Hue, the Impala shell, or other SQL clients that use JDBC or ODBC, the query is handled by the coordinator process. First, the coordinator generates an execution plan for the query. The execution plan includes an estimate of the memory required to run the query.
  • Executor group: Then the coordinator locates an executor group that has enough available memory to run the query. Each executor group is limited by the number of queries and the memory available to run them. Currently, executor groups can handle up to 3 large queries. Executors can handle more queries that are simpler and that do not utilize concurrency on the executor. If there are no executor groups with enough available resources to handle a query, it is queued until resources become available.
  • Auto-scaler: When the auto-scaler detects that there are queued queries, it adds executor groups to the Virtual Warehouse to execute the queries. The auto-scaler starts scaling up and down the number of executor groups according to what is set in the auto-scale settings:
    • Scale Up Delay: Sets the length of time in seconds that the system waits before adding more executors when it detects queries waiting in the queue to execute. The time to auto-scale is affected by how the underlying Kubernetes system is configured.
    • Scale Down Delay: Sets the length of time in seconds that the system waits before it removes executors when it detects idle executor groups. As with the Scale Up Delay setting, the time to auto-scale is affected by how the underlying Kubernetes system is configured.

When the auto-scaler has scaled back to the last executor group, which contains the default number of executors for the Virtual Warehouse, and those executors are idle, the Virtual Warehouse is suspended. You can override this behavior by selecting the Disable Auto Suspend option. The time CDW waits, after there are no longer any queries running and before the Virtual Warehouse is auto-suspended, is determined by the value set for AutoSuspend Timeout. The default number of executors per executor group is based on the number of executor nodes contained by the original size of the Virtual Warehouse when it was created. For example, if the Virtual Warehouse was created as MEDIUM-sized, which has 20 executor nodes, then each executor group contains 20 executors.

If a query is executed on a suspended Virtual Warehouse, then the coordinator queues the query. When the auto-scaler detects a queued query, it immediately adds an executor group that can run the query on the Virtual Warehouse.