Kusto can run sandboxes for specific flows that must be run in a secure and isolated environment. Examples of these flows are user-defined scripts that run using the Python plugin or the R plugin.

Sandboxes are run locally (meaning, processing is done close to the data), with no extra latency for remote calls.

Prerequisites and limitations


  • A sandboxed query operator may use one or more sandboxes for its execution.
    • A sandbox is only used for a single query and is disposed of once that query completes.
    • When a node is restarted, for example, as part of a service upgrade, all running sandboxes on it are disposed of.
  • Each node maintains a predefined number of sandboxes that are ready for running incoming requests.
    • Once a sandbox is used, a new one is automatically made available to replace it.
  • If there are no pre-allocated sandboxes available to serve a query operator, it will be throttled until new sandboxes are available. For more information, see Errors. New sandbox allocation could take up to 10-15 seconds per sandbox, depending on the SKU and available resources on the data node.

Sandbox parameters

Some of the parameters can be controlled using a cluster-level sandbox policy, for each kind of sandbox.

  • Number of sandboxes per node: The number of sandboxes per node is limited.
    • Requests that are made when there's no available sandbox will be throttled.
  • Initialize on startup: if set to false (default), sandboxes are lazily initialized on a node, the first time a query requires a sandbox for its execution. Otherwise, if set to true, sandboxes are initialized as part of service startup.
    • This means that the first execution of a plugin that uses sandboxes on a node will include a short warm-up period.
  • CPU: The maximum rate of CPU a sandbox can consume of its host's processors is limited (default is 50%).
    • When the limit is reached, the sandbox's CPU use is throttled, but execution continues.
  • Memory: The maximum amount of RAM a sandbox can consume of its host's RAM is limited (default is 20GB).
    • Reaching the limit results in termination of the sandbox, and a query execution error.

Sandbox limitations

  • Network: A sandbox can't interact with any resource on the virtual machine (VM) or outside of it.
    • A sandbox can't interact with another sandbox.


The resources used with sandbox depend not only on the size of the data being processed as part of the request, but also on the logic that runs in the sandbox, and the implementation of libraries being used by it. For example, for the python and r plugins, the latter means the user-provided script and the Python or R libraries it consumes at runtime.


ErrorCode Status Message Potential reason
E_SB_QUERY_THROTTLED_ERROR TooManyRequests (429) The sandboxed query was aborted because of throttling. Retrying after some backoff might succeed There are no available sandboxes on the target node. New sandboxes should become available in a few seconds
E_SB_QUERY_THROTTLED_ERROR TooManyRequests (429) Sandboxes of kind '{kind}' haven't yet been initialized The sandbox policy has recently changed. New sandboxes obeying the new policy will become available in a few seconds
InternalServiceError (520) The sandboxed query was aborted due to a failure in initializing sandboxes An unexpected infrastructure failure.

VM Sizes supporting nested virtualization

The following table lists all modern VM sizes that support Hyper-V sandbox technology.

Name Category
Standard_L8s_v3 storage-optimized
Standard_L16s_v3 storage-optimized
Standard_L8as_v3 storage-optimized
Standard_L16as_v3 storage-optimized
Standard_E8as_v5 storage-optimized
Standard_E16as_v5 storage-optimized
Standard_E8s_v4 storage-optimized
Standard_E16s_v4 storage-optimized
Standard_E8s_v5 storage-optimized
Standard_E16s_v5 storage-optimized
Standard_E2ads_v5 compute-optimized
Standard_E4ads_v5 compute-optimized
Standard_E8ads_v5 compute-optimized
Standard_E16ads_v5 compute-optimized
Standard_E2d_v4 compute-optimized
Standard_E4d_v4 compute-optimized
Standard_E8d_v4 compute-optimized
Standard_E16d_v4 compute-optimized
Standard_E2d_v5 compute-optimized
Standard_E4d_v5 compute-optimized
Standard_E8d_v5 compute-optimized
Standard_E16d_v5 compute-optimized