Create a formula to automatically scale compute nodes in a Batch pool

Azure Batch can automatically scale pools based on parameters that you define, saving you time and money. With automatic scaling, Batch dynamically adds nodes to a pool as task demands increase, and removes compute nodes as task demands decrease.

To enable automatic scaling on a pool of compute nodes, you associate the pool with an autoscale formula that you define. The Batch service uses the autoscale formula to determine how many nodes are needed to execute your workload. These nodes can be dedicated nodes. Batch periodically reviews service metrics data and uses it to adjust the number of nodes in the pool based on your formula and at an interval that you define.

You can enable automatic scaling when you create a pool, or apply it to an existing pool. Batch lets you evaluate your formulas before assigning them to pools and to monitor the status of automatic scaling runs. Once you configure a pool with automatic scaling, you can make changes to the formula later.

Autoscale formulas

An autoscale formula is a string value that you define that contains one or more statements. The autoscale formula is assigned to a pool's autoScaleFormula element (Batch REST) or CloudPool.AutoScaleFormula property (Batch .NET). The Batch service uses your formula to determine the target number of compute nodes in the pool for the next interval of processing. The formula string can't exceed 8 KB, can include up to 100 statements that are separated by semicolons, and can include line breaks and comments.

You can think of automatic scaling formulas as a Batch autoscale "language." Formula statements are free-formed expressions that can include both service-defined variables, which are defined by the Batch service, and user-defined variables. Formulas can perform various operations on these values by using built-in types, operators, and functions. For example, a statement might take the following form:

$myNewVariable = function($ServiceDefinedVariable, $myCustomVariable);

Formulas generally contain multiple statements that perform operations on values that are obtained in previous statements. For example, first you obtain a value for variable1, then pass it to a function to populate variable2:

$variable1 = function1($ServiceDefinedVariable);
$variable2 = function2($OtherServiceDefinedVariable, $variable1);

Include these statements in your autoscale formula to arrive at a target number of compute nodes. Dedicated nodes have their own target settings. An autoscale formula can include a target value for dedicated nodes.

The target number of nodes might be higher, lower, or the same as the current number of nodes of that type in the pool. Batch evaluates a pool's autoscale formula at specific automatic scaling intervals. Batch adjusts the target number of each type of node in the pool to the number that your autoscale formula specifies at the time of evaluation.

Sample autoscale formulas

The following examples show two autoscale formulas, which can be adjusted to work for most scenarios. The variables startingNumberOfVMs and maxNumberofVMs in the example formulas can be adjusted to your needs.

Pending tasks

With this autoscale formula, the pool is initially created with a single VM. The $PendingTasks metric defines the number of tasks that are running or queued. The formula finds the average number of pending tasks in the last 180 seconds and sets the $TargetDedicatedNodes variable accordingly. The formula ensures that the target number of dedicated nodes never exceeds 25 VMs. As new tasks are submitted, the pool automatically grows. As tasks complete, VMs become free and the autoscaling formula shrinks the pool.

This formula scales dedicated nodes.

startingNumberOfVMs = 1;
maxNumberofVMs = 25;
pendingTaskSamplePercent = $PendingTasks.GetSamplePercent(180 * TimeInterval_Second);
pendingTaskSamples = pendingTaskSamplePercent < 70 ? startingNumberOfVMs : avg($PendingTasks.GetSample(180 * TimeInterval_Second));
$TargetDedicatedNodes=min(maxNumberofVMs, pendingTaskSamples);
$NodeDeallocationOption = taskcompletion;

Variables

You can use both service-defined and user-defined variables in your autoscale formulas.

The service-defined variables are built in to the Batch service. Some service-defined variables are read-write, and some are read-only.

User-defined variables are variables that you define. In the previous example, $TargetDedicatedNodes and $PendingTasks are service-defined variables, while startingNumberOfVMs and maxNumberofVMs are user-defined variables.

The following tables show the read-write and read-only variables defined by the Batch service.

Read-write service-defined variables

You can get and set the values of these service-defined variables to manage the number of compute nodes in a pool.

Variable	Description
$TargetDedicatedNodes	The target number of dedicated compute nodes for the pool. Specified as a target because a pool might not always achieve the desired number of nodes. For example, if the target number of dedicated nodes is modified by an autoscale evaluation before the pool has reached the initial target, the pool might not reach the target. A pool in an account created in Batch service mode might not achieve its target if the target exceeds a Batch account node or core quota. A pool in an account created in user subscription mode might not achieve its target if the target exceeds the shared core quota for the subscription.
$NodeDeallocationOption	The action that occurs when compute nodes are removed from a pool. Possible values are: - requeue: The default value. Ends tasks immediately and puts them back on the job queue so that they're rescheduled. This action ensures the target number of nodes is reached as quickly as possible. However, it might be less efficient, because any running tasks are interrupted and then must be restarted. - terminate: Ends tasks immediately and removes them from the job queue. - taskcompletion: Waits for currently running tasks to finish and then removes the node from the pool. Use this option to avoid tasks being interrupted and requeued, wasting any work the task has done. - retaineddata: Waits for all the local task-retained data on the node to be cleaned up before removing the node from the pool.

Read-only service-defined variables

You can get the value of these service-defined variables to make adjustments that are based on metrics from the Batch service.

Variable	Description
$CPUPercent	The average percentage of CPU usage.
$ActiveTasks	The number of tasks that are ready to execute but aren't yet executing. This includes all tasks that are in the active state and whose dependencies have been satisfied. Any tasks that are in the active state but whose dependencies haven't been satisfied are excluded from the $ActiveTasks count. For a multi-instance task, $ActiveTasks includes the number of instances set on the task.
$RunningTasks	The number of tasks in a running state.
$PendingTasks	The sum of $ActiveTasks and $RunningTasks.
$SucceededTasks	The number of tasks that finished successfully.
$FailedTasks	The number of tasks that failed.
$TaskSlotsPerNode	The number of task slots that can be used to run concurrent tasks on a single compute node in the pool.
$CurrentDedicatedNodes	The current number of dedicated compute nodes.

Types

Autoscale formulas support the following types:

• double
• doubleVec
• doubleVecList
• string
• timestamp--a compound structure that contains the following members:
  • year
  • month (1-12)
  • day (1-31)
  • weekday (in the format of number; for example, 1 for Monday)
  • hour (in 24-hour number format; for example, 13 means 1 PM)
  • minute (00-59)
  • second (00-59)
• timeinterval
  • TimeInterval_Zero
  • TimeInterval_100ns
  • TimeInterval_Microsecond
  • TimeInterval_Millisecond
  • TimeInterval_Second
  • TimeInterval_Minute
  • TimeInterval_Hour
  • TimeInterval_Day
  • TimeInterval_Week
  • TimeInterval_Year

Operations

These operations are allowed on the types that are listed in the previous section.

Operation	Supported operators	Result type
double operator double	+, -, *, /	double
double operator timeinterval	*	timeinterval
doubleVec operator double	+, -, *, /	doubleVec
doubleVec operator doubleVec	+, -, *, /	doubleVec
timeinterval operator double	*, /	timeinterval
timeinterval operator timeinterval	+, -	timeinterval
timeinterval operator timestamp	+	timestamp
timestamp operator timeinterval	+	timestamp
timestamp operator timestamp	-	timeinterval
operator double	-, !	double
operator timeinterval	-	timeinterval
double operator double	<, <=, ==, >=, >, !=	double
string operator string	<, <=, ==, >=, >, !=	double
timestamp operator timestamp	<, <=, ==, >=, >, !=	double
timeinterval operator timeinterval	<, <=, ==, >=, >, !=	double
double operator double	&&, ||	double

Testing a double with a ternary operator (double ? statement1 : statement2), results in nonzero as true, and zero as false.

Functions

You can use these predefined functions when defining an autoscale formula.

Function	Return type	Description
avg(doubleVecList)	double	Returns the average value for all values in the doubleVecList.
ceil(double)	double	Returns the smallest integer value not less than the double.
ceil(doubleVecList)	doubleVec	Returns the component-wise ceil of the doubleVecList.
floor(double)	double	Returns the largest integer value not greater than the double.
floor(doubleVecList)	doubleVec	Returns the component-wise floor of the doubleVecList.
len(doubleVecList)	double	Returns the length of the vector that is created from the doubleVecList.
lg(double)	double	Returns the log base 2 of the double.
lg(doubleVecList)	doubleVec	Returns the component-wise lg of the doubleVecList.
ln(double)	double	Returns the natural log of the double.
ln(doubleVecList)	doubleVec	Returns the component-wise ln of the doubleVecList.
log(double)	double	Returns the log base 10 of the double.
log(doubleVecList)	doubleVec	Returns the component-wise log of the doubleVecList.
max(doubleVecList)	double	Returns the maximum value in the doubleVecList.
min(doubleVecList)	double	Returns the minimum value in the doubleVecList.
norm(doubleVecList)	double	Returns the two-norm of the vector that is created from the doubleVecList.
percentile(doubleVec v, double p)	double	Returns the percentile element of the vector v.
rand()	double	Returns a random value between 0.0 and 1.0.
range(doubleVecList)	double	Returns the difference between the min and max values in the doubleVecList.
round(double)	double	Returns the nearest integer value to the double (in floating-point format), rounding halfway cases away from zero.
round(doubleVecList)	doubleVec	Returns the component-wise round of the doubleVecList.
std(doubleVecList)	double	Returns the sample standard deviation of the values in the doubleVecList.
stop()		Stops evaluation of the autoscaling expression.
sum(doubleVecList)	double	Returns the sum of all the components of the doubleVecList.
time(string dateTime="")	timestamp	Returns the time stamp of the current time if no parameters are passed, or the time stamp of the dateTime string if that is passed. Supported dateTime formats are W3C-DTF and RFC 1123.
val(doubleVec v, double i)	double	Returns the value of the element that is at location i in vector v, with a starting index of zero.

Some of the functions that are described in the previous table can accept a list as an argument. The comma-separated list is any combination of double and doubleVec. For example:

doubleVecList := ( (double | doubleVec)+(, (double | doubleVec) )* )?

The doubleVecList value is converted to a single doubleVec before evaluation. For example, if v = [1,2,3], then calling avg(v) is equivalent to calling avg(1,2,3). Calling avg(v, 7) is equivalent to calling avg(1,2,3,7).

Metrics

You can use both resource and task metrics when you define a formula. You adjust the target number of dedicated nodes in the pool based on the metrics data that you obtain and evaluate. For more information on each metric, see the Variables section.

Metric	Description
Resource	Resource metrics are based on the CPU, the bandwidth, the memory usage of compute nodes, and the number of nodes. These service-defined variables are useful for making adjustments based on node count: - $TargetDedicatedNodes - $TargetLowPriorityNodes - $CurrentDedicatedNodes - $SampleNodeCount These service-defined variables are useful for making adjustments based on node resource usage: - $CPUPercent - $WallClockSeconds - $MemoryBytes - $DiskBytes - $DiskReadBytes - $DiskWriteBytes - $DiskReadOps - $DiskWriteOps - $NetworkInBytes - $NetworkOutBytes
Task	Task metrics are based on the status of tasks, such as Active, Pending, and Completed. The following service-defined variables are useful for making pool-size adjustments based on task metrics: - $ActiveTasks - $RunningTasks - $PendingTasks - $SucceededTasks - $FailedTasks

Obtain sample data

The core operation of an autoscale formula is to obtain task and resource metrics data (samples), and then adjust pool size based on that data. As such, it's important to have a clear understanding of how autoscale formulas interact with samples.

Methods

Autoscale formulas act on samples of metric data provided by the Batch service. A formula grows or shrinks the pool compute nodes based on the values that it obtains. Service-defined variables are objects that provide methods to access data that's associated with that object. For example, the following expression shows a request to get the last five minutes of CPU usage:

$CPUPercent.GetSample(TimeInterval_Minute * 5)

The following methods can be used to obtain sample data about service-defined variables.

Method	Description
GetSample()	The GetSample() method returns a vector of data samples. A sample is 30 seconds worth of metrics data. In other words, samples are obtained every 30 seconds. But as noted below, there's a delay between when a sample is collected and when it's available to a formula. As such, not all samples for a given time period might be available for evaluation by a formula. - doubleVec GetSample(double count): Specifies the number of samples to obtain from the most recent samples that were collected. GetSample(1) returns the last available sample. For metrics like $CPUPercent, however, GetSample(1) shouldn't be used, because it's impossible to know when the sample was collected. It could be recent, or, because of system issues, it might be much older. In such cases, it's better to use a time interval as shown below. - doubleVec GetSample((timestamp or timeinterval) startTime [, double samplePercent]): Specifies a time frame for gathering sample data. Optionally, it also specifies the percentage of samples that must be available in the requested time frame. For example, $CPUPercent.GetSample(TimeInterval_Minute * 10) would return 20 samples if all samples for the last 10 minutes are present in the CPUPercent history. If the last minute of history wasn't available, only 18 samples would be returned. In this case $CPUPercent.GetSample(TimeInterval_Minute * 10, 95) would fail because only 90 percent of the samples are available, but $CPUPercent.GetSample(TimeInterval_Minute * 10, 80) would succeed. - doubleVec GetSample((timestamp or timeinterval) startTime, (timestamp or timeinterval) endTime [, double samplePercent]): Specifies a time frame for gathering data, with both a start time and an end time. As mentioned above, there's a delay between when a sample is collected and when it becomes available to a formula. Consider this delay when you use the GetSample method. See GetSamplePercent below.
GetSamplePeriod()	Returns the period of samples that were taken in a historical sample data set.
Count()	Returns the total number of samples in the metrics history.
HistoryBeginTime()	Returns the time stamp of the oldest available data sample for the metric.
GetSamplePercent()	Returns the percentage of samples that are available for a given time interval. For example, doubleVec GetSamplePercent( (timestamp or timeinterval) startTime [, (timestamp or timeinterval) endTime] ). Because the GetSample method fails if the percentage of samples returned is less than the samplePercent specified, you can use the GetSamplePercent method to check first. Then you can perform an alternate action if insufficient samples are present, without halting the automatic scaling evaluation.

Samples

The Batch service periodically takes samples of task and resource metrics and makes them available to your autoscale formulas. These samples are recorded every 30 seconds by the Batch service. However, there's typically a delay between when those samples were recorded and when they're made available to (and read by) your autoscale formulas. Additionally, samples might not be recorded for a particular interval because of factors such as network or other infrastructure issues.

Sample percentage

When samplePercent is passed to the GetSample() method or the GetSamplePercent() method is called, percent refers to a comparison between the total possible number of samples recorded by the Batch service and the number of samples that are available to your autoscale formula.

Let's look at a 10-minute time span as an example. Because samples are recorded every 30 seconds within that 10-minute time span, the maximum total number of samples recorded by Batch would be 20 samples (2 per minute). However, due to the inherent latency of the reporting mechanism and other issues within Azure, there might be only 15 samples that are available to your autoscale formula for reading. So, for example, for that 10-minute period, only 75 percent of the total number of samples recorded might be available to your formula.

GetSample() and sample ranges

Your autoscale formulas grow and shrink your pools by adding or removing nodes. Because nodes cost you money, be sure that your formulas use an intelligent method of analysis that's based on sufficient data. It's recommended that you use a trending-type analysis in your formulas. This type grows and shrinks your pools based on a range of collected samples.

To do so, use GetSample(interval look-back start, interval look-back end) to return a vector of samples:

$runningTasksSample = $RunningTasks.GetSample(1 * TimeInterval_Minute, 6 * TimeInterval_Minute);

When Batch evaluates the above line, it returns a range of samples as a vector of values. For example:

$runningTasksSample=[1,1,1,1,1,1,1,1,1,1];

After you collect the vector of samples, you can then use functions like min(), max(), and avg() to derive meaningful values from the collected range.

To exercise extra caution, you can force a formula evaluation to fail if less than a certain sample percentage is available for a particular time period. When you force a formula evaluation to fail, you instruct Batch to cease further evaluation of the formula if the specified percentage of samples isn't available. In this case, no change is made to the pool size. To specify a required percentage of samples for the evaluation to succeed, specify it as the third parameter to GetSample(). Here, a requirement of 75 percent of samples is specified:

$runningTasksSample = $RunningTasks.GetSample(60 * TimeInterval_Second, 120 * TimeInterval_Second, 75);

Because there might be a delay in sample availability, you should always specify a time range with a look-back start time that's older than one minute. It takes approximately one minute for samples to propagate through the system, so samples in the range (0 * TimeInterval_Second, 60 * TimeInterval_Second) might not be available. Again, you can use the percentage parameter of GetSample() to force a particular sample percentage requirement.

Write an autoscale formula

You build an autoscale formula by forming statements that use the above components, then combine those statements into a complete formula. In this section, you create an example autoscale formula that can perform real-world scaling decisions and make adjustments.

First, let's define the requirements for our new autoscale formula. The formula should:

• Increase the target number of dedicated compute nodes in a pool if CPU usage is high.
• Decrease the target number of dedicated compute nodes in a pool when CPU usage is low.
• Always restrict the maximum number of dedicated nodes to 400.
• When reducing the number of nodes, don't remove nodes that are running tasks; if necessary, wait until tasks have finished before removing nodes.

The first statement in the formula increases the number of nodes during high CPU usage. You define a statement that populates a user-defined variable ($totalDedicatedNodes) with a value that is 110 percent of the current target number of dedicated nodes, but only if the minimum average CPU usage during the last 10 minutes was above 70 percent. Otherwise, it uses the value for the current number of dedicated nodes.

$totalDedicatedNodes =
    (min($CPUPercent.GetSample(TimeInterval_Minute * 10)) > 0.7) ?
    ($CurrentDedicatedNodes * 1.1) : $CurrentDedicatedNodes;

To decrease the number of dedicated nodes during low CPU usage, the next statement in the formula sets the same $totalDedicatedNodes variable to 90 percent of the current target number of dedicated nodes, if average CPU usage in the past 60 minutes was under 20 percent. Otherwise, it uses the current value of $totalDedicatedNodes populated in the statement above.

$totalDedicatedNodes =
    (avg($CPUPercent.GetSample(TimeInterval_Minute * 60)) < 0.2) ?
    ($CurrentDedicatedNodes * 0.9) : $totalDedicatedNodes;

Now, limit the target number of dedicated compute nodes to a maximum of 400.

$TargetDedicatedNodes = min(400, $totalDedicatedNodes);

Finally, ensure that nodes aren't removed until their tasks are finished.

$NodeDeallocationOption = taskcompletion;

Here's the complete formula:

$totalDedicatedNodes =
    (min($CPUPercent.GetSample(TimeInterval_Minute * 10)) > 0.7) ?
    ($CurrentDedicatedNodes * 1.1) : $CurrentDedicatedNodes;
$totalDedicatedNodes =
    (avg($CPUPercent.GetSample(TimeInterval_Minute * 60)) < 0.2) ?
    ($CurrentDedicatedNodes * 0.9) : $totalDedicatedNodes;
$TargetDedicatedNodes = min(400, $totalDedicatedNodes);
$NodeDeallocationOption = taskcompletion;

Automatic scaling interval

By default, the Batch service adjusts a pool's size according to its autoscale formula every 15 minutes. This interval is configurable by using the following pool properties:

• CloudPool.AutoScaleEvaluationInterval (Batch .NET)
• autoScaleEvaluationInterval (REST API)

The minimum interval is five minutes, and the maximum is 168 hours. If an interval outside this range is specified, the Batch service returns a Bad Request (400) error.

Create an autoscale-enabled pool with Batch SDKs

Pool autoscaling can be configured using any of the Batch SDKs, the Batch REST API Batch PowerShell cmdlets, and the Batch CLI. In this section, you can see examples for both .NET and Python.

.NET

To create a pool with autoscaling enabled in .NET, follow these steps:

1. Create the pool with BatchClient.PoolOperations.CreatePool.
2. Set the CloudPool.AutoScaleEnabled property to true.
3. Set the CloudPool.AutoScaleFormula property with your autoscale formula.
4. (Optional) Set the CloudPool.AutoScaleEvaluationInterval property (default is 15 minutes).
5. Commit the pool with CloudPool.Commit or CommitAsync.

The following example creates an autoscale-enabled pool in .NET. The pool's autoscale formula sets the target number of dedicated nodes to 5 on Mondays, and to 1 on every other day of the week. The automatic scaling interval is set to 30 minutes. In this and the other C# snippets in this article, myBatchClient is a properly initialized instance of the BatchClient class.

CloudPool pool = myBatchClient.PoolOperations.CreatePool(
                    poolId: "mypool",
                    virtualMachineSize: "standard_d1_v2",
                    VirtualMachineConfiguration: new VirtualMachineConfiguration(
                        imageReference: new ImageReference(
                                            publisher: "MicrosoftWindowsServer",
                                            offer: "WindowsServer",
                                            sku: "2019-datacenter-core",
                                            version: "latest"),
                        nodeAgentSkuId: "batch.node.windows amd64");
pool.AutoScaleEnabled = true;
pool.AutoScaleFormula = "$TargetDedicatedNodes = (time().weekday == 1 ? 5:1);";
pool.AutoScaleEvaluationInterval = TimeSpan.FromMinutes(30);
await pool.CommitAsync();

Python

To create an autoscale-enabled pool with the Python SDK:

1. Create a pool and specify its configuration.
2. Add the pool to the service client.
3. Enable autoscale on the pool with a formula you write.

The following example illustrates these steps.

# Create a pool; specify configuration
new_pool = batch.models.PoolAddParameter(
    id="autoscale-enabled-pool",
    virtual_machine_configuration=batchmodels.VirtualMachineConfiguration(
        image_reference=batchmodels.ImageReference(
          publisher="Canonical",
          offer="UbuntuServer",
          sku="20.04-LTS",
          version="latest"
            ),
        node_agent_sku_id="batch.node.ubuntu 20.04"),
    vm_size="STANDARD_D1_v2",
    target_dedicated_nodes=0,
    target_low_priority_nodes=0
)
batch_service_client.pool.add(new_pool) # Add the pool to the service client

formula = """$curTime = time();
             $workHours = $curTime.hour >= 8 && $curTime.hour < 18;
             $isWeekday = $curTime.weekday >= 1 && $curTime.weekday <= 5;
             $isWorkingWeekdayHour = $workHours && $isWeekday;
             $TargetDedicated = $isWorkingWeekdayHour ? 20:10;""";

# Enable autoscale; specify the formula
response = batch_service_client.pool.enable_auto_scale(pool_id, auto_scale_formula=formula,
                                            auto_scale_evaluation_interval=datetime.timedelta(minutes=10),
                                            pool_enable_auto_scale_options=None,
                                            custom_headers=None, raw=False)

Enable autoscaling on an existing pool

Each Batch SDK provides a way to enable automatic scaling. For example:

• BatchClient.PoolOperations.EnableAutoScaleAsync (Batch .NET)
• Enable automatic scaling on a pool (REST API)

When you enable autoscaling on an existing pool, keep in mind:

• If autoscaling is currently disabled on the pool, you must specify a valid autoscale formula when you issue the request. You can optionally specify an automatic scaling interval. If you don't specify an interval, the default value of 15 minutes is used.
• If autoscaling is currently enabled on the pool, you can specify a new formula, a new interval, or both. You must specify at least one of these properties.
  • If you specify a new automatic scaling interval, the existing schedule is stopped and a new schedule is started. The new schedule's start time is the time at which the request to enable autoscaling was issued.
  • If you omit either the autoscale formula or interval, the Batch service continues to use the current value of that setting.

This C# example uses the Batch .NET library to enable autoscaling on an existing pool.

// Define the autoscaling formula. This formula sets the target number of nodes
// to 5 on Mondays, and 1 on every other day of the week
string myAutoScaleFormula = "$TargetDedicatedNodes = (time().weekday == 1 ? 5:1);";

// Set the autoscale formula on the existing pool
await myBatchClient.PoolOperations.EnableAutoScaleAsync(
    "myexistingpool",
    autoscaleFormula: myAutoScaleFormula);

Update an autoscale formula

To update the formula on an existing autoscale-enabled pool, call the operation to enable autoscaling again with the new formula. For example, if autoscaling is already enabled on myexistingpool when the following .NET code is executed, its autoscale formula is replaced with the contents of myNewFormula.

await myBatchClient.PoolOperations.EnableAutoScaleAsync(
    "myexistingpool",
    autoscaleFormula: myNewFormula);

Update the autoscale interval

To update the autoscale evaluation interval of an existing autoscale-enabled pool, call the operation to enable autoscaling again with the new interval. For example, to set the autoscale evaluation interval to 60 minutes for a pool that's already autoscale-enabled in .NET:

await myBatchClient.PoolOperations.EnableAutoScaleAsync(
    "myexistingpool",
    autoscaleEvaluationInterval: TimeSpan.FromMinutes(60));

Evaluate an autoscale formula

You can evaluate a formula before applying it to a pool. This lets you test the formula's results before you put it into production.

Before you can evaluate an autoscale formula, you must first enable autoscaling on the pool with a valid formula, such as the one-line formula $TargetDedicatedNodes = 0. Then, use one of the following to evaluate the formula you want to test:

• BatchClient.PoolOperations.EvaluateAutoScale or EvaluateAutoScaleAsync

  These Batch .NET methods require the ID of an existing pool and a string containing the autoscale formula to evaluate.

• Evaluate an automatic scaling formula

  In this REST API request, specify the pool ID in the URI, and the autoscale formula in the autoScaleFormula element of the request body. The response of the operation contains any error information that might be related to the formula.

The following Batch .NET example evaluates an autoscale formula. If the pool doesn't already use autoscaling, enable it first.

// First obtain a reference to an existing pool
CloudPool pool = await batchClient.PoolOperations.GetPoolAsync("myExistingPool");

// If autoscaling isn't already enabled on the pool, enable it.
// You can't evaluate an autoscale formula on a non-autoscale-enabled pool.
if (pool.AutoScaleEnabled == false)
{
    // You need a valid autoscale formula to enable autoscaling on the
    // pool. This formula is valid, but won't resize the pool:
    await pool.EnableAutoScaleAsync(
        autoscaleFormula: "$TargetDedicatedNodes = $CurrentDedicatedNodes;",
        autoscaleEvaluationInterval: TimeSpan.FromMinutes(5));

    // Batch limits EnableAutoScaleAsync calls to once every 30 seconds.
    // Because you want to apply our new autoscale formula below if it
    // evaluates successfully, and you *just* enabled autoscaling on
    // this pool, pause here to ensure you pass that threshold.
    Thread.Sleep(TimeSpan.FromSeconds(31));

    // Refresh the properties of the pool so that we've got the
    // latest value for AutoScaleEnabled
    await pool.RefreshAsync();
}

// You must ensure that autoscaling is enabled on the pool prior to
// evaluating a formula
if (pool.AutoScaleEnabled == true)
{
    // The formula to evaluate - adjusts target number of nodes based on
    // day of week and time of day
    string myFormula = @"
        $curTime = time();
        $workHours = $curTime.hour >= 8 && $curTime.hour < 18;
        $isWeekday = $curTime.weekday >= 1 && $curTime.weekday <= 5;
        $isWorkingWeekdayHour = $workHours && $isWeekday;
        $TargetDedicatedNodes = $isWorkingWeekdayHour ? 20:10;
    ";

    // Perform the autoscale formula evaluation. Note that this code does not
    // actually apply the formula to the pool.
    AutoScaleRun eval =
        await batchClient.PoolOperations.EvaluateAutoScaleAsync(pool.Id, myFormula);

    if (eval.Error == null)
    {
        // Evaluation success - print the results of the AutoScaleRun.
        // This will display the values of each variable as evaluated by the
        // autoscale formula.
        Console.WriteLine("AutoScaleRun.Results: " +
            eval.Results.Replace("$", "\n    $"));

        // Apply the formula to the pool since it evaluated successfully
        await batchClient.PoolOperations.EnableAutoScaleAsync(pool.Id, myFormula);
    }
    else
    {
        // Evaluation failed, output the message associated with the error
        Console.WriteLine("AutoScaleRun.Error.Message: " +
            eval.Error.Message);
    }
}

Successful evaluation of the formula shown in this code snippet produces results similar to:

AutoScaleRun.Results:
    $TargetDedicatedNodes=10;
    $NodeDeallocationOption=requeue;
    $curTime=2016-10-13T19:18:47.805Z;
    $isWeekday=1;
    $isWorkingWeekdayHour=0;
    $workHours=0

Get information about autoscale runs

It's recommended to periodically check the Batch service's evaluation of your autoscale formula. To do so, get (or refresh) a reference to the pool, then examine the properties of its last autoscale run.

In Batch .NET, the CloudPool.AutoScaleRun property has several properties that provide information about the latest automatic scaling run performed on the pool:

• AutoScaleRun.Timestamp
• AutoScaleRun.Results
• AutoScaleRun.Error

In the REST API, information about a pool includes the latest automatic scaling run information in the autoScaleRun property.

The following C# example uses the Batch .NET library to print information about the last autoscaling run on pool myPool.

await Cloud pool = myBatchClient.PoolOperations.GetPoolAsync("myPool");
Console.WriteLine("Last execution: " + pool.AutoScaleRun.Timestamp);
Console.WriteLine("Result:" + pool.AutoScaleRun.Results.Replace("$", "\n  $"));
Console.WriteLine("Error: " + pool.AutoScaleRun.Error);

Sample output from the preceding example:

Last execution: 10/14/2016 18:36:43
Result:
  $TargetDedicatedNodes=10;
  $NodeDeallocationOption=requeue;
  $curTime=2016-10-14T18:36:43.282Z;
  $isWeekday=1;
  $isWorkingWeekdayHour=0;
  $workHours=0
Error:

Get autoscale run history using pool autoscale events

You can also check automatic scaling history by querying PoolAutoScaleEvent. Batch emits this event to record each occurrence of autoscale formula evaluation and execution, which can be helpful to troubleshoot potential issues.

Sample event for PoolAutoScaleEvent:

{
    "id": "poolId",
    "timestamp": "2020-09-21T23:41:36.750Z",
    "formula": "...",
    "results": "$TargetDedicatedNodes=10;$NodeDeallocationOption=requeue;$curTime=2016-10-14T18:36:43.282Z;$isWeekday=1;$isWorkingWeekdayHour=0;$workHours=0",
    "error": {
        "code": "",
        "message": "",
        "values": []
    }
}

Example autoscale formulas

Let's look at a few formulas that show different ways to adjust the amount of compute resources in a pool.

Example 1: Time-based adjustment

Suppose you want to adjust the pool size based on the day of the week and time of day. This example shows how to increase or decrease the number of nodes in the pool accordingly.

The formula first obtains the current time. If it's a weekday (1-5) and within working hours (8 AM to 6 PM), the target pool size is set to 20 nodes. Otherwise, it's set to 10 nodes.

$curTime = time();
$workHours = $curTime.hour >= 8 && $curTime.hour < 18;
$isWeekday = $curTime.weekday >= 1 && $curTime.weekday <= 5;
$isWorkingWeekdayHour = $workHours && $isWeekday;
$TargetDedicatedNodes = $isWorkingWeekdayHour ? 20:10;
$NodeDeallocationOption = taskcompletion;

$curTime can be adjusted to reflect your local time zone by adding time() to the product of TimeZoneInterval_Hour and your UTC offset. For instance, use $curTime = time() + (-6 * TimeInterval_Hour); for Mountain Daylight Time (MDT). Keep in mind that the offset needs to be adjusted at the start and end of daylight saving time, if applicable.

Example 2: Task-based adjustment

In this C# example, the pool size is adjusted based on the number of tasks in the queue. Both comments and line breaks are included in the formula strings.

// Get pending tasks for the past 15 minutes.
$samples = $PendingTasks.GetSamplePercent(TimeInterval_Minute * 15);
// If you have fewer than 70 percent data points, use the last sample point,
// otherwise use the maximum of last sample point and the history average.
$tasks = $samples < 70 ? max(0,$PendingTasks.GetSample(1)) : max( $PendingTasks.GetSample(1), avg($PendingTasks.GetSample(TimeInterval_Minute * 15)));
// If number of pending tasks is not 0, set targetVM to pending tasks, otherwise
// half of current dedicated.
$targetVMs = $tasks > 0? $tasks:max(0, $TargetDedicatedNodes/2);
// The pool size is capped at 20, if target VM value is more than that, set it
// to 20. This value should be adjusted according to your use case.
$TargetDedicatedNodes = max(0, min($targetVMs, 20));
// Set node deallocation mode - let running tasks finish before removing a node
$NodeDeallocationOption = taskcompletion;

Example 3: Accounting for parallel tasks

This C# example adjusts the pool size based on the number of tasks. This formula also takes into account the TaskSlotsPerNode value that's been set for the pool. This approach is useful in situations where parallel task execution has been enabled on your pool.

// Determine whether 70 percent of the samples have been recorded in the past
// 15 minutes; if not, use last sample
$samples = $ActiveTasks.GetSamplePercent(TimeInterval_Minute * 15);
$tasks = $samples < 70 ? max(0,$ActiveTasks.GetSample(1)) : max( $ActiveTasks.GetSample(1),avg($ActiveTasks.GetSample(TimeInterval_Minute * 15)));
// Set the number of nodes to add to one-fourth the number of active tasks
// (the TaskSlotsPerNode property on this pool is set to 4, adjust
// this number for your use case)
$cores = $TargetDedicatedNodes * 4;
$extraVMs = (($tasks - $cores) + 3) / 4;
$targetVMs = ($TargetDedicatedNodes + $extraVMs);
// Attempt to grow the number of compute nodes to match the number of active
// tasks, with a maximum of 3
$TargetDedicatedNodes = max(0,min($targetVMs,3));
// Keep the nodes active until the tasks finish
$NodeDeallocationOption = taskcompletion;

Example 4: Setting an initial pool size

This example shows a C# example with an autoscale formula that sets the pool size to a specified number of nodes for an initial time period. After that, it adjusts the pool size based on the number of running and active tasks.

Specifically, this formula does the following:

• Sets the initial pool size to four nodes.
• Doesn't adjust the pool size within the first 10 minutes of the pool's lifecycle.
• After 10 minutes, obtains the max value of the number of running and active tasks within the past 60 minutes.
  • If both values are 0, indicating that no tasks were running or active in the last 60 minutes, the pool size is set to 0.
  • If either value is greater than zero, no change is made.

string now = DateTime.UtcNow.ToString("r");
string formula = string.Format(@"
    $TargetDedicatedNodes = {1};
    lifespan         = time() - time(""{0}"");
    span             = TimeInterval_Minute * 60;
    startup          = TimeInterval_Minute * 10;
    ratio            = 50;

    $TargetDedicatedNodes = (lifespan > startup ? (max($RunningTasks.GetSample(span, ratio), $ActiveTasks.GetSample(span, ratio)) == 0 ? 0 : $TargetDedicatedNodes) : {1});
    ", now, 4);

Next steps

• Learn how to execute multiple tasks simultaneously on the compute nodes in your pool. Along with autoscaling, this can help to lower job duration for some workloads, saving you money.
• Learn how to query the Azure Batch service efficiently.