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How To: Author a ForgeTree
How-To: Author a ForgeTree
This page provides a detailed guide how to author a ForgeTree and utilize the various features of Forge.
Overview of ForgeTree Properties
How Forge TreeWalker Walks the ForgeTree
{
"RootTreeNodeKey": "Root",
"Tree": {
"Root": {
"Type": "Selection",
"ChildSelector": [
{
"Label": "Container",
"ShouldSelect": "C#|UserContext.ResourceType == \"Container\"",
"Child": "Container"
},
{
"Label": "Node",
"ShouldSelect": "C#|UserContext.ResourceType == \"Node\"",
"Child": "Node"
}
]
},
"Container": {
"Type": "Action",
"Actions": {
"CollectDiagnosticsAction_Container": {
"Action": "CollectDiagnosticsAction"
}
},
"ChildSelector": [
{
"Label": "Tardigrade",
"ShouldSelect": "C#|Session.GetLastActionResponse().Status == \"Success\"",
"Child": "Tardigrade"
}
]
},
"Tardigrade": {
"Type": "Action",
"Actions": {
"TardigradeAction_Tardigrade": {
"Action": "TardigradeAction",
"Input": {
"Context": "ContainerFault",
"EnableV2": true,
"DiagnosticData": "C#|Session.GetLastActionResponse().Output"
}
}
},
"ChildSelector": [
{
"Label": "Tardigrade_Success",
"ShouldSelect": "C#|(await Session.GetOutputAsync(\"Tardigrade_TardigradeAction\")).Status == \"Success\"",
"Child": "Tardigrade_Success"
},
{
"Label": "Tardigrade_Failure",
"Child": "Tardigrade_Failure"
}
]
},
"Tardigrade_Failure": {
"Type": "Leaf"
},
"Tardigrade_Success": {
"Type": "Leaf",
"Actions": {
"LeafNodeSummaryAction_Tardigrade_Success": {
"Action": "LeafNodeSummaryAction",
"Input": {
"Status": "C#|string.Format(\"{0}_{1}\", \"ContainerFaultScenario\", (await Session.GetLastActionResponseAsync()).Status)",
"StatusCode": 0,
"Output": {
"ActionOutput": "C#|(await Session.GetLastActionResponseAsync()).Output",
"DiagnosticsOutput": "C#|(await Session.GetOutputAsync(\"Container_CollectDiagnosticsAction\")).Output"
}
}
}
}
},
"Node": {
"Type": "Selection",
"Properties": {
"Notes": "Decision to Reboot or Evacuate is decided in UserContext.ShouldReboot()."
},
"ChildSelector": [
{
"Label": "Reboot",
"ShouldSelect": "C#|UserContext.ShouldReboot()",
"Child": "Reboot"
},
{
"Label": "Evacuate",
"Child": "Evacuate"
}
]
},
"Reboot": {
"Type": "Action",
"Actions": {
"RebootAction_Reboot": {
"Action": "RebootAction",
"RetryPolicy": {
"Type": "FixedCount",
"MaxRetryCount": 3,
"MinBackoffMs": 1000
},
"ContinuationOnRetryExhaustion": true
}
}
},
"Evacuate": {
"Type": "Action",
"Actions": {
"EvacuateAction_Evacuate": {
"Action": "EvacuateAction",
"Timeout": 60000,
"RetryPolicy": {
"Type": "ExponentialBackoff",
"MinBackoffMs": 1000,
"MaxBackoffMs": 30000
},
"ContinuationOnTimeout": true
},
"NotifyCustomerAction_Evacuate": {
"Action": "NotifyCustomerAction",
"Properties": "Notify customer in parallel with the impact."
}
},
"Timeout": "C#|UserContext.GetTimeoutForEvacuatingAndNotifyingCustomer()"
}
}
}{
"RootTreeNodeKey": "Root",
"Tree": {
"Root": {Tree is a dictionary that maps unique TreeNodeKey strings to TreeNodes. In the example we see several TreeNodeKeys, such as: Root, Container, Node, etc..
RootTreeNodeKey defines the suggested "Root" TreeNodeKey that should be visited first when walking the tree. Without this property, the callers of WalkTree would not know which TreeNodeKey to visit first since the TreeNodes are organized in a dictionary.
The default value of RootTreeNodeKey is "Root."
"Root": {
"Type": "Selection",
"ChildSelector": [
{
"Label": "Container",
"ShouldSelect": "C#|UserContext.ResourceType == \"Container\"",
"Child": "Container"
},
{
"Label": "Node",
"ShouldSelect": "C#|UserContext.ResourceType == \"Node\"",
"Child": "Node"
}
]
},Let's look at our first TreeNode, which is a Selection node. There are 4 TreeNodeTypes: Selection, Action, Leaf, and Subroutine. Selection type nodes have the following behavior:
- ChildSelector property is defined. Attempts to select a child node to visit next.
- Does not execute Actions.
- Does not consume the TreeNode Timeout.
ChildSelectors define the connections to child TreeNodes, as well as the conditions required to visit each child. Several TreeNodeTypes can use ChildSelectors, including: Selection, Action, and Subroutine. Let's break down each property:
- Label - This is used only in ForgeEditor for visualization purposes. It is the text that hovers above each child TreeNode. Recommend using this to describe the ShouldSelect statement. This gives context to why the child is being visited.
- ShouldSelect - A string code-snippet that can be parsed and evaluated to a boolean value. If the expression is true, visit the attached Child TreeNode. If the expression is empty, evaluate to true by default. We'll dive more into "C#|" and Roslyn further down. For now, you can read the first ShouldSelect statement as follows: If the ResourceType equals Container, then visit the Container TreeNode.
- Child - The string TreeNodeKey that will be visited if the attached ShouldSelect expression evaluates to true.
"Container": {
"Type": "Action",
"Actions": {
"CollectDiagnosticsAction_Container": {
"Action": "CollectDiagnosticsAction"
}
},
"ChildSelector": [
{
"Label": "Tardigrade",
"ShouldSelect": "C#|Session.GetLastActionResponse().Status == \"Success\"",
"Child": "Tardigrade"
}
]
},Our next TreeNode is an Action node. Action type nodes have the following behavior:
- Executes Actions. Must contain at least one Action.
- If multiple Actions are defined, executes them all in parallel.
- Cannot execute a SubroutineAction.
- Optionally, ChildSelector can be defined. Child selection happens after executing Actions, as long as there are no unhandled exceptions/timeouts.
- Optionally, TreeNode-level Timeout can be defined. This is the timeout in milliseconds for executing all TreeActions. If the timeout is hit, a TimeoutException will be thrown and the tree walker session will be cancelled.
Actions is a dictionary that maps unique TreeActionKey strings to TreeActions. In the example we see several TreeActionKeys, such as: CollectDiagnosticsAction_Container, TardigradeAction_Tardigrade, etc..
TreeActionKeys must be unique across each ForgeTree. This is required because Forge uses the TreeActionKey when persisting some state. The application owner could also decide to enforce globally unique TreeActionKeys. Global uniqueness allows TreeActionKeys by themselves to be a strong key, instead of having to couple it with TreeNode or TreeName.
"Tardigrade": {
"Type": "Action",
"Actions": {
"TardigradeAction_Tardigrade": {
"Action": "TardigradeAction",
"Input": {
"Context": "ContainerFault",
"EnableV2": true,
"DiagnosticData": "C#|Session.GetLastActionResponse().Output"
}
}
},
"ChildSelector": [
{
"Label": "Tardigrade_Success",
"ShouldSelect": "C#|(await Session.GetOutputAsync(\"Tardigrade_TardigradeAction\")).Status == \"Success\"",
"Child": "Tardigrade_Success"
},
{
"Label": "Tardigrade_Failure",
"Child": "Tardigrade_Failure"
}
]
},The string name that maps to a ForgeAction. In the example we see several ActionNames, such as: CollectDiagnosticsAction, TardigradeAction, LeafNodeSummaryAction, etc.. These all map to classes that have been tagged with the ForgeActionAttribute and inherit from Forge's BaseAction class.
More details here:
The Input property becomes the ActionInput object passed to the corresponding ForgeAction. Like other dynamic properties in the ForgeTree, this can be any supported JSON type including object, string, number, dictionary, array, etc.. Forge tree walker will dynamically evaluate the Input property while walking the tree, instantiate the object as the specified Type, and pass it to the ForgeAction.
The recommended way to utilize ActionInput is for the ForgeAction to specify the desired InputType in the ForgeActionAttribute. This allows Forge tree walker to create the desired Type object, and fill its properties from the TreeAction.Input. This has the benefit of type safety for the ForgeAction author and ForgeTree author, since they are using the same data contract.
The not recommended to utilize ActionInput is for the ForgeAction to not specify any InputType, but still allow the TreeAction.Input to be used. (Note: ForgeAction authors can choose which ForgeTree properties and values are allowed by utilizing the ForgeSchemaValidationRules if the application is utilizing that feature.) In this case, Forge will create a dynamic JObject from the TreeAction.Input. This is not recommended because you lose the type safety and data contract between ForgeAction author and ForgeTree author.
ForgeActions can specify to not have any InputType, like we saw in the CollectDiagnosticsAction.
More details here:
In this example, we see the Input for TardigradeAction with 3 properties: Context, EnableV2, and DiagnosticsData. So the TardigradeInput class could be defined like this:
[ForgeAction(InputType: typeof(TardigradeInput))]
public class TardigradeAction : BaseAction { ... }
public class TardigradeInput
{
public string Context { get; set; }
public bool EnableV2 { get; set; }
public DiagnosticData DiagnosticData { get; set; }
public long PollingIntervalInMilliseconds { get; set; } = 1000;
public string AdditionalDetails { get; set; }
}Few things to note:
- Notice the Types were aligned for the 3 properties specified in TreeAction.Input. Context given as a string, EnableV2 as a bool, and DiagnosticData given as DiagnosticData. Unexpected properties or types will likely result in an exception. E.g. An exception will be thrown if the TreeAction.Input tried to set DiagnosticData as a string, or tried to add an undefined property.
- DiagnosticData came from a Roslyn expression that gets the ActionResponse.Output object from the CollectDiagnosticsAction. Not shown in the example is the ForgeAction defining the Output object as type DiagnosticData.
- It is a common pattern in Forge to use either the results of previous ForgeActions or data from the UserContext as ActionInput.
- Not all properties of TardigradeInput were used in the TreeAction.Input. Since Forge initializes the TardigradeInput object, the default value of PollingIntervalInMilliseconds will be set without having to specify in the TreeAction.Input.
- AdditionalDetails has no default value and was not specified in the TreeAction.Input, so it will be null. The ForgeAction is expected to handle this gracefully, or the author should require the property to be specified in TreeAction.Input by utilizing the ForgeSchemaValidationRules.
"Tardigrade_Failure": {
"Type": "Leaf"
},Our next TreeNode is a Leaf node. Leaf type nodes have the following behavior:
- Only TreeNodeType that does not allow ChildSelectors.
- Optionally can execute the native LeafNodeSummaryAction. No other ForgeActions can be executed on Leaf nodes, and only one LeafNodeSummaryAction can be executed.
- Does not consume the TreeNode Timeout.
Since they cannot have children, Leaf nodes are used as a clear indicator for the end of tree paths.