Houdini 20.0 Nodes TOP nodes

Service Start TOP node

Starts a PDG service.

Since 19.5

This node creates work items that will start up a PDG service.


You can also start up PDG services from the PDG Services panel.


Service Start

Open PDG Service Panel

Opens the PDG Services panel.

Generate When

Determines when this node will generate work items. You should generally leave this set to “Automatic” unless you know the node requires a specific generation mode, or that the work items need to be generated dynamically.

All Upstream Items are Generated

This node will generate work items once all of the input nodes have generated their work items.

All Upstream Items are Cooked

This node will generate work items once all of the input nodes have cooked their work items.

Each Upstream Item is Cooked

This node will generate work items each time a work item in an input node is cooked.


The generation mode is selected based on the generation mode of the input nodes. If any of the input nodes are generating work items when their inputs cook, this node will be set to Each Upstream Item is Cooked. Otherwise, it will be set to All Upstream Items are Generated.

Start Service During

Determines when the service is started.

Cook (In-Process)

Starts the service during the cook stage in the same process as the Houdini session.


Starts the service when the work item is generating.

Copy Inputs to Outputs

When on, work items in this node copy input files to their output file list.

Service Already Running

Determines how the node operates when the specified service (as defined by the Service Name parameter) is already running.


The node does nothing and the work item will successfully cook.

Report Warning

The node reports a warning and the work item will successfully cook.

Report Error

The node reports an error and the work item will fail.

Service Name

Specifies the name of the service to start. Type the name in this field or select the service from the drop-down list. If you want to start multiple services at once, then you can specify an expression.


TOP Scheduler Override

This parameter overrides the TOP scheduler for this node.

Work Item Label

Determines how the node should label its work items. This parameter allows you to assign non-unique label strings to your work items which are then used to identify the work items in the attribute panel, task bar, and scheduler job names.

Use Default Label

The work items in this node will use the default label from the TOP network, or have no label if the default is unset.

Inherit From Upstream Item

The work items inherit their labels from their parent work items.

Custom Expression

The work item label is set to the Label Expression custom expression which is evaluated for each item.

Node Defines Label

The work item label is defined in the node’s internal logic.

Label Expression

When on, this parameter specifies a custom label for work items created by this node. The parameter can be an expression that includes references to work item attributes or built-in properties. For example, $OS: @pdg_frame will set the label of each work item based on its frame value.

Work Item Priority

This parameter determines how the current scheduler prioritizes the work items in this node.

Inherit From Upstream Item

The work items inherit their priority from their parent items. If a work item has no parent, its priority is set to 0.

Custom Expression

The work item priority is set to the value of Priority Expression.

Node Defines Priority

The work item priority is set based on the node’s own internal priority calculations.

This option is only available on the Python Processor TOP, ROP Fetch TOP, and ROP Output TOP nodes. These nodes define their own prioritization schemes that are implemented in their node logic.

Priority Expression

This parameter specifies an expression for work item priority. The expression is evaluated for each work item in the node.

This parameter is only available when Work Item Priority is set to Custom Expression.

See also

TOP nodes