Depending on the process definition, an instance of it can be created in the following ways
- CreateProcessInstance commands
- timer event handler
- message event
A process instance can be created by sending a command specifying the BPMN process id or the unique key of the process. There are two commands to create a process instance.
A process that has a none start event can be started explicitly using CreateProcessInstance. This command creates a new process instance and immediately responds with the process instance id. The execution of the process happens after the response is send.
Create a process instance:
Typically, process creation and execution are decoupled. However, there are use-cases that need to collect the results of a process when it's execution is completed. CreateProcessInstanceWithResult allows you to “synchronously” execute processes and receive the results via a set of variables. The response is sent when the process execution is completed.
This command is typically useful for short-running processes and processes that collect information. If the process mutates system state, or further operations rely on the process outcome response to the client, take care to consider and design your system for failure states and retries. Note that when the client resends the command, it creates a new process instance.
Create a process instance and await results:
Response: (Note that the variables in the response depend on the process.)
Failure scenarios that are applicable to other commands are applicable to this command, as well. Clients may not get a response in the following cases even if the process execution is completed successfully.
- Connection timeout: If the gRPC deadlines are not configured for long request timeout, the connection may be closed before the process is completed.
- Network connection loss: This can happen at several steps in the communication chain
- Failover: When the node that is processing this process crashed, another node continues the processing. But that other node does not send the response because the request is registered on the first one.
- Gateway failure: If the gateway to which the client is connected fails, nodes inside the cluster cannot send the response to the client.
Process instances are also created implicitly via various start events. Camunda Cloud supports message start events and timer start events.
A process with a message start event can be started by publishing a message with the name that matches the message name of the start event. For each new message a new instance is created.
A process can also have one or more timer start events. An instance of the process is created when the associated timer is triggered. Timers can also trigger periodically.