External Task Client
- Complete External Tasks
- Extend the lock duration of External Tasks
- Unlock External Tasks
- Report BPMN errors and failures
- Share variables with the Workflow Engine
Bootstrapping the Client
The client allows to handle service tasks of type “external”. In order to configure and instantiate the client, all supported implementations offer a convenient interface. The communication between the client and the Camunda Workflow Engine is HTTP. Hence, the respective URL of the REST API is a mandatory information.
To add additional HTTP headers to the performed REST API requests, the request interceptor method can be used. This becomes necessary, in the context of e.g. authentication.
In some cases it is necessary to secure the REST API of the Camunda Workflow Engine via Basic Authentication. For such situations a Basic Authentication implementation is provided by the client. Once configured with user credentials, the basic authentication header is added to each REST API request.
Custom interceptors can be added while bootstrapping the client. For more details regarding the implementation please check the documentation related to the client of interest.
If a Service Task of the type “External” is placed inside a workflow, a topic name must be specified. The corresponding BPMN 2.0 XML could look as follows:
... <serviceTask id="checkCreditScoreTask" name="Check credit score" camunda:type="external" camunda:topic="creditScoreChecker" /> ...
As soon as the Workflow Engine reached an External Task in a BPMN process, a corresponding activity instance is created, which is waiting to be fetched and locked by a client.
The client subscribes to the topic and fetches continuously for newly appearing External Tasks provided by the Workflow Engine. Each fetched External Task is marked with a temporary lock. Like this, no other clients can work on this certain External Task in the meanwhile. A lock is valid for the specified period of time and can be extended.
When setting up a new topic subscription, it is mandatory to specify the topic name and a handler function. Once a topic has been subscribed, the client can start receiving work items by polling the process engine’s API.
Handlers can be used to implement custom methods which are invoked whenever an External Task is fetched and locked successfully. For each topic subscription an External Task handler interface is provided.
The handlers are invoked sequentially for each fetched-and-locked external task.
Once the custom methods specified in the handler are completed, the External Task can be completed. This means for the Workflow Engine that the execution will
move on. For this purpose, all supported implementations have a
complete method which can be called within the handler function. However, the
External Task can only be completed, if it is currently locked by the client.
Extending the Lock Duration of Tasks
Sometimes the completion of custom methods takes longer than expected. In this case the lock duration needs to be extended.
This action can be performed by calling an
extendLock method passing the new lock duration.
The lock duration can only be extended, if the External Task is currently locked by the client.
If an External Task is supposed to be unlocked so that other clients are allowed to fetch and lock this task again,
unlock method can be called. The External Task can only be unlocked, if the task is currently locked by the client.
If the client faces a problem that makes it impossible to complete the External Task successfully, this problem can be reported to the Workflow Engine. A failure can only be reported, if the External Task is currently locked by the client. You can find a detailed documentation about this action in the Camunda BPM User Guide.
Reporting BPMN Errors
Error boundary events are triggered by BPMN errors. A BPMN error can only be reported, if the External Task is currently locked by the client. You can find a detailed documentation about this action in the Camunda BPM User Guide.
Both external tasks clients are compatible with all data types the Camunda Engine supports. Variables can be accessed/altered using typed or the untyped API.
Process and Local Variables
Variables can be treated as process or local variables. The former is set on the highest possible hierarchy of the variable scope and available to its child scopes in the entire process. If a variable, in contrast, is supposed to be set exactly on the provided execution scope, the local type can be used.
Note: setting variables does not make sure that variables are persisted. Variables which were set locally on client-side are only available during runtime and get lost if they are not shared with the Workflow Engine by successfully completing the External Task of the current lock.
Untyped variables are stored by using the respective type of their values. It is possible to store/retrieve only a single variable or multiple variables at once.
Setting typed variables requires the type to be specified explicitly. Typed variables can also be retrieved, the received object provides a variety of information besides type and value. Of course it is also possible to set and get multiple typed variables.
The client implementations support logging various events in the client lifecycle. Hence situations like the following can be reported:
- External Tasks could not be fetched and locked successfully
- An exception occurred…
- while invoking a handler
- while deserializing variables
- while invoking a request interceptor
For more details, please check the documentation related to the client of interest.
External task throughput
For a high throughput of external tasks, you should balance between the number of external task instances, the number of clients and the duration of handling the work.
A rule of thump for long running tasks (maybe more than 30 secs) would be, to fetch-and-lock the tasks one by one (maskTasks = 1) and adjust the Long Polling interval to your needs (maybe 60 secs, asyncResponseTime = 60000). The Java client supports exponential backoff, default by 500 ms with factor 2, limited by 60000 ms. This could be shorted to your needs, too.
As the external task clients didn’t use any threading internally, you should start as many clients as needed and balance the load with your operating system.