This page describes Pipelets, ProcessingServices and their lifecycle.
Pipelets and ProcessingServices are reusable Java Components in a BPEL workflow and can be orchestrated like any regular BPEL Service. Both are used to process the data contained in Records.
A Pipelet is a POJO that implements the interface org.eclipse.smila.processing.SimplePipelet. It's lifecycle and configuration are managed by the workflow engine. An instance of a Pipelet is not shared by multiple Pipelines (workflows), even multiple invocations of a Pipelet in the same Pipeleline do not share the same instance. Each <invokePipelet> Pipeline has it's own instance. An instance may still be accessed by multiple threads, for example if the same Pipeline is executed in parallel. The configuration of each Pipelet instance is included in the <invokePipelet> call in the BPEL pipeline. Technical details on Pipelet development can be found in the tutorial How to write a Pipelet.
A ProcessingService is an OSGi service (preferably a Declarative Service) that implements the interface org.eclipse.smila.processing.ProcessingService. It's lifecycle and configuration are NOT managed by the workflow engine, but by the OSGi runtime. An instance of a ProcessingService can be shared between multiple Pipelines and so is frequently accessed by multiple threads. The configuration for a ProcessingServices is not contained within the <invokeService> call. Each ProcessingService can have it's own configuration file(s). Typically these are located in a folder equal to the ProcessingServices bundle name within the global configuration folder. For simple configuration options a ProcessingService can use the same XML format used to configure Pipelets. There are ready to use classes for reading and parsing such configuration files available. Technical details on ProcessingServices development can be found in the tutorial How to write a ProcessingService.
The following diagram shows the lifecycle of Pipelets.
Pipelets live inside the workflow engine. When the engine starts, it reads the pipeline definitions (i.e. BPEL workflows) from the ConfigurationHelper. The pipelines are introspected for pipelet invocations (invokePipelet extension activities) and the pipelet configurations that are contained in the invocation XML elements. For each invocation it creates an instance of the specified pipelet class, parses the configuration from the BPEL document and injects it into the pipelet instance. The pipelet instance is stored in the workflow engine as long as the engine is not stopped (and as long as the bundle providing the pipelet is available, of course). So for each single pipelet invocation occurring in the pipelines a different pipelet instance exists with a single configuration.
The following diagram shows the lifecycle of ProcessingServices.
ProcessingServices live independent from the workflow engine. They are created, activated and registered by the OSGi Declarative Services runtime just like the engine itself (of course, they can also be started and registered using bundle activators or other code, if declaring them as a DS is not appropriate for some reason). They read their configuration by themselves, e.g. by using the ConfigurationHelper. Eventually, the DS runtime binds all correctly registered ProcessingServices to the workflow engine so that it can invoke them when it reaches an invokeService extension activity during execution of a pipeline.
The following diagram shows the lifecycle of both Pipelets and ProcessingServices.
Of course, pipelines can combine the use of Pipelets and ProcessingServices. The main purpose of this figure is to emphasize the main difference between Pipelets and ProcessingServices: Pipelets live inside the workflow engine and their instances are not shared, even if they have the same class and configuration. ProcessingServices live independent from the engine, manage their configuration on their own and instances can be shared by multiple pipelines. This makes them the more appropriate integration model if functionality to be integrated needs a complex internal model uses a lot of memory during runtime, or if resources should not be duplicated if used in multiple pipelines.
When to use a ProcessingService
Technically there are no limitations on Pipelets compared to ProcessingServices. The same functionality can be implemented using both approaches. There are no strict rules when to use which technology. However, ProcessingServices offer some benefits you may want to make use of. Here are some rules of thumb when to implement functionality as a ProcessingService instead as a Pipelet:
- Lower Memory Usage: a shared ProcessingService uses less memory than multiple instances of Pipelets. If your functionality needs lots of memory you should implement it as a ProcessingService
- Preserve Internal State: if your component has an internal state that should be preserved independently from the lifecycle of the workflow engine then implement it as a ProcessingService
- Reuse Functionality: if your functionality should not only be used inside of a pipeline but also from other services, then you should implement it as a ProcessingService. Being an OSGi service it offers the possibility to provide not only the ProcessingService interface but any other interface you like. For example this feature is used in the SimpleMimeTypeIdentifier.
- Not sharable Resources: if your components accesses resources that cannot be shared among multiple clients implement it as a ProcessingService as then only one instance exists and can be reused in multiple pipelines
- Flexible/Complex Configuration: if your component needs configuration options that are not supported by the simple PipeletConfiguration options implement it as a ProcessingService
- Use of OSGi Services: if your component wants to use one or more OSGi services than it's easier to implement it as a ProcessingService. References to other OSGi services can be easily configured on the component description of the ProcessingService. Otherwise you have to make use of the ServiceTracker to find references to services.