SimpleStorage {#guide-simplestorage}

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Overview # {#guide-simplestorage-overview}

A Simple storage service is the simplest possible abstraction for a service that can store and provide workflow files. It has a certain storage capacity, and provides write, read, and delete operations on files. Writes and Reads can be done synchronously or asynchronously. In addition, higher-level semantics such as copying a file directly from a storage service to another are provided.

Creating a Simple storage service # {#guide-simplestorage-creating}

In WRENCH, a Simple storage service represents a storage service (wrench::StorageService), which is defined by the wrench::SimpleStorage class. An instantiation of a Simple storage service requires the following parameters:

• The name of a host on which to start the service (this is the entry point to the service);
• A list of mount points (corresponding to disks attached to the host)
• Maps (std::map) of configurable properties (wrench::BatchComputeServiceProperty) and configurable message payloads (wrench::BatchComputeServiceMessagePayload).

The example below shows how to create an instance of a Simple storage service that runs on host "Gateway", has access to 64TiB of storage. Furthermore, the number of maximum concurrent data connections supported by the service is configured to be 8:

auto storage_service = simulation->add(
          new wrench::SimpleStorageService("Gateway", 
                                  {"/data/", "/home/"},
                                       {{wrench::SimpleStorageProperty::MAX_NUM_CONCURRENT_DATA_CONNECTIONS, "8"}}
                                      );

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Using a simple storage service # {#guide-simplestorage-using}

One can interact directly with a simple storage service to check whether the service hosts a particular file or to delete a file:

wrench::WorkflowFile *some_file;

[...]

if (wrench::StorageService::lookupFile(wrench::FileLocation::LOCATION(storage_service, "/data/")) {
  std::cerr << "File is there! Let's delete it...\n";
  storage_service->deleteFile(some_file);
}

While there are few other direct interactions are possible (see the documentation of the wrench::StorageService and wrench::SimpleStorageService classes), many interactions are via a data movement manager (an instance of the wrench::DataMovementManager class). This is a helper process that makes it possible interact asynchronously and transparently with storage services. For instance, here is an example of a synchronous file copy:

wrench::WorkflowFile *some_file;
wrench::StorageService *src, *dst;

[...]

// Create a data movement manager
auto data_movement_manager = this->createDataMovementManager();

data_movement_manager->doSynchronousFileCopy(some_file, 
                                             wrench::FileLocation::LOCATION(src), 
                                             wrench::FileLocation::LOCATION(dst));

The above call blocks until the file copy has completed. An asynchronous file copy would work as follows:

wrench::WorkflowFile *some_file;
wrench::StorageService *src, *dst;

[...]

// Create a job manager
auto job_manager = this->createJobManager();

job_manager->initiateAsynchronousFileCopy(some_file, 
                                          wrench::FileLocation::LOCATION(src), 
                                          wrench::FileLocation::LOCATION(dst));

[...]

// Wait for a workflow execution event
auto event = this->getWorkflow()->waitForNextExecutionEvent();

if (event->type == wrench::WorkflowExecutionEvent::FILE_COPY_COMPLETION) {
  std::cerr << "Success!\n";
} else if (event->type == wrench::WorkflowExecutionEvent::FILE_COPY_COMPLETION) {
  std::cerr << "Failure!\n";
}

See the documentation of the wrench::DataMovementManager class for all available API methods.

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