GridFS files and file reference attributes

Table of Contents

• Introduction
• Configuration
• File reference attributes
  • Reading a file stored in object attribute
  • Saving a file into object attribute

Introduction

MemoryX use MongoDB implementation of GridFS to store files in a database.

Configuration

Following properties are supported:

• <ArmarX|MemoryX>.CachePath: Files will be cached in $CachePath/files directory

The default cache directory is

~/.armarx/mongo/.cache/files

File reference attributes

Amongst other data types, EntityAttribute can have a file as its value. One common usage is an inventor model file required to visualize an object.

MemoryX handles this case as follows: the actual file resides in GridFS, whereas the attribute itself holds only a reference to it (s. memoryx::MongoDBRef class for details).

memoryx::GridFileManager class provides a convenient way to "dereference" such attributes and get the file content. Moreover, it performs local caching of retrieved files "under the hood".

Reading a file stored in object attribute

Init short-term memory

// get a proxy
memoryPrx = getProxy<WorkingMemoryInterfacePrx>("WorkingMemory");
// get a memory segment containing object instances
WorkingMemorySegmentBasePrx instancesSegment = memoryPrx->getObjectInstancesSegment();
// fetch an entity
const EntityBasePtr entity = instancesSegment->getEntityById(1);

Init GridFileManager

// CommonStorage proxy must be provided as a parameter
GridFileManagerPtr fileManager ( new GridFileManager(memoryPrx->getCommonStorage()) );

Option 1. Get an opened file stream

std::ifstream fs;
if (fileManager->getFileStream(entity->getAttribute("IvFile"), fs))
{
    // read filestream as usual
}

Option 2. Get a local file name

std::string localFileName;
if (fileManager->ensureFileInCache(entity->getAttribute("IvFile"), localFileName))
{
    // open an ifstream for localFileName, pass it to your library function etc
}

Both methods will retrieve the whole file and cache it locally, making available for further processing with usual filesystem tools.

Saving a file into object attribute

// create object instance
ObjectInstancePtr inst = new ObjectInstance();
// create attribute
EntityAttributePtr fileAttr = new EntityAttribute("IvFile");
// Store file in GridFS and put a reference to it into attribute
fileManager->storeFileToAttr(
    "mydb",                         //  Mongo database to store file into
    "/home/alex/data/table.iv",     //  File name on the locale filesystem
    fileAttr,                       //  Attribute to store reference into
    "table.iv");                    //  File name in Mongo GridFS (optional)
// add file attribute to instance
inst->putAttribute(fileAttr);
// save instance into working memory
instancesSegment->putEntity(inst);