Draft, in work.
Major Xtc Library Classes
In the Xtc library, the most important data containers and associated classes are:
- XtcData:Dgram, which is the root container, essentially an envelope, for other containers.
- XtcData:Xtc, the parent data container class for detector data and descriptive metadata.
- XtcData:TypeId is used to identify which flavor of Xtc container listed below is being worked with when recording or parsing.
- XtcData:Names, derived Xtc container for storing named metadata about data types and data dimensionality.
- XtcData:ShapesData, a derived convenience Xtc container with methods to access detector data.
- XtcData:Shapes, a derived Xtc container containing metadata about the structure of the raw detector data.
- XtcData:Data, a derived Xtc container for raw detector data itself.
The DAQ Side: Generating Xtc
For tutorial purposes, the following streamlined example shows XXX. For an example involving more detectors, formats and algorithms, see xtcdata/xtcdata/app/xtcwriter.cc.
A Notional DAQ Harness
Let's motivate the example. Assume we're developing a an Xtc writer class to output HSD data. This class will get plugged in to a (notional) DAQ framework.
// %sample:intro:cpp% // Assume some totally made-up data acquisition callback framework named DAQFramework // Developers of the psdaq package are welcome to come in and change this toy example // to match the real DAQ API. HSDXtcWriter hsdwriter() DAQFramework.registerHandler("configure", hsdwriter.setup) DAQFramework.registerHandler("runstart", std::bind(hsdwriter.openXtcFile, filename)) // Assume we get a "readout" message per event DAQFramework.registerHandler("readout", hsdwriter.writeFeature) DAQFramework.registerHandler("runend", hsdwriter.closeXtcFile) //%endsample%
"setup" Method: Set Up The Names Structure
Register the algorithms and "shape" of the data associated with detector elements so the processing pipeline knows what to do with it.
- First some plumbing. Here, for example, in a constructor:
- Declare an XtcData:Dgram as the root container for metadata (see %codesample1:14,15,38,39,45:setupexamples%).
- And call a utility method to actually create and set up this metadata Dgram (see %codesample1:14,15,38,39,43,44,45,46-58:setupexamples%).
- Set aside a vector to store some XtcData:Names we use to annotate this data (see %codesample1:14,15,40,45:setupexamples%).
- In the setup method itself, call an internal method to actually populate this configuration structure. See %codesample1:76-80,59-74:setupexamples%.
- We create an Xtc:Alg to record a specific feature extraction algorithm name, "fex", and version, 4.5.6, needed to process this data.
- We create an Xtc:Alg to record a specific feature extraction algorithm name, "fex", and version, 4.5.6, needed to process this data.
- XtcData::Dgram is your root container for metadata (Names and Shapes) as well as the data itself
- Sub-class from XtcData::VarDef to define data shapes
- VarDef gives you a NameVec with push_back() method to xxxx
"writeFeature" Method: Respond to Readout Events and Add Readout ShapesData and Data
(Introduce CreateData and its set_value (assign values explicitly) and allocate (set aside structured memory for the data then full in the structure with values) methods here. What about set_array_shape? With FexDef, we're positing that the data coming out has a well-defined shape, i.e. definable as a record, so per-event, you will update and emit this data record into the output stream)
Another list of steps/big ideas this time about appending data.
- .
- A chunk of code summary.
- Use CreateData for XYZ applications:
- set_value() for scalar data
- allocate() for data vectors or matrices
- Use DescribedData for XYZ applications
The Pipeline Side: Parsing Whole Xtc Files, Using Small Data Files
xtcreader.cc and XtcIterator.hh
A Notional Offline Pipeline Harness
Notes for Real-World Code
Realistic Xtc Writers
(Bullet lists of gotchas and recommendations)
- Thread safety issue #1
- Thread safety issue #2
- (You'll likely need to sub-class from XYZ in order to ABC)
Realistic Xtc Readers
(Bullet lists of gotchas and recommendations)
- Thread safety issue #1
- Thread safety issue #2
- (You'll likely need to sub-class from XYZ in order to ABC)
Full "Hello Xtc" Code Listings
HSDXtcWriter Example
// %sample:codesample1:cpp% #include "xtcdata/xtc/ShapesData.hh" #include "xtcdata/xtc/DescData.hh" #include "xtcdata/xtc/Dgram.hh" #include "xtcdata/xtc/TypeId.hh" #include "xtcdata/xtc/XtcIterator.hh" #include "xtcdata/xtc/VarDef.hh" using namespace XtcData; #define BUFSIZE 0x4000000 HSDXtcWriter::HSDXtcWriter() { // Define the "signature" of the data in // our feature extraction data set class FexDef:public VarDef { public: enum index { floatFex, arrayFex, intFex }; FexDef() { NameVec.push_back({"floatFex",Name::DOUBLE}); NameVec.push_back({"arrayFex",Name::FLOAT,2}); NameVec.push_back({"intFex",Name::INT64}); } }; // Define some instance variables. FILE *this->xtcoutfile; // An Xtc::Dgram to store metadata about the output Dgram& this->metadataDgram; std::vector<NameIndex>& this->namesVec; this->FexDef = new FexDef(); // Now initialize various things _initializeMetadataDgram(); } void HSDXtcWriter::_initializeMetadataDgram() { TypeId tid(TypeId::Parent, 0); void* configbuf = malloc(BUFSIZE); this->metadataDgram = *(Dgram*)configbuf; this->metadataDgram.xtc.contains = tid; this->metadataDgram.xtc.damage = 0; // Check if Xtc::Xtc auto-alloc will take care of // updating this extent as we go? this->metadataDgram.xtc.extent = sizeof(Xtc); } void HSDXtcWriter::_addNames(Xtc& parent, std::vector<NameIndex>& aNamesVec) { // Instantiate an Alg to define metadata for // a feature extraction ("fex") algorithm Alg hsdFexAlg("fex",4,5,6); // Create a Names structure that identifies // the location, proper decoding algorithm, detector type // and detectorID. Names& fexNames = *new(parent) Names("xpphsd", hsdFexAlg, "hsd", "detnum1234") // And now we add the Names structure to the // parent Xtc metadata container along with // the FexDef data signature. fexNames.add(parent, this->FexDef); aNamesVec.push_back(NameIndex(this->fexNames)); } void HSDXtcWriter::setup() { // Add a Names structure to the configuration Dgram _addNames(this->metadataDgram.xtc, this->namesVec); } void HSDXtcWriter::openXtcFile(xtcoutfilename) { this->xtcoutfile = fopen(xtcoutfilename, "w"); } void HSDXtcWriter::writeFeature() { // Need some makebelieve code here to show how this // callback method unpacks data from DAQ // to cram into these Xtc structures... // Talk to Chris. Is this event by event? Would you want // to destruct this CreateData after each event? CreateData fex(parent, NamesVec, nameId); // set_value() lets you set scalar values in the data record. // The 'floatFex' ID is defined in our custom FexDef class. fex.set_value(FexDef::floatFex, (double)41.0); // Use allocate() to set up data that's better represented as // vectors or matrices. // The 'arrayFex' ID is defined in our custom FexDef class. unsigned shape[MaxRank] = {2,3}; //MaxRank is an Xtc library //global upper limit on data complexity. Array<float> arrayT = fex.allocate<float>(this->FexDef::arrayFex, shape); for(unsigned i=0; i<shape[0]; i++){ for (unsigned j=0; j<shape[1]; j++) { arrayT(i,j) = 142.0+i*shape[1]+j; } }; // Another scalar value setter. // The 'intFex' ID is defined in our custom FexDef class. fex.set_value(FexDef::intFex, (int64_t) 42); } void HSDXtcWriter::closeXtcFile() { fclose(this->xtcoutfile); } // ... // %endsample%
HSDXtcReader Example
// %sample:codesample2:cpp% cout<<"Hi world?" // %endsample%