\page refCCDB Module 'CCDB'

The Conditions and Calibration DataBase provides a REST API which can be used to list, store and retrieve objects.

The CCDB API class (CcdbApi) is implemented using libcurl and gives C++ API access to the CCDB via its REST api. The API can be also used to create a snapshot of conditions objects on the local disc and retrieve the objects therefrom. This can be useful in circumstances of reduced or no network connectivity.

There are currently 2 different kinds of store/retrieve functions, which we expect to unify in the immediate future: 2. storeAsTFile/retrieveFromTFile API serializing a TObject in a ROOT TFile with the advantage of keeping the data together with the ROOT streamer info in the same place. 3. A strongly-typed storeAsTFileAny<T>/retrieveFromTFileAny<T> API allowing to handle any type T having a ROOT dictionary. We encourage to use this API by default.

There is a test central CCDB at http://ccdb-test.cern.ch:8080. Feel free to use it. If you prefer to use a local instance, you can follow the instructions here.

If you access the CCDB with a web browser, add /browse at the end of the URL to have a user readable interface. Moreover, using /browse/?report=true will provide details on the number of files and the size of the folders (e.g. http://ccdb-test.cern.ch:8080/browse/?report=true).

• storing / retrieving arbitrary (non TObject) classes

// init
CcdbApi api;
map<string, string> metadata; // can be empty
api.init("http://ccdb-test.cern.ch:8080"); // or http://localhost:8080 for a local installation
// store abitrary user object in strongly typed manner
auto deadpixels = new o2::FOO::DeadPixelMap();
api.storeAsTFileAny(deadpixels, "FOO/DeadPixels", metadata);
// read like this (you have to specify the type)
auto deadpixelsback = api.retrieveFromTFileAny<o2::FOO::DeadPixelMap>("FOO/DeadPixels", metadata);

• creating a local snapshot and fetching objects therefrom

// init
CcdbApi api;
map<string, string> metadata; // can be empty
api.init("http://ccdb-test.cern.ch:8080"); // or http://localhost:8080 for a local installation
// create a local snapshot of everthing in or below the FOO folder valid for timestamp 12345
api.snapshot("FOO", "/tmp/CCDBSnapshot/", 12345);

// read from snapshot by saying
CcdbApi snapshotapi;
snaptshotapi.init("file:///tmp/CCDBSnapshot");

// reading still works just like this (you have to specify the type)
auto deadpixelsback = snapshotapi.retrieveFromTFileAny<o2::FOO::DeadPixelMap>("FOO/DeadPixels", metadata);

• offer API without need to pass metadata object
• deprecate TMessage based API
• code reduction or delegation between various storeAsTFile APIs
• eventually just call the functions store/retrieve once TMessage is disabled

A basic higher level class BasicCCDBManager is offered for convenient access to the CCDB from user code. This class

• Encapsulates the timestamp.
• Offers a more convenient get function to retrieve objects.
• Is a singleton which is initialized once and can be used from any detector code.

The class was written for the use-case of transport MC simulation. Typical usage should be like

// setup manager once (at start of processing) 
auto& mgr = o2::ccdb::BasicCCDBManager::instance();
mgr.setURL("http://ourccdbserverver.cern.ch");
mgr.setTimestamp(timestamp_which_we_want_to_anchor_to);


// in some FOO detector code (detector initialization)
auto& mgr = o2::ccdb::BasicCCDBManager::instance();
// just give the correct path and you will be served the object
auto alignment = mgr.get<o2::FOO::GeomAlignment>("/FOO/Alignment");

• offer improved error handling / exceptions
• do we need a store method?