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. 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.

Central and local instances of the CCDB

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.

Access with a browser

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).

Example Usage

• 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); 
// read like this to get the headers as well, and thus the metadata attached to the object 
map<string, string> headers;
auto deadpixelsback = api.retrieveFromTFileAny<o2::FOO::DeadPixelMap>("FOO/DeadPixels", metadata /* constraint the objects retrieved to those matching the metadata */, -1 /* timestamp */, &headers /* the headers attached to the returned object */); 
// finally, use this method to retrieve only the headers (and thus the metadata)
std::map<std::string, std::string> headers = f.api.retrieveHeaders("FOO/DeadPixels", f.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);

Local object caching and testing feature

A simple mechanism is offered that allows caching CCDB objects on disc. In this mode, the CcdbApi will look if an object is already located on the disc. If not, the request is made to the server and the response automically saved locally. If yes, we just take the local file. The feature can be activated by exporting a shell variable export ALICEO2_CCDB_LOCALCACHE=$PWD/.ccdb which is set to a path on the disc where objects should be put. This mechanism is useful to reduce calls to the server in case multiple processes need the same objects (and the object can be regarded as valid for all requests).

Note that this mechanism also allows for local development and test cycles. Say, some algorithm needs access to object /Foo/Bar/. Then it suffices to put the ROOT file containing the ccdb-object as filename snapshot.root inside the /Foo/Bar/ directory structure, inside the ALICEO2_CCDB_LOCALCACHE folder (so, something like /home/user/.ccdb/Foo/Bar/snapshot.root). Then testing can proceed without actually having to upload the CCDB object to a server.

BasicCCDBManager

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");

By default loaded object are cached in the BasicCCDBManager and owned by it, therefore user should not delete retrieved objects. If the query is done for the object which is already in the cache, its pointer is returned instead of loading again the object from the CCDB server, unless the validity range of the cached object does not match to requested timestamp. In case user wants to enforce a fresh copy loading, the cache for particular CCDB path can be cleaned by invoking mgr.clear(<path>). One can also reset whole cache using mgr.clear().

Uncached mode can be imposed by invoking mgr.setCaching(false), in which case every query will retrieve a new copy of object from the server and the user should take care himself of deleting retrieved objects to avoid memory leaks.

Upper and lower limits on the object creation time can be set by mgr.setCreatedNotAfter(upper_limit_timestamp) and mgr.setCreatedNotBefore(lower_limit_timestamp), it specifies the fields "If-Not-After" and "If-Not-Before" when retrieving an object from CCDB. This feature is useful to avoid using newer objects if the CCDB is updated in parallel to the task execution.

In cached mode, the manager can check that local objects are still valid by requiring mgr.setLocalObjectValidityChecking(true), in this case a CCDB query is performed only if the cached object is no longer valid.

Future ideas / todo:

• offer improved error handling / exceptions
• do we need a store method?
• offer API without need to pass metadata object
• code reduction or delegation between various storeAsTFile APIs
• eventually just call the functions store/retrieve once TMessage is disabled

Note on the use of TFile to store data

The reason for using a TFile to store the data in the CCDB is because it has the advantage of keeping the data together with the ROOT streamer info in the same place and because it makes it easy to open objects directly from a ROOT session. The streamers enable class schema evolution.

Command line tools

A few prototypic command line tools are offered. These can be used in scriptable generic workflows and facilitate the following tasks:

1. Upload and annotate a generic C++ object serialized in a ROOT file

   o2-ccdb-upload -f myRootFile.root --key histogram1 --path /Detector1/QA/ --meta "Description=Foo;Author=Person1;Uploader=Person2"

   This will upload the object serialized in myRootFile.root under the key histogram1. Object will be put to the CCDB path /Detector1/QA. For full list of options see o2-ccdb-upload --help.

2. Download a CCDB object to a local ROOT file (including its meta information)

   o2-ccdb-downloadccdbfile --path /Detector1/QA/ --dest /tmp/CCDB --timestamp xxx

   This will download the CCDB object under path given by --path to a directory given by --dest on the disc. (The final filename will be /tmp/CCDB/Detector1/QA/snapshot.root for the moment). All meta-information as well as the information associated to this query will be appended to the file.

   For full list of options see o2-ccdb-downloadccdbfile --help.

3. Inspect the content of a ROOT file and print summary about type of contained (CCDB) objects and its meta information

   o2-ccdb-inspectccdbfile filename

   Lists all keys and stored types in a ROOT file (downloaded with tool o2-ccdb-downloadccdbfile) and prints a summary about the attached meta-information.

TODO command line tools

• combine all tools into a single swiss-knife executable?
• offer more tools mapping all REST functionality of the server: be able to browse, list, query online content, etc.
• provide error diagnostics; different level of verbosity
• provide json interaction modes (give meta-information; retrieve meta-information)
• use meta-info filters when downloading