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CCDB_caching.md

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CCDB caching

Info from AliceO2

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.

Command line tools

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.

CCDB mirror

alien.py ccdb -mirror enable possibility to mirror a given selection content on disk. The on-disk format respects CCDB reference and can be served by a java based http server: ccdb-local After the server is started, the configured endpoint should be used as CCDB server name.