Tutorial: creating conditions objects

Complete: 3
Detailed Review status

Goals of this page:

This page explains how create new objects to be stored in the conditions database:

  • creating the corresponding C++ classes;
  • filling in the database;
  • retrieving the data.

The tutorial is aimed at software developers of conditions-related data.


CMSSW provides an interface to database implementations for the storage of conditions data. The relevant packages are located in the various Cond... subsystems of CMSSW. This interface allows to store objects as C++ class in various types of database.

In this tutorial, we define a simple data structure (C++ class) MyPedestals and all the components the interface needs. We then store data in one of the supported DB schemas and retrieve it back.


Your Goal is to store objects of a given type (in this example MyPedestals) in the condition database each associated to a given Time Interval of Validity and be able to retrieve the one valid for a given event during reconstruction (or analysis) using the standard EventSetup mechanism i.e.

MyAnalizer::analyze(const edm::Event& e, const edm::EventSetup& setup){
    edm::ESHandle<MyPedestals> pPeds;
    MyPedestals const * myped=pPeds.product();

Setup your environment

This tutorial works with CMSSW release CMSSW_7_6_5.

Create a local developer area:

cmsrel CMSSW_7_6_5
cd CMSSW_7_6_5/src

We also the package that includes all records (where the framework actually stores the conditions data, see below):

git cms-addpkg CondFormats/DataRecord
(do a scram b -j 8 at this point to gain time)

How to create a new conditions DB object

We first have to create the package containing our object. This has to be put in the CondFormats subsystem:

mkdir -p CondFormats/MyPedestals/{interface,src,test}
cd CondFormats/MyPedestals

caveat: this tutorial is based on code actually existing in the pakage CondFormats/Calibration

The object class

We then define a simple container class for the data (see some restrictions here). Since we want to keep it simple, it all fits in the header file interface/MyPedestals.h:

#include <vector>
class MyPedestals {
      struct Item {
       float m_mean;
       float m_variance;

    std::vector<Item>  m_pedestals;

For a class to be stored in DB, it has to be serializable.

To make it serializable we add the COND_SERIALIZABLE macro:

#include "CondFormats/Serialization/interface/Serializable.h"

#include <vector>

class MyPedestals{
  struct Item {
    float m_mean;
    float m_variance;

  std::vector<Item> m_pedestals;

This of course adds "CondFormats/Serialization" as a dependency.

See other examples in CMSSW Git

We also need a src/headers.h file, which holds include statements to all our serializable classes. This file is read by the serialization system to find the serializable classes and generate serialization code for them.

Our src/headers.h

#include "CondFormats/MyPedestals/interface/MyPedestals.h"

Now, we can write a test program for the class's serialization, using built-in tests. Here's test/test_serialization.cc:

#include "CondFormats/Serialization/interface/Test.h"

#include "../src/headers.h"

int main()

The test/BuildFile.xml:

<bin file="test_serialization.cpp">
    <use name="CondFormats/MyPedestals"/>

If we want our class to be available as EventSetup data (and in this case we do), we have to have an LCG dictionary made for it, which is needed for the class to be used in ROOT.

This is done through two files to be put in the src of our project: classes_def.xml and classes.h.

src/classes.h holds the class definitions:

#include "CondFormats/MyPedestals/interface/MyPedestals.h"

In this case, the contents are the same as headers.h, but this might not be always the case.

The rules for the dictionary are the very same as for any other persistent class: for instance those forming the event. There are only two additional attributes specific to the DB: class_version and mapping.

src/classes_def.xml contains the names of all the classes we want to store and their constituents, as well as class template instantiations. In our case, here is how it might look like this:

       <class name="MyPedestals" class_version="0"/>
       <class name="MyPedestals::Item"/>
       <class name="std::vector<MyPedestals::Item>"/>

If you really want more details on the dictionary generation, have a look here

For our class to be useable as EventSetup data, it needs to be registered into the CMSSW framework as such. The framework will then handle it as any EventSetup data. This is done by convention in a corresponding file T_EventSetup_ClassName.cc. In our case src/T_EventSetup_MyPedestals.cc:

// T_EventSetup_MyPedestals.cc

#include "CondFormats/MyPedestals/interface/MyPedestals.h"
#include "FWCore/Utilities/interface/typelookup.h"


This, along with LCG dict, adds a "FWCore/Utilities" dependency to our BuildFile.xml .

See here for more details about the EventSetup.

Now, our project needs a BuildFile.xml to tell scram how to compile it. For our component BuildFile.xml shall be:

<use name="FWCore/Utilities"/>
<use name="CondFormats/Serialization"/>

  <lib name="1"/>

Now we can compile this first component (you will see the dictionary and serialization generation):

scram b

More complex classes

Classes with a complex structure and objects with a large number of data items gives performance problems in storage and retrieval. In such cases we advice to use a blob representation of the complex or large data member. This can be easily achieved adding in the dictionary the attribute "mapping=blob". For instance for our MyPedestals class the declaration in the dictionary to store the vector as blob will look like this:

       <class name="MyPedestals" class_version="0">
         <field name="m_pedestals" mapping="blob" />
       <class name="MyPedestals::Item"/>
       <class name="std::vector<MyPedestals::Item>"/>

Complex data structures will inevitably produce performance degradation at runtime. For persistent objects please try to optimize data structures for simple Write-Once Read-Many use cases: usually flat vector-like structures suffices. Consult the Condition core team if you think you need to implement more complex structures to represent your data.

The EventSetup record

As we just said, the conditions data is handled by the framework through the EventSetup. We then have to define a Record where the objects are actually stored and from which it can be retrieved. The record class must be defined in CondFormats/DataRecord. This is easily done with the mkrecord script provided by the framework:

cd $CMSSW_BASE/src/CondFormats/DataRecord/interface
mkrecord MyPedestalsRcd
mv MyPedestalsRcd.cc ../src

This should compile straight away:

cd ..
scram b

caveat for conditions each record can contain objects of only one super type (single-inheritence).

The DataProxy plugin

The last component we need will tell the DB interface which data do associate to which record. This has to be located under CondCore, in a package specific to each task. Here, we define it as CondCore/MyPedestalsPlugins:

cd $CMSSW_BASE/src/CondCore
mkdir -p MyPedestalsPlugin/src
cd MyPedestalsPlugin/src

and we create the file plugin.cc in there:

#include "CondCore/PluginSystem/interface/registration_macros.h"
#include "CondFormats/DataRecord/interface/MyPedestalsRcd.h"
#include "CondFormats/MyPedestals/interface/MyPedestals.h"


Note one can register several such pairs in the same plugin. See some examples here.

The corresponding src/BuildFile.xml is:

<use name="CondCore/ESSources"/>
<use name="CondFormats/CondTest"/>
<use name="CondFormats/DataRecord"/>

<flags EDM_PLUGIN="1"/>

We can now build our plugin system (scram b). After succesful compilation, we check that the proxy for the EventSetup has been correctly built:

$ edmPluginDump | grep MyPedestalsRcd

How to fill the DB with data

In order to actually write our data into the database, we need a small module that will create the data. We then configure the generic PoolDBOutputService which will write them to the database. (In production this HAS to be done using the CMS.PopCon infrastructure)

The data maker

We first create a new (dummy) subsystem:

cd $CMSSW_BASE/src
mkdir MyCalibrations
cd MyCalibrations
and we use the mkedanlzr script to create a skeleton of our maker:
mkedanlzr MyPedestalsMaker

The resulting file MyCalibrations/MyPedestalsMaker/src/MyPedestalsMaker.cc needs to be edited in the analyze method, to create the desired data. In our example, we will not create anything: we just save a dummy object through the DB interface.

Here's how the analyze method then looks like:

MyPedestalsMaker::analyze(const edm::Event& iEvent, const edm::EventSetup& iSetup)
  MyPedestals* pMyPedestals = new MyPedestals();

  // Form the data here

  edm::Service<cond::service::PoolDBOutputService> poolDbService;
  if( poolDbService.isAvailable() )
      poolDbService->writeOne( pMyPedestals, poolDbService->currentTime(),
                                                "MyPedestalsRcd"  );
      throw std::runtime_error("PoolDBService required.");


A few comments (more will come later):

  • The interface is called as a framework service. We first have to check that it is available (i.e. it was configured), otherwise we throw an exception.
    • Using an unavailable interface would throw an exception as well.
  • Data is tagged in the database (see configuration below): we then check if the tag already exists.
    • If the tag does not exist: a new interval of validity (IOV) for this tag is created, valid till "end of time".
    • If the tag already exists: the IOV of the previous data is stopped at "current time" and we register new data valid from now on (currentTime is the time of the current event!).
  • The data is then registered in the service for writing out (done at the end of the job).

Everything you want to know about IOV is available at SWGuideIOVAndIOVMetaDataInANutshell

This producer will compile with the following BuildFile.xml:

<use name="FWCore/Framework"/>
<use name="FWCore/PluginManager"/>
<use name="FWCore/ParameterSet"/>
<use name="CoralBase"/>
<use name="CondFormats/MyPedestals"/>
<use name="CondCore/DBOutputService"/>
<flags EDM_PLUGIN="1"/>

Do not forget to compile everything again. Just do it:

cd $CMSSW_BASE/src/MyCalibrations
And execute this build command:
scram b
If this build fail, try this one:
scram b -f
it will skip reading the cache.

Configuring the DB output service

We now want to run the analyzer and create an output file. We will configure the DB service to write to a SQLite file. Here's the test-maker_cfg.py file (for structure's sake, place it in MyPedestalsMaker/test):

import FWCore.ParameterSet.Config as cms

process = cms.Process("MyPedestalsMakerwrite")

# Load CondDB service

# output database (in this case local sqlite file)
process.CondDB.connect = 'sqlite_file:MyPedestals.db'

# A data source must always be defined. We don't need it, so here's a dummy one.
process.source = cms.Source("EmptyIOVSource",
    timetype = cms.string('runnumber'),
    firstValue = cms.uint64(1),
    lastValue = cms.uint64(1),
    interval = cms.uint64(1)

# We define the output service.
process.PoolDBOutputService = cms.Service("PoolDBOutputService",
    timetype = cms.untracked.string('runnumber'),
    toPut = cms.VPSet(cms.PSet(
        record = cms.string('MyPedestalsRcd'),
        tag = cms.string('myPedestal_test')

process.pedestals_maker = cms.EDAnalyzer("MyPedestalsMaker",
    record = cms.string('MyPedestalsRcd'),
    loggingOn= cms.untracked.bool(True),

process.path = cms.Path(process.pedestals_maker)

  • We first include the standard settings CondDB_cfi.py (have a look at it in Git).
  • We customize it:
    • the connect parameter defines the output file; sqlite_file is the protocol to use (other possibilities are oracle and frontier);
  • We then configure the PoolDBOutputService:
    • We use the previous process.CondDB for the connection.
    • the record name is a local keyword to associate the object to a tag.
    • the tag string is the tag we want to attach to our data (see also above, when storing the data).

To run the configuration file test/test-maker_cfg.py:

cmsRun test/test-maker_cfg.py
It should create the file we asked for MyPedestals.db.

How to retrieve the data

Now that we have a DB file, we can also retrieve the data. This again requires the configuration of the DB interface, this time using the ESSource, in a way very similar to the output service.

In order to check that we can retrieve the data through the EventSetup, we will use a framework module that exactly checks that, the EventSetupRecordDataGetter.

Here's test/test-retrieve_cfg.py:

import FWCore.ParameterSet.Config as cms

process = cms.Process("MyPedestalsMakerRetrieveTest")

# input database (in this case the local sqlite file)
process.CondDB.connect = 'sqlite_file:MyPedestals.db'

process.PoolDBESSource = cms.ESSource("PoolDBESSource",
    toGet = cms.VPSet(cms.PSet(
        record = cms.string('MyPedestalsRcd'),
        tag = cms.string("myPedestal_test")

process.get = cms.EDAnalyzer("EventSetupRecordDataGetter",
    toGet = cms.VPSet(cms.PSet(
        record = cms.string('MyPedestalsRcd'),
        data = cms.vstring('MyPedestals')
    verbose = cms.untracked.bool(True)

# A data source must always be defined. We don't need it, so here's a dummy one.
process.source = cms.Source("EmptyIOVSource",
    timetype = cms.string('runnumber'),
    firstValue = cms.uint64(1),
    lastValue = cms.uint64(1),
    interval = cms.uint64(1)

process.path = cms.Path(process.get)

Now run test/test-retrieve_cfg.py, and you should get:

$ cmsRun test/test-retrieve_cfg.py
%MSG-s DataGetter:  EventSetupRecordDataGetter:get@streamBeginRun  25-Jul-2016 18:10:02 CEST Run: 1
got data of type "MyPedestals" with name "" in record MyPedestalsRcd


How to read/write the data to the offline conditions DB

Now, what next?

  1. Write a data object that does what you need.
  2. Test it the same way as above with sqlite.
  3. Also test it in a more realistic environment by actually using the data in your algorithm.
  4. Then try to store/retrieve it through ORACLE on cmsprep. Using the following parameters in your _cfg.py
               process.CondDB.connect = "oracle://cmsprep/CMS_COND_TASKNAME"                                         
    Note: replace "CMS_COND_TASKNAME" with the account name allocated to your task.
               process.CondDB.DBParameters.authenticationPath = 'where_it_is'
    Note: parameter authenticationPath should point to the location of your authentication.xml. For example, at CERN the AFS location is "/afs/cern.ch/cms/DB/conddb"
  5. Try the Frontier access on the same. Instruction can be found here
  6. Get your data validated (for DB input/output) by the DB core-team. More on oracle production setup can be found here
    • New oracle accounts shall be requested to the DB coordinator
    • Insertion of the validated data in the integration/production DB shall be coordinated with the PopCon responsible
    • Use in production shall be coordinated with ALCA coordination (for the global-tag) and release management (for the code)

Various tools

Some command-line tools are presented in SWGuideCondToolCommand

Give me the source

As a closing remark, you can find the whole source (the state at which you should be now) here:

Review status

Reviewer/Editor and Date (copy from screen) Comments
VytautasMickus - 2016-07-25 Update to 7_6_5
VincenzoInnocente - 19 May 2010 Update after the release of 3_7_0
FredericRonga - 03 Apr 2007 Update after the release of 1_3_0
ZhenXie - 20 Feb 2007 Review section How to read/write the data to the offline conditions DB
FredericRonga - 16 Feb 2007 First complete version
FredericRonga - 14 Feb 2007 First draft

Responsible: VincenzoInnocente
Last reviewed by: ZhenXie - 20 Feb 2007

Topic attachments
I Attachment History Action Size Date Who Comment
Texttxt MyPedestals.h.txt r1 manage 0.2 K 2007-02-16 - 11:21 FredericRonga DB object C++ class
Texttxt MyPedestalsMaker.cc.txt r2 r1 manage 2.8 K 2007-02-16 - 11:45 FredericRonga Data maker
Texttxt MyPedestalsRcd.cc.txt r1 manage 0.4 K 2007-02-16 - 11:21 FredericRonga Data record definition
Texttxt T_EventSetup_Pedestals.cc.txt r1 manage 0.2 K 2007-02-16 - 16:12 FredericRonga DB object registration
Texttxt classes.h.txt r1 manage 0.1 K 2007-02-16 - 11:22 FredericRonga LCG dictionary header
Texttxt classes_def.xml.txt r2 r1 manage 0.1 K 2010-05-19 - 15:17 VincenzoInnocente LCG dictionary
Texttxt plugin.cc.txt r1 manage 0.2 K 2007-02-16 - 11:23 FredericRonga Plugin
Unknown file formatgz src.tar.gz r1 manage 40.0 K 2016-07-25 - 18:44 VytautasMickus The complete tutorial source.
Texttxt test-maker.cfg.txt r2 r1 manage 0.7 K 2010-05-19 - 15:35 VincenzoInnocente Configuration file to make object
Texttxt test-retrieve.cfg.txt r2 r1 manage 0.9 K 2010-05-19 - 15:38 VincenzoInnocente Test the retrieval of DB data
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Topic revision: r31 - 2017-04-19 - AivarasSilale

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