QC-199 update doc about storage (#229)

Author: Barthelemy

README.md

@@ -24,24 +24,22 @@ For a general overview of our (O2) software, organization and processes, please
          * [QC architecture](doc/ModulesDevelopment.md#qc-architecture)
          * [DPL](doc/ModulesDevelopment.md#dpl)
          * [Data Sampling](doc/ModulesDevelopment.md#data-sampling)
-            * [Custom Data Sampling Condition](doc/ModulesDevelopment.md#custom-data-sampling-condition)
-            * [Bypassing the Data Sampling](doc/ModulesDevelopment.md#bypassing-the-data-sampling)
          * [Code Organization](doc/ModulesDevelopment.md#code-organization)
          * [User-defined modules](doc/ModulesDevelopment.md#user-defined-modules)
       * [Module creation](doc/ModulesDevelopment.md#module-creation)
       * [Test run](doc/ModulesDevelopment.md#test-run)
       * [Modification of a Task](doc/ModulesDevelopment.md#modification-of-a-task)
       * [Addition of a Check](doc/ModulesDevelopment.md#addition-of-a-check)
       * [Commit Code](doc/ModulesDevelopment.md#commit-code)
-      * [DPL workflow customization](doc/ModulesDevelopment.md#dpl-workflow-customization)
-      * [Plugging Data Sampling and QC into an existing DPL workflow](doc/ModulesDevelopment.md#usage-of-ds-and-qc-in-an-existing-dpl-workflow)
+      * [Details on data storage](doc/ModulesDevelopment.md#details-on-data-storage)
    * [Advanced topics](doc/Advanced.md)
+      * [Access conditions from the CCDB](doc/Advanced.md#access-conditions-from-the-ccdb)
+      * [Definition and access of task-specific configuration](doc/Advanced.md#definition-and-access-of-task-specific-configuration)
+      * [Custom QC object metadata](doc/Advanced.md#custom-qc-object-metadata)
       * [Data Inspector](doc/Advanced.md#data-inspector)
       * [Use MySQL as QC backend](doc/Advanced.md#use-mysql-as-qc-backend)
       * [Local CCDB setup](doc/Advanced.md#local-ccdb-setup)
       * [Local QCG (QC GUI) setup](doc/Advanced.md#local-qcg-qc-gui-setup)
-      * [Custom QC object metadata](doc/Advanced.md#custom-qc-object-metadata)
-      * [Access conditions from the CCDB](doc/Advanced.md#access-conditions-from-the-ccdb)
       * [Information Service](doc/Advanced.md#information-service)
       * [Configuration files details](doc/Advanced.md#configuration-files-details)
    * [Frequently Asked Questions](doc/FAQ.md)

doc/ModulesDevelopment.md

@@ -17,6 +17,8 @@
       * [Modification of a Task](#modification-of-a-task)
       * [Addition of a Check](#addition-of-a-check)
       * [Commit Code](#commit-code)
+      * [Details on data storage](#details-on-data-storage)
+         * [Storage before v0.14 and ROOT 6.18](#storage-before-v014-and-root-618)
 
 <!-- Added by: bvonhall, at:  -->
 
@@ -220,6 +222,19 @@ For a new feature, just create a new branch for it and use the same procedure. D
 
 General ALICE Git guidelines can be accessed [here](https://alisw.github.io/git-tutorial/).
 
+## Details on data storage
+
+Each MonitorObject is stored as a TFile in the CCDB (see section [Details on data storage](doc/ModulesDevelopment.md#details-on-data-storage)
+). It is therefore possible to easily open it with root loaded with alienv. It also seamlessly supports class schema evolution. 
+
+The quality is stored as a metadata on the object. 
+
+### Storage before v0.14 and ROOT 6.18
+
+Before September 2019, objects were serialized with TMessage and stored as _blobs_ in the CCDB. The main drawback was the loss of the corresponding streamer infos leading to problems when the class evolved or when accessing the data outside the QC framework. 
+
+The QC framework is nevertheless backward compatible and can handle the old and the new storage system. 
+
 ---
 
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