Quattor package management with yum-based ncm-spma
The update of ncm-spma, Quattor's package management component, to a version that uses yum instead SPMA and rpmt-py, has significantly simplified the configuration of package lists, changed the requirements on software repositories and changed the workflow of updating software.
The major advantage of the new component lies in the simplification of the software that needs to be installed. Where the original version required every package and all of its dependent packages to be exactly specified, often referred to as the RPM dependency hell, the new component only needs the packages that are desired (version can be specified if required, but that is not needed). The dependencies are handled automatically by yum, just as yum will take care of the selection of the optimal or most recent version of each package. Consequently, there will be less need for modifications in the software installation templates to upgrade and the trial-and-error approach to satisfy all dependencies will become an unpleasant relic of the past. Furthermore, since package versions do not need to be specified exactly any more, the painful process to generate template for all package groups when a a new minor version of the distribution is released should also become simpler.
There is a modest price to pay for the simplification of the software templates: part of the complexity goes into maintaining the software repositories and one loses the compile-time verification of the presence of the packages to be installed in the available repositories. The latter may lead to unexpected problems when deploying.
One of Quattor's was the ability to roll back an installation or configuration to a previous state. Performing a roll back to an earlier set of packages is not trivial when using yum. Nevertheless, it is very desirable not to lose this feature. By using snapshots of software repositories, it is possible to let yum consider an earlier set of packages when managing the installation, implying that downgrading to previous versions is required.
This article gives a description of the new situation.
