We are using OpenShift. I have a confusion between buildconfig file vs jenkinsfile. Do we need both of them or one is sufficient. I have seen examples where in jenkinsfile docker build is defined using buildconfig file. In some cases buildconfig file is using jenkinsfile as the build strategy. Can some one please clarify on this
BuildConfig is the base type for all builds, there are different build strategies that can be used in a build config, by running oc explain buildconfig.spec.strategy you can see them all. If you want to do a docker build you use the dockerStrategy, if you want to build from source code using source2image you specify the sourceStrategy.
Sometimes you have more complex needs than simply running a build with an output image, let's say you want to run the build, wait for that image to be deployed to some environment and then run some automated GUI tests. In this case you need a pipeline. If you want to trigger and configure this pipeline from the OpenShift Web Console you would use the jenkinsPipelineStrategy in your BuildConfig. In the OpenShift 3.x web console such BuildConfigs are presented as Pipelines and not Builds even though they are all really BuildConfigs.
Any BuildConfig with the jenkinsPipelineStrategy will be executed by the Jenkins Build Server running inside the project. That Jenkins instance could also have other pipelines that are not mapped or visible in the OpenShift Web Console, there does not need to be a BuildConfig for every Jenkinsfile if you don't see the benefit of them appearing in the OpenShift Web Console.
The difference of running builds inside a Jenkinsfile and a BuildConfig with some non-jenkinsfile-strategy is that the build is actually executed inside the jenkins build agent rather than a normal OpenShift build pod.
At our company we utilize a combination of jenkinsFile pipelines and BuildConfigs with the sourceStrategy. Instead of running builds in our Jenkinsfile pipelines directly inside the Jenkins build agent we let the pipeline call the OpenShift API and tell it to execute the BuildConfig with sourceStrategy. So basically we still use s2i for building the images but the Jenkinsfile as our CI/CD pipeline engine. You can find some examples of this at https://github.com/openshift/jenkins-client-plugin.