Deployment Scenarios in AEM

Hi,"

- Can we have a tarMK cluster at repository level , like one publish instance but repository cluster ?

I dont understand the above question. There is no clustering with TarMK, they are individual instances. You can use TarMK Standby for a "cold standby" type of deployment. There is also no reccomended option for clustering on the publish tier. If you decided to cluster using MongoDB on the author, then you can still use TarMK for the publish instances.

- What is the difference between using a cluster and multiple AEM publish instances 

Clustering is not reccomended for publish instances, a cluster is required when you need to keep all the nodes in synch. On the publish instances, you simply get updates from an author instance, essentially all the publish instances should have the same content. As there are no updates being done on the publish instances, keeping them in synch via a cluster is no longer required.

On the author side, clustering is used for scalability and failover[1]. With TarMK Standby, you can have a fail-over with a small amount of downtime.

- Can an OAK instance exist without having an AEM publish or with one publish can there be two OAK instances 

Are you saying can an Author instance exist without a publish instance? Then yes.

A publish instance could have content replicated from two author instances. But would require careful administration.

Regards,

Opkar 

[1]http://docs.adobe.com/docs/en/aem/6-1/deploy/recommended-deploys.html