Dryad is Microsoft's answer to Google's map-reduce. What's the question: How do you process really large amounts of data? My initial impression of Dryad is it's like a giant Unix command line filter on steroids. There are lots of inputs, outputs, tees, queues, and merge sorts all connected together by a master exec program. What else does Dryad have to offer the scalable infrastructure wars?
Dryad models programs as the execution of a directed acyclic graph. Each vertex is a program and edges are typed communication channels (files, TCP pipes, and shared memory channels within a process). Map-reduce uses a different model. It's more like a large distributed sort where the programmer defines functions for mapping, partitioning, and reducing. Each approach seems to borrow from the spirit of its creating organization. The graph approach seems a bit too complicated and map-reduce seems a bit too simple. How ironic, in the Alanis Morissette sense.
Dryad is a middleware layer that executes graphs for you, automatically taking care of scheduling, distribution, and fault tolerance. It's written in C++, but apparently few write directly to this layer, most people use higher layer interfaces. A Job Manager runs the program. It's a library you link in and it loads and executes the graph. A daemon runs on each machine to run jobs. A name server provides access to cluster resources.
The DAG is a multigraph so you can have multiple edges between vertices. A DAG was chosen because it's not too cold, or too hot, the porridge is just right. Cycles are too hard. Simpler isn't as useful. DAGs support relational algebra and can split multiple inputs and outputs nicely. One interesting aspect is a a channel is a sequence of structure items that are C++ objects. This means pointers can be passed directly so you don't have to worry about serialization overhead. No restrictions are put on the data model. Graphs are dynamically changeable at runtime which allows for a lot of optimizations.
Several case studies were provided. It's probably just me, but I didn't really understand what was going on. Google's example is much better. Everyone can relate to counting words in a document. My thoughts while watching is that the graph stuff sounds cool and general, but it's hard to map it efficiently to solutions when the problems have large numbers of inputs. You have to manually optimize for available RAM and CPUs. The system should do all this work for you.
But the graph approach is powerful. The programmer provide the bits of atomic behaviour and the system can then try various optimizations. The code doesn't have to change because the graph can be manipulated abstractly on its own. So you can write something like a SQL query. Then something like a query planner figures out how to execute the query on Dryad.