Page History

...

• The JM should keep track of the number of ExecutionVertexes which have blocking input edges that have not been fully read.
• Each ExecutionVertex should notify JM after it finishes reading all its blocking input edges.
• The JM should enable checkpoint only after all ExecutionVertexes have finished reading the blocking input edges.
• Records buffered in the upstream node due to blocking edges can be removed after its downstream nodes have made the first checkpoint. 

5) An A blocking input edge with pending records is same as a source with isBacklog=true when an operator determines its RecordAttributes for downstream nodes.

...