Channels are the atomic connectors in Reo. A channel is a medium of communication with exactly two ends. A channel can connect two components or be composed with other channels using Reo nodes to build more complex connectors.

A channel has exactly two directed ends. There are two types of channel ends: source and sink (terminology). An source channel end accepts data into its channel. A sink channel end dispenses data out of its channel.

Channel types

• Synchronous - the I/O operations on both ends of a synchronous channel succeed only atomically (i.e., we can imagine they succeed simultaneously). An I/O operation on one channel end is delayed until it can succeed simultaneously with an I/O operation on the other channel end.
• Asynchronous - the I/O operations performed on the channel ends of an asynchronous channel may or may not succeed simultaneously. The success of an I/O operation at one of the channel ends may exclude the possibility of an I/O operation to simultaneously succeed at its other end.
• Buffered - the channel has a bounded or an unbounded buffer to hold data.
• Ordered - a buffered channel that imposes a certain order on the delivery of its buffer contents, (e.g., FIFO, STACK, etc.).
• Lossy - a channel that delivers only some of the data items it receives and loses the rest. 
• Spout - a channel that creates and delivers data.