Bitswap is the data trading module for ipfs, it manages requesting and sending blocks to and from other peers in the network. Bitswap has two main jobs, the first is to acquire blocks requested by the client from the network. The second is to judiciously send blocks in its posession to other peers who want them.
Bitswap is a message based protocol, as opposed to response-reply. All messages contain wantlists, or blocks. Upon receiving a wantlist, a node should consider sending out wanted blocks if they have them. Upon receiving blocks, the node should send out a notification called a 'Cancel' signifying that they no longer want the block. At a protocol level, bitswap is very simple.
Internally, when a message with a wantlist is received, it is sent to the decision engine to be considered, and blocks that we have that are wanted are placed into the peer request queue. Any block we possess that is wanted by another peer has a task in the peer request queue created for it. The peer request queue is a priority queue that sorts available tasks by some metric, currently, that metric is very simple and aims to fairly address the tasks of each other peer. More advanced decision logic will be implemented in the future. Task workers pull tasks to be done off of the queue, retreive the block to be sent, and send it off. The number of task workers is limited by a constant factor.
Client requests for new blocks are handled by the want manager, for every new block (or set of blocks) wanted, the 'WantBlocks' method is invoked. The want manager then ensures that connected peers are notified of the new block that we want by sending the new entries to a message queue for each peer. The message queue will loop while there is work available and do the following: 1) Ensure it has a connection to its peer, 2) grab the message to be sent, and 3) send it. If new messages are added while the loop is in steps 1 or 3, the messages are combined into one to avoid having to keep an actual queue and send multiple messages. The same process occurs when the client receives a block and sends a cancel message for it.