Using transactions to minimize churn in a distributed network control system

For a recipient controller for managing a network comprising a plurality of managed forwarding elements that forward data in the network, a method for computing forwarding state using inputs from a set of source controllers, the method comprising:

• receiving a plurality of groups of inputs from the set of source controllers, each group of inputs for being processed together and separately from another group of inputs;
• when the groups of inputs meet a certain condition, processing two or more of the groups of inputs together to generate a set of outputs;
• and
• when the groups of inputs do not meet the certain condition, processing the groups of inputs by processing one group of inputs together at a time to generate a set of outputs;
• and
• sending the generated set of outputs to a set of managed forwarding elements.

A non-transitory machine readable medium for storing a first controller which when executed by at least one processing unit manages a network comprising a plurality of managed forwarding elements that forward data in the network, the network controller comprising sets of instructions for:

• receiving a request for information about a logical forwarding element that logically connects a set of end machines;
• identifying a set of second controllers that manage a set of managed forwarding elements that implement the logical forwarding element;
• distributing the request to the set of second controllers;
• receiving a response from each of the set of second controllers, the response including at least a portion of the requested information;
• and
• when the set of responses meets certain criteria, combining the responses received from the set of physical controllers and sending the combined responses to a source of the request.

For a first controller for managing a network comprising a plurality of managed forwarding elements that forward data in a network, a method for computing forwarding state to send to a second controller, the method comprising:

• generating a set of output changes that include the forwarding state that defines forwarding behaviors of a set of managed forwarding elements;
• sending each output change of the set of output changes to the second controller as the output change is generated;
• and
• sending an indicator, which indicates an end of the set of output changes, to the second controller,
• wherein the second controller generates its own indicator for a set of outputs that the second controller generates only after receiving the indicator from the first controller.