Core components

Frames

Frames are self-contained sections of the Poset. They are composed of Roots and regular Poset Events, where Roots are the base on top of which Events can be inserted. Basically, Frames form a valid foundation for a new Poset, such that gossip-about-gossip routines are not discontinued, while earlier records of the gossip history are ignored.

type Frame struct { Round int //RoundReceived Roots []Root //[participant ID] => Root Events []Event //Events with RoundReceived = Round }

A Frame corresponds to a Poset consensus round. Indeed, the consensus algorithm commits Events in batches, which we map onto Frames, and finally onto a Blockchain. This is an evolution of the previously defined :ref:blockchain mapping <blockchain>. Block headers now contain a Frame hash. As we will see later, this is useful for security. The Events in a Frame are the Events of the corresponding batch, in consensus order.

.. image:: assets/dag_frames_bx.png

Roots

Frames also contain Roots. To get an understanding for why this is necessary, we must consider the initial state of a Poset, i.e., the base on top of which the first Events are inserted.

The Poset is an interlinked chain of Events, where each Event contains two references to anterior Events (SelfParent and OtherParent). Upon inserting an Event in the Poset, we check that its references point to existing Events (Events that are already in the Poset) and that at least the SelfParent reference is not empty. This is partially illustrated in the following picture where Event A2 cannot be inserted because its references are unknown.

.. image:: assets/roots_0.png

So what about the first Event? Until now, we simply implemented a special case, whereby the first Event for any participant, could be inserted without checking its references. In fact the above picture shows that Events A0, B0, and C0, have empty references, and yet they are part of the Poset. This special case is fine as long as we do not expect to initialize Posets from a 'non-zero' state.

We introduced the concept of Roots to remove the special case and handle more general situations. They make it possible to initialize a Poset from a section of an existing Poset, and discard everything below it.

.. image:: assets/roots_1.png

A Root is a data structure containing condensed information about the ancestors of the first Events to be added to the Poset. Each participant has a Root, containing a SelfParent - the direct ancestor of the first Event for the corresponding participant - and Others - a map of Event hashes to the corresponding Other-Parents. These parents are instances of the RootEvent object, which is a minimal version of the Poset Event. RootEvents contain information about the Index, Round, and LamportTimestamp of the corresponding Events. The Root itself contains a NextRound field, which helps in calculating the Round of its direct descendant.

type Root struct { NextRound int SelfParent RootEvent Others map[string]RootEvent }

type RootEvent struct { Hash string CreatorID int Index int LamportTimestamp int Round int }