ESPGossip

Since	Origin / Contributor	Maintainer	Source
2020-01-20	alexandruantochi	alexandruantochi	gossip.lua

This module is based on the gossip protocol and usefull for use cases when multiple controllers are on the same network and they need to know about each other's state or when a single point of failure (such as an entity that queries each controller in particular) is to be avoided. It runs using UDP, thus providing faster communication and less network noise.

Usage

config = {
    seedList = { '192.168.0.1', '192.168.0.15' },
    debug = true,
    debugOutput = print
}
gossip = require ("gossip")
gossip.setConfig(config)
gossip.start()

Strategy

Each controller will randomly pick an IP from it's seed list. It will send a SYN request to that IP and set recieving node's state to an intermediary state between Up and Suspect. The node that recieves the SYN request will compute a diff on the received networkState vs own networkState. It will then send that diff as an ACK request. If there is no data to send, it will only send an ACK. When the ACK is received, the sender's state will revert to Up and the receiving node will update it's own networkState using the diff (based on the ACK reply).

Gossip will establish if the information received from another node has fresher data by first comparing the revision, then the heartbeat and lastly the state. States that are closer to DOWN have priority as an offline node does not update it's heartbeat.

Currently there is no implemented deletion for nodes that are down except for the fact that their status is signaled as REMOVE.

Terms

revision : generation of the node; if a node restarts, the revision will be increased by one. The revision data is stored as a file to provide persistency

heartBeat : the node uptime in seconds (tmr.time()). This is used to help the other nodes figure out if the information about that particular node is newer.

networkState : the list with the state of the network composed of the ip as a key and revision, heartBeat and state as values packed in a table.

state : all nodes start with a state set to UP and when a node sends a SYN request, it will mark the destination node in an intermediary state until it receives an ACK or a SYN from it. Basicaly, if a node receives any message, it will mark that senders IP as UP as this provides proof that the node is online.

setConfig()

Syntax

gossip.setConfig(config)

Sets the configuration for gossip. The available options are:

seedList : the list of seeds gossip will start with; this will be updated as new nodes are discovered. Note that it's enough for all nodes to start with the same IP in the seedList, as once they have one seed in common, the data will propagate

roundInterval: interval in milliseconds at which gossip will pick a random node from the seed list and send a SYN request

comPort : port for the listening UDP socket

debug : flag that will provide debugging messages

debugOutput : if debug is set to true, then this method will be used as a callback with the debug message as the first parameter

config = {
    seedList = {'192.168.0.54','192.168.0.55'},
    roundInterval = 10000,
    comPort = 5000,
    debug = true,
    debugOutput = function(message) print('Gossip says: '..message); end
}

If any of them is not provided, the values will default:

seedList : nil

roundInterval: 10000 (10 seconds)

comPort : 5000

debug : false

debugOutput : print

start()

Syntax

gossip.start()

Starts gossip, sets the started flag to true and initiates the revision. The revision (generation) main purpose is like a persisten heartbeat, as the heartbeat (measured by uptime in seconds) will obviously revert to 0.

setRevManually()

Syntax

gossip.setRevManually(number)

The only scenario when rev should be set manually is when a new node is added to the network and has the same ip. Having a smaller revision than the previous node with the same ip would make gossip think the data it received is old, thus ignoring it.

getNetworkState()

Syntax

networkState = gossip.getNetworkState()
print(networkState)

The network state can be directly accessed as a Lua table : gossip.networkState or it can be received as a JSON with this method.

Returns

JSON formatted string regarding the network state.

Example:

{
  "192.168.0.53": {
    "state": 3,
    "revision": 25,
    "heartbeat": 2500
  },
  "192.168.0.75": {
    "state": 0,
    "revision": 4,
    "heartbeat": 6500
  }
}

##ToDo:

• add function for handling REMOVE nodes
• add callback function for network update