A leader election program written in Go(golang) using Bully leader election algorithm.

NOTE: This program is indented to be used within LAN among small number of nodes. It is vulnerable to attack if it is exposed to outside network and it may consume a lot of bandwith on a large cluster.

• Download and install:

    go get github.com/monnand/bully
  

• Suppose we want to run the program on two machines whose IPs are 192.168.1.67 and 192.168.1.68, respectively.

• Run the following command on both machines:

    bully -port=8117 -nodes="192.168.1.67:8117,192.168.1.68:8117" -http=0.0.0.0:8080
  

• To know who is the leader, use HTTP to connect one of the machines with path /leader:

    curl http://192.168.1.67:8080/leader
  

Let's start with a real example. Again, we have two nodes at first, 192.168.1.67 and 192.168.1.68

On 192.168.1.67, I first execute the following command:

    bully -port=8117 -nodes="192.168.1.67:8117,192.168.1.68:8117" -http=0.0.0.0:8080

This command contains three arguments, port, nodes and http.

• port: tells bully to listen on port 8117 waiting for other candidates' connection.
• http: tells bully to set up a web service on port 8080 accepting connections from any client. The client will ask who is the leader using HTTP.
• nodes: tells bully the address (IP and port) of the initial set of candidates.

Note that by executing the command above, bully will complain by printing the following message at begining:

    192.168.1.68:8117 cannot be added: dial tcp 192.168.1.68:8117: connection refused

This message says that it cannot connect to 192.168.1.68, which is correct because we haven't started it yet.

Now, let's ask it who is the leader:

    $ curl http://192.168.1.67:8080/leader
    192.168.1.67

The returned value contains the IP address of the elected leader. In this case, there is only one candidate, and apparently, it is the leader.

Now let's move on to 192.168.1.68 and start another bully instance with same command:

    bully -port=8117 -nodes="192.168.1.67:8117,192.168.1.68:8117" -http=0.0.0.0:8080

Since both candidates specified in nodes argument are on-line, there is no more complain.

Then we can ask both 192.168.1.67 and 192.168.1.68 who is the leader. And we can always get the same answer:

    $ curl http://192.168.1.67:8080/leader
    192.168.1.68
    $ curl http://192.168.1.68:8080/leader
    192.168.1.68

The leader now is 192.168.1.68. In this case, they elected another node as the leader. But this is not always the case. Every time a new node join, there will be a new election and the winner of the election may or may not be the previours leader.

Let's say we want to add one more node into this cluster, whose IP is 192.168.1.69. Only thing we need to do is to start a new instance of bully on 192.168.1.69 with almost the same command:

    bully -port=8117 -nodes="192.168.1.67:8117,192.168.1.68:8117,192.168.1.69:8117" -http=0.0.0.0:8080

The only changed part is the nodes parameter. Since we added one more node, then we need to add it as an initial candidate. The good part is that you don't need to stop the other two instances running on 192.168.1.67 and 192.168.1.68. They will automatically update the candidate list and elect another leader.

Once the new instance started, we can query any of one of them about the leader using HTTP:

    $ curl http://192.168.1.67:8080/leader
    192.168.1.68
    $ curl http://192.168.1.68:8080/leader
    192.168.1.68
    $ curl http://192.168.1.69:8080/leader
    192.168.1.68

This time, the leader is same as previours leader, which is 192.168.1.68.

What if the leader node crashed? The rest of the candidates will elect a new leader on the fly.

Suppose we killed the leader in previours example and ask who is the leader to the rest of the group:

    $ curl http://192.168.1.67:8080/leader
    192.168.1.67
    $ curl http://192.168.1.69:8080/leader
    192.168.1.67

They elected a new leader, which is 192.168.1.67.

If the old leader recovered, then it can join the group again by executing the following command:

    bully -port=8117 -nodes="192.168.1.67:8117,192.168.1.68:8117,192.168.1.69:8117" -http=0.0.0.0:8080

The good part of bully is that once we decided the set of candidates, we can use exactly the same command with same parameters on any candidate node to start bully. This is very nice because we can use the same virtual machine image to start several VMs on the cloud.

Imaging we have a service needs a node to do some management work. Such work may not be heavy but critical. So we need to have some backup servers in case of the manager down. Normally, 3 or 5 node will be enough. Since all nodes are same, we need to select a leader node to do the job, let others keep running and wait the leader die. In this case, we can use bully to decide who is the leader, and elect another leader when the old one down or new one comes.

Apache 2.0