Program to broadcast messages based on a pseudo-random number sequence.
Use Stack to install program:
stack setup
stack build
To see help info, run program with flag -h:
stack exec -- hcloud run -h
The following options are available for hcloud run:
--send-for SEC Sending period, in seconds
--wait-for SEC Grace period, in seconds
--with-seed INT Seed value for PRNG
--config FILE Configuration file with nodes
-h,--help Show this help text
Example command:
stack exec -- hcloud run --send-for 8 --wait-for 2 --with-seed 1 --config node.conf
Here, messages will be sent during 8 seconds. After that, during a grace period of 2 seconds, nodes will print out results. The node configuration is given by the node.conf file.
A configuration file contains a list of nodes in format HOST:PORT. For instance, a three-node configuration is given by
127.0.0.1:12301
127.0.0.1:12302
127.0.0.1:12303
The model of communication follows that of Raft Consensus Algorithm. At any given time each server is in one of three states: leader, follower, or candidate. In normal operation there is exactly one leader and all of the other servers are followers. Followers are passive: they issue no requests on their own but simply respond to requests from leaders and candidates. The leader handles all client requests. The third state, candidate, is used to elect a new leader.
The key safety property for Raft is the State Machine Safety Property: if a server has applied a log entry at a given index to its state machine, no other server will ever apply a different log entry for the same index.
Messages are produced by client using the Xorshift32 pseudo-random number generator (PRNG), which gives us a smooth process of message creation: it is enough to know the seed of the last message to generate a new one. Then, a deterministic random number from range (0, 1] is given by the division of seed value by the maximum 32-bit number. It should be noticed that such a random sequence, starting from a non-zero seed value, will never become zero.
After sending period expires, we ask all nodes to send their results in the form of the following tuple:
|m|
( |m|, Σ i * m(i) )
i=1
where m(i) is the i-th message received by a node. To demonstrate the State Machine Safety Property, the node's result contains messages applied to state machine only (in the terminology of Raft). Then, in normal situation, a majority of servers gives the same result.
Raft consensus algorithm is fully functional (available) as long as any majority of the servers are operational and can communicate with each other and with clients. Thus, a typical cluster of five servers can tolerate the failure of any two servers. Servers are assumed to fail by stopping; they may later recover from state on stable storage and rejoin the cluster.
There is the hcloud test command which realizes a testing scenario with multiple server failures and recovers from state on stable storage. It uses the same options as hcloud run does.