When to use Paxos (real practical use cases)?

Question

Could someone give me a list of real use cases of Paxos. That is real problems that require consensus as part of a bigger problem.

Is the following a use case of Paxos?<

Answer 1

Update 2018: Mysql High Availability uses paxos: https://mysqlhighavailability.com/the-king-is-dead-long-live-the-king-our-homegrown-paxos-based-consensus/

Real world example:

Cassandra uses Paxos to ensure that clients connected to different cluster nodes can safely perform write operations by adding "IF NOT EXISTS" to write operations. Cassandra has no master node so two conflicting operations can to be issued concurrently at multiple nodes. When using the if-not-exists syntax the paxos algorithm is used order operations between machines to ensure only one succeeds. This can then be used by clients to store authoritative data with an expiration lease. As long as a majority of Cassandra nodes are up it will work. So if you define the replication factor of your keyspace to be 3 then 1 node can fail, of 5 then 2 can fail, etc.

For normal writes Caassandra allows multiple conflicting writes to be accepted by different nodes which may be temporary unable to communicate. In that case doesn't use Paxos so can loose data when two Writes occur at the same time for the same key. There are special data structures built into Cassandra that won't loose data which are insert-only.

Poker and Paxos:

As other answers note poker is turn based and has rules. If you allow one master and many replicas then the master arbitrates the next action. Let's say a user first clicks the "check" button then changes their mind and clicks "fold". Those are conflicting commands only the first should be accepted. The browser should not let them press the second button it will disable it when they pressed the first button. Since money is involved the master server should also enforce the rules and only allow one action per player per turn. The problem comes when the master crashes during the game. Which replica can become master and how do you enforce that only one replica becomes master?

One way to handle choosing a new master is to use an external strong consistently service. We can use Cassandra to create a lease for the master node. The replicas can timeout on the master and attempt to take the lease. As Cassandra is using Paxos it is fault tolerant; you can still read or update the lease even if Cassandra nodes crash.

In the above example the poker master and replicas are eventually consistent. The master can send heartbeats so the replicas know that they are still connected to the master. That is fast as messages flow in one direction. When the master crashes there may be race conditions in replicas trying to be the master. Using Paxos at that point gives you strong consistently on the outcome of which node is now the master. This requires additional messages between nodes to ensure a consensus outcome of a single master.