paxos

X marks intersection points
Cliens want to write to a shared file
All acceptors and learners start with no one ('null') having access

ClientA   ClientB   Proposer  Acceptor1   Acceptor2   Acceptor3   LearnerX  LearnerY
  |         |         |          |            |           |           |         |
  X------------------>|          |            |           |           |         |       Request(ClientA)
  |         |         X--------->|----------->|---------->|           |         |       Prepare(transaction_id=1)
  |         |         |<---------X------------X-----------X           |         |       Promise(transaction_id=1, {null, null, null}) // Acceptors answer with their current value, i.e. who currently has write access
  |         |         X--------->|----------->|---------->|           |         |       Accept(transaction_id=1, ClientA) // The proposer decides to let ClientA access the file
  |         |         |<---------X------------X-----------X---------->|-------->|       Accepted(transaction_id=1, ClientA)
  |<------------------------------------------------------------------X---------X       Reponse(accepted) // Convey the information to ClientA
  |         |         |          |            |           |           |         |
  |         X-------->|          |            |           |           |         |       Request(ClientB)
  |         |         X--------->|----------->|---------->|           |         |       Prepare(transaction_id=2)
  |         |         |<---------X------------X-----------X           |         |       Promise(transaction_id=2, {ClientA, ClientA, ClientA})
  |         |         X--------->|----------->|---------->|           |         |       Accept(transaction_id=2, ClientA) // The proposer decides ClientA still has access to the file 
  |         |         |<---------X------------X-----------X---------->|-------->|       Accepted(transaction_id=2, ClientA)
  |         |<--------------------------------------------------------X---------X       Reponse(denied) // Convey the information to ClientA
  |         |         |          |            |           |           |         |
  X------------------>|          |            |           |           |         |       Request(null) // ClientA doesn't want the file anymore
  |         |         X--------->|----------->|---------->|           |         |       Prepare(transaction_id=3)
  |         |         |<---------X------------X-----------X           |         |       Promise(transaction_id=3, {ClientA, ClientA, ClientA})
  |         |         X--------->|----------->|---------->|           |         |       Accept(transaction_id=3, null)
  |         |         |<---------X------------X-----------X---------->|-------->|       Accepted(transaction_id=3, null)
  |<------------------------------------------------------------------X---------X       Reponse(accept) // Convey the information to ClientA
  |         |         |          |            |           |           |         |
  |         X-------->|          |            |           |           |         |       Request(ClientB)
  |         |         X--------->|----------->|---------->|           |         |       Prepare(transaction_id=4)
  |         |         |<---------X------------X-----------X           |         |       Promise(transaction_id=4, {null, null, null})
  |         |         X--------->|----------->|---------->|           |         |       Accept(transaction_id=4, ClientB) // The proposer decides ClientB can now access the file 
  |         |         |<---------X------------X-----------X---------->|-------->|       Accepted(transaction_id=4, ClientB)
  |         |<--------------------------------------------------------X---------X       Reponse(accept) // Convey the information to ClientA