Why paxos in mysql group replication jump prepare phase？

I see such code segment in proposer_task(xcom_base.c)

if(threephase || ep->p->force_delivery){
    push_msg_3p(ep->site, ep->p, ep->prepare_msg, ep->msgno, normal);
}else{
    push_msg_2p(ep->site, ep->p);
}

the threepahse is int const threephase = 0 and force_delivery == 0 here

push_msg_eq is normal paxos include prepare, accept and learn phase

but push_msg_2p will skip prepare phase and directly send accept request

I want to know why, Thanks a lot.

If you look at the paper Paxos Made Simple page 10 paragraph 3 says:

A newly chosen leader executes phase 1 for infinitely many instances of the consensus algorithm [...]

Then paragraph 4:

Since failure of the leader and election of a new one should be rare events, the effective cost of executing a state machine command—that is, of achieving consensus on the command/value—is the cost of executing only phase 2 of the consensus algorithm. It can be shown that phase 2 of the Paxos consensus algorithm has the minimum possible cost of any algorithm for reaching agreement in the presence of faults. Hence, the Paxos algorithm is essentially optimal.

This is saying that a leader only issues a prepare during a leader failover. After that it streams accept messages. It then has "optimal messaging" in that the leader only needs one round trip to know a value is chosen (the accept message and its acknowledgment).

In a three node cluster, a leader self-accepts instantaneously, then only needs one accept acknowledgment from a second node to have a majority. It then knows the value is chosen without having to await the response from the 3rd node (which could be down). That is as efficient as you can get. The value is known to be accepted at a second node with strong consistency.

Given that is how paxos works to get maximum efficiency we should expect that mysql xcom has a mode that skips the prepare message phase in steady state.

You can read more about the Paxos Made Simple techniques on my blog here.

You might be interested to know about the latest developments of Paxos where you don't need a majority response for accept messages in the cluster using FPaxos and tricks like the even nodes optimization.