Whenever a Raft configuration change is performed, `raft::server` calls
`raft_rpc::add_server`/`raft_rpc::remove_server`. Our `raft_rpc`
implementation has a function, `_on_server_update`, passed in the
constructor, which it called in `add_server`/`remove_server`;
that function would update the set of endpoints detected by the
direct failure detector. `_on_server_update` was passed an IP address
and that address was added to / removed from the failure detector set
(there's another translation layer between the IP addresses and internal
failure detector 'endpoint ID's; but we can ignore it for the purposes
of this commit).
Therefore: the failure detector was pinging a certain set of IP
addresses. These IP addresses were updated during Raft configuration
changes.
To implement the `is_alive(raft::server_id)` function (required by
`raft::failure_detector` interface), we would translate the ID using
the Raft address map, which is currently also updated during
configuration changes, to an IP address, and check if that IP address is
alive according to the direct failure detector (which maintained an
`_alive_set` of type `unordered_set<gms::inet_address>`).
This all works well but it assumes that servers can be identified using
IP addresses - it doesn't play well with the fact that servers may
change their IP addresses. The only immutable identifier we have for a
server is `raft::server_id`. In the future, Raft configurations will not
associate IP addresses with Raft servers; instead we will assume that IP
addresses can change at any time, and there will be a different
mechanism that eventually updates the Raft address map with the latest
IP address for each `raft::server_id`.
To prepare us for that future, in this commit we no longer operate in
terms of IP addresses in the failure detector, but in terms of
`raft::server_id`s. Most of the commit is boilerplate, changing
`gms::inet_address` to `raft::server_id` and function/variable names.
The interesting changes are:
- in `is_alive`, we no longer need to translate the `raft::server_id` to
an IP address, because now the stored `_alive_set` already contains
`raft::server_id`s instead of `gms::inet_address`es.
- the `ping` function now takes a `raft::server_id` instead of
`gms::inet_address`. To send the ping message, we need to translate
this to IP address; we do it by the `raft_address_map` pointer
introduced in an earlier commit.
Thus, there is still a point where we have to translate between
`raft::server_id` and `gms::inet_address`; but observe we now do it at
the last possible moment - just before sending the message. If we
have no translation, we consider the `ping` to have failed - it's
equivalent to a network failure where no route to a given address was
found.
Closes#11759
* github.com:scylladb/scylladb:
direct_failure_detector: get rid of complex `endpoint_id` translations
service/raft: ping `raft::server_id`s, not `gms::inet_address`es
service/raft: store `raft_address_map` reference in `direct_fd_pinger`
gms: gossiper: move `direct_fd_pinger` out to a separate service
gms: gossiper: direct_fd_pinger: extract generation number caching to a separate class
a9063f9582