1. run touch_member from the main loop
2. move code which takes care about long tasks into separate class
3. change format of data stored in a DCS: use json instead of url
4. change Member class: from now it deserialize everything into data property
5. rework API: from now it takes into account state of the current node in a dcs
Implementation is done on top of feature/is-healthiest-via-api and
feature/api branches.
In order to trigger manual failover one has to create 'failover' key in
a configuration store with the value in following format:
'leader_name:member_name'
leader_name can be empty or should match with the name of current leader
member_name can be empty or should match with the name one of cluster
nodes
Leader always checks that either desired member (if specified) or one of
the memners is accessible and healthy before demote.
After leader has deomted himself other nodes are performig checks that
desired node is healthy. If it is not they are participating in a leader
race. In some cases (when accidently there is no healthy nodes) former
leader can also participate in a leader race.
Current implementation does not provide REST API endpoint for a manual
failover.
POST /restart -- will restart postgres
You you are restartung leader node, lock would be maintained during
restart.
POST /reinitialize -- will reinitialize node from the leader.
It's not possible to reinitialize current leader.
Command will fail when the leader is unknown.
query method in an api.py also needs retry in some cases (for example
when we are running is_healthiest_node check).
In all cases we should retry only when connection is closed or broken.
BUT, the connection status must be checked via cursor.connection (old
implementation was using general connection object for that). For
multi-threaded applications this is not appropriate, because some other
thread might restore connection.
In addition to that I've changed most of the unit tests to use `Mock` and
`patch` where it is possible.
`Cluster.leader` is not reference to `Member` anymore, but to `Leader`
`Leader` class contains field `index` (update index). This field is very
useful for watching for events which changing leader key. Also `Leader`
contains `member` field, which should reference real member.
Also check presence of trigger_file on master after promotion when
pg_is_in_recovery() = false and if it is there - remove it.
Plus check presence of trigger_file on a new slave (after running
pg_basebackup) and if it is there - also remove it.
Alexander Kukushkin