Dear all,
I am trying to construct a state model with the following transition
diagram:
OFFLINE -> BOOTSTRAPPING <---> SLAVE <-----> MASTER
<-----------------------------------
That is, an offline mode can go into a bootstraping state, from the
bootstrap state it can go into a slave state,
from slave it can go from master, from master to slave and from slave it
can go offline.
Assume that if I have a partition with two nodes pf1 and pf2 and a
partition partition_0 with the following ideal state:
partition_0: pf2: MASTER pf1: SLAVE,
and that currently pf1 is serving as a master. When pf2 boots, Helix will
issue, almost simultaneously, two commands:
for pf1: transition from MASTER to SLAVE
for pf2: transition from BOOTSTRAPPING to SLAVE
My understanding is that this happens since Helix is trying to execute as
many commands in parallel and since the last state
has pf2 as master. However, the transition from BOOTSTRAPPING to SLAVE for
pf2 involves a long data copy step, so
I would like to keep pf1 as a master in the meanwhile. I tried prioritizing
the transition from BOOTSTRAPPING to SLAVE
over the transition from MASTER to SLAVE, however Helix still issues them
in parallel (as it should).
I was wondering what my options would be in order to keep the master up
while the future master is bootstrapping. Could
a throttling in the number of transitions be enforced at partition level?
Could I somehow specify that a state with a slave
and a bootstrapping node is undesirable?
As a note, I have also looked at the RSync-replicateed filesystem example.
The reason for not using the OfflineOnline or the
MasterSlave model in my application is that I would like the bootstrapping
node to receive updates from clients, i.e. be visible
during the bootstrap. For this reason, I am introducing the new
BOOTSTRAPPING phase in-between OFFLINE and SLAVE.
Regards,
Vlad
PS: The state model definition is as follows:
builder.addState(MASTER, 1);
builder.addState(SLAVE, 2);
builder.addState(BOOTSTRAP, 3);
builder.addState(OFFLINE);
builder.addState(DROPPED);
// Set the initial state when the node starts
builder.initialState(OFFLINE);
// Add transitions between the states.
builder.addTransition(OFFLINE, BOOTSTRAP, 4);
builder.addTransition(BOOTSTRAP, SLAVE, 5);
builder.addTransition(SLAVE, MASTER, 6);
builder.addTransition(MASTER, SLAVE, 3);
builder.addTransition(SLAVE, OFFLINE, 2);
builder.addTransition(OFFLINE, DROPPED, 1);
// set constraints on states.
// static constraint
builder.upperBound(MASTER, 1);
// dynamic constraint, R means it should be derived based on
the replication
// factor.
builder.dynamicUpperBound(SLAVE, "R");
