Zhen Zhang created HELIX-541:
--------------------------------
Summary: Possible "livelock" in Helix controller
Key: HELIX-541
URL: https://issues.apache.org/jira/browse/HELIX-541
Project: Apache Helix
Issue Type: Bug
Reporter: Zhen Zhang
We discover a "livelock" bug in Helix controller. This leads to HELIX-540.
Assuming we have 3 partitions and 2 nodes, using LeaderStandby state model,
FULL_AUTO mode, and replica is 2. When reaching stable mapping, we might have
the following:
{noformat}
partition_0:
node_0: LEADER
node_1: STANDBY
partition_1:
node_1: LEADER
node_0: STANDBY
partition_2:
node_0: LEADER
node_1: STANDBY
{noformat}
Later we add a new node (node_2) to the cluster and rebalancer decides that
node_2 should become LEADER for partition_2. So controller first sends
OFFLINE->STANDBY transition to node_2, and the mapping becomes:
{noformat}
partition_0:
node_0: LEADER
node_1: STANDBY
partition_1:
node_1: LEADER
node_0: STANDBY
partition_2:
node_0: LEADER
node_1: STANDBY
node_2: STANDBY
{noformat}
Note that given LEADER state count is 1 and STANDBY state count is R, where
R=2, it is implying the following state constraints:
{noformat}
LEADER: upper_bound=1
STANDBY: upper_bound=2
{noformat}
Helix controller now enters the "livelock": it can't send STANDBY->LEADER to
node_2, since this will violate LEADER upper bound; it can't send
LEADER->STANDBY to node_0 either, since will violate STANDBY upper bound.
We can solve the problem in several ways:
1) State count definition is ambiguous. For some state, like LEADER, when we
say state_count=1, that means we can't violate this constraint at any time.
However, for some other state, like STANDBY, when we say state_count=R, that
means in stable mapping, there should be R-1 STANDBY replicas, but we don't
care the count in any transient state. In this case, we can set STANDBY
upper_bound to be larger than 2. Note that this doesn't solve the problem in
general. We may have some state model that has a restrict requirement on
STANDBY state.
2) Using state transition priority. If we define LEADER->STANDBY transition
should have a higher priority than OFFLINE->STANDBY transition, it will solve
the "livelock". But this doesn't solve the problem in general either, because
when the state model gets complicated, it's hard to define and prove proper
transition priorities that avoid "livelock" in any situation.
3) The root cause of the problem is that Helix controller uses a greedy
algorithm that only looks one step ahead. In the example, if Helix controller
can look two steps further, then it will find out that sending OFFLINE->STANDBY
transition to node_2 will lead to a dead end, therefore it should choose to
send LEADER->STANDBY to node_0 instead. In general we might need to do a
DFS/BFS and it's hard if state model is complicated and system is large.
In practice, most systems use simple state models with less than 5 states and
have strict state constraint on a single state (e.g MASTER, LEADER) only. We
can avoid "livelock" by carefully choosing state constraints.
--
This message was sent by Atlassian JIRA
(v6.3.4#6332)
