Xing Lin created HDFS-17055:
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Summary: Export HAState as a metric from Namenode for monitoring
Key: HDFS-17055
URL: https://issues.apache.org/jira/browse/HDFS-17055
Project: Hadoop HDFS
Issue Type: Improvement
Components: hdfs
Affects Versions: 3.4.0, 3.3.9
Reporter: Xing Lin
We'd like measure the uptime for Namenodes: percentage of time when we have the
active/standby/observer node available (up and running). We could monitor the
namenode from an external service, such as ZKFC. But that would require the
external service to be available 100% itself. And when this third-party
external monitoring service is down, we won't have info on whether our
Namenodes are still up.
We propose to take a different approach: we will emit Namenode state directly
from namenode itself. Whenever we miss a data point for this metric, we
consider the corresponding namenode to be down/not available. In other words,
we assume the metric collection/monitoring infrastructure to be 100% reliable.
One implementation detail: in hadoop, we have the _NameNodeMetrics_ class,
which is used to emit all metrics for {_}NameNode.java{_}. However, we don't
think that is a good place to emit NameNode HAState. HAState is stored in
NameNode.java and we should directly emit it from NameNode.java. Otherwise, we
basically duplicate this info in two classes and we would have to keep them in
sync. Besides, _NameNodeMetrics_ class does not have a reference to the
_NameNode_ object which it belongs to. An _NameNodeMetrics_ is created by a
_static_ function _initMetrics()_ in {_}NameNode.java{_}. We shouldn't emit HA
state from FSNameSystem.java either, as it is initialized from NameNode.java
and all state transitions are implemented in NameNode.java.
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