Github user ahgittin commented on a diff in the pull request:
https://github.com/apache/brooklyn-server/pull/879#discussion_r149839084
--- Diff:
policy/src/main/java/org/apache/brooklyn/policy/ha/ElectPrimaryPolicy.java ---
@@ -0,0 +1,223 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements. See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership. The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied. See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.apache.brooklyn.policy.ha;
+
+import java.util.Collection;
+import java.util.List;
+import java.util.Map;
+
+import org.apache.brooklyn.api.entity.Entity;
+import org.apache.brooklyn.api.entity.Group;
+import org.apache.brooklyn.api.mgmt.Task;
+import org.apache.brooklyn.api.sensor.AttributeSensor;
+import org.apache.brooklyn.api.sensor.EnricherSpec;
+import org.apache.brooklyn.api.sensor.Sensor;
+import org.apache.brooklyn.api.sensor.SensorEvent;
+import org.apache.brooklyn.api.sensor.SensorEventListener;
+import org.apache.brooklyn.config.ConfigKey;
+import org.apache.brooklyn.core.config.ConfigKeys;
+import org.apache.brooklyn.core.effector.Effectors;
+import org.apache.brooklyn.core.entity.AbstractEntity;
+import org.apache.brooklyn.core.entity.Attributes;
+import org.apache.brooklyn.core.entity.Entities;
+import org.apache.brooklyn.core.entity.EntityInternal;
+import org.apache.brooklyn.core.entity.StartableApplication;
+import org.apache.brooklyn.core.entity.lifecycle.Lifecycle;
+import org.apache.brooklyn.core.mgmt.BrooklynTaskTags;
+import org.apache.brooklyn.core.policy.AbstractPolicy;
+import org.apache.brooklyn.core.sensor.Sensors;
+import org.apache.brooklyn.entity.group.DynamicGroup;
+import org.apache.brooklyn.util.collections.MutableList;
+import org.apache.brooklyn.util.collections.QuorumCheck.QuorumChecks;
+import org.apache.brooklyn.util.core.task.DynamicTasks;
+import org.apache.brooklyn.util.exceptions.Exceptions;
+import org.apache.brooklyn.util.exceptions.UserFacingException;
+import org.apache.brooklyn.util.text.Strings;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import com.google.common.base.Preconditions;
+import com.google.common.base.Throwables;
+import com.google.common.reflect.TypeToken;
+
+/**
+
+The ElectPrimaryPolicy acts to keep exactly one of its children or members
as primary, promoting and demoting them when required.
+
+A simple use case is where we have two children, call them North and
South, and we wish for North to be primary. If North fails, however, we want
to promote and fail over to South. This can be done by:
+
+* adding this policy at the parent
+* setting ` ha.primary.weight` on North
+* optionally defining `promote` on North and South (if action is required
there to promote it)
+* observing the `primary` sensor to see which is primary
+* optionally setting `propagate.primary.sensors: [ main.uri ]` to publish
`main.uri` from whichever of North or South is active
+* optionally setting `primary.selection.mode: best` to switch back to
North if it comes back online
+
+The policy works by listening for service-up changes in the target pool
(children or members) and listening for `ha.primary.weight` sensor values from
those elements.
+On any change, it invokes an effector to perform the primary election.
+By default, the effector invoked is `electPrimary`, but this can be
changed with the `primary.election.effector` config key. If this effector does
not exist, the policy will add a default behaviour using
`ElectPrimaryEffector`. Details of the election are described in that
effector, but to summarize, it will find an appropriate primary from the target
pool and publish a sensor indicating who the new primary is.
+If the effector is not defined this policy will add one with the standard
election behaviour
+({@link ElectPrimaryEffector}).
+That effector will also invoke `promote` and `demote` on the relevant
entities.
+
+All the `primary.*` parameters accepted by that effector can be defined on
this policy
+and will be passed to the effector, along with an `event` parameter
indicating the sensor which triggered the election.
+
+If no quorum.up or quorum.running is set on the entity, both will be set
to a constant 1.
+
+ */
+public class ElectPrimaryPolicy extends AbstractPolicy implements
ElectPrimaryConfig {
+
+ private static final Logger log =
LoggerFactory.getLogger(ElectPrimaryPolicy.class);
+
+ public static ConfigKey<String> EFFECTOR_NAME =
ConfigKeys.newStringConfigKey("primary.election.effector",
+ "The effector to invoke to perform the scan; if not set, it will
use electPrimary and create if necessary",
+ "electPrimary");
+
+ @SuppressWarnings("serial")
+ public static ConfigKey<Collection<? extends Object>>
PROPAGATE_PRIMARY_SENSORS = ConfigKeys.newConfigKey(
+ new TypeToken<Collection<? extends Object>>() {},
+ "propagate.primary.sensors");
+
+ @Override
+ public void setEntity(@SuppressWarnings("deprecation")
org.apache.brooklyn.api.entity.EntityLocal entity) {
+ super.setEntity(entity);
+
+ checkAndMaybeAddEffector(entity);
+ checkQuorums(entity);
+ addSubscriptions(entity);
--- End diff --
That got me a little concerned about races / inefficiency if we get many
sensors coming in. I've set it up so we have `rescanRequest` and `rescanImpl`
where the former applies JS `_.once` semantics.
---
