Github user neykov commented on a diff in the pull request:

    https://github.com/apache/brooklyn-server/pull/382#discussion_r83198850
  
    --- Diff: 
core/src/main/java/org/apache/brooklyn/policy/InvokeEffectorOnSetChange.java ---
    @@ -0,0 +1,203 @@
    +/*
    + * 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;
    +
    +import static com.google.common.base.Preconditions.checkArgument;
    +import static com.google.common.base.Preconditions.checkNotNull;
    +
    +import java.util.Collection;
    +import java.util.Collections;
    +import java.util.HashSet;
    +import java.util.Map;
    +import java.util.Set;
    +
    +import org.apache.brooklyn.api.effector.Effector;
    +import org.apache.brooklyn.api.entity.EntityLocal;
    +import org.apache.brooklyn.api.sensor.AttributeSensor;
    +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.policy.AbstractPolicy;
    +import org.apache.brooklyn.core.sensor.BasicAttributeSensor;
    +import org.apache.brooklyn.util.collections.MutableMap;
    +import org.apache.brooklyn.util.core.flags.TypeCoercions;
    +import org.apache.brooklyn.util.guava.Maybe;
    +import org.apache.brooklyn.util.text.Strings;
    +import org.slf4j.Logger;
    +import org.slf4j.LoggerFactory;
    +
    +import com.google.common.collect.Sets;
    +import com.google.common.reflect.TypeToken;
    +
    +/**
    + * Subscribes to events on a collection {@link AttributeSensor} and 
invokes the named
    + * effectors for each element that was added and removed.
    + * <p>
    + * The policy only detects <em>replacements</em> of the collection; it 
does not act on
    + * modifications. If the sensor has value <code>A</code> and an element is 
added &ndash;
    + * value <code>A'</code> &ndash; the on-added effector is not invoked. If 
the sensor is
    + * later set to <code>B</code> the delta is made between <code>A</code> 
and <code>B</code>,
    + * not <code>A'</code> and <code>B</code>.
    + * <p>
    + * To simplify the detection of additions and removals the collection is 
converted to a
    + * {@link Set}. This means that only a single event will fire for 
duplicate elements in
    + * the collection.
    + * <p>
    + * The effectors are provided the elements that changed in their parameter 
map. If the
    + * sensor is a collection of maps the elements are provided with their 
keys coerced to
    + * strings and their values unchanged. Otherwise the elements are provided 
in a
    + * single-entry map keyed by the value for {@link #PARAMETER_NAME}.
    + */
    +public class InvokeEffectorOnSetChange extends AbstractPolicy implements 
SensorEventListener<Collection<?>> {
    +
    +    private static final Logger LOG = 
LoggerFactory.getLogger(InvokeEffectorOnSetChange.class);
    +
    +    public static final ConfigKey<AttributeSensor<? extends 
Collection<?>>> TRIGGER_SENSOR = ConfigKeys.newConfigKey(
    +            new TypeToken<AttributeSensor<? extends Collection<?>>>() {},
    +            "sensor",
    +            "Sensor to be monitored.");
    +
    +    public static final ConfigKey<String> PREVIOUS_SENSOR_NAME = 
ConfigKeys.newStringConfigKey(
    +            "previousSensorName",
    +            "The name under which the previous value for the trigger 
sensor should be published. " +
    +                    "If unset \".previous\" will be appended to the name 
of the trigger sensor.");
    +
    +    public static final ConfigKey<String> ON_ADDED_EFFECTOR_NAME = 
ConfigKeys.newStringConfigKey(
    +            "onAdded",
    +            "Name of the effector to invoke when entries are added to the 
collection.");
    +
    +    public static final ConfigKey<String> ON_REMOVED_EFFECTOR_NAME = 
ConfigKeys.newStringConfigKey(
    +            "onRemoved",
    +            "Name of the effector to invoke when entries are removed from 
the collection.");
    +
    +    public static final ConfigKey<String> PARAMETER_NAME = 
ConfigKeys.newStringConfigKey(
    +            "parameterName",
    +            "The name of the parameter to supply to the effectors",
    +            "value");
    +
    +    /** The previous version of the set against which events will be 
compared. */
    +    private Set<Object> previous = Collections.emptySet();
    +
    +    /** Guards accesses of previous. */
    +    private final Object[] updateLock = new Object[0];
    +
    +    @Override
    +    public void setEntity(EntityLocal entity) {
    +        super.setEntity(entity);
    +        Sensor<? extends Collection<?>> sensor =
    +                checkNotNull(getConfig(TRIGGER_SENSOR), "Value required 
for " + TRIGGER_SENSOR.getName());
    +
    +        checkArgument(Strings.isNonBlank(getConfig(PARAMETER_NAME)), 
"Value required for " + PARAMETER_NAME.getName());
    +
    +        // Fail straight away if neither effector is found.
    +        if (getEffector(getOnAddedEffector()).isAbsentOrNull() &&
    +                getEffector(getOnRemovedEffector()).isAbsentOrNull()) {
    +            throw new IllegalArgumentException("Value required for one or 
both of " + ON_ADDED_EFFECTOR_NAME.getName() +
    +                    " and " + ON_REMOVED_EFFECTOR_NAME.getName());
    +        }
    +
    +        // Initialise `present` before subscribing.
    +        Collection<?> current = entity.sensors().get(getTriggerSensor());
    +        synchronized (updateLock) {
    +            previous = (current != null) ? new HashSet<>(current) : 
Collections.emptySet();
    +        }
    +        subscriptions().subscribe(entity, sensor, this);
    +    }
    +
    +    @Override
    +    public void onEvent(SensorEvent<Collection<?>> event) {
    +        final Set<Object> added = new HashSet<>(), removed = new 
HashSet<>();
    +        Set<Object> newValue = new HashSet<>(event.getValue());
    +        synchronized (updateLock) {
    +            // Not using .immutableCopy() in case either set contains 
`null`.
    +            Sets.difference(newValue, previous).copyInto(added);
    +            Sets.difference(previous, newValue).copyInto(removed);
    --- End diff --
    
    You can hold the lock just for the following actions.
      * copying the `previous` reference in a local var
      * updating `previous` to `newValue`
      * moving `entity.sensors().set(getPreviousSensor(), previous);` up here
      * using the local variable to work out the differences


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