timoninmaxim commented on code in PR #12301:
URL: https://github.com/apache/ignite/pull/12301#discussion_r2444359041


##########
modules/core/src/main/java/org/apache/ignite/internal/processors/rollingupgrade/RollingUpgradeProcessor.java:
##########
@@ -0,0 +1,180 @@
+/*
+ * 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.ignite.internal.processors.rollingupgrade;
+
+import java.util.concurrent.atomic.AtomicReference;
+import org.apache.ignite.IgniteCheckedException;
+import org.apache.ignite.IgniteException;
+import org.apache.ignite.internal.GridKernalContext;
+import org.apache.ignite.internal.processors.GridProcessorAdapter;
+import 
org.apache.ignite.internal.processors.metastorage.DistributedMetaStorage;
+import 
org.apache.ignite.internal.processors.metastorage.DistributedMetastorageLifecycleListener;
+import 
org.apache.ignite.internal.processors.metastorage.ReadableDistributedMetaStorage;
+import org.apache.ignite.internal.util.lang.IgnitePair;
+import org.apache.ignite.internal.util.typedef.F;
+import org.apache.ignite.internal.util.typedef.internal.A;
+import org.apache.ignite.internal.util.typedef.internal.LT;
+import org.apache.ignite.lang.IgniteProductVersion;
+import org.jetbrains.annotations.Nullable;
+
+import static org.apache.ignite.internal.IgniteNodeAttributes.ATTR_BUILD_VER;
+import static 
org.apache.ignite.internal.processors.metastorage.DistributedMetaStorage.IGNITE_INTERNAL_KEY_PREFIX;
+
+/** Rolling upgrade processor. Manages current and target versions of cluster. 
*/
+public class RollingUpgradeProcessor extends GridProcessorAdapter {
+    /** Key for the distributed property that holds current and target 
versions. */
+    private static final String ROLLING_UPGRADE_VERSIONS_KEY = 
IGNITE_INTERNAL_KEY_PREFIX + "rolling.upgrade.versions";
+
+    /** Metastorage with the write access. */
+    @Nullable private volatile DistributedMetaStorage metastorage;
+
+    /** Distributed property that holds current and target version. */
+    private final AtomicReference<IgnitePair<IgniteProductVersion>> 
verPairHolder = new AtomicReference<>();
+
+    /**
+     * @param ctx Context.
+     */
+    public RollingUpgradeProcessor(GridKernalContext ctx) {
+        super(ctx);
+    }
+
+    /** {@inheritDoc} */
+    @Override public void start() throws IgniteCheckedException {
+        
ctx.internalSubscriptionProcessor().registerDistributedMetastorageListener(new 
DistributedMetastorageLifecycleListener() {
+            @Override public void 
onReadyForRead(ReadableDistributedMetaStorage metastorage) {
+                try {
+                    IgnitePair<IgniteProductVersion> pair = 
metastorage.read(ROLLING_UPGRADE_VERSIONS_KEY);
+
+                    if (verPairHolder.compareAndSet(null, pair) && 
log.isDebugEnabled())
+                        log.debug("Read current and target versions from 
metastore: " + pair);
+                }
+                catch (IgniteCheckedException e) {
+                    throw new IgniteException(e);
+                }
+
+                metastorage.listen(ROLLING_UPGRADE_VERSIONS_KEY::equals, (key, 
oldVal, newVal) ->
+                    
verPairHolder.compareAndSet((IgnitePair<IgniteProductVersion>)oldVal, 
(IgnitePair<IgniteProductVersion>)newVal)
+                );
+            }
+
+            @Override public void onReadyForWrite(DistributedMetaStorage 
metastorage) {
+                RollingUpgradeProcessor.this.metastorage = metastorage;
+            }
+        });
+    }
+
+    /**
+     * Enables rolling upgrade with specified target version.
+     *
+     * @param target Target version.
+     * @throws IgniteCheckedException If versions are incorrect or metastorage 
is not available.
+     */
+    public void enable(IgniteProductVersion target) throws 
IgniteCheckedException {
+        A.notNull(metastorage, "Metastorage not ready. Node not started?");
+
+        String curBuildVer = 
ctx.discovery().localNode().attribute(ATTR_BUILD_VER);
+        IgniteProductVersion curtVer = 
IgniteProductVersion.fromString(curBuildVer);
+
+        if (checkVersions(curtVer, target)) {
+            metastorage.write(ROLLING_UPGRADE_VERSIONS_KEY, F.pair(curtVer, 
target));
+
+            if (log.isInfoEnabled())
+                log.info("Rolling upgrade enabled [current=" + curtVer + ", 
target=" + target + ']');
+        }
+    }
+
+    /**
+     * Disables rolling upgrade.
+     *
+     * @throws IgniteCheckedException If metastorage is not available.
+     */
+    public void disable() throws IgniteCheckedException {

Review Comment:
   It must be prohibited to disable RU if there are different versions in 
cluster



##########
modules/core/src/main/java/org/apache/ignite/internal/processors/rollingupgrade/RollingUpgradeProcessor.java:
##########
@@ -0,0 +1,180 @@
+/*
+ * 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.ignite.internal.processors.rollingupgrade;
+
+import java.util.concurrent.atomic.AtomicReference;
+import org.apache.ignite.IgniteCheckedException;
+import org.apache.ignite.IgniteException;
+import org.apache.ignite.internal.GridKernalContext;
+import org.apache.ignite.internal.processors.GridProcessorAdapter;
+import 
org.apache.ignite.internal.processors.metastorage.DistributedMetaStorage;
+import 
org.apache.ignite.internal.processors.metastorage.DistributedMetastorageLifecycleListener;
+import 
org.apache.ignite.internal.processors.metastorage.ReadableDistributedMetaStorage;
+import org.apache.ignite.internal.util.lang.IgnitePair;
+import org.apache.ignite.internal.util.typedef.F;
+import org.apache.ignite.internal.util.typedef.internal.A;
+import org.apache.ignite.internal.util.typedef.internal.LT;
+import org.apache.ignite.lang.IgniteProductVersion;
+import org.jetbrains.annotations.Nullable;
+
+import static org.apache.ignite.internal.IgniteNodeAttributes.ATTR_BUILD_VER;
+import static 
org.apache.ignite.internal.processors.metastorage.DistributedMetaStorage.IGNITE_INTERNAL_KEY_PREFIX;
+
+/** Rolling upgrade processor. Manages current and target versions of cluster. 
*/
+public class RollingUpgradeProcessor extends GridProcessorAdapter {
+    /** Key for the distributed property that holds current and target 
versions. */
+    private static final String ROLLING_UPGRADE_VERSIONS_KEY = 
IGNITE_INTERNAL_KEY_PREFIX + "rolling.upgrade.versions";
+
+    /** Metastorage with the write access. */
+    @Nullable private volatile DistributedMetaStorage metastorage;
+
+    /** Distributed property that holds current and target version. */
+    private final AtomicReference<IgnitePair<IgniteProductVersion>> 
verPairHolder = new AtomicReference<>();
+
+    /**
+     * @param ctx Context.
+     */
+    public RollingUpgradeProcessor(GridKernalContext ctx) {
+        super(ctx);
+    }
+
+    /** {@inheritDoc} */
+    @Override public void start() throws IgniteCheckedException {
+        
ctx.internalSubscriptionProcessor().registerDistributedMetastorageListener(new 
DistributedMetastorageLifecycleListener() {
+            @Override public void 
onReadyForRead(ReadableDistributedMetaStorage metastorage) {
+                try {
+                    IgnitePair<IgniteProductVersion> pair = 
metastorage.read(ROLLING_UPGRADE_VERSIONS_KEY);
+
+                    if (verPairHolder.compareAndSet(null, pair) && 
log.isDebugEnabled())
+                        log.debug("Read current and target versions from 
metastore: " + pair);
+                }
+                catch (IgniteCheckedException e) {
+                    throw new IgniteException(e);
+                }
+
+                metastorage.listen(ROLLING_UPGRADE_VERSIONS_KEY::equals, (key, 
oldVal, newVal) ->
+                    
verPairHolder.compareAndSet((IgnitePair<IgniteProductVersion>)oldVal, 
(IgnitePair<IgniteProductVersion>)newVal)

Review Comment:
   Add log here too



##########
modules/core/src/main/java/org/apache/ignite/internal/processors/rollingupgrade/RollingUpgradeProcessor.java:
##########
@@ -0,0 +1,180 @@
+/*
+ * 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.ignite.internal.processors.rollingupgrade;
+
+import java.util.concurrent.atomic.AtomicReference;
+import org.apache.ignite.IgniteCheckedException;
+import org.apache.ignite.IgniteException;
+import org.apache.ignite.internal.GridKernalContext;
+import org.apache.ignite.internal.processors.GridProcessorAdapter;
+import 
org.apache.ignite.internal.processors.metastorage.DistributedMetaStorage;
+import 
org.apache.ignite.internal.processors.metastorage.DistributedMetastorageLifecycleListener;
+import 
org.apache.ignite.internal.processors.metastorage.ReadableDistributedMetaStorage;
+import org.apache.ignite.internal.util.lang.IgnitePair;
+import org.apache.ignite.internal.util.typedef.F;
+import org.apache.ignite.internal.util.typedef.internal.A;
+import org.apache.ignite.internal.util.typedef.internal.LT;
+import org.apache.ignite.lang.IgniteProductVersion;
+import org.jetbrains.annotations.Nullable;
+
+import static org.apache.ignite.internal.IgniteNodeAttributes.ATTR_BUILD_VER;
+import static 
org.apache.ignite.internal.processors.metastorage.DistributedMetaStorage.IGNITE_INTERNAL_KEY_PREFIX;
+
+/** Rolling upgrade processor. Manages current and target versions of cluster. 
*/
+public class RollingUpgradeProcessor extends GridProcessorAdapter {
+    /** Key for the distributed property that holds current and target 
versions. */
+    private static final String ROLLING_UPGRADE_VERSIONS_KEY = 
IGNITE_INTERNAL_KEY_PREFIX + "rolling.upgrade.versions";
+
+    /** Metastorage with the write access. */
+    @Nullable private volatile DistributedMetaStorage metastorage;
+
+    /** Distributed property that holds current and target version. */
+    private final AtomicReference<IgnitePair<IgniteProductVersion>> 
verPairHolder = new AtomicReference<>();
+
+    /**
+     * @param ctx Context.
+     */
+    public RollingUpgradeProcessor(GridKernalContext ctx) {
+        super(ctx);
+    }
+
+    /** {@inheritDoc} */
+    @Override public void start() throws IgniteCheckedException {
+        
ctx.internalSubscriptionProcessor().registerDistributedMetastorageListener(new 
DistributedMetastorageLifecycleListener() {
+            @Override public void 
onReadyForRead(ReadableDistributedMetaStorage metastorage) {
+                try {
+                    IgnitePair<IgniteProductVersion> pair = 
metastorage.read(ROLLING_UPGRADE_VERSIONS_KEY);
+
+                    if (verPairHolder.compareAndSet(null, pair) && 
log.isDebugEnabled())
+                        log.debug("Read current and target versions from 
metastore: " + pair);

Review Comment:
   This log is written when node is started in moment of RU. Looks like this 
log can be useful, let's make it INFO



##########
modules/core/src/main/java/org/apache/ignite/internal/processors/rollingupgrade/RollingUpgradeProcessor.java:
##########
@@ -0,0 +1,180 @@
+/*
+ * 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.ignite.internal.processors.rollingupgrade;
+
+import java.util.concurrent.atomic.AtomicReference;
+import org.apache.ignite.IgniteCheckedException;
+import org.apache.ignite.IgniteException;
+import org.apache.ignite.internal.GridKernalContext;
+import org.apache.ignite.internal.processors.GridProcessorAdapter;
+import 
org.apache.ignite.internal.processors.metastorage.DistributedMetaStorage;
+import 
org.apache.ignite.internal.processors.metastorage.DistributedMetastorageLifecycleListener;
+import 
org.apache.ignite.internal.processors.metastorage.ReadableDistributedMetaStorage;
+import org.apache.ignite.internal.util.lang.IgnitePair;
+import org.apache.ignite.internal.util.typedef.F;
+import org.apache.ignite.internal.util.typedef.internal.A;
+import org.apache.ignite.internal.util.typedef.internal.LT;
+import org.apache.ignite.lang.IgniteProductVersion;
+import org.jetbrains.annotations.Nullable;
+
+import static org.apache.ignite.internal.IgniteNodeAttributes.ATTR_BUILD_VER;
+import static 
org.apache.ignite.internal.processors.metastorage.DistributedMetaStorage.IGNITE_INTERNAL_KEY_PREFIX;
+
+/** Rolling upgrade processor. Manages current and target versions of cluster. 
*/
+public class RollingUpgradeProcessor extends GridProcessorAdapter {
+    /** Key for the distributed property that holds current and target 
versions. */
+    private static final String ROLLING_UPGRADE_VERSIONS_KEY = 
IGNITE_INTERNAL_KEY_PREFIX + "rolling.upgrade.versions";
+
+    /** Metastorage with the write access. */
+    @Nullable private volatile DistributedMetaStorage metastorage;
+
+    /** Distributed property that holds current and target version. */
+    private final AtomicReference<IgnitePair<IgniteProductVersion>> 
verPairHolder = new AtomicReference<>();
+
+    /**
+     * @param ctx Context.
+     */
+    public RollingUpgradeProcessor(GridKernalContext ctx) {
+        super(ctx);
+    }
+
+    /** {@inheritDoc} */
+    @Override public void start() throws IgniteCheckedException {
+        
ctx.internalSubscriptionProcessor().registerDistributedMetastorageListener(new 
DistributedMetastorageLifecycleListener() {
+            @Override public void 
onReadyForRead(ReadableDistributedMetaStorage metastorage) {
+                try {
+                    IgnitePair<IgniteProductVersion> pair = 
metastorage.read(ROLLING_UPGRADE_VERSIONS_KEY);
+
+                    if (verPairHolder.compareAndSet(null, pair) && 
log.isDebugEnabled())
+                        log.debug("Read current and target versions from 
metastore: " + pair);
+                }
+                catch (IgniteCheckedException e) {
+                    throw new IgniteException(e);
+                }
+
+                metastorage.listen(ROLLING_UPGRADE_VERSIONS_KEY::equals, (key, 
oldVal, newVal) ->
+                    
verPairHolder.compareAndSet((IgnitePair<IgniteProductVersion>)oldVal, 
(IgnitePair<IgniteProductVersion>)newVal)
+                );
+            }
+
+            @Override public void onReadyForWrite(DistributedMetaStorage 
metastorage) {
+                RollingUpgradeProcessor.this.metastorage = metastorage;
+            }
+        });
+    }
+
+    /**
+     * Enables rolling upgrade with specified target version.
+     *
+     * @param target Target version.
+     * @throws IgniteCheckedException If versions are incorrect or metastorage 
is not available.
+     */
+    public void enable(IgniteProductVersion target) throws 
IgniteCheckedException {
+        A.notNull(metastorage, "Metastorage not ready. Node not started?");
+
+        String curBuildVer = 
ctx.discovery().localNode().attribute(ATTR_BUILD_VER);
+        IgniteProductVersion curtVer = 
IgniteProductVersion.fromString(curBuildVer);
+
+        if (checkVersions(curtVer, target)) {
+            metastorage.write(ROLLING_UPGRADE_VERSIONS_KEY, F.pair(curtVer, 
target));
+
+            if (log.isInfoEnabled())
+                log.info("Rolling upgrade enabled [current=" + curtVer + ", 
target=" + target + ']');
+        }
+    }
+
+    /**
+     * Disables rolling upgrade.
+     *
+     * @throws IgniteCheckedException If metastorage is not available.
+     */
+    public void disable() throws IgniteCheckedException {
+        A.notNull(metastorage, "Metastorage not ready. Node not started?");
+
+        metastorage.remove(ROLLING_UPGRADE_VERSIONS_KEY);
+
+        if (log.isInfoEnabled())
+            log.info("Rolling upgrade disabled");
+    }
+
+    /**
+     * Returns a pair containing the current and target versions of the 
cluster.
+     * <p>
+     * This method returns {@code null} if rolling upgrade has not been 
enabled yet
+     * or if version information has not been read from the distributed 
metastorage.
+     *
+     * @return A pair where:
+     *     <ul>
+     *         <li><b>First element</b> — current version of the cluster.</li>
+     *         <li><b>Second element</b> — target version to which the cluster 
is being upgraded.</li>
+     *     </ul>
+     *     or {@code null} if rolling upgrade is not active.
+     */
+    public IgnitePair<IgniteProductVersion> versions() {
+        return verPairHolder.get();
+    }
+
+    /** Сhecks whether the cluster is in the rolling upgrade mode. */
+    public boolean isRollingUpgradeEnabled() {

Review Comment:
   We don't use 'is' prefix for internal packages



##########
modules/core/src/main/java/org/apache/ignite/internal/processors/rollingupgrade/RollingUpgradeProcessor.java:
##########
@@ -0,0 +1,180 @@
+/*
+ * 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.ignite.internal.processors.rollingupgrade;
+
+import java.util.concurrent.atomic.AtomicReference;
+import org.apache.ignite.IgniteCheckedException;
+import org.apache.ignite.IgniteException;
+import org.apache.ignite.internal.GridKernalContext;
+import org.apache.ignite.internal.processors.GridProcessorAdapter;
+import 
org.apache.ignite.internal.processors.metastorage.DistributedMetaStorage;
+import 
org.apache.ignite.internal.processors.metastorage.DistributedMetastorageLifecycleListener;
+import 
org.apache.ignite.internal.processors.metastorage.ReadableDistributedMetaStorage;
+import org.apache.ignite.internal.util.lang.IgnitePair;
+import org.apache.ignite.internal.util.typedef.F;
+import org.apache.ignite.internal.util.typedef.internal.A;
+import org.apache.ignite.internal.util.typedef.internal.LT;
+import org.apache.ignite.lang.IgniteProductVersion;
+import org.jetbrains.annotations.Nullable;
+
+import static org.apache.ignite.internal.IgniteNodeAttributes.ATTR_BUILD_VER;
+import static 
org.apache.ignite.internal.processors.metastorage.DistributedMetaStorage.IGNITE_INTERNAL_KEY_PREFIX;
+
+/** Rolling upgrade processor. Manages current and target versions of cluster. 
*/
+public class RollingUpgradeProcessor extends GridProcessorAdapter {
+    /** Key for the distributed property that holds current and target 
versions. */
+    private static final String ROLLING_UPGRADE_VERSIONS_KEY = 
IGNITE_INTERNAL_KEY_PREFIX + "rolling.upgrade.versions";
+
+    /** Metastorage with the write access. */
+    @Nullable private volatile DistributedMetaStorage metastorage;
+
+    /** Distributed property that holds current and target version. */
+    private final AtomicReference<IgnitePair<IgniteProductVersion>> 
verPairHolder = new AtomicReference<>();
+
+    /**
+     * @param ctx Context.
+     */
+    public RollingUpgradeProcessor(GridKernalContext ctx) {
+        super(ctx);
+    }
+
+    /** {@inheritDoc} */
+    @Override public void start() throws IgniteCheckedException {
+        
ctx.internalSubscriptionProcessor().registerDistributedMetastorageListener(new 
DistributedMetastorageLifecycleListener() {
+            @Override public void 
onReadyForRead(ReadableDistributedMetaStorage metastorage) {
+                try {
+                    IgnitePair<IgniteProductVersion> pair = 
metastorage.read(ROLLING_UPGRADE_VERSIONS_KEY);
+
+                    if (verPairHolder.compareAndSet(null, pair) && 
log.isDebugEnabled())
+                        log.debug("Read current and target versions from 
metastore: " + pair);
+                }
+                catch (IgniteCheckedException e) {
+                    throw new IgniteException(e);
+                }
+
+                metastorage.listen(ROLLING_UPGRADE_VERSIONS_KEY::equals, (key, 
oldVal, newVal) ->
+                    
verPairHolder.compareAndSet((IgnitePair<IgniteProductVersion>)oldVal, 
(IgnitePair<IgniteProductVersion>)newVal)
+                );
+            }
+
+            @Override public void onReadyForWrite(DistributedMetaStorage 
metastorage) {
+                RollingUpgradeProcessor.this.metastorage = metastorage;
+            }
+        });
+    }
+
+    /**
+     * Enables rolling upgrade with specified target version.
+     *
+     * @param target Target version.
+     * @throws IgniteCheckedException If versions are incorrect or metastorage 
is not available.
+     */
+    public void enable(IgniteProductVersion target) throws 
IgniteCheckedException {
+        A.notNull(metastorage, "Metastorage not ready. Node not started?");
+
+        String curBuildVer = 
ctx.discovery().localNode().attribute(ATTR_BUILD_VER);
+        IgniteProductVersion curtVer = 
IgniteProductVersion.fromString(curBuildVer);
+
+        if (checkVersions(curtVer, target)) {
+            metastorage.write(ROLLING_UPGRADE_VERSIONS_KEY, F.pair(curtVer, 
target));
+
+            if (log.isInfoEnabled())
+                log.info("Rolling upgrade enabled [current=" + curtVer + ", 
target=" + target + ']');
+        }
+    }
+
+    /**
+     * Disables rolling upgrade.
+     *
+     * @throws IgniteCheckedException If metastorage is not available.
+     */
+    public void disable() throws IgniteCheckedException {
+        A.notNull(metastorage, "Metastorage not ready. Node not started?");
+
+        metastorage.remove(ROLLING_UPGRADE_VERSIONS_KEY);
+
+        if (log.isInfoEnabled())
+            log.info("Rolling upgrade disabled");
+    }
+
+    /**
+     * Returns a pair containing the current and target versions of the 
cluster.
+     * <p>
+     * This method returns {@code null} if rolling upgrade has not been 
enabled yet
+     * or if version information has not been read from the distributed 
metastorage.
+     *
+     * @return A pair where:
+     *     <ul>
+     *         <li><b>First element</b> — current version of the cluster.</li>
+     *         <li><b>Second element</b> — target version to which the cluster 
is being upgraded.</li>
+     *     </ul>
+     *     or {@code null} if rolling upgrade is not active.
+     */
+    public IgnitePair<IgniteProductVersion> versions() {
+        return verPairHolder.get();
+    }
+
+    /** Сhecks whether the cluster is in the rolling upgrade mode. */
+    public boolean isRollingUpgradeEnabled() {
+        return versions() != null;
+    }
+
+    /**
+     * Checks cur and target versions.
+     *
+     * @param cur Current cluster version.
+     * @param target Target cluster version.
+     * @return {@code false} if there is no need to update versions {@code 
true} otherwise.
+     * @throws IgniteCheckedException If versions are incorrect.
+     */
+    private boolean checkVersions(IgniteProductVersion cur, 
IgniteProductVersion target) throws IgniteCheckedException {
+        IgnitePair<IgniteProductVersion> pair = verPairHolder.get();
+
+        if (pair != null && pair.get2().equals(target))
+            return false;
+
+        if (cur.major() != target.major()) {
+            String errMsg = "Major versions are different.";
+
+            LT.warn(log, errMsg);

Review Comment:
   no need in log throttling here



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