Prober circular list (#218)

* Fixed changes suggested by CydeWeys * Fixed Javadoc and comments for ActionHandler * Fixed comments and JavaDoc on other files * EOL added * Removed Unnecessary Files * fixed .gradle files styles * Removed outbound message from ActionHandler's fields and renamed Marker Interfaces * Fixed javadoc for Marker Interfaced * Modified Comments on ActionHandler * Removed LocalAddress from Protocol * Fixed Travis Build Issues * Rebased to Master and added in modified Handlers and ProbingAction * Added missing license headers and JavaDoc * Minor fix in NewChannelAction JavaDoc * Minor Style Fix * Full WebWhoIs Sequence Added * fixed build issues * Refactored by responses suggested by jianglai. * Minor Style Fixes * Updated build.gradle file * Modified license header dates * Updated WebWhois tests. * Refactored WebWhois to accomodate jianglai's suggested changes and modified tests to reflect this refactoring * SpotlessApply run to fix style issues * Added license header and newline where appropriate. * Javadoc style fix in tests and removed unused methods * Refactored ProbingAction to minimize number of unnecessary methods * Initial Commit. * Initial Commit. * Deleted unfinished features. Added ActionHandler and its Unit Tests. * Included prober subproject in settings.gradle * Added Protocol Class and its Basic Unit Tests * Added Protocol Class and its Basic Unit Tests * Added Changes Suggested by jianglai * Fixed Gitignore to take out AutoValue generated code * Fixed Gitignore to take out AutoValue generated code * Removed AutoValue java files * Added gitignore within prober * Removed all generated java * Final Changes in .gitignore * Final Changes in .gitignore * Added Ssl and WebWhois Action Handlers and their unit tests in addition to the ProbingAction class * Fixed build.gradle changes requested * Removed Files irrelevant to current pull request * Fixed changes suggested by CydeWeys * Fixed changes suggested by CydeWeys * Fixed changes suggested by CydeWeys * Fixed changes suggested by CydeWeys * Minor fixes to ActionHandler, as responded in comments, removed package-info, and updated settings.gradle * Minor fixes to ActionHandler, as responded in comments, removed package-info, and updated settings.gradle * Fully Updated ActionHandler (missing updated JavaDoc) * Added changed Protocol and both Inbound and Outbound Markers * Removed AutoVaue ignore clause from .gitignore * Removed AutoVaue ignore clause from .gitignore * removed unneccessary dependencies in build.gradle * Fixed Javadoc and comments for ActionHandler * Fixed comments and JavaDoc on other files * EOL added * Removed Unnecessary Files * fixed .gradle files styles * Removed outbound message from ActionHandler's fields and renamed Marker Interfaces * Fixed javadoc for Marker Interfaced * Modified Comments on ActionHandler * Removed LocalAddress from Protocol * Fixed Travis Build Issues * Rebased to Master and added in modified Handlers and ProbingAction * Rebased to Master and added in modified Handlers and ProbingAction * Rebased to Master and added in modified Handlers and ProbingAction * Rebased to Master and added in modified Handlers and ProbingAction * Rebased to Master and added in modified Handlers and ProbingAction * Added missing license headers and JavaDoc * Added missing license headers and JavaDoc * Added missing license headers and JavaDoc * Added missing license headers and JavaDoc * Added missing license headers and JavaDoc * Minor fix in NewChannelAction JavaDoc * Minor fix in NewChannelAction JavaDoc * Minor fix in NewChannelAction JavaDoc * Minor fix in NewChannelAction JavaDoc * Minor fix in NewChannelAction JavaDoc * Minor Style Fix * Minor Style Fix * Minor Style Fix * Minor Style Fix * Full WebWhoIs Sequence Added * Full WebWhoIs Sequence Added * Full WebWhoIs Sequence Added * Full WebWhoIs Sequence Added * Full WebWhoIs Sequence Added * Full WebWhoIs Sequence Added * fixed build issues * fixed build issues * fixed build issues * fixed build issues * fixed build issues * Refactored by responses suggested by jianglai. * Refactored by responses suggested by jianglai. * Refactored by responses suggested by jianglai. * Refactored by responses suggested by jianglai. * Refactored by responses suggested by jianglai. * Refactored by responses suggested by jianglai. * Minor Style Fixes * Minor Style Fixes * Minor Style Fixes * Minor Style Fixes * Minor Style Fixes * Updated build.gradle file * Updated build.gradle file * Updated build.gradle file * Updated build.gradle file * Modified license header dates * Modified license header dates * Modified license header dates * Modified license header dates * Modified license header dates * Updated WebWhois tests. * Updated WebWhois tests. * Updated WebWhois tests. * Updated WebWhois tests. * Refactored WebWhois to accomodate jianglai's suggested changes and modified tests to reflect this refactoring * Refactored WebWhois to accomodate jianglai's suggested changes and modified tests to reflect this refactoring * Refactored WebWhois to accomodate jianglai's suggested changes and modified tests to reflect this refactoring * Refactored WebWhois to accomodate jianglai's suggested changes and modified tests to reflect this refactoring * Refactored WebWhois to accomodate jianglai's suggested changes and modified tests to reflect this refactoring * Refactored WebWhois to accomodate jianglai's suggested changes and modified tests to reflect this refactoring * SpotlessApply run to fix style issues * SpotlessApply run to fix style issues * SpotlessApply run to fix style issues * SpotlessApply run to fix style issues * SpotlessApply run to fix style issues * Added license header and newline where appropriate. * Added license header and newline where appropriate. * Added license header and newline where appropriate. * Added license header and newline where appropriate. * Added license header and newline where appropriate. * Javadoc style fix in tests and removed unused methods * Javadoc style fix in tests and removed unused methods * Javadoc style fix in tests and removed unused methods * Javadoc style fix in tests and removed unused methods * Javadoc style fix in tests and removed unused methods * Refactored ProbingAction to minimize number of unnecessary methods * Refactored ProbingAction to minimize number of unnecessary methods * Refactored ProbingAction to minimize number of unnecessary methods * Refactored ProbingAction to minimize number of unnecessary methods * Modified tests for WebWhois according to changes suggested by laijiang. * Modified tests for WebWhois according to changes suggested by laijiang. * Modified tests for WebWhois according to changes suggested by laijiang. * Modified tests for WebWhois according to changes suggested by laijiang. * Modified tests for WebWhois according to changes suggested by laijiang. * Removed TestProvider from TestUtils. * Removed TestProvider from TestUtils. * Removed TestProvider from TestUtils. * Removed TestProvider from TestUtils. * Removed TestProvider from TestUtils. * Rebased to master * Rebased to master * Updated issues in rebasing * Updated issues in rebasing * Minor style change on prober/build.gradle * Minor style change on prober/build.gradle * Fixed warnings for java compilation * Fixed warnings for java compilation * Fixed files to pass all style tests * Fixed files to pass all style tests * Fixed files to pass all style tests * Minor syle fixes after succesful rebase onto master * Fixed changes suggested by CydeWeys * Fixed changes suggested by CydeWeys * Fixed changes suggested by CydeWeys * Rebased to Master and added in modified Handlers and ProbingAction * Rebased to Master and added in modified Handlers and ProbingAction * Rebased to Master and added in modified Handlers and ProbingAction * Added missing license headers and JavaDoc * Added missing license headers and JavaDoc * Added missing license headers and JavaDoc * Minor fix in NewChannelAction JavaDoc * Minor fix in NewChannelAction JavaDoc * Minor fix in NewChannelAction JavaDoc * Minor Style Fix * Minor Style Fix * Minor Style Fix * Full WebWhoIs Sequence Added * Full WebWhoIs Sequence Added * Full WebWhoIs Sequence Added * fixed build issues * fixed build issues * fixed build issues * Refactored by responses suggested by jianglai. * Refactored by responses suggested by jianglai. * Refactored by responses suggested by jianglai. * Minor Style Fixes * Minor Style Fixes * Updated build.gradle file * Updated build.gradle file * Updated build.gradle file * Modified license header dates * Modified license header dates * Modified license header dates * Updated WebWhois tests. * Updated WebWhois tests. * Updated WebWhois tests. * Refactored WebWhois to accomodate jianglai's suggested changes and modified tests to reflect this refactoring * Refactored WebWhois to accomodate jianglai's suggested changes and modified tests to reflect this refactoring * SpotlessApply run to fix style issues * Added circular linked list to utils * Added circular linked list to utils * Added circular linked list to utils * Added circular linked list to utils * Added circular linked list to utils * License Header added * License Header added * License Header added * License Header added * License Header added * Added license header and newline where appropriate. * Added license header and newline where appropriate. * Refactored probing sequence to be circular linked list iterator * Refactored probing sequence to be circular linked list iterator * Refactored probing sequence to be circular linked list iterator * Refactored probing sequence to be circular linked list iterator * Javadoc style fix in tests and removed unused methods * Javadoc style fix in tests and removed unused methods * Javadoc style fix in tests and removed unused methods * Modified ProbingStep tests to reflect new ProbingStep structure. * Modified ProbingStep tests to reflect new ProbingStep structure. * Refactored ProbingAction to minimize number of unnecessary methods * Refactored ProbingAction to minimize number of unnecessary methods * Refactored ProbingAction to minimize number of unnecessary methods * Modified tests for WebWhois according to changes suggested by laijiang. * Modified tests for WebWhois according to changes suggested by laijiang. * Modified tests for WebWhois according to changes suggested by laijiang. * Removed TestProvider from TestUtils. * Removed TestProvider from TestUtils. * Rebased to master * ProbingStepTest modified to have fewer unnecessary helper methods * ProbingStepTest modified to have fewer unnecessary helper methods * Updated issues in rebasing * Updated issues in rebasing * Added missing license header to DefaultCircularLinkedListIterator * Fixed max column length to be 100 * Fixed max column length to be 100 * Minor changes to pass style tests * Successful rebase onto finished web-whois branch * Removed need for TestTokens with Mockito mocks of Tokens * Fixed style issues in DefaultCircularLinkedListIterator and AbstractCircularLinkedListIterator * Modified CircularList according to changes suggested by jianglai. * Merge branch 'master' into prober-circular-list * Modified ProbingSequenceTest to not expect unnecessary NullPointerException * ProbingSequence and tests modified to reflect addition of UnrecoverableStateException and restarts on failures * Modified ProbingSequence and its tests to reflect action generation and calling being put in the same try catch block
2026-09-19 14:34:18 +00:00 · 2019-08-12 18:41:50 -04:00
parent 89a44f176c
commit df5f450435
10 changed files with 697 additions and 281 deletions
@@ -135,12 +135,12 @@ public abstract class ProbingAction implements Callable<ChannelFuture> {
   * when the connection future is successful.</p>
   *
   * <p>Once the connection is successful, we establish which of the handlers in the pipeline is
-   * the {@link ActionHandler}.From that, we can obtain a future that is marked as a success when
-   * we receive an expected response from the server.</p>
+   * the {@link ActionHandler}.From that, we can obtain a future that is marked as a success when we
+   * receive an expected response from the server.</p>
   *
   * <p>Next, we set a timer set to a specified delay. After the delay has passed, we send the
-   * {@code outboundMessage} down the channel pipeline, and when we observe a success or failure,
-   * we inform the {@link ProbingStep} of this.</p>
+   * {@code outboundMessage} down the channel pipeline, and when we observe a success or failure, we
+   * inform the {@link ProbingStep} of this.</p>
   *
   * @return {@link ChannelFuture} that denotes when the action has been successfully performed.
   */
@@ -14,14 +14,25 @@

 package google.registry.monitoring.blackbox;

+import com.google.common.flogger.FluentLogger;
+import google.registry.monitoring.blackbox.exceptions.UnrecoverableStateException;
 import google.registry.monitoring.blackbox.tokens.Token;
+import google.registry.util.CircularList;
 import io.netty.bootstrap.Bootstrap;
 import io.netty.channel.AbstractChannel;
+import io.netty.channel.ChannelFuture;
 import io.netty.channel.EventLoopGroup;

 /**
 * Represents Sequence of {@link ProbingStep}s that the Prober performs in order.
 *
+ * <p>Inherits from {@link CircularList}, with element type of
+ * {@link ProbingStep} as the manner in which the sequence is carried out is similar to the {@link
+ * CircularList}. However, the {@link Builder} of {@link ProbingSequence} override
+ * {@link CircularList.AbstractBuilder} allowing for more customized flows, that are looped, but
+ * not necessarily entirely circular.
+ * Example: first -> second -> third -> fourth -> second -> third -> fourth -> second -> ...
+ * </p>
 *
 * <p>Created with {@link Builder} where we specify {@link EventLoopGroup}, {@link AbstractChannel}
 * class type, then sequentially add in the {@link ProbingStep.Builder}s in order and mark which one
@@ -31,78 +42,202 @@ import io.netty.channel.EventLoopGroup;
 * the first one is activated with the requisite {@link Token}, the {@link ProbingStep}s do the rest
 * of the work.</p>
 */
-public class ProbingSequence {
+public class ProbingSequence extends CircularList<ProbingStep> {

-  private ProbingStep firstStep;
+  private static final FluentLogger logger = FluentLogger.forEnclosingClass();

  /**
   * Each {@link ProbingSequence} requires a start token to begin running.
   */
  private Token startToken;

-  private ProbingSequence(ProbingStep firstStep, Token startToken) {
-    this.firstStep = firstStep;
+  /**
+   * Each {@link ProbingSequence} is considered to not be the last step unless specified by the
+   * {@link Builder}.
+   */
+  private boolean lastStep = false;
+
+  /**
+   * {@link ProbingSequence} object that represents first step in the sequence.
+   */
+  private ProbingSequence first;
+
+
+  /**
+   * Standard constructor for {@link ProbingSequence} in the list that assigns value and token.
+   */
+  private ProbingSequence(ProbingStep value, Token startToken) {
+    super(value);
    this.startToken = startToken;
  }

+  /**
+   * Method used in {@link Builder} to mark the last step in the sequence.
+   */
+  private void markLast() {
+    lastStep = true;
+  }
+
+  /**
+   * Obtains next {@link ProbingSequence} in sequence instead of next {@link CircularList}.
+   */
+  @Override
+  public ProbingSequence next() {
+    return (ProbingSequence) super.next();
+  }
+
+  /**
+   * Starts ProbingSequence by calling first {@code runStep} with {@code startToken}.
+   */
  public void start() {
-    // calls the first step with startToken;
-    firstStep.accept(startToken);
+    runStep(startToken);
+  }
+
+  /**
+   * Generates new {@link ProbingAction} from {@link ProbingStep}, calls the action, then retrieves
+   * the result of the action.
+   *
+   * @param token - used to generate the {@link ProbingAction} by calling {@code
+   * get().generateAction}.
+   *
+   * <p>Calls {@code runNextStep} to have next {@link ProbingSequence} call {@code runStep}
+   * with next token depending on if the current step is the last one in the sequence.
+   *
+   * <p>If unable to generate the action, or the calling the action results in an immediate error,
+   * we note an error. Otherwise, if the future marked as finished when the action is completed is
+   * marked as a success, we note a success. Otherwise, if the cause of failure will either be a
+   * failure or error. </p>
+   */
+  private void runStep(Token token) {
+    ProbingAction currentAction;
+    ChannelFuture future;
+
+    try {
+      // Attempt to generate new action. On error, move on to next step.
+      currentAction = get().generateAction(token);
+
+      // Call the generated action.
+      future = currentAction.call();
+
+    } catch (UnrecoverableStateException e) {
+      // On an UnrecoverableStateException, terminate the sequence.
+      logger.atSevere().withCause(e).log(
+          "Unrecoverable error in generating or calling action.");
+      return;
+
+    } catch (Exception e) {
+      // On any other type of error, restart the sequence at the very first step.
+      logger.atWarning().withCause(e).log("Error in generating or calling action.");
+
+      // Restart the sequence at the very first step.
+      restartSequence();
+      return;
+    }
+
+    future.addListener(f -> {
+      if (f.isSuccess()) {
+        // On a successful result, we log as a successful step, and note a success.
+        logger.atInfo().log(String.format("Successfully completed Probing Step: %s", this));
+
+      } else {
+        // On a failed result, we log the failure and note either a failure or error.
+        logger.atSevere().withCause(f.cause()).log("Did not result in future success");
+
+        // If not unrecoverable, we restart the sequence.
+        if (!(f.cause() instanceof UnrecoverableStateException)) {
+          restartSequence();
+        }
+        // Otherwise, we just terminate the full sequence.
+        return;
+      }
+
+      if (get().protocol().persistentConnection()) {
+        // If the connection is persistent, we store the channel in the token.
+        token.setChannel(currentAction.channel());
+      }
+
+      //Calls next runStep
+      runNextStep(token);
+
+    });
+  }
+
+  /**
+   * Helper method to first generate the next token, then call runStep on the next {@link
+   * ProbingSequence}.
+   */
+  private void runNextStep(Token token) {
+    token = lastStep ? token.next() : token;
+    next().runStep(token);
+  }
+
+  /**
+   * Helper method to restart the sequence at the very first step, with a channel-less {@link
+   * Token}.
+   */
+  private void restartSequence() {
+    // Gets next possible token to insure no replicated domains used.
+    Token restartToken = startToken.next();
+
+    // Makes sure channel from original token isn't passed down.
+    restartToken.setChannel(null);
+
+    // Runs the very first step with starting token.
+    first.runStep(restartToken);
  }

  /**
   * Turns {@link ProbingStep.Builder}s into fully self-dependent sequence with supplied {@link
   * Bootstrap}.
   */
-  public static class Builder {
+  public static class Builder extends CircularList.AbstractBuilder<ProbingStep, ProbingSequence> {

-    private ProbingStep currentStep;
-    private ProbingStep firstStep;
-    private ProbingStep firstRepeatedStep;
+    private ProbingSequence firstRepeatedSequenceStep;

    private Token startToken;

+    /**
+     * This Builder must also be supplied with a {@link Token} to construct a {@link
+     * ProbingSequence}.
+     */
    public Builder(Token startToken) {
      this.startToken = startToken;
    }

-    /**
-     * Adds {@link ProbingStep}, which is supplied with {@link Bootstrap}, built, and pointed to by
-     * the previous {@link ProbingStep} added.
-     */
-    public Builder addStep(ProbingStep step) {
-
-      if (currentStep == null) {
-        firstStep = step;
-      } else {
-        currentStep.nextStep(step);
-      }
-
-      currentStep = step;
-      return this;
-    }
-
    /**
     * We take special note of the first repeated step.
     */
    public Builder markFirstRepeated() {
-      firstRepeatedStep = currentStep;
+      firstRepeatedSequenceStep = current;
      return this;
    }

+    @Override
+    public Builder add(ProbingStep value) {
+      super.add(value);
+      current.first = first;
+
+      return this;
+    }
+
+    @Override
+    protected ProbingSequence create(ProbingStep value) {
+      return new ProbingSequence(value, startToken);
+    }
+
    /**
-     * Points last {@link ProbingStep} to the {@code firstRepeatedStep} and calls private
+     * Points last {@link ProbingStep} to the {@code firstRepeatedSequenceStep} and calls private
     * constructor to create {@link ProbingSequence}.
     */
+    @Override
    public ProbingSequence build() {
-      if (firstRepeatedStep == null) {
-        firstRepeatedStep = firstStep;
+      if (firstRepeatedSequenceStep == null) {
+        firstRepeatedSequenceStep = first;
      }

-      currentStep.nextStep(firstRepeatedStep);
-      currentStep.lastStep();
-      return new ProbingSequence(this.firstStep, this.startToken);
+      current.markLast();
+      current.setNext(firstRepeatedSequenceStep);
+      return first;
    }
  }
 }
-
@@ -15,13 +15,10 @@
 package google.registry.monitoring.blackbox;

 import com.google.auto.value.AutoValue;
-import com.google.common.flogger.FluentLogger;
 import google.registry.monitoring.blackbox.exceptions.UndeterminedStateException;
 import google.registry.monitoring.blackbox.messages.OutboundMessageType;
 import google.registry.monitoring.blackbox.tokens.Token;
 import io.netty.bootstrap.Bootstrap;
-import io.netty.channel.ChannelFuture;
-import java.util.function.Consumer;
 import org.joda.time.Duration;

 /**
@@ -34,15 +31,7 @@ import org.joda.time.Duration;
 * Token} and from that, generates a new {@link ProbingAction} to call.</p>
 */
@AutoValue
-public abstract class ProbingStep implements Consumer<Token> {
-
-  private static final FluentLogger logger = FluentLogger.forEnclosingClass();
-
-  /**
-   * Necessary boolean to inform when to obtain next {@link Token}
-   */
-  protected boolean isLastStep = false;
-  private ProbingStep nextStep;
+public abstract class ProbingStep {

  public static Builder builder() {
    return new AutoValue_ProbingStep.Builder();
@@ -69,22 +58,10 @@ public abstract class ProbingStep implements Consumer<Token> {
   */
  abstract Bootstrap bootstrap();

-  void lastStep() {
-    isLastStep = true;
-  }
-
-  void nextStep(ProbingStep step) {
-    this.nextStep = step;
-  }
-
-  ProbingStep nextStep() {
-    return this.nextStep;
-  }
-
  /**
   * Generates a new {@link ProbingAction} from {@code token} modified {@link OutboundMessageType}
   */
-  private ProbingAction generateAction(Token token) throws UndeterminedStateException {
+  public ProbingAction generateAction(Token token) throws UndeterminedStateException {
    OutboundMessageType message = token.modifyMessage(messageTemplate());
    ProbingAction.Builder probingActionBuilder = ProbingAction.builder()
        .setDelay(duration())
@@ -101,71 +78,6 @@ public abstract class ProbingStep implements Consumer<Token> {
    return probingActionBuilder.build();
  }

-  /**
-   * On the last step, gets the next {@link Token}. Otherwise, uses the same one.
-   */
-  private Token generateNextToken(Token token) {
-    return isLastStep ? token.next() : token;
-  }
-
-  /**
-   * Generates new {@link ProbingAction}, calls the action, then retrieves the result of the
-   * action.
-   *
-   * @param token - used to generate the {@link ProbingAction} by calling {@code generateAction}.
-   *
-   * <p>If unable to generate the action, or the calling the action results in an immediate error,
-   * we note an error. Otherwise, if the future marked as finished when the action is completed is
-   * marked as a success, we note a success. Otherwise, if the cause of failure will either be a
-   * failure or error. </p>
-   */
-  @Override
-  public void accept(Token token) {
-    ProbingAction currentAction;
-    //attempt to generate new action. On error, move on to next step
-    try {
-      currentAction = generateAction(token);
-    } catch (UndeterminedStateException e) {
-      logger.atWarning().withCause(e).log("Error in Action Generation");
-      nextStep.accept(generateNextToken(token));
-      return;
-    }
-
-    ChannelFuture future;
-    try {
-      //call the generated action
-      future = currentAction.call();
-    } catch (Exception e) {
-      //On error in calling action, log error and note an error
-      logger.atWarning().withCause(e).log("Error in Action Performed");
-
-      //Move on to next step in ProbingSequence
-      nextStep.accept(generateNextToken(token));
-      return;
-    }
-
-    future.addListener(f -> {
-      if (f.isSuccess()) {
-        //On a successful result, we log as a successful step, and not a success
-        logger.atInfo().log(String.format("Successfully completed Probing Step: %s", this));
-
-      } else {
-        //On a failed result, we log the failure and note either a failure or error
-        logger.atSevere().withCause(f.cause()).log("Did not result in future success");
-      }
-
-      if (protocol().persistentConnection()) {
-        //If the connection is persistent, we store the channel in the token
-        token.setChannel(currentAction.channel());
-      }
-
-      //Move on the the next step in the ProbingSequence
-      nextStep.accept(generateNextToken(token));
-
-
-    });
-  }
-
  @Override
  public final String toString() {
    return String.format("ProbingStep with Protocol: %s\n"
@@ -175,7 +87,7 @@ public abstract class ProbingStep implements Consumer<Token> {
  }

  /**
-   * Default {@link AutoValue.Builder} for {@link ProbingStep}.
+   * Standard {@link AutoValue.Builder} for {@link ProbingStep}.
   */
  @AutoValue.Builder
  public abstract static class Builder {
@@ -190,7 +102,5 @@ public abstract class ProbingStep implements Consumer<Token> {

    public abstract ProbingStep build();
  }
-
 }

-
@@ -23,6 +23,7 @@ import google.registry.monitoring.blackbox.handlers.WebWhoisActionHandler;
 import google.registry.monitoring.blackbox.handlers.WebWhoisMessageHandler;
 import google.registry.monitoring.blackbox.messages.HttpRequestMessage;
 import google.registry.monitoring.blackbox.tokens.WebWhoisToken;
+import google.registry.util.CircularList;
 import io.netty.bootstrap.Bootstrap;
 import io.netty.channel.Channel;
 import io.netty.channel.ChannelHandler;
@@ -189,7 +190,7 @@ public class WebWhoisModule {
      WebWhoisToken webWhoisToken) {

    return new ProbingSequence.Builder(webWhoisToken)
-        .addStep(probingStep)
+        .add(probingStep)
        .build();
  }

@@ -205,8 +206,10 @@ public class WebWhoisModule {
  @Singleton
  @Provides
  @WebWhoisProtocol
-  ImmutableList<String> provideTopLevelDomains() {
-    return ImmutableList.of("how", "soy", "xn--q9jyb4c");
+  CircularList<String> provideTopLevelDomains() {
+    return new CircularList.Builder<String>()
+        .add("how", "soy", "xn--q9jyb4c")
+        .build();
  }

  @Provides
@@ -244,6 +247,5 @@ public class WebWhoisModule {
  public @interface WebWhoisProtocol {

  }
-
-
 }
+
@@ -0,0 +1,30 @@
+// Copyright 2019 The Nomulus Authors. All Rights Reserved.
+//
+// Licensed 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 google.registry.monitoring.blackbox.exceptions;
+
+/**
+ * Exception thrown when error is severe enough that sequence cannot recover, and should be
+ * terminated as a result.
+ */
+public class UnrecoverableStateException extends UndeterminedStateException {
+
+  public UnrecoverableStateException(String msg) {
+    super(msg);
+  }
+
+  public UnrecoverableStateException(Throwable e) {
+    super(e);
+  }
+}
@@ -18,7 +18,7 @@ import com.google.common.collect.ImmutableList;
 import google.registry.monitoring.blackbox.WebWhoisModule.WebWhoisProtocol;
 import google.registry.monitoring.blackbox.exceptions.UndeterminedStateException;
 import google.registry.monitoring.blackbox.messages.OutboundMessageType;
-import java.util.Iterator;
+import google.registry.util.CircularList;
 import javax.inject.Inject;

 /**
@@ -38,26 +38,20 @@ public class WebWhoisToken extends Token {
  /**
   * {@link ImmutableList} of all top level domains to be probed.
   */
-  private final Iterator<String> topLevelDomainsIterator;
-
-  /**
-   * Current index of {@code topLevelDomains} that represents tld we are probing.
-   */
-  private String currentDomain;
+  private CircularList<String> topLevelDomainsList;

  @Inject
-  public WebWhoisToken(@WebWhoisProtocol ImmutableList<String> topLevelDomains) {
+  public WebWhoisToken(@WebWhoisProtocol CircularList<String> topLevelDomainsList) {

-    topLevelDomainsIterator = topLevelDomains.iterator();
-    currentDomain = topLevelDomainsIterator.next();
+    this.topLevelDomainsList = topLevelDomainsList;
  }

  /**
-   * Increments {@code domainsIndex} or resets it to reflect move to next top level domain.
+   * Moves on to next top level domain in {@code topLevelDomainsList}.
   */
  @Override
  public WebWhoisToken next() {
-    currentDomain = topLevelDomainsIterator.next();
+    topLevelDomainsList = topLevelDomainsList.next();
    return this;
  }

@@ -76,7 +70,7 @@ public class WebWhoisToken extends Token {
   */
  @Override
  public String host() {
-    return PREFIX + currentDomain;
+    return PREFIX + topLevelDomainsList.get();
  }
 }