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I am working on a project in which I construct a URL with a valid hostname (but not a blocked hostname) and then execute that URL using RestTemplate from my main application thread. I also have a single background thread in my application which parses the data from the url and extracts the block list of hostnames from it.

So if any block list of hostnames is present, then I won't make a call to that hostname from the main thread and I will try making a call to another hostname.

By block list, I mean whenever any server is down, its hostname is on the block list.

Below is my background thread code. It will get the data from my service URL and keep on running every 10 minutes once my application has started up. It will then parse the data coming from the URL and store it in a ClientData class variable -

TempScheduler
public class TempScheduler {

    // .. scheduledexecutors service code to start the background thread

    // call the service and get the data and then parse 
    // the response.
    private void callServiceURL() {
        String url = "url";
        RestTemplate restTemplate = new RestTemplate();
        String response = restTemplate.getForObject(url, String.class);
        parseResponse(response);

    }

    // parse the response and store it in a variable
    private void parseResponse(String response) {
        //...       

        // get the block list of hostnames
        Map<String, List<String>> coloExceptionList = gson.fromJson(response.split("blocklist=")[1], Map.class);
        List<String> blockList = new ArrayList<String>();
        for(Map.Entry<String, List<String>> entry : coloExceptionList.entrySet()) {
            for(String hosts : entry.getValue()) {
                blockList.add(hosts);
            }
        }

        // store the block list of hostnames which I am not supposed to make a call
        // from my main application
        ClientData.replaceBlockedHosts(blockList);
    }
}

Below is my ClientData class. replaceBlockedHosts method will only be called by a background thread meaning only one writer. But isHostBlocked will be called by main application threads multiple times to check whether the hostname is blocked or not. And also blockHost method might be called from catch block multiple times to add the down host in the blockedHosts list so I need to make sure all the read threads can see the consistent data and are not making calls to that down host, instead they are making calls to next host in the hostnames linked list.

ClientData
public class ClientData {

    // .. some other variables here which in turn used to decide the  list of hostnames

    private static final AtomicReference<ConcurrentHashMap<String, String>> blockedHosts = 
            new AtomicReference<ConcurrentHashMap<String, String>>(new ConcurrentHashMap<String, String>());

    public static boolean isHostBlocked(String hostName) {
        return blockedHosts.get().containsKey(hostName);
    }

    public static void blockHost(String hostName) {
        blockedHosts.get().put(hostName, hostName);
    }

    public static void replaceBlockedHosts(List<String> hostNames) {
        ConcurrentHashMap<String, String> newBlockedHosts = new ConcurrentHashMap<>();
        for (String hostName : hostNames) {
            newBlockedHosts.put(hostName, hostName);
        }
        blockedHosts.set(newBlockedHosts);
    }
}

And below is my main application thread code in which I am finding all the hostnames on which I can make a call and then iterate the hostnames list to make a call.

If that hostname is null or in the block list category, then I won't make a call to that particular hostname and will try next hostname in the list.

@Override
public DataResponse call() throws Exception {

    List<String> hostnames = new LinkedList<String>();

    // .. some separate code here to populate the hostnames list
    // from ClientData class

    for (String hostname : hostnames) {     

        // If host name is null or host name is in block list category, skip sending request to this host
        if (hostname == null || ClientData.isHostBlocked(hostname)) {
            continue;
        }

        try {
            String url = generateURL(hostname);

            response = restTemplate.getForObject(url, String.class);

            break;
        } catch (RestClientException ex) {
            // does this call will be thread safe for blockHost method in ClientData class?
            ClientData.blockHost(hostname);
        }
    }
}

I don't need to make a call to the hostname whenever it is down from the main thread. And my background thread gets these detail from one of my service, whenever any server is down, it will have the list of hostnames which are block hosts and whenever they are up, that list will get updated.

And also, whenever any RestClientException is being thrown, I will add that hostname in the blockedHosts concurrentmap since my background thread is running every 10 minutes so that map won't have this hostname until 10 minutes is done. And whenever this server came back up, my background will update this list automatically.

Should I have fully atomic operations so that all the main threads should see consistent data from isHostBlocked method?

I'm opting for code review to see whether full atomicity is maintained or not in my above code.

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To answer the issue you raised in a comment:

I have this code running for a while in production and the problem I am seeing is as soon as hostA goes down, I start seeing lot of java.util.concurrent.TimeoutException on my client as you can see I am using Callable so which means I am doing future.get somewhere and it throws TimeoutException as soon as any hosts goes down but if all the hosts are up then everything works like a charm without any java.util.concurrent.TimeoutException which makes me think that something is wrong when any host goes down and gets added to block list so this is puzzling me a little bit

(Also, holy run-on sentences, Batman!)

A possible scenario for the time-outs is as follows:

  • You have 4 threads, A through D, connecting to server api-2.acme.com.
  • Thread B throws a RestException for some reason, and marks api-2.acme.com as unreachable.
  • However, threads A, C, and D are not notified of this, and continue to attempt connecting to api-2.acme.com, leading either to a time-out or an exception of their own.

You could try cancelling the attempts of A, C, and D through Future.cancel(true), which usually issues a thread interrupt that, in turn, would lead to an InterruptedIOException from the socket. Or you could leave them be: maybe the error was transient—a hiccup—and the other threads make it through just fine.


Thread-safe?

Your code is thread-safe in the sense that it is maintains consistency under concurrency, but it runs the risk of lost updates.

replaceBlockedHosts builds a new map and then replaces the current map with it. But any intermediate changes made to the old map are lost when this happens, so if a thread just reported failure, it's gone. I see two quick ways to deal with that issue:

  1. Block until the replacement is complete.

  2. Add timestamps to your blocklists. Ignore commands with an earlier timestamp than your list/map currently holds; after all, it is already holding more recent data.

Timestamping failed attempts allows you more fine-grained control with retry policies. It also obviates the need for a timed check-and-replace, so that saves you from needing a replace function at all.

I've added a rudimentary implementation that simply marks the host as unavailable for 10 minutes. Note that setHostUnavailable uses atomic retrieve-and-replace calls to handle concurrency.

static final long OUTAGE_RETRY_MILLIS = TimeUnit.MILLISECONDS.convert(10, TimeUnit.MINUTES);

final ConcurrentMap<String, Long> failedAttempts;

/* For now, return a constant value.  Change this in case we need to have variable retry delays,
 * for instance, depending on recent failures. */
public long getRetryDelayInMillis(String host) {
  return OUTAGE_RETRY_MILLIS;
}

public boolean isHostBlocked(String host, long currentTickMillis) {
  final Long lastOutageMillis = failedAttempts.get(host);
  return lastOutageMillis != null && (currentTickMillis - lastOutageMillis) < getRetryDelayInMillis(host);
}

public void setHostUnavailable(String host, long currentTickMillis) {
  /* Loop until result written away without interference. */
  while (true) {
    final Long lastOutageMillis = failedAttempts.get(host);
    if ( lastOutageMillis == null ) {
      if ( failedAttempts.putIfAbsent(host, currentTickMillis) == null ) {
        break;
      }
    } else if ( lastOutageMillis < currentTickMillis ) {
      if ( failedAttempts.replace(host, lastOutageMillis, currentTickMillis) ) {
        break;
      }
    } else { // lastOutageMillis >= currentTickMillis
      break; // another thread seems to have written more recent results
    }
  }
}

Replacing the blocklist

I will be calling setHostUnavailable from my background thread right? [My] background thread will have a list of block hostnames so I need to pass the list of block hostnames to setHostUnavailable which accepts String as of now.

It depends on the semantics of the check your background thread (the every-ten-minutes one) does. If the idea is that these hosts should be added to the blocklist, then it is a simple matter of looping over setHostUnavailable. If additionally hosts not in the line-up need to be unblocked, then you need a way to remove them from the blacklist:

public void replaceBlockedHosts(Set<String> hosts, long currentTickMillis) {
  for ( final String hostToBlock : hosts ) {
    setHostUnavailable(blockedHost, currentTickMillis);
  }

  // Now unblock other hosts
  final Set<String> hostsToUnblock = new Hashset<>(failedAttempts.keySet());
  hostsToUnblock.removeAll(hosts);
  for ( final String blockedHost : hostsToUnblock ) {
    setHostAvailable(blockedHost, currentTickMillis);
  }
}

public void setHostAvailable(String host, long currentTickMillis) {
  /* Loop until result written away without interference. */
  while (true) {
    final Long lastOutageMillis = failedAttempts.get(host);
    if ( lastOutageMillis == null || lastOutageMillis > currentTickMillis ) {
      break; // no outage or outdated results, break out
    } else { // lastOutageMillis <= currentTickMillis
      if ( failedAttempts.remove(host, lastOutageMillis) ) {
        break;
      }
    }
  }
}
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  • \$\begingroup\$ @JvR Thanks for suggestion. Quick question, I will be calling setHostUnavailable from my background thread right? And also, do you think I should have Http Request timeout as well on my RestTemplate along with future timeout? \$\endgroup\$ – arsenal Sep 6 '14 at 15:51
  • \$\begingroup\$ @Webby Call setHostUnavailable from any thread that has just discovered the host is unreachable. \$\endgroup\$ – JvR Sep 6 '14 at 16:35
  • \$\begingroup\$ @JvR sure but in my case background thread will have a list of block hostnames so I need to pass the list of block hostnames to setHostUnavailable which accepts String as of now. \$\endgroup\$ – arsenal Sep 7 '14 at 3:08
  • \$\begingroup\$ @Webby I've (hopefully) addressed your question in the answer above. \$\endgroup\$ – JvR Sep 7 '14 at 14:17
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Your code is thread-safe as it is. [But not really... see the comments below.]

  • Be careful with method and variable names. TempScheduler does not really have anything temporary. callServiceURL does a lot more than calling a URL. blockHost(hostname) would be better as addToBlockedHosts(hostname). call is non-descriptive. ClientData only contains information related to host names.

  • Try to have each method doing only one thing. For example, callServiceURL should just return some json. Then parseResponse should take that json and return a list of host names. Finally, some other functions should add the host names to the set of blocked hosts.

  • volatile has the same effect as AtomicReference and makes the code simpler. See this post.

  • Instead of either AtomicReference or volatile you could just have a final ConcurrentHashMap which you would clear when you receive a new list.

  • Your non-class ClientData with only static methods makes me cringe. Please make it a normal class. Create an instance of ClientData in the main thread and pass it to the secondary thread when you create it.

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  • \$\begingroup\$ Thanks toto for the great suggestion. I will work on making the method names better and class name as well. One thing which is bothering me, do you think the blockHost operation is atomic? Meaning if let's say hostA got added to block list by ThreadA, I want to make sure no other thread makes a call to hostA then so somehow I have a feeling it is not fully atomic? \$\endgroup\$ – arsenal Sep 3 '14 at 3:04
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    \$\begingroup\$ You are using a ConcurrentHashMap with an AtomicReference so it is absolutely thread-safe. \$\endgroup\$ – toto2 Sep 3 '14 at 4:14
  • \$\begingroup\$ hmmm, I have this code running for a while in production and the problem I am seeing is as soon as hostA goes down, I start seeing lot of java.util.concurrent.TimeoutException on my client as you can see I am using Callable so which means I am doing future.get somewhere and it throws TimeoutException as soon as any hosts goes down but if all the hosts are up then everything works like a charm without any java.util.concurrent.TimeoutException which makes me think that something is wrong when any host goes down and gets added to block list so this is puzzling me a little bit.. \$\endgroup\$ – arsenal Sep 3 '14 at 4:59
  • \$\begingroup\$ ..may be I thought blockHost operation is not fully atomic so that's why I am seeing this issue? \$\endgroup\$ – arsenal Sep 3 '14 at 5:00
  • \$\begingroup\$ I think I understand now. blockHost has to get the set and then it adds the value to it. It is possible that the set in the underlying reference is swapped during a call to blockHost. For your application, it is not an issue as you have "sloppy" requirements: The blocked hosts which you know about are never quite exactly the true set of blocked hosts. If it does happen that the set is swapped during a call to blockHost, it is effectively equivalent to having a new blocked host added to the set a fraction of a second before the set is swapped. It's not something you would care abou \$\endgroup\$ – toto2 Sep 3 '14 at 18:17
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Let's get something straight:

  1. One thread iterates over a list of hosts to run a request for each, and if the request fails it adds to a list of blocked hosts
  2. Another thread runs in the background, and every 10 minutes it gets the list of blocked hosts from a service and replaces the current list

In the current code these actions are not synchronized: while the background thread gets the new list of blocked hosts, the first thread may add hosts. If those hosts are not on the new list, then the replace operation will effectively remove them.

This might not be a real problem. It depends on if you really want to get every update or not. If you are ok with missing a few updates, that an unreachable host was not correctly added (because the replace operation wiped it out), then it's fine. If you don't want to miss updates, then you need to synchronize these two actions.

Essentially, this is the same thing that @JvR said already. And I agree with him that adding timestamps to the block lists would be a better solution than blocking during the replacement. I suggest to work with his code snippet.

Another good thing about that you were not using the values of your ConcurrentHashMap anyway.


You didn't include the code that populates the LinkedList<String> hostnames in the call method. Keep in mind that if you iterate over the block-list, if during the iteration the block-list gets replaced, there are no guarantees about the iterator in progress. The iterator's state corresponds to the elements that existed at the time of its creation. It may or may not get the updates, so you may be iterating over a list that's out of date. See this related answer for more details.


It's not related to your problem, but I find this null check a bit strange:

if (hostname == null || ClientData.isHostBlocked(hostname)) {

How would a null hostname end up in your list? I suggest to improve your code in a way that avoids adding null hostnames to the list, which doesn't make sense. And then you can drop this null check.


You could replace the AtomicReference with the volatile keyword:

  • a ConcurrentHashMap already ensures thread safety of the hashmap operations
  • the replacement of the hashmap can be safe with the volatile keyword
  • none of these can help with the logical disconnect between your 2 threads

An AtomicReference would help if you wanted to do compare-and-set operations, which is not the case here.

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  • \$\begingroup\$ Thanks janos for the suggestion. One quick thing, do you think I should have Http Request timeout as well on my RestTemplate along with future timeout? \$\endgroup\$ – arsenal Sep 6 '14 at 15:45

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