Key-Value Intro (Java)

Key-Value Intro in Key-Value

The key-value (KV) capability in NATS is an abstraction over a stream which models message subjects as keys. It uses a standard set of stream configuration to be optimized for KV workloads.

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$ nbe run kv/intro/java

View the source code or learn how to run this example yourself

Code

package example;


import io.nats.client.*;
import io.nats.client.api.*;
import io.nats.client.impl.Headers;


import java.io.IOException;
import java.util.List;


public class Main {
  public static void main(String[] args) {
    String natsURL = System.getenv("NATS_URL");
    if (natsURL == null) {
      natsURL = "nats://127.0.0.1:4222";
    }


    try (Connection nc = Nats.connect(natsURL)) {

Bucket basics

A key-value (KV) bucket is created by specifying a bucket name. Java returns a KeyValueStatus object upon creation

      KeyValueManagement kvm = nc.keyValueManagement();


      KeyValueConfiguration kvc = KeyValueConfiguration.builder()
          .name("profiles")
          .build();


      KeyValueStatus keyValueStatus = kvm.create(kvc);

Retrieve the Key Value context once the bucket is created.

      KeyValue kv = nc.keyValue("profiles");

As one would expect, the KeyValue interface provides the standard Put and Get methods. However, unlike most KV stores, a revision number of the entry is tracked.

      kv.put("sue.color", "blue".getBytes());
      KeyValueEntry entry = kv.get("sue.color");
      System.out.printf("%s %d -> %s\n", entry.getKey(), entry.getRevision(), entry.getValueAsString());


      kv.put("sue.color", "green");
      entry = kv.get("sue.color");
      System.out.printf("%s %d -> %s\n", entry.getKey(), entry.getRevision(), entry.getValueAsString());

A revision number is useful when you need to enforce optimistic concurrency control on a specific key-value entry. In short, if there are multiple actors attempting to put a new value for a key concurrently, we want to prevent the “last writer wins” behavior which is non-deterministic. To guard against this, we can use the kv.Update method and specify the expected revision. Only if this matches on the server, will the value be updated.

      try {
        kv.update("sue.color", "red".getBytes(), 1);
      }
      catch (JetStreamApiException e) {
        System.out.println(e);
      }


      long lastRevision = entry.getRevision();
      kv.update("sue.color", "red".getBytes(), lastRevision);
      entry = kv.get("sue.color");
      System.out.printf("%s %d -> %s\n", entry.getKey(), entry.getRevision(), entry.getValueAsString());

Stream abstraction

Before moving on, it is important to understand that a KV bucket is light abstraction over a standard stream. This is by design since it enables some powerful features which we will observe in a minute.

How exactly is a KV bucket modeled as a stream? When one is created, internally, a stream is created using the KV_ prefix as convention. Appropriate stream configuration are used that are optimized for the KV access patterns, so you can ignore the details.

      JetStreamManagement jsm = nc.jetStreamManagement();


      List<String> streamNames = jsm.getStreamNames();
      System.out.println(streamNames);

Since it is a normal stream, we can create a consumer and fetch messages. If we look at the subject, we will notice that first token is a special reserved prefix, the second token is the bucket name, and remaining suffix is the actually key. The bucket name is inherently a namespace for all keys and thus there is no concern for conflict across buckets. This is different from what we need to do for a stream which is to bind a set of public subjects to a stream.

      JetStream js = nc.jetStream();


      PushSubscribeOptions pso = PushSubscribeOptions.builder()
          .stream("KV_profiles").build();
      JetStreamSubscription sub = js.subscribe(">", pso);


      Message m = sub.nextMessage(100);
      System.out.printf("%s %d -> %s\n", m.getSubject(), m.metaData().streamSequence(), new String(m.getData()));

Let’s put a new value for this key and see what we get from the subscription.

      kv.put("sue.color", "yellow".getBytes());
      m = sub.nextMessage(100);
      System.out.printf("%s %d -> %s\n", m.getSubject(), m.metaData().streamSequence(), new String(m.getData()));

Unsurprisingly, we get the new updated value as a message. Since it’s a KV interface, we should be able to delete a key as well. Does this result in a new message?

      kv.delete("sue.color");
      m = sub.nextMessage(100);
      System.out.printf("%s %d -> %s\n", m.getSubject(), m.metaData().streamSequence(), new String(m.getData()));

🤔 That is useful to get a message that something happened to that key, and that this is considered a new revision. However, how do we know if the new value was set to be nil or the key was deleted? To differentiate, delete-based messages contain a header. Notice the KV-Operation: DEL header.

      System.out.println("Headers:");
      Headers headers = m.getHeaders();
      for (String key : headers.keySet()) {
        System.out.printf("  %s:%s\n", key, headers.getFirst(key));
      }

Watching for changes

Although one could subscribe to the stream directly, it is more convenient to use a KeyValueWatcher which provides a deliberate API and types for tracking changes over time.

      KeyValueWatcher watcher = new KeyValueWatcher() {
        @Override
        public void watch(KeyValueEntry entry) {
          System.out.printf("Watcher: %s %d -> %s\n", entry.getKey(), entry.getRevision(), entry.getValueAsString());
        }

The end of data signal can be useful to known when the watcher has caught up with the current updates before tracking the new ones.

        @Override
        public void endOfData() {
          System.out.println("Watcher: Received End Of Data Signal");
        }
      };

Notice that we can use a wildcard for watching keys.

      kv.watch("sue.*", watcher, KeyValueWatchOption.UPDATES_ONLY);

Even though we deleted the key, of course we can put a new value. In Java, there are a variety of Put signatures also, so here just put a string.

      kv.put("sue.color", "purple");

To finish this short intro, since we know that keys are subjects under the covers, if we put another key, we can observe the change through the watcher. One other detail to call out is notice the revision for this new key is not 1. It relies on the underlying stream’s message sequence number to indicate the revision. The guarantee being that it is always monotonically increasing, but numbers will be shared across keys (like subjects) rather than sequence numbers relative to each key.

      kv.put("sue.food", "pizza");

Sleep this thread a little so the program has time to receive all the messages before the program quits.

      Thread.sleep(500);
    } catch (InterruptedException | IOException | JetStreamApiException e) {
      e.printStackTrace();
    }
  }
}

Output

sue.color 1 -> blue
sue.color 2 -> green
io.nats.client.JetStreamApiException: wrong last sequence: 2 [10071]
sue.color 3 -> red
[KV_profiles]
$KV.profiles.sue.color 3 -> red
$KV.profiles.sue.color 4 -> yellow
$KV.profiles.sue.color 5 -> 
Headers:
  KV-Operation:DEL
Watcher: Received End Of Data Signal
Watcher: sue.color 6 -> purple
Watcher: sue.food 7 -> pizza

Recording

Note, playback is half speed to make it a bit easier to follow.