Collecting Information and Logging in the Java SDK with Couchbase Server

    The Java SDK logs events and also provides an event bus that transmits information about the behavior of your database system, including system and metric events. It has no hard dependency on a specific logger implementation, but you should add one you are comfortable with.


    The Couchbase Java SDK has no hard dependency on a specific logger implementation. It tries to find a logger on the class path and uses that logger if it is supported by the SDK. If no logger implementation is found, the standard JDK logger is used.

    The following loggers are supported (and tried in this order):

    1. SLF4J

    2. Log4J

    3. JDK Logger (java.util.logging)

    Configuring SLF4J

    To enable SLF4J, put it on the class path, as well as one of the support logger implementations (like logback). If you want to use logback and include logback-classic, it will be pulled in automatically:


    By default, the log level for logback is set to DEBUG, but with the addition of a logback configuration this can be configured (for example, as a logback.xml in the resources folder):

      <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
          <pattern>%d{HH:mm:ss.SSS} [%thread] %-5level %logger{36} - %msg%n</pattern>
      <root level="info">
        <appender-ref ref="STDOUT" />

    Consult the SLF4J documentation for advanced configuration.

    Configuring Log4J

    Log4J is also automatically found when put on the classpath:


    If no configuration is applied, the following message appears:

    log4j:WARN No appenders could be found for logger (com.couchbase.client.core.logging.CouchbaseLoggerFactory).
    log4j:WARN Please initialize the log4j system properly.
    log4j:WARN See for more info.

    This log4j.xml sets it to INFO level:

    <?xml version="1.0" encoding="UTF-8" ?>
    <!DOCTYPE log4j:configuration SYSTEM "log4j.dtd">
    <log4j:configuration xmlns:log4j=''>
      <appender name="CA" class="org.apache.log4j.ConsoleAppender">
        <layout class="org.apache.log4j.PatternLayout">
          <param name="ConversionPattern" value="%-4r [%t] %-5p %c %x - %m%n" />
        <level value="info" />
        <appender-ref ref="CA" />

    Alternatively, you can provide a file with similar settings:

    log4j.rootLogger=INFO, CA
    log4j.appender.CA.layout.ConversionPattern=%-4r [%t] %-5p %c %x - %m%n

    Consult the Log4J documentation for more information and advanced configuration options.

    Configuring the JDK Logger

    If no logging library is found on the class path, the JDK logger (also known as JUL from java.util.logging) is used as a fallback.

    By default it logs INFO level and above. If you want to set it to DEBUG (or the JUL equivalent: Fine) you can do it like this programmatically before initializing the CouchbaseCluster object:

    Logger logger = Logger.getLogger("com.couchbase.client");
    for(Handler h : logger.getParent().getHandlers()) {
    	if(h instanceof ConsoleHandler){

    You should not use JUL in production because SLF4J and Log4J provide better configuration options and performance.

    Manual Overriding

    If different loggers are on the class path and you want to force a different one (for example Log4J instead of SLF4J) you can set the default one directly:

    CouchbaseLoggerFactory.setDefaultFactory(new Log4JLoggerFactory());

    There are a few loggers you may have particular interest in with Couchbase. These are available at the uncommitted interface level:

    # the following forces logging of BucketManager's N1QL index watcher steps
    # the following avoid seeing Node connect/disconnect logs

    The Event Bus

    Log files are neither fun to wade through, nor do they have any kind of real-time aspect. To make them usable, normally their content is piped into systems such as Graphite or Logstash. Since most setups interleave all different kinds of log messages, it makes it very hard to see whats going on, let alone perform post-disaster analysis.

    To make the situation better and ultimately improve supportability, the Java SDK provides you with the following:

    • An event bus that allows you to receive system and metric events as they are created

    • System and metric events generated by the SDK that allow near real-time insight and consumability

    It may not be visible instantly that this provides a huge leap forward in detecting and reacting to different kinds of situations. You can subscribe to the event bus, receive and react to events as they are happening; not when someone parses the logs, sends them into another system where an alarm is triggered and eventually a sysadmin will check whats going on. The time delta between an event happening and reacting to it can be massively decreased.

    The event bus provides the foundational piece of code through which the SDK publishes events and the user receives them. Because Observable semantics fit really well here, it is exposed as such through the environment.

    The following code subscribes to the event bus and prints out all events that are published on it:

    // Create the Environment
    CouchbaseEnvironment env = DefaultCouchbaseEnvironment.create();
    // Subscribe to the Event Bus and print out all events
    // Connect to the cluster, open the bucket and then disconnect to generate events.
    Cluster cluster = CouchbaseCluster.create(env, "");
    Bucket bucket = cluster.openBucket("travel-sample");

    Connected to a three-node cluster, this will print the following events (in your IDE probably interleaved with the regular logs):

       clusterNodes=[vnode1/, vnode2/, vnode3/]}
    ConfigUpdatedEvent{bucketNames=[], clusterNodes=[]}

    We recommend filtering on the specific events you are interested in, since most of the time only a subset of the published ones will be of use to you. You can use the Observable#filter method to help with that. Of course you can subscribe as often as you want if you need to move different events to different downstream systems.

    // Only care about system events (no metrics)
        .filter(ev -> ev.type().equals(EventType.SYSTEM))


    Printing events is not that fancy, so the SDK comes bundled with a few consumers (with the plan to add more in the future). They are sample implementations for common use cases and also provide inspiration to write your own. The only rule to keep in mind is: do not block in your subscriber without moving it over to a different scheduler. This is true for RxJava in general, but if you add a subscriber and then use a blocking API to push the data into a remote system, you can probably hold up the whole thing. So either use non-blocking code all the way down or make sure to move it onto a different scheduler (for example through.observeOn(

    The SDK ships with the following consumers:

    • LoggingConsumer: all events will be logged to a file on the configured log level. In addition, if not configured otherwise all events will be turned into JSON so it is both human and machine readable.

    • BucketConsumer: all events will be written into a Couchbase Bucket as JSON documents. You can use this to demo the event bus or write to a bucket that collects actual log data.

    Using them is easy, just subscribe with the consumer:

    // Log system events
        .filter(ev -> ev.type().equals(EventType.SYSTEM))
    // Store metric events in a couchbase bucket
        .filter(ev -> ev.type().equals(EventType.METRIC))

    We are planning on more out-of-the-box consumers (mainly based on user feedback and requests), especially one that exposes the data over MXBeans.

    Default metric logging consumer

    If we wouldn’t enable them by default in some way, the whole thing would entirely miss the point. System events are logged anyways, but metric events would never be visible.

    In order to mitigate that, a logging consumer which listens on the metric events and prints them to the configured logger every hour is run by default. If you want to tune the intervals, see the next sections on configuration.

    The logger itself can be tuned (or disabled) on the environment. The following configuration changes the compact JSON format to something more verbose, but decreases the default log level (INFO) to debug:

    CouchbaseEnvironment env = DefaultCouchbaseEnvironment
        .defaultMetricsLoggingConsumer(true, CouchbaseLogLevel.DEBUG, LoggingConsumer.OutputFormat.JSON_PRETTY)

    If you want to disable it completely, set the first argument to false (and just set any logging level you want, it won’t be used:)

    CouchbaseEnvironment env = DefaultCouchbaseEnvironment
        .defaultMetricsLoggingConsumer(false, CouchbaseLogLevel.INFO)

    This default logger is really meant to have something reusable in the application logs. If you are planning to do something more fancy with metric information anyways, you can disable the default one and then attach your custom consumers (or one that comes with the SDK).

    Configurations and defaults

    By default, system events will be published as they happen and metric events are published every hour (to not flood the logs in production systems).

    This section describes the configuration options available, the next sections dig into the actual events and the data you can gather from them.

    If you need a more recent insight into metric events you can tune them through the environment. Currently you can tune the settings for the following metric collectors (which emit the events eventually):

    • RuntimeMetricsCollector

    • NetworkLatencyMetricsCollector

    Configuring the RuntimeMetricsCollector

    The RuntimeMetricsCollector collects metrics from the JVM runtime and can be configured on the CouchbaseEnvironment. By default it will emit an event every hour, but you can tune the emit frequency and its timeout very easily.

    The following code sets it from once per hour to once per minute:

    CouchbaseEnvironment env = DefaultCouchbaseEnvironment
        .runtimeMetricsCollectorConfig(DefaultMetricsCollectorConfig.create(1, TimeUnit.MINUTES))

    You can also easily disable collection and emission completely:

    CouchbaseEnvironment env = DefaultCouchbaseEnvironment

    Configuring the NetworkLatencyMetricsCollector

    The NetworkLatencyMetricsCollector collects operation timings on a very granular basis, so there is a little more you can configure in addition to the emit interval. If you just want to do that, it is identical to the RuntimeMetricsCollector:

    CouchbaseEnvironment env = DefaultCouchbaseEnvironment
        .networkLatencyMetricsCollectorConfig(DefaultLatencyMetricsCollectorConfig.create(1, TimeUnit.MINUTES))

    You can also use the disabled method for easy disabling of the whole collection process. In addition to the emit interval, you can tune parameters such as the target percentiles and the target unit. For this the same builder API is available like on the environment itself. So if you want to use milliseconds instead of microseconds (the default) and you only care about the 99 percentile (instead of 50.0, 90.0, 95.0, 99.0 and 99.9 as the default), you can configure it like this:

    CouchbaseEnvironment env = DefaultCouchbaseEnvironment
            .targetPercentiles(new Double[] { 99.0 })

    The great thing about this approach is that because the data is aggregated in a histogram, you can pick whatever percentiles and target unit you need for your analysis.

    System events

    System events are emitted as they happen inside the SDK. They are intended to provide insight into what the "system" is doing. If you are only interested in system events, you can filter them by their type: EventType.SYSTEM. Right now, the following events can be emitted, depending on the current state of the client:

    Event Description Contains


    A bucket has been opened.

    The bucket name.


    A bucket has been closed.

    The bucket name.


    A node has been connected.

    The InetAddr of the node.


    A node has been disconnected.

    The InetAddr of the node.


    A new configuration has been applied on the client.

    All open buckets and all known nodes.

    Since the bucket events are most of the time triggered by the application itself, they are mainly used for debugging. Especially theNodeDisconnectedEvent can be helpful to send over to a monitoring system, but keep in mind that even during a planned rebalance out at some point this event will be raised once the node is gone.

    Metric events

    Metric events (in contrast to system events) are emitted at a fixed frequency that can be tuned by the user. Right now these metric events are emitted:

    • RuntimeMetricsEvent

    • NetworkLatencyMetricsEvent

    The RuntimeMetricsEvent

    Run-time metrics are designed to capture the run-time state at the time of emission. The metric contains information about GC, Memory and Threads which it collects through the systems MBeans. It makes use of the internal Diagnostics class which collects the information in a transparent way and then creates the RuntimeMetricsEvent for you to consume.

    Note that if the collection is disabled on the environment, there is no run-time overhead whatsoever (since the collection is triggered on emission every interval).

    The following output is from the LoggingConsumer, which turns the message itself into a JSON object so it is nicely formatted. Of course, you can access all of that through the event object itself.

      "heap.used" : {
        "init" : 268435456,
        "used" : 17541192,
        "committed" : 257425408,
        "max" : 3817865216
      " marksweep.collectionTime" : 0,
      " scavenge.collectionTime" : 23,
      " scavenge.collectionCount" : 3,
      "thread.count" : 31,
      "offHeap.used" : {
        "init" : 2555904,
        "used" : 29304560,
        "committed" : 29949952,
        "max" : -1
      " marksweep.collectionCount" : 0,
      "heap.pendingFinalize" : 0,
      "thread.peakCount" : 31,
      "event" : {
        "name" : "RuntimeMetrics",
        "type" : "METRIC"
      "thread.startedCount" : 32

    Gathering this information over a longer period of time gives you insight in system trends even if you don’t have full blown monitoring on every application server.

    The NetworkLatencyMetricsEvent

    The NetworkLatencyMetricsEvent is the most involved event of all when it comes to creation. Because latency metrics need to be gathered on every request/response and then aggregated on emission, the SDK uses some excellent libraries under the covers (HdrHistogram and the corresponding LatencyUtils) and repackages them so you don’t run into classpath issues.

    The I/O event loops will track timestamps after encoding and before decoding and then send them into individual histograms located in a central collector. Note that the collected times still include time in the netty event loops, the JVM and the operating system. Real network time can only be collected by external parties through tools like tcpdump. At emission time, the data is taken, aggregated and the event is published onto the event bus. Note that because the way the underlying histograms work, the data is cleaned on every emission, meaning that if you do 10 second interval emits you exclusively get the data for the last 10 second interval.

    To provide as much insight as possible into what’s going on in the layer below, the data is collected an formatted in a tree-like structure. The format always is:

    Node > Service Type > Request Type > Response > Status

    The components in this format are:

    • Node: The host name of the target server in the cluster.

    • Service Type: The service on each node. Right now there are BINARY (key/value), VIEW, QUERY (N1QL), CONFIG (internal 8091 communication) and DCP.

    • Request Type: The type of request you are sending, for example GetRequest, or UpsertRequest.

    • Response Status: The raw response status the server returned, for example SUCCESS, EXISTS or many more types. You can find all of them on the com.couchbase.client.core.message.ResponseStatus enum.

    Because metrics are collected at a very low level, some mappings might not feel familiar to you, because the java client maps them into proper exceptions and so forth. Also, you’ll see requests in the dump which you didn’t command but that are part of the underlying communication, for example a GetBucketConfigurationRequest and so forth.

    So taking our new knowledge to the test, here is some output from the metrics, collected over a 30 second interval. What can we gather from it?

      "": {
        "BINARY": {
          "GetRequest": {
            "NOT_EXISTS": {
              "metrics": {
                "percentiles": {
                  "50.0": 124,
                  "90.0": 199,
                  "95.0": 234,
                  "99.0": 489,
                  "99.9": 3784
                "min": 69,
                "max": 175112,
                "count": 74864,
                "timeUnit": "MICROSECONDS"
          "UpsertRequest": {
            "SUCCESS": {
              "metrics": {
                "percentiles": {
                  "50.0": 1269,
                  "90.0": 1835,
                  "95.0": 2670,
                  "99.0": 6160,
                  "99.9": 6389
                "min": 561,
                "max": 6389,
                "count": 145,
                "timeUnit": "MICROSECONDS"
          "GetBucketConfigRequest": {
            "SUCCESS": {
              "metrics": {
                "percentiles": {
                  "50.0": 741,
                  "90.0": 741,
                  "95.0": 741,
                  "99.0": 741,
                  "99.9": 741
                "min": 737,
                "max": 741,
                "count": 1,
                "timeUnit": "MICROSECONDS"
      "": {
        "BINARY": {
          "GetRequest": {
            "NOT_EXISTS": {
              "metrics": {
                "percentiles": {
                  "50.0": 123,
                  "90.0": 198,
                  "95.0": 236,
                  "99.0": 6060,
                  "99.9": 40460
                "min": 69,
                "max": 1520430,
                "count": 73996,
                "timeUnit": "MICROSECONDS"
          "GetBucketConfigRequest": {
            "SUCCESS": {
              "metrics": {
                "percentiles": {
                  "50.0": 366,
                  "90.0": 4489,
                  "95.0": 4489,
                  "99.0": 4489,
                  "99.9": 4489
                "min": 364,
                "max": 4489,
                "count": 2,
                "timeUnit": "MICROSECONDS"
      "event": {
        "name": "NetworkLatencyMetrics",
        "type": "METRIC"

    If you take a look at the numbers, here is what you can infer from these metrics:

    1. Our get requests are evenly distributed over the 2 nodes, but they are all not finding the document (NOT_EXISTS). In the Java API, this means returning null on a get operation.

    2. We have a very high get to mutation ratio. In this interval we had over 148900 get requests, but only 145 upsert requests.

    3. Get requests on have significant outliers (check out the max), while on they don’t. At this point we might want to start investigate the server side logs and stats and see what’s going on this specific node.

    4. The Upsert operations did all go through well and don’t have latency outliers, but they all seem to be against one node and not two.

    Looking at the sample code that produces the metrics, you can see that items 2 and 4 in the previous list are explainable while items 1 and 3 were discovered based on the metrics (and can point to potential issues).

    while(true) {
        for (int i = 0; i < 1024; i++) {
            metricsBucket.get("key" +i); // this doc has never been created
            // once every loop we create the document, but always with the same id
            // so it goes to the same node, naturally.
            if (i == 500) {