Hazelcast IMDG Standard Support has expired. Extended support for version 4.1 ends in April 2024. Extended support for version 4.2 ends in September 2024.

We recommend that you try Hazelcast Platform.

In Hazelcast Platform, we’ve combined the in-memory storage of IMDG with the stream processing power of Jet. Find out more in our Platform documentation.

The following topics are a good place to start:

Instance Tracking

Instance tracking is a feature which, when enabled, writes a file on the instance startup at the configured location. The file contains metadata about the instance, such as version, product name and process ID. This file can then later be used by other programs to detect the kinds of Hazelcast instances that have been running on a particular machine by inspecting the file contents. This feature supports both IMDG Open Source and Enterprise members and clients as well as Hazelcast Jet, and is disabled by default. Failing to write the file only generates a warning, and the instance is allowed to start.

The name and content of the file are configurable and may contain placeholders. The placeholders used for instance tracking have a prefix so that they can be distinguished from the other ones like XML placeholders. We use the same style as the EncryptionReplacer by adding a "namespace" to the placeholder prefix; for example, $HZ_INSTANCE_TRACKING{start_timestamp} (the namespace here being HZ_INSTANCE_TRACKING).

In addition to the above, the Hazelcast instance overwrites any existing file in the configured location. To prevent this, you can configure the file location using the placeholders in the same way they can be used when defining the file contents. For example, if the file name is configured as Hazelcast-$HZ_INSTANCE_TRACKING{pid}-$HZ_INSTANCE_TRACKING{start_timestamp}.process, it contains the process ID and the creation time, making it unique every time the instance is started. The created file is not deleted on member shutdown. As such, it leaves a trace of instances started on a particular machine. The file creation process also is fail-safe meaning that the instance will proceed with starting even though it is unable to write the tracking file and the instance will only log a warning.

Auditing the Instance Tracking File

When you enable the instance tracking feature and its file is created on a member startup, the full path of the file with its name is set into a system property of the JVM running the Hazelcast member, i.e., hazelcast.config.instance.tracking.file.

You can audit that all your running Hazelcast members in your environment have the instance tracking file name set correctly. For this, you can use the jcmd utility As shown below.

jcmd <PID> VM.system_properties | grep hazelcast.config.instance.tracking.file

PID here is the process ID of your JVM on which Hazelcast member runs. It will give you an output similar to following:


See below for the example content of an instance tracking file.

Configuring Instance Tracking

Here is an example of programmatic member-side Java configuration:

Config config = new Config();

The equivalent declarative configuration is as follows:

  • XML

  • YAML

<hazelcast xmlns="http://www.hazelcast.com/schema/config"

    <instance-tracking enabled="true">

    enabled: true
    file-name: /tmp/hz-tracking.txt
    format-pattern: $HZ_INSTANCE_TRACKING\{product}:$HZ_INSTANCE_TRACKING\{version}

You can use this configuration to enable the instance tracking feature, specify the file name and the pattern for the file contents. By default, the feature is disabled, the file name is Hazelcast.process in the OS temporary directory as returned by System.getProperty("java.io.tmpdir") and the file contents are JSON-formatted key-value pairs of all available metadata.

The client configuration is analogous and only differs in the name of the outer configuration block or configuration instance containing the instance tracking configuration.

Here is an example when running a client instance:

{"product":"Hazelcast", "version":"4.1.7", "pid":27746, "mode":"client", "start_timestamp":1595851430741, "licensed":0}

Here is an example when running a member instance in the "server" mode:

{"product":"Hazelcast", "version":"4.1.7", "pid":27746, "mode":"server", "start_timestamp":1595851430741, "licensed":1}

And here is an example when running a member instance in the "embedded" mode:

{"product":"Hazelcast", "version":"4.1.7", "pid":27746, "mode":"embedded", "start_timestamp":1595851430741, "licensed":1}

You can specify a custom format by using a predefined set of available metadata keys an example of which is shown below:

String format = "mode: $HZ_INSTANCE_TRACKING{mode}\n"
        + "product: $HZ_INSTANCE_TRACKING\{product}\n"
        + "licensed: $HZ_INSTANCE_TRACKING{licensed}\n"
        + "missing: $HZ_INSTANCE_TRACKING{missing}\n"
        + "broken: $HZ_INSTANCE_TRACKING{broken ";

This should produce a file with the following content:

mode: embedded
product: Hazelcast
licensed: 0
missing: $HZ_INSTANCE_TRACKING{missing}
broken: $HZ_INSTANCE_TRACKING{broken

As you can see, once we encounter a broken placeholder, all subsequent placeholders are ignored. On the other hand, missing placeholders are skipped and subsequent placeholders are resolved.

The currently valid metadata placeholders and their possible values are as follows:

  • product: Instance product name, e.g., "Hazelcast" or "Hazelcast Enterprise".

  • version: Instance version.

  • mode: Instance mode, e.g., "server", "embedded" or "client"

  • start_timestamp: the timestamp of when the instance was started as the difference, measured in milliseconds, between the current time and midnight, January 1, 1970 UTC.

  • licensed: Specifies whether the instance is using a license or not. The value 0 signifies that there is no license set and the value 1 signifies that a license is in use.

  • pid: Attempts to get the process ID value. The algorithm does not guarantee to get the process ID on all JVMs and operating systems so please test before use. In case we are unable to get the PID, the value is -1.

The possible values for the product placeholder: Hazelcast, Hazelcast Enterprise, Hazelcast Client, Hazelcast Client Enterprise, Hazelcast Jet, Hazelcast Jet Enterprise.

The possible values for the mode placeholder:

  • server: This value is used when the instance was started using the start.sh or start.bat scripts.

  • client: This instance is a Hazelcast client instance.

  • embedded: This instance is embedded in another Java program.