Non-Persistable Objects & Garbage Collecting

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1 Introduction

This page explains the life cycle of both persistable and non-persistable objects, and how they flow through the platform memory. In order to understand the behavior of non-persistable objects there are a few facts that you need to be aware of:

  • A non-persistable object is an object that is considered temporary and only exists in memory
  • Changed persistable objects that are not committed only exist in memory and behave similarly to non-persistable objects
  • The Mendix Platform will remove objects automatically when they are no longer “used” (the definition of “used” will be explained later)

2 Behavior

Non-persistable objects in Mendix are not kept in the Runtime Server, but maintained in the Mendix Client. This means there is no server-side garbage collection. This simplifies the handling of objects on the server side because an object will not be garbage collected while it exists on the server.

Objects will be returned to the client together with the response to a request. Objects created outside the context of a request (like Scheduled Event execution) will automatically be discarded when the scheduled event has finished.

2.1 Influencing the Impact on Response Size

As the objects that are still available are returned with the server call automatically, it is possible to reduce the response size by deleting non-persistable objects that are not useful for the client or subsequent requests. This can happen by deleting non-persistable objects or rolling back changed persistable objects.

3 Client Side Garbage Collection

The Mendix Client has a garbage collector. This garbage collector will automatically free up memory by deleting objects that are no longer used or necessary to keep in memory. Objects are used when they are visible in a widget. For non-persistable objects it also means that they are seen as in-use when other used objects refer to them. Unchanged persistable objects are removed from memory when they aren’t used because they can be reloaded from the Mendix Database when necessary.

3.1 Exceptional Cases

3.1.1 Objects Associated with Current User or Session

When non-persistable objects are associated with the current user or the current session, they (and any non-persistable objects associated with them) are not garbage collected. As such, this can function as a way for objects to survive requests, although this should be used with care as it can easily lead to a growing state.

3.1.2 Objects Which Are Parameters of Web Pages

Objects which are the parameter of a page which is closed in a web browser are only garbage collected after five new pages have been opened. This means that the end-user can use the back button in their browser (a limited number of times) and still see the same page they saw before, even if the parameter is non-persistable.

4 Tracking State Growth

As state is managed by the client, it can be hard to get an overview of all the state used by all clients in Mendix (it is not available in one place, but distributed over all the clients). However, there are means in Mendix to track state growth by observing the log files.

4.1 Observing State Growth by Session

By enabling TRACE level logging on the RequestStatistics log node, Mendix Runtime will log a message for every request that contains information about state. This information is logged in the form of a JSON structure, allowing it to be used in tooling to create graphs over time. See this example of a log statement (formatted for readability in this case):

TRACE: Request-State statistics: {
  session: "fd0771fe-8c12-49cf-8667-921058b116a3",
  action: "execute-action",
  total: 5,
  details: {
    "MyModule.MyEntity": 3,
    "AnotherModule.SomeEntity": 2

In the details section you find the number of instances per entity type available in the state of a request.

4.2 Detecting Requests with Large State

By default the Mendix Runtime will log a WARNING on the RequestStatistics log node when the request state exceeds the configured threshold. See this example of a log statement:

WARNING: Request state size of 551 objects exceeds the threshold of 500 objects. Request details: type `execute-action` in session `fd0771fe-8c12-49cf-8667-921058b116a3`. State consists of:
 * MyModule.MyEntity: 421 objects
 * AnotherModule.SomeEntity: 130 objects

This threshold can be configured with the custom setting com.mendix.webui.StateSizeWarningThreshold (the value is a number that reflects the total number of objects in the request state).

4.2.1 Choosing a Correct Threshold Level

Choosing the right level for the threshold is crucial because when it is set too low it will trigger too often and setting it too high will cause the detection of problems too late. It is meant to detect state memory leakage, which means that state grows to certain levels and does not get properly garbage collected. In some apps it is possible that some pages use a large number of non-persistable objects to show the data on-screen. In that case the threshold should be larger then the number of objects that are normally shown on this screen to prevent this warning from being logged too often.

4.2.2 Acting on Large Request State Problems

When the request state exceeds the configured threshold, you can look at the following list of possible causes (or a combination of them):

  • A problem in a widget (for example, if the widget does not unsubscribe itself from updates on objects which it showed previously)
  • Too many objects are associated with the current session or user
  • Non-persistable objects are associated with an object shown in a widget in a layout (meaning that this object stays in use as long as this layout is shown, usually a long time)

In order to find the root cause of this state size, you need to press Ctrl + Alt + G to make a state overview dump using the developer tools in the client. The results are shown in the browser console. This allows you to see the objects that are in the state and why they are not garbage collected.

5 Server-Side Memory Management

For every request to the Mendix Runtime — be it from the client or via web service calls — objects are cleaned up at the end of the request. This means that if you create a lot of temporary objects in a microflow, they will occupy Runtime memory until the end of the request.

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