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Why are changes in a producer not reflected in the consumers? (aka Library Hell)

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  • If one is developing in multiple eSpaces at a time, it may happen that a change is made to a producer eSpace (e.g. at an integration level) and there is the need to see the change reflected in a consumer eSpace. In some situations, simply republishing the consumer eSpace solves the problem; but in others, the changes don't get reflected in the consumers.

    This situation can be time-consuming and lead to frustration by the developer - who may believe there is a problem with his code and trigger a wild goose chase.

    Most of the times this problem happens is because of a reason: Library Hell. This is something that can happen as your OutSystems application becomes more complex, with more levels of consumer/producer eSpaces. Library Hell is a generalization of concepts already in use in the software industry, which are the root cause of the behavior (particularly DLL Hell and Jar Hell).

    This article explains:

    • What the concept means (both in the context of the native technologies and from an OutSystems applications perspective)

    • Common pitfalls and dealing with them;

    • Avoiding or dealing with the problem.

    The basics of deployment for an OutSystems application

    OutSystems applications are standard .NET or Java applications. In .NET, applications are deployed in IIS by deploying each eSpace as a separate Virtual Directory or VDir; in Java, they are deployed in either JBoss or Weblogic by deploying each eSpace as a separate WAR application.

    Understanding how the platform deploys eSpaces

    In OutSystems it is possible to create dependencies between applications by creating references between eSpaces; it is also possible to use Extensions to create integrations with native code and import data from external systems.

    When references are used, the eSpace or Extension that exposes the references is called a Producer; the eSpace that imports the reference is called a Consumer.

    When an OutSystems application using references is deployed, the libraries of each producer are deployed with the consumer. In .NET, libraries are DLL files - a set for each producer; in Java, libraries are JAR files - again, a set for each producer.

    A practical example

    For better understanding these concepts, a practical example can be useful. In this document, we will use a sample application with producers and consumers. The sample application is available here.

    The structure of the application is as follows:

    Image 1: Application structure

    Explaining the application:

    • APIProvider is a bottom-line producer: it exposes calls to public API

    • DataLayer is a bottom-line producer: it exposes the data of the application

    • BrokerLogic consumes both APIProvider and DataLayer to provide actions to list products

    • Purchasing consumes both APIProvider and DataLayer to provide actions to buy products

    • UI_Public is the front-end used by customers

    Based on the example above, we will now describe some scenarios and explain what happens when the platform deploys the above application.

    What libraries exist in each eSpace?

    In an IIS VDir / Java WAR, for each of the eSpaces of the sample application, the following libraries are present:

    • APIProvider: only itself

    • DataLayer: only itself

    • BrokerLogic: itself, APIProvider, DataLayer

    • Purchasing: itself, APIProvider, DataLayer

    • UI_Public: itself, APIProvider, DataLayer, BrokerLogic, Purchasing

    A library being present means that the DLL (.NET) or JAR (Java) are present in the deployment unit (IIS VDir / Java WAR).

    Below are three examples:

    • APIProvider: its library is in side the running folder for APIProvider, inside bin:

    Image 2: APIProvider library in the eSpace's IIS VDir / Java WAR

    • BrokerLogic: in a consumer, producer libraries are inside a bin2 folder. Here you can see that the libraries for APIProvider and DataLayer , the producers for BrokerLogic:

    Image 3: Producer eSpaces' libraries in BrokerLogic eSpace's IIS VDir / Java WAR

    • UI_Public: again in the bin2 folder; in this one, all the libraries are there: for APIProvider, DataLayer, BrokerLogic and Purchasing:

    Image 4: Producer eSpaces' libraries in UI_Public eSpace's IIS VDir / Java WAR

    In conclusion: for any eSpace in the system, libraries for it can be found:

    • For the own eSpace IIS VDir / Java WAR, in the bin folder (IIS VDir) or under WEB-INF/lib(Java WAR);

    • In each of the consumer eSpaces IIS VDir / Java WAR, in the bin2 folder (IIS VDir) or under WEB-INF/lib(Java WAR).

    When a request is running in the context of a consumer eSpace, calls to actions from the producers use the local copies of libraries (under bin2) and not the original copy in the producer eSpace's IIS VDir / Java War. In the example above, a call to APIProvider from BrokerLogic uses the file in Image 3; but if that call is made from UI_Public, then the file in Image 4 is used instead. If you delete the APIProvider file from Image 4, calls to APIProvider logic from UI_Public start to fail, but calls from BrokerLogic will still work.

    Changing producers

    Because of how code from producers is called, if I change the code of a producer, the change is only reflected in a consumer after that consumer has been republished.

    Meaning: if I change APIProvider and publish it, UI_Public does not get refreshed automatically - it is still using the previous library for it. Technically, only the IIS VDir / Java WAR for APIProvider was changed; the IIS VDir / Java WAR for UI_Public remains unchanged, with the previous versions of all libraries.

    To test this scenario using the sample application:

    • Access http://<myserver>/UI_Public/ . There is an option to purchase items. Upon purchase, a feedback message is returned. This message is obtained from using the Echo action from APIProvider. In this version, Echo returns the input as output;

    • Change the output of Echo in APIProvider to prepend "Simon says:" at the beginning of the output. Publish the new version APIProvider;

    • Access http://<myserver>/UI_Public/ again. A new purchase still does not echo "Simon says". This happens because the library used in UI_Public is still the previous one.

    To try to fix this scenario, the obvious solution could be to simply publish UI_Public again, so the change gets reflected. However, this is not true.

    You can test it yourself:

    • In Service Center, republish the latest version of UI_Public.

    • Access http://<myserver>/UI_Public/ again. The echo still does not have the "Simon says" part.

    You can repeat publish a few more times - it does not change anything.

    Multiple levels of producers

    The reason republishing UI_Provider does not fix this problem is because: UI_Provider is not a direct consumer of APIProvider.

    This is relevant because of the way the platform obtains libraries of producer eSpaces:

    • When the platform publishes an eSpace, it obtains dependency libraries from its direct producers. In our example: since the two producers of UI_Public are BrokerLogic and Purchasing, the platform will obtain libraries from them;

    • If the platform needs further libraries (producer of producer) they will be obtained from the direct producers. In our example: all the libraries needed by UI_Public will be obtained directly from either BrokerLogic and Purchasing, not anywhere else.

    Since there was no republish of BrokerLogic or Purchasing, their version of APIProvider library is still the old one. That version is the one included in UI_Public when it is published, and so it continues to use the outdated version.

    Service Center warns about this: when republishing UI_Public, one gets warnings that both BrokerLogic and Purchasing are outdated. You also get the warning that UI_Public will be outdated when you published APIProvider initially.

    An important question arises:

    From which producer (*BrokerLogic or *Purchasing) is the platform obtaining the libraries for APIProvider?

    The answer is: in this case, it is undefined.

    How does it work generically for the platform?

    • If an eSpace / Extension is a direct producer, the library is obtained directly from it;

    • If an eSpace / Extension is not a direct producer, but is only a producer of one of the direct producers, that is the place where the library comes from. In our example: if only BrokerLogic consumed APIProvider, the library for APIProvider would come from BrokerLogic;

    • If (as our example) the eSpace / Extension is not a direct producer and is a producer of more than one of the direct producers, then the library can come from any of the producers that consume it.

    In our example, since there are only two direct producers, if you republish the two and then republish UI_Public the change gets reflected and purchases start echoing "Simon says".

    But in more complex reference scenarios, this may be extremely complex or even impossible (for example, in certain scenarios of circular references, which OutSystems allows using).

    How to solve a producer library inconsistency?

    To solve producer library inconsistencies, the use of "solution packs" is required.

    Publishing a "solution pack" publishes all the involved modules (eSpaces and Extensions) in an atomic operation: all published modules get exactly the same versions of all libraries being deployed.

    Solution packs are the technical mechanism used for:

    • Solution publish in Service Center (via OSP files)

    • Application publish of Forge components or apps downloaded in Service Center (via OAP packages)

    • LifeTime deployments (which uses OSP files underneath).

    Dealing with frequent changes to "core producers"

    There are development scenarios in which one needs to frequently change a "core producer" (in our example, APIProvider) and need to see the change reflected in the top-level Consumers (in our example, UI_Public).

    The appropriate solution is using "solutions packs". This can however be time-consuming and impractical.

    To avoid constantly publishing solution packs in these scenarios, there are two options:

    1. The architecture change: deal with these dependencies as independent "micro-services" by using loose referencing. From an architecture standpoint, this means treating the dependency as an external micro-service with a separate lifecycle, rather than a simple dependency of your project. This article explains how to obtain a Micro-services architecture in OutSystems. In practical terms, instead of referencing through public action, this means referencing by consuming a web-service / REST service call. That way, one simply need to publish the service producer and the change is immediately reflected in the consumer - no publish required. From a platform perspective, a web-service/REST service call is an external integration - so the consumer is not referencing any libraries of the producer.

    This solution may pose other concerns: it is not transactional (the web-service logic runs in a separate transaction, so uncommitted changes from the original logic will not be visible), and can incur a performance penalty for the web-service call (especially if the service is called multiple times in the same request).

    1. The temporary hack: add a direct reference from your top-level consumer (e.g. UI_Public) to your core producer (e.g. APIProvider). Even if that reference is not used in your code, by having this reference the platform is forced to obtain the library directly from the producer. This should be seen as an emergency solution only: widespread use will make the reference model unmanageable. Wear with care!
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