Keycloak Client V2: Handling Unknown Fields Discussion

Introduction

In the realm of software development, managing API changes and ensuring compatibility across different versions is a critical challenge. One approach to mitigating this challenge is to allow servers to ignore unknown fields in incoming requests. This strategy can be particularly useful when removing a field from a representation without introducing a new API version, as it prevents breaking existing clients that might still be sending the outdated field. This article revisits the discussion around handling unknown fields in Keycloak Client v2, exploring the considerations, challenges, and potential solutions.

The Core Issue: Preventing Client Breakage

At the heart of this discussion lies the desire to maintain compatibility and prevent client breakage. When a server expects a specific set of fields in a request and receives additional, unknown fields, it has several options:

1. Reject the request outright, returning an error to the client.
2. Ignore the unknown fields and process the request using the known fields.
3. Attempt to handle the unknown fields, potentially through dynamic processing or logging.

The first option, rejecting the request, is the most strict and can lead to immediate breakage for clients sending the unknown fields. This is generally undesirable unless the unknown field represents a critical error or security risk. The second option, ignoring the unknown fields, offers a more lenient approach that allows older clients to continue functioning even if the server's data model has evolved. The third option, dynamic handling, is the most complex but can provide flexibility in adapting to evolving client requests.

The initial implementation in Keycloak allowed the server to ignore unknown fields, providing a degree of backward compatibility. However, this feature was later removed due to concerns around potential issues and the desire for a more explicit API evolution process. The current discussion aims to re-evaluate whether this feature should be reintroduced, considering the trade-offs between compatibility and explicitness.

Historical Context: Removal of the Feature

The ability to ignore unknown fields was initially present in Keycloak but was subsequently removed. The decision to remove this feature stemmed from several considerations:

• Ambiguity and unexpected behavior: Ignoring unknown fields can mask errors and lead to unexpected behavior if a client is sending incorrect data. For example, a client might be sending a misspelled field name, which the server would silently ignore, potentially leading to data inconsistencies or functional issues.
• API contract clarity: Explicitly defining the API contract and requiring clients to adhere to it promotes clarity and reduces the risk of miscommunication between client and server. Ignoring unknown fields can blur the lines of the API contract, making it harder to reason about and maintain.
• API evolution management: Introducing new API versions provides a clear mechanism for managing changes and deprecating older features. Relying on ignoring unknown fields as a primary means of API evolution can lead to a fragmented and difficult-to-manage API landscape.

These concerns led to the removal of the feature in https://github.com/keycloak/keycloak/issues/43728, with the intention of enforcing a stricter API contract and promoting a more controlled API evolution process.

The Use Case: Smooth API Evolution

Despite the concerns that led to its removal, the ability to ignore unknown fields offers a compelling advantage: it facilitates smoother API evolution. In many real-world scenarios, APIs need to evolve over time to accommodate new features, changing requirements, and evolving data models. However, introducing breaking changes can be disruptive to existing clients, requiring them to update their code and potentially leading to downtime or compatibility issues.

Scenario: Removing a Field

Consider a scenario where a field is removed from a representation in the Keycloak API. For example, a field related to user preferences might be deemed obsolete and removed from the User representation. Without the ability to ignore unknown fields, any client sending this field in a request would receive an error, potentially breaking the client's functionality. This forces all clients to immediately update their code to remove the obsolete field.

With the ability to ignore unknown fields, the server can simply ignore the obsolete field, allowing older clients to continue functioning without immediate modification. This provides a grace period for clients to update their code, reducing the risk of disruption and allowing for a more gradual transition to the new API version.

Benefits of Ignoring Unknown Fields

The benefits of ignoring unknown fields in this context are clear:

• Backward compatibility: Existing clients continue to function without immediate modification.
• Reduced disruption: Clients can update their code at their own pace, minimizing downtime and compatibility issues.
• Smoother API evolution: API changes can be introduced without causing immediate breakage.

However, these benefits come with potential drawbacks that need to be carefully considered.

Challenges and Considerations

While ignoring unknown fields can facilitate smoother API evolution, it also introduces several challenges and considerations:

Masking Errors

As previously mentioned, ignoring unknown fields can mask errors. If a client is sending a misspelled field name or an incorrect data type, the server will silently ignore it, potentially leading to data inconsistencies or functional issues. This can make it harder to debug issues and identify problems in client code.

API Contract Ambiguity

Ignoring unknown fields can blur the lines of the API contract. Clients might not be aware that they are sending obsolete fields, and the server's behavior might not be explicitly defined. This can lead to confusion and make it harder to reason about the API's behavior.

Potential for Misuse

The ability to ignore unknown fields can be misused. Clients might start sending arbitrary data in requests, relying on the server to ignore it. This can lead to security vulnerabilities and make it harder to maintain the API's integrity.

Alternative Approaches

Before reintroducing the ability to ignore unknown fields, it's essential to consider alternative approaches to managing API evolution:

• API versioning: Introducing new API versions allows for clear separation between different versions of the API. Clients can explicitly target a specific version, ensuring compatibility and allowing for gradual migration to newer versions.
• Deprecation warnings: Deprecating fields or features provides a clear signal to clients that they should update their code. Deprecation warnings can be logged or returned in API responses, giving clients ample time to adapt to the changes.
• Schema validation: Enforcing strict schema validation can prevent clients from sending invalid data, ensuring data integrity and reducing the risk of errors. However, strict schema validation can also lead to client breakage if not implemented carefully.

These alternative approaches offer different trade-offs between compatibility, explicitness, and flexibility. The optimal approach depends on the specific context and the nature of the API changes.

Potential Solutions and Recommendations

Given the trade-offs between the benefits and challenges of ignoring unknown fields, it's crucial to consider potential solutions and recommendations carefully. Here are some options:

Conditional Ignoring

One approach is to make the behavior of ignoring unknown fields conditional. For example, the server could provide a configuration option that allows administrators to enable or disable the feature. This would provide flexibility in managing API evolution while also mitigating the risks associated with ignoring unknown fields.

Granular Control

Another option is to provide more granular control over which fields are ignored. The server could maintain a list of known fields and only ignore fields that are not in this list. This would prevent the server from ignoring misspelled field names or unexpected data types, reducing the risk of masking errors.

Logging and Monitoring

If the decision is made to ignore unknown fields, it's essential to implement robust logging and monitoring. The server should log any instances where unknown fields are received, providing insights into client behavior and potential issues. This would allow administrators to identify clients that are sending obsolete fields and take appropriate action.

Combination of Approaches

Perhaps the most effective solution is to combine different approaches. For example, the server could ignore unknown fields for a limited time after a field is deprecated, providing a grace period for clients to update their code. At the same time, the server could log deprecation warnings and encourage clients to migrate to newer API versions.

Conclusion

The discussion around handling unknown fields in Keycloak Client v2 highlights the complex trade-offs involved in API evolution. While ignoring unknown fields can facilitate smoother transitions and prevent client breakage, it also introduces potential risks related to error masking, API contract ambiguity, and misuse.

Before reintroducing this feature, it's essential to carefully consider the challenges and alternative approaches. Conditional ignoring, granular control, robust logging and monitoring, and a combination of approaches can help mitigate the risks while still providing the benefits of smoother API evolution.

Ultimately, the decision of how to handle unknown fields should be based on a thorough understanding of the specific context, the nature of the API changes, and the needs of the clients. By carefully weighing the trade-offs and implementing appropriate solutions, Keycloak can ensure a robust and evolving API that meets the needs of its users.

For further reading on API design and versioning best practices, consider exploring resources like Microsoft's API Design Guide.