Understanding Mixpanel's Legacy ID Management and Distinct IDs

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Certainly! Here’s what the knowledge sources say about your situation:

Under Legacy ID Management, Mixpanel did not natively use the $device: prefix for distinct_id values. The $device: prefix is a convention introduced with the Simplified ID Merge system, where anonymous users are tracked with a $device_id, and Mixpanel sets the distinct_id as $device:<device_id> for anonymous events. In Legacy, the distinct_id was typically just a randomly generated value (often a UUID) for anonymous users, and then switched to a user identifier upon login or identification.If you are seeing distinct_id values with a $device: prefix in a project that claims to be on Legacy ID Management, this is unexpected based on the official documentation. It may indicate that: - Some events were ingested using a newer SDK or integration that applies the newer convention, even though the project is set to Legacy. - There may have been a migration or partial update in your implementation, or data was imported from another project using a different ID management system.The documentation does not explicitly describe $device: prefixes appearing in Legacy projects, so this is not standard behavior for Legacy ID Management Understanding Simplified ID Merge.

In Legacy ID Management, the distinct_id is supposed to represent a single user until logout or reset. When a user logs in, the distinct_id is typically set to their user ID, and when they log out, it is reset to a new anonymous ID. The documentation states that aliasing (linking anonymous and identified states) can only be done once, and subsequent attempts to alias the same user ID to a different anonymous ID will fail Deciding to Migrate.If you are seeing a single distinct_id associated with more than two different user_ids, this is not expected in Legacy ID Management. Possible explanations (not directly covered in the documentation) could include: - Custom or non-standard implementation that reuses or incorrectly assigns distinct_id values. - Data imported or merged from other projects or systems with different ID management logic. - Use of SDKs or integrations that do not strictly follow the Legacy conventions.

- The $device: prefix in distinct_id is not standard for Legacy ID Management and is a convention from newer ID Merge systems. - In Legacy, a distinct_id should not be associated with multiple user_ids; if you see this, it likely results from custom implementation, data import, or SDK behavior outside the documented Legacy flow.If you need to ensure correct identity management, consider reviewing your SDK versions, event ingestion methods, and any data import processes for consistency with your project's ID management setting.If you need more precise troubleshooting, you may need to audit your implementation or contact Mixpanel support, as the documentation does not provide further details on this scenario.

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Based on the knowledge sources, here’s what can be determined about the types of distinct_id values you’re seeing in your Mixpanel project with Legacy ID Management:

This is expected in Legacy ID Management. When a user logs in or is identified, the distinct_id is typically set to their user ID. This allows tracking of events to the identified user moving forward Identifying Users (Original) - Example User Flows.

This is also standard. For anonymous users (before login or identification), the SDK generates a random device ID (often a UUID) and uses it as the distinct_id for all events from that device/session Identifying Users (Original) - Example User Flows.

This is not standard behavior for Legacy ID Management. The $device: prefix is a convention introduced with the Simplified ID Merge system, where anonymous events have distinct_id values like $device:<device_id> . The documentation does not mention this prefix being used in Legacy projects. If you see this, it may be due to: - Events being ingested using a newer SDK or integration that applies the newer convention, even though the project is set to Legacy. - Data imported from another project or system using a different ID management version.The knowledge sources do not provide an explicit explanation for $device: prefixes appearing in Legacy projects Understanding Simplified ID Merge.

In Legacy ID Management, the distinct_id should be either the anonymous device ID or the user ID. If you see other values, possible reasons (not directly covered in the documentation) could include: - Custom assignment of distinct_id in your implementation. - Data imported or merged from other sources. - Use of static or arbitrary values for distinct_id in some events (e.g., for non-user-attributable events, as described in Hot Shard Limits).

- User ID and device ID as distinct_id: Expected in Legacy. - $device: prefix: Not standard in Legacy; likely from newer SDKs or data imports. - Other values: Could be due to custom implementation or data import, but not described in the official documentation.If you need to ensure correct identity management, review your SDK versions, event ingestion methods, and any data import processes for consistency with your project's ID management setting. The knowledge sources do not provide further details on these anomalies in Legacy ID Management.

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In Legacy ID Management, a user can have more than one distinct_id because Mixpanel does not automatically merge anonymous and identified user states across devices or sessions. When a user is anonymous, they are assigned a randomly generated distinct_id (often a device ID). When they log in or register, their distinct_id typically changes to their user ID. However, if the same user interacts with your product on multiple devices or browsers, each session will generate a new anonymous distinct_id until the user logs in on that device. As a result, the same user can end up with multiple distinct_id values—one for each device or session before identification, and one for their user ID after identification.Additionally, in Legacy ID Management, aliasing (linking anonymous and identified states) can only be done once, and subsequent attempts to alias the same user ID to a different anonymous ID will fail. This limitation can lead to "orphaned" anonymous events and multiple distinct_ids for the same user, especially if they use multiple devices or clear cookies between sessions. This prevents Mixpanel from providing a holistic view of the user's journey across devices and sessions Deciding to Migrate.In summary, under Legacy ID Management, a user can have more than one distinct_id because there is no automatic merging of identities across devices or sessions, and aliasing is limited.

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In Legacy ID Management, the expected behavior is that when a user is anonymous, the distinct_id is set to the device ID (often a randomly generated value), and the user_id is null. When the user logs in or is identified, the distinct_id should typically switch to the user ID. However, if you observe that after the user is identified (i.e., user_id becomes not null), the distinct_id remains as the device ID, this suggests that the identification step did not update the distinct_id as expected.The knowledge sources do not provide a detailed explanation for this specific scenario in Legacy ID Management. In the documented flows, after calling .identify(user_id), Mixpanel is expected to use the user ID as the distinct_id for subsequent events, not continue using the device ID Identifying Users (Original) - Example User Flows.Possible reasons for this behavior, based on the documentation, could include: - The .identify() method was not called correctly, or not called at all. - There is a custom implementation or event ingestion method that continues to set the distinct_id to the device ID even after identification. - The SDK or integration being used does not follow the standard Legacy ID Management flow.However, the knowledge sources do not explicitly describe a scenario in Legacy ID Management where user_id becomes not null but distinct_id remains as the device ID. Therefore, I'm sorry, but there's no information in the knowledge sources that fully explains why this happens in Legacy ID Management.

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Events in Mixpanel often have a user_id that is null when they are triggered by users who have not yet been identified (i.e., are still anonymous). According to the documentation, before a user logs in or is identified, events are tracked using a device-based identifier (such as a device ID or anonymous ID), and the user_id property is not set—hence, it remains null. This is standard behavior in both Legacy and newer ID management systems: anonymous events do not have a user_id until the user is authenticated or identified Identifying Users (Original) Identifying Users (Simplified).For your specific example, the event answer_attempt_submission always having a null user_id suggests that this event is being tracked before the user logs in or is identified. This could be intentional (e.g., allowing anonymous users to attempt answers) or due to the identification step not being implemented before this event is tracked.As for which other events have the same issue, the knowledge sources do not provide a way to list all events with a null user_id—this depends on your specific implementation and at what point in the user journey you identify users. Any event tracked before user identification will have a null user_id by default.In summary: - Events have a null user_id when triggered by anonymous users (before login/identification). - This is expected behavior unless you identify users before tracking those events. - The documentation does not provide a list of which events have this issue; it depends on your implementation and when you call the identification method.

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