Engage merge protection algorithm protects your identity graph from unnecessary merges by finding and removing untrusted external IDs. Here's an example:

![Merge protection](images/merge_protection.png"An image representing the merge protection flow")

In this example,`anonymous_id: a1` is not reset during a`User Logout`. Without merge protection rules, Segment would merge`user_id u1` and`user_id u2`. Instead, the identity resolution algorithm detects that such a merge would break`user_id` uniqueness and prevents the merge.

This is especially helpful for preventing "blob users" that are merged together by non-unique anonymous IDs or by common group emails like`team@company.com`.

Most destinations on the Segment Platform are built up around a user model. They assume that a user will have a single userId. Further, most Destinations are not built to handle anonymous traffic.

If you want Identity Graph to operate on a different custom ID, you can pass it in using`context.externalIds` on an`identify()` or`track()`. If you're interested in this feature, contact your CSM to discuss the best way to implement this feature.

Each incoming event is analyzed and external IDs are extracted (`user_id`,`anonymous_id`,`email`). The simplified algorithm works as follows:

4. If Segment finds multiple matching profiles, Segment applies the identity resolution settings for merge protection. Specifically, Segment uses identifier limits and priorities to add the correct identifiers to the profile.

5. Segment then applies[Profile limits](/docs/profiles/profile-api-limits/) to ensure profiles remain under these limits. Segment doesn't add any further merges or mappings if the profile is at either limit, but event resolution for the profile will continue.

{% comment %}

![Identity graph merging](images/merging_1.png"Flowchart of Segment receiving an incoming event")

![Identity graph merging](images/merging_2.png"Flowchart of Segment searching for profiles by external ID")

![Identity graph merging](images/merging_3.png"Flowchart of Segment merging profiles")

{% endcomment %}

All Profile matching is deterministic and based on first-party data that you've collected.

| name| Limit| Details|

| -----------------| -------------| ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|

| Identity Merges| 100 merges| Engage supports up to 100 merges per profile in its identity graph. Merges occur when a common `external_id` joins two existing profiles. For example, if a user initiates a mobile session but then signs in through a web application, a common identifier like`user_id` will join the two user profiles.Segment dropsadditionalmessage merge attempts, which usually indicate corrupt profiles. Once thelimit is reached, Segment rejects additional events.|

| Identity Mappings| 1000 mappings| Engage supports up to 1000 mappings per profile in its identity graph. Mappings are external identifier values like a`user_id`, email, mobile advertising`id`, or any custom identifier.Segment dropsadditionalmessage mapping attempts, which usually indicate corrupt profiles. This limit counts mappings across all merged profiles.|

| Identify calls| 300 traits| Engage rejects Identify events with 300 or more traits. If your use case requires more than 300 traits, you can split the traits into multiple Identify calls.|

| name| Limit| Details|

| -----------------| -------------| ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|

| Identity Merges| 100 merges| Engage supports up to 100 merges per profile in its identity graph. Mergesoccur when a common`external_id`joins two existing profiles. For example, if a user initiates a mobile session but then signs in througha web application, a common identifier like`user_id` will join the two user profiles.Noadditionalmerges will be added once the profile reaches thislimit. Event resolution for the profile, however, will continue.|

| Identity Mappings| 1000 mappings| Engage supports up to 1000 mappings per profile in its identity graph. Mappings are external identifier values like a`user_id`, email, mobile advertising`id`, or any custom identifier.Noadditionalmappings will be added once the profile reaches this limit. Event resolution for the profile, however, will continue.|

| Identify calls| 300 traits| Engage rejects Identify events with 300 or more traits. If your use case requires more than 300 traits, you can split the traits into multiple Identify calls.|