409 Conflict error using multi-write regions

Question

409 Conflict error using multi-write regions

João Pereira 0

Hello,

I'm currently trying to understand how to implement conflict resolution in my project.

I'm trying to create conflicts to test my stored procedures but I keep receiving the error 409 Conflict with the following message: "Entity with the specified id already exists in the system.".

I'm inserting two items with the same ID on different regions on purpose, trying to create a conflict, but when I fetch all the conflicts it says that there are no conflicts in the system.

My question is why do I get the conflict error but the system does not store the conflicts for the stored procedures to solve? I'm using the Nodejs SDK in case that's relevant.

Oury Ba-MSFT 20,926 Reputation points Microsoft Employee Moderator

2025-03-26T16:22:45.7466667+00:00

@João Pereira

I haven't heard back from you since my last comment. Please let me know if you have any additional questions or concerns. I will be more than happy to assit you further.

Regards,

Oury
João Pereira 0 Reputation points

2025-03-27T15:12:24.7066667+00:00

Hello @Oury Ba-MSFT

First of all, thank you for your reply.

Currently I changed my approach. Now I'm using the Java SDK in order to use the Direct Connection Mode. However I'm still unable to generate conflicts, even with Direct mode enabled. I've also enabled the Custom Conflict Resolution, in order to store the conflicts.

If someone could help me or indicate an example I could look at it on how to simulate conflicts in Cosmos DB I would be very thankful.

4 answers

Your answer

Oury Ba-MSFT 20,926 Reputation points Microsoft Employee Moderator

2025-03-26T16:22:45.7466667+00:00

@João Pereira

I haven't heard back from you since my last comment. Please let me know if you have any additional questions or concerns. I will be more than happy to assit you further.

Regards,

Oury
João Pereira 0 Reputation points

2025-03-27T15:12:24.7066667+00:00

Hello @Oury Ba-MSFT

First of all, thank you for your reply.

Currently I changed my approach. Now I'm using the Java SDK in order to use the Direct Connection Mode. However I'm still unable to generate conflicts, even with Direct mode enabled. I've also enabled the Custom Conflict Resolution, in order to store the conflicts.

If someone could help me or indicate an example I could look at it on how to simulate conflicts in Cosmos DB I would be very thankful.

Answer 1

Vinodh247 34,666 MVP Volunteer Moderator

Hi ,

Thanks for reaching out to Microsoft Q&A.

It sounds like you are running into cosmos db's behavior in multi-write regions, but there are a few key points to consider:

Why are you getting a 409 conflict but not seeing conflicts stored?

Default "last writer wins" mode
- In a multi-region write-enabled cosmos db setup, the default conflict resolution strategy is last writer wins (LWW). This means that when a conflict occurs, the system automatically resolves it based on the defined conflict resolution policy.
- If you haven't explicitly set a custom conflict resolution strategy (custom stored procedure based resolution), conflicts will be automatically resolved without being recorded in the conflict feed.
1. The conflict feed is empty
  - The conflict feed is only populated if you set custom conflict resolution using stored procedures.
    - If your container is using the default lww strategy, conflicts will never appear in the conflict feed because cosmos db resolves them automatically.
    1. 409 conflict vs. Conflict feed
      - A 409 conflict error occurs at the sdk level when you try to insert a document with an already existing id.
        
        However, this is not the same as a conflict that is eligible for conflict resolution. If two regions try to insert different versions of the same document at the same time, and there’s a conflict, cosmos db handles it based on your conflict resolution policy.

If you're getting a 409 Conflict, it's due to an attempt to insert an existing ID, which is an SDK level constraint.
Cosmos DB doesn't store conflicts unless conflict resolution is explicitly set to "Custom".
If you want stored procedures to resolve conflicts, you must configure a custom conflict resolution policy and query the conflict feed.

Please feel free to click the 'Upvote' (Thumbs-up) button and 'Accept as Answer'. This helps the community by allowing others with similar queries to easily find the solution.

João Pereira 0 Reputation points

2025-03-14T15:02:35.1366667+00:00

First of all, thank you for the reply.

I have just a small doubt. I have everything set the way it's described above, but since I need to insert two items with the same ID in order to simulate concurrent operations, I will always get a 409 error right?
Vinodh247 34,666 Reputation points MVP Volunteer Moderator

2025-03-16T08:27:59.55+00:00
Yes you will always get a 409 Conflict error when inserting two items with the same ID in a multi-write region setup because Cosmos DB enforces unique IDs within a container.

However if your goal is to simulate a realworld conflict that will be handled by the conflict resolution mechanism, you need to perform updates to the same document in different regions instead of trying to insert it twice.

How to Properly Simulate a Conflict?

Instead of inserting the same ID twice (which results in an immediate 409 due to unique constraints), do this:

Insert the document once in one region.

Update the same document simultaneously in two different regions.

Retrieve conflicts from the conflict feed.
Sai Raghunadh M 4,640 Reputation points Microsoft External Staff Moderator

2025-03-17T01:01:53.4433333+00:00

Hi @ João Pereira

We haven’t heard from you on the last response and was just checking back to see if you have a resolution yet. In case if you have any resolution please do share that same with the community as it can be helpful to others. Otherwise, will respond with more details and we will try to help.
João Pereira 0 Reputation points

2025-03-17T01:14:28.7266667+00:00

Hello,

I did not came with a valid solution. It appears that Cosmos DB is not detecting any conflict at all. I'm using Promise.all() to perform simultaneous writes but it does not seem to work.

Answer 2

Sai Raghunadh M 4,640 Microsoft External Staff Moderator

Hi @ João Pereira

If Promise.all() performs simultaneous writes to the same region, it may not replicate the multi-region update scenario needed to trigger conflicts.

This is because simultaneous writes in the same region aren't considered conflicts; they are resolved through the unique ID constraint

Please try the below Suggestions:

Verify that the container’s conflict resolution policy is set to "custom" and not the default LWW mode.
Use the readConflicts() method in the SDK to fetch conflicts and debug the process.
When testing, make sure writes occur in different regions. You can specify the target region in your SDK configuration to explicitly direct traffic.

João Pereira 0 Reputation points

2025-03-17T14:08:37.8533333+00:00

Hello,

I've been testing by writing in different regions but still with no success. I was wondering if the distance between regions matters since I'm using two regions in Europe (UK South and North Europe).

The behavior I'm experiencing replaces each item in each region but it looks like the item is overwritten without generating a conflict.

Answer 3

PratikLad 1,830 Microsoft External Staff Moderator

Hi @João Pereira,

My question is why do I get the conflict error, but the system does not store the conflicts for the stored procedures to solve?

When Do Conflicts Appear in the Conflict Feed?

As per MS Document Conflicts will only appear in the conflict feed under the following conditions:

Automatic Conflict Resolution Fails:
- If the system has an automatic conflict resolution policy (such as "Last Write Wins"), it tries to resolve conflicts automatically.
- If automatic resolution succeeds, the conflict does not get logged in the conflict feed.
- If automatic resolution fails, the conflict is stored in the feed.
Stored Procedure Fails to Resolve Conflict:
- If your system uses a stored procedure for conflict resolution but that stored procedure encounters an error or fails to execute, the conflict is logged in the feed.
Custom Conflict Resolution Without a Stored Procedure:
- If your conflict resolution policy is set to custom, but you have not specified a stored procedure to handle conflicts, the system does not resolve them automatically, and conflicts are logged in the conflict feed.

Why You Might Get a Conflict Error Without Seeing It in the Conflict Feed?

If your system is set to Last Write Wins, conflicts are resolved automatically, meaning they never appear in the conflict feed.
If you use a custom policy but have defined a proper stored procedure, conflicts may be resolved before they need to be stored.

João Pereira 0 Reputation points

2025-03-18T12:38:21.3266667+00:00

Hello @PratikLad

First of all, thank you for the reply.

What is happening in my project is that Cosmos DB is not capturing conflicts. I have everything well configured (I've double checked) so I guess my problem is generating conflicting writes in the code. I'm using the Nodejs SDK and I'm writing in Typescript.

My approach is to use Promise.all() to replace the same object in two different regions, but that is not generating a conflict, it seems that the writes are not being executed simultaneously.

Another question that I have is why are these conflicts not being generated when the consistency level is set to Eventual? A write in a region might conflict with another write in another region when that write is replicated to another region.

If someone has an example using the Nodejs SDK on how to generate conflicts to test the conflict resolution policies I would be very grateful.
PratikLad 1,830 Reputation points Microsoft External Staff Moderator

2025-03-18T14:49:39.0966667+00:00
Hi @João Pereira,

Thanks for the detailed explanation! It sounds like the main issue is ensuring that true conflicting writes occur simultaneously across regions. Since you're using Promise.all(), there's a chance that the writes are not happening at exactly the same time due to network delays or the way the SDK handles requests.

A few things you might want to check:

Consistency Level – Since you're using Eventual consistency, conflicts may not occur as expected because writes can be applied in different orders across regions without strict conflict detection. You might want to try Session, Bounded Staleness, or Strong consistency to force conflict resolution.

Write to Different Regions Explicitly – Instead of using the same Cosmos client, initialize separate clients for each region and write from them concurrently.

Force a Conflict with Different Versions – When writing the same document from different regions, ensure each write has a distinct version number or timestamp to help trigger a conflict.

Would you be able to try using two separate Cosmos clients (one per region) and executing the writes from both explicitly? If needed, I can also share a small example in Node.js that may help test this scenario.
João Pereira 0 Reputation points

2025-03-18T17:44:10.3666667+00:00
Hello, thank you for your reply again.

I've been using two different clients, one for each region. This is how I've set up the clients:

const clientNorthEurope: CosmosClient = new CosmosClient({ endpoint: process.env.CONFIGURATION__AZURECOSMOSDB__ENDPOINT_NORTH!, key: process.env.AUTH_KEY!, connectionPolicy: { preferredLocations: ["North Europe"], useMultipleWriteLocations: true } });

In terms of consistency models, I've been using the Session model, but in the future I might also try to use Eventual consistency.

If you have a small example I could look at I would be very thankful.

Answer 4

Oury Ba-MSFT 20,926 Microsoft Employee Moderator

@João Pereira Thank you for reaching out.

Inserting a document with same id is not a conflict - it is a unique id violation, you are getting the expected error. To simulate a conflict, you have to update the same record on two different regions at the same time. That would result in a conflict, and as Vinodh247 mentioned in their response, by default Cosmos DB would not store the conflict as the default conflict resolution strategy is last writer wins (LWW).

Regards,

Oury

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409 Conflict error using multi-write regions

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