Multi-Region Writes and Conflict Resolution

When writes need to be everywhere

Jake’s decision: single-write or multi-write?

🛒 Jake at GlobeCart runs in 12 regions. His product catalogue is read-heavy (99% reads), so single-write works fine — read replicas handle the load. But the order processing system is write-heavy: customers place orders from everywhere simultaneously.

Jake enables multi-region writes for the orders database to get:

• Local write latency for checkout speed
• 99.999% availability SLA
• No write outage during regional failures

Single-write vs multi-write

Aspect	Single-Write	Multi-Write
Write location	One designated region	Any configured region
Write latency	Low locally, high remotely	Low everywhere
Availability SLA	99.99%	99.999%
Consistency levels	All 5 available	Session, Consistent Prefix, Eventual only
Conflict resolution	Not needed	Required (LWW default)
Cost	Standard per-region RU	Higher — write RU in every region
Failover behaviour	Promote read region to write	No failover needed — all regions write
Best for	Read-heavy, simple architecture	Write-heavy, global low-latency writes

Conflict resolution: Last-Writer-Wins (LWW)

LWW is the default conflict resolution policy. When two regions write to the same item simultaneously, the write with the highest value on the conflict resolution path wins.

Default conflict resolution path: _ts (timestamp)

Region A writes product price = $29.99 at _ts = 1000
Region B writes product price = $24.99 at _ts = 1001

Winner: Region B ($24.99) — higher _ts wins
Loser:  Region A's write is discarded silently

Custom LWW path

You can use any numeric property instead of _ts:

// Use a custom numeric property for LWW
ContainerProperties properties = new ContainerProperties("orders", "/customerId")
{
    ConflictResolutionPolicy = new ConflictResolutionPolicy
    {
        Mode = ConflictResolutionMode.LastWriterWins,
        ResolutionPath = "/priority"  // higher numeric value wins
    }
};

💡 Exam tip: LWW path must be numeric

The LWW conflict resolution path must point to a numeric property. If you specify a path that doesn’t exist or isn’t numeric, Cosmos DB falls back to _ts. The exam may test this edge case — an invalid path doesn’t throw an error; it silently uses _ts.

Custom conflict resolution (stored procedure)

For complex merge logic, register a stored procedure that runs when conflicts are detected:

ContainerProperties properties = new ContainerProperties("inventory", "/warehouseId")
{
    ConflictResolutionPolicy = new ConflictResolutionPolicy
    {
        Mode = ConflictResolutionMode.Custom,
        ResolutionProcedure = "dbs/globecart/colls/inventory/sprocs/mergeInventory"
    }
};

The stored procedure receives both conflicting documents and must return the merged result. If the procedure fails or isn’t registered, conflicts go to the conflict feed for manual resolution.

The conflict feed

When conflicts can’t be auto-resolved (custom mode without a procedure, or procedure failure), they land in the conflict feed — a system container you can read programmatically:

FeedIterator<ConflictProperties> conflicts = container.Conflicts
    .GetConflictQueryIterator<ConflictProperties>();

while (conflicts.HasMoreResults)
{
    FeedResponse<ConflictProperties> response = await conflicts.ReadNextAsync();
    foreach (ConflictProperties conflict in response)
    {
        Console.WriteLine($"Conflict on {conflict.Id}, type: {conflict.OperationType}");
    }
}

💡 Exam tip: conflict feed expiration

Conflicts in the conflict feed are retained based on the container’s TTL setting. If you don’t process them, they expire. In LWW mode, the conflict feed is empty because conflicts are auto-resolved — only Custom mode uses the feed (when the stored procedure is missing or fails).

Impact on consistency

Enabling multi-region writes restricts your consistency options:

Consistency Level	Available?	Why
Strong	❌	Requires single write source for linearizability
Bounded Staleness	❌	Requires single write source for staleness bounds
Session	✅	Read-your-own-writes per session still works
Consistent Prefix	✅	Write ordering within a region is preserved
Eventual	✅	No ordering guarantees needed

Sophie’s tip: 🗄️ Sophie points out that if Jake’s order system previously relied on Strong consistency for inventory checks, switching to multi-region writes means redesigning that flow to handle Session consistency — possibly using optimistic concurrency with ETags instead.

🎬 Video walkthrough

🎬 Video coming soon

Multi-Region Writes and Conflict Resolution — DP-420 Module 14

Multi-Region Writes and Conflict Resolution — DP-420 Module 14

~14 min

Flashcards

Question

What is the default conflict resolution policy in multi-region writes?

Click or press Enter to reveal answer

Answer

Last-Writer-Wins (LWW) using the _ts (timestamp) property. The write with the highest _ts value wins. You can configure a custom numeric path instead of _ts.

Click to flip back

💡 Show hint

Question

What happens if the LWW resolution path points to a non-existent or non-numeric property?

Click or press Enter to reveal answer

Answer

Cosmos DB silently falls back to _ts (timestamp) as the conflict resolution path. It does NOT throw an error. This is an exam trap — invalid paths don't fail, they default.

Click to flip back

💡 Show hint

Question

Where do unresolved conflicts go in Custom conflict resolution mode?

Click or press Enter to reveal answer

Answer

To the conflict feed — a system container that stores both conflicting versions for manual or programmatic resolution. Conflicts expire based on the container's TTL setting.

Click to flip back

💡 Show hint

Knowledge Check

Knowledge Check

Jake enables multi-region writes for GlobeCart's order system. Two customers in different regions update the same order simultaneously. The account uses the default conflict resolution policy. Which write wins?

AThe first write to reach the serverBThe write with the higher _ts (timestamp) valueCBoth writes are merged automaticallyDThe write is rejected with a 409 Conflict error

Check Answer

Knowledge Check

Jake wants his inventory system to merge conflicting stock updates rather than picking one winner. What should he configure?

ALWW with a custom path pointing to the stock quantity fieldBCustom conflict resolution with a stored procedure that merges stock countsCStrong consistency to prevent conflicts entirelyDBounded staleness with a 5-second window

Check Answer

Knowledge Check

After enabling multi-region writes, Jake's team discovers their code relies on Strong consistency for critical reads. What's the impact?

ANo impact — Strong consistency works the same with multi-region writesBStrong consistency is downgraded to Session automaticallyCStrong consistency is not available — the code will fail until they change the consistency levelDStrong consistency still works but at 4× RU cost

Check Answer

---

Next up: Change Feed with Azure Functions and Processors — how to react to data changes in real time by streaming inserts and updates from your Cosmos DB containers.