Cosmos DB Consistency

Introduction

• Cosmos DB maintains two extra replicas of a database apart from the primary database.
• All reads are distributed across three copies.
• All writes to the primary are replicated across two secondary replicas.
• The process of updating replicas takes time.
• If read request goes to replica yet to update, the client may get an older data or inconsistent data.

Consistency Models

Cosmos DB allows developers to choose between five well-defined consistency models along the consistency spectrum.

• Strong,
• Bounded staleness
• Session (Default Consistency)
• Consistent prefix
• Eventual

Strong Consistency

• This is the strongest form of the consistency.
• It ensures that the requested data is most recent.
• Read requests are guaranteed to return the latest version of an item.
• The cost of the read operation is higher in terms of RUs (Request Units).
• Performance is lowest in comparison to other consistency policies as read needs to wait until all writes are committed.
• It is useful in critical applications, like Stock Market, Banking, Airline Reservation.

Bounded staleness

• A client might see old data but one can specify a limit for how old that data can be (e.g. 2 sec).
• It ensures all the updates on replicas happen in the order they are received.
• The "staleness window" can be configured in two ways: Maximum Number of Operations, Maximum duration.
• The cost of a read operation is lower than that in Strong Consistency if the request is within "staleness window".
• Performance is better than Strong Consistency when a request is within "staleness window".
  
  

Session

• Session consistency is scoped to client session.
• It is default consistency provided by Cosmos DB.
• It provided Read Your Own (RYW) guarantee.
• The client reads its own writes, but other clients reading this same might see older values.
• The cost of a read operation is lower than both the previous Consistency policies.
• Performance is much better than other two policies mentioned above.

Consistent Prefix

• This can not guarantee the freshness of data but makes sure that reads never see out of order writes.
• If the write is happening in order of A, B, C, then a client may read either A or AB or ABC and not the out of order.
• The cost of the read operation is lower than all three policies mentioned above.
• Performance is also better than other three policies.

Eventual

• Eventual consistency is the weakest form of consistency where a client may get the values that are older than the ones it had seen before.
• The client might see old data for as long as it takes a write to propagate to all replicas.
• Highest performance and availability is ensured but a client might sometimes read out-of-date information or see updates out of order.

How to set Consistency?

• Log in to Azure Portal.
• Go to Cosmos DB account.
• Click on "Default Consistency" option.
• Set "Consistency" as per requirement.
• Click on "Save".