Top 46 Cosmos DB Interview Questions And Answers

Q: What's _Logical Partition_ in CosmosDB?

A **logical partition** consists of a set of items that have the _same partition key_. Selecting a partition key with a wide range of possible values ensures that the container is able to scale. Cosmos DB is designed to scale horizontally based on the distribution of data between Physical Partitions (PP) (think of it as separately deployable underlaying self-sufficient nodes) and **logical partition** - a bucket of documents with the same characteristic _(partition key)_ which is supposed to be stored fully on the same PP. So LP can't have part of the data on PP1 and another on PP2. A logical partition also defines the _scope_ of database _transactions_. You can update items within a logical partition by using a transaction with _snapshot isolation_. ![https://miro.medium.com/v2/resize:fit:720/format:webp/0*ODo2Z1ckdU26mkSH.png](https://miro.medium.com/v2/resize:fit:720/format:webp/0*ODo2Z1ckdU26mkSH.png)

Q: Is using _CurrentDate_ a good partition key?

- For a **write-heavy** workload using the date as a partition key is a **bad choice** because you will always have a hot partition on the _current date_. However, if the amount of data being written is consistent and the write volume is low, then it can be used and you will have_ good distribution_ of data on storage. - In **read-heavy** scenarios, the date can be a **good** partition key if it is used to answer most of the queries in the app. The value for `id` must be _unique per partition key_ value so for your data model to work you can only have one _id_ value per day.

Question 1

What is the _rule of thumb_ for choosing the _ideal partitioning key_ in CosmosDB?

Accepted Answer

An **ideal partition key** is one that:
- appears _frequently_ as a _filter_ in your queries and 
- has _sufficient cardinality_ to ensure your solution is scalable.

Question 2

What is the difference between _Partition Key_ and _Primary Key_ in Cosmos DB?

Accepted Answer

**Partition key** is not unique for each document as the primary key is unique for each record in relational databases. The partition key is _unique_ for each _logical partition_. Accordingly, all the documents in a logical partition will have the same partition key.

Question 3

Can we change the _Partition Key_ in Cosmos DB?

Accepted Answer

Changing the partition key is not supported. You would need to create a new collection.

Question 4

What is Partitioning in Cosmos DB?

Accepted Answer

Partitioning denotes how our data gets distributed. Mainly there are two types of partitions:
- **Logical Partition** is a virtual bucket where the data gets wrapped around based on the partition key we select.
- **Physical partitions** are the physical servers available in order to store our documents in azure cosmos db.

Question 5

What is _Partition Key_ in Cosmos DB?

Accepted Answer

The **PartitionKey** is a property that will exist on every single object that is best used to group similar objects together. Good examples include Location (like City), Customer Id, Team, and more. Partition key acts as a _logical partition_.

Question 6

What is _RU_ in Cosmos DB?

Accepted Answer

Azure Cosmos DB normalizes the cost of all database operations using **Request Units** (or RUs, for short). The request unit is a performance currency abstracting the system resources such as CPU, IOPS, and memory that are required to perform the database operations supported by Azure Cosmos DB.

The cost to do a point read (fetching a single item by its ID and partition key value) for a 1-KB item is one Request Unit (or one RU).

Question 7

How is data stored in different partitions in a Cosmos DB container?

Accepted Answer

For every container, a partition key must be pre-defined. The partition key is used to divide a _container_ into many subsets, AKA _logical partitions_, and each logical partition is used to store items with the same partition key values.

- A container can have an unlimited number of logical partitions
- Each logical partition has a storage limit of 20G

![https://trycatch.me/images/cosmos-structure.png](https://trycatch.me/images/cosmos-structure.png)

Question 8

What is the purpose of _synthetic partition key_ in Cosmos DB?

Accepted Answer

You can form a partition key by concatenating multiple property values into a single artificial (synthetic) `partitionKey` property. These keys are referred to as **synthetic keys**.

```js
{
  "deviceId": "abc-123",
  "date": 2018,
  "partitionKey": "abc-123-2018" // <-- synthetic partition key
}
```

Question 9

What's _Logical Partition_ in CosmosDB?

Accepted Answer

A **logical partition** consists of a set of items that have the _same partition key_. Selecting a partition key with a wide range of possible values ensures that the container is able to scale.

Cosmos DB is designed to scale horizontally based on the distribution of data between Physical Partitions (PP) (think of it as separately deployable underlaying self-sufficient nodes) and **logical partition** - a bucket of documents with the same characteristic _(partition key)_ which is supposed to be stored fully on the same PP. So LP can't have part of the data on PP1 and another on PP2.

A logical partition also defines the _scope_ of database _transactions_. You can update items within a logical partition by using a transaction with _snapshot isolation_.

![https://miro.medium.com/v2/resize:fit:720/format:webp/0*ODo2Z1ckdU26mkSH.png](https://miro.medium.com/v2/resize:fit:720/format:webp/0*ODo2Z1ckdU26mkSH.png)

Question 10

Is using _CurrentDate_ a good partition key?

Accepted Answer

- For a **write-heavy** workload using the date as a partition key is a **bad choice** because you will always have a hot partition on the _current date_. However, if the amount of data being written is consistent and the write volume is low, then it can be used and you will have_ good distribution_ of data on storage.

- In **read-heavy** scenarios, the date can be a **good** partition key if it is used to answer most of the queries in the app.

The value for `id` must be _unique per partition key_ value so for your data model to work you can only have one _id_ value per day.

Top 46 Cosmos DB Interview Questions

What is Partition Key in Cosmos DB?

Can we change the Partition Key in Cosmos DB?

What is Partitioning in Cosmos DB?

What is the difference between Partition Key and Primary Key in Cosmos DB?

What is RU in Cosmos DB?

What is the rule of thumb for choosing the ideal partitioning key in CosmosDB?

How is data stored in different partitions in a Cosmos DB container?

Name types of the Indexes in Cosmos DB

What's Logical Partition in CosmosDB?

What does "Cosmos DB automatically indexes the documents" mean? Explain.

Explain different indexing modes in Cosmos DB

When will you use Unique Keys constraints in Cosmos DB?

Explain the use of Change Feed in Cosmos DB. What use cases will you use it for?

Explain how Transactions are supported in Cosmos DB?

Can you mix different document types in the same container in Cosmos DB? Why would you do it?

How to do a transaction with two collections on Azure CosmosDB?

When would you create a new container instead of adding data of different type to the one already created in Cosmos DB?

Is having the same Partition Key in multiple containers a good idea?

What are the consequences of the wrong Partition Key choice?

Explain the use of Store Procedures, Triggers and UDF in Cosmos DB

What’s the difference between Logical and Physical partitions in Cosmos DB?

Name and define some Consistency Models/Levels in Azure Cosmos DB

Name some RUs Throughput Modes used in Cosmos DB

Explain the difference between Point Reads and Queries in Cosmos DB?

Explain what is the Point Read in Cosmos DB?

Name some pros and cons of using GUID as Partition Key in Cosmos DB

What are cross-partition queries in Cosmos DB?

How would you select the right partition key for the container in CosmosDB?

What is the purpose of synthetic partition key in Cosmos DB?

What is the use of ETag property in Cosmos DB?

How would you choose Partition Key for read-heavy containers in CosmosDB?

How would you choose/design a muti-tenant architecture with Azure Cosmos DB?

Why not put users, posts and comments in just one container in Cosmos DB?

When and how would you avoid/prevent cross-partition queries in Cosmos DB?

Explain the use of Bounded Staleness consistency model in Cosmos DB

When and why would you exclude property paths from indexing in Cosmos DB?

When would you use None indexing mode in Cosmos DB?

Explain the difference between two models below. Which one is more performant for reading?

How OOC is working for accounts with multi-region writes in Cosmos DB?

What are some of the Security Options provided by Cosmos DB?

Why large logical partitions are a terrible choice for growing Cosmos DB?

What are some limitations of Physical Partitions and why is it important to monitor?

Explain what is Optimistic Concurrency Control (OCC) and how is it implemented in Cosmos DB?

Can we use Strong Consistency with multiple write regions for Cosmos DB? Explain

How would you optimise this query to have lower latency and consume fewer RUs?

Is using CurrentDate a good partition key?