Use system-assigned managed identities to access Azure Cosmos DB data

How-to
11/01/2023

APPLIES TO: NoSQL

In this article, you'll set up a robust, key rotation agnostic solution to access Azure Cosmos DB keys by using managed identities and data plane role-based access control. The example in this article uses Azure Functions, but you can use any service that supports managed identities.

You'll learn how to create a function app that can access Azure Cosmos DB data without needing to copy any Azure Cosmos DB keys. The function app will trigger when an HTTP request is made and then list all of the existing databases.

Prerequisites

An Azure account with an active subscription. Create an account for free.
An existing Azure Cosmos DB API for NoSQL account. Create an Azure Cosmos DB API for NoSQL account
An existing Azure Functions function app. Create your first function in the Azure portal
- A system-assigned managed identity for the function app. Add a system-assigned identity
Azure Functions Core Tools
To perform the steps in this article, install the Azure CLI and sign in to Azure.

Prerequisite check
1. In a terminal or command window, store the names of your Azure Functions function app, Azure Cosmos DB account and resource group as shell variables named functionName, cosmosName, and resourceGroupName.
```
# Variable for function app name
functionName="msdocs-function-app"

# Variable for Azure Cosmos DB account name
cosmosName="msdocs-cosmos-app"

# Variable for resource group name
resourceGroupName="msdocs-cosmos-functions-dotnet-identity"
```
  Note
  
  These variables are reused in later steps. This example assumes your Azure Cosmos DB account name is msdocs-cosmos-app, your function app name is msdocs-function-app and your resource group name is msdocs-cosmos-functions-dotnet-identity.
2. View the function app's properties using the az functionapp show command.
```
az functionapp show \
    --resource-group $resourceGroupName \
    --name $functionName
```
3. View the properties of the system-assigned managed identity for your function app using az webapp identity show.
```
az webapp identity show \
    --resource-group $resourceGroupName \
    --name $functionName
```
4. View the Azure Cosmos DB account's properties using az cosmosdb show.
```
az cosmosdb show \
    --resource-group $resourceGroupName \
    --name $cosmosName
```

Create Azure Cosmos DB API for NoSQL databases

In this step, you create two databases.

In a terminal or command window, create a new products database using az cosmosdb sql database create.

az cosmosdb sql database create \
    --resource-group $resourceGroupName \
    --name products \
    --account-name $cosmosName

Create a new customers database.

az cosmosdb sql database create \
    --resource-group $resourceGroupName \
    --name customers \
    --account-name $cosmosName

Get Azure Cosmos DB API for NoSQL endpoint

In this step, you query the document endpoint for the API for NoSQL account.

Use az cosmosdb show with the query parameter set to documentEndpoint. Record the result. You'll use this value in a later step.

az cosmosdb show \
    --resource-group $resourceGroupName \
    --name $cosmosName \
    --query documentEndpoint

cosmosEndpoint=$(
    az cosmosdb show \
        --resource-group $resourceGroupName \
        --name $cosmosName \
        --query documentEndpoint \
        --output tsv
)

echo $cosmosEndpoint

Note

This variable is reused in a later step.

Grant access to your Azure Cosmos DB account

In this step, you assign a role to the function app's system-assigned managed identity. Azure Cosmos DB has multiple built-in roles that you can assign to the managed identity for control-plane access. For data-plane access, you create a new custom role with access to read metadata.

Tip

For more information about the importance of least privilege access, see the Lower exposure of privileged accounts article.

Use az cosmosdb show with the query parameter set to id. Store the result in a shell variable named scope.

scope=$(
    az cosmosdb show \
        --resource-group $resourceGroupName \
        --name $cosmosName \
        --query id \
        --output tsv
)

echo $scope

Note

This variable is reused in a later step.

Use az webapp identity show with the query parameter set to principalId. Store the result in a shell variable named principal.

principal=$(
    az webapp identity show \
        --resource-group $resourceGroupName \
        --name $functionName \
        --query principalId \
        --output tsv
)

echo $principal

Create a new JSON file with the configuration of the new custom role.

{
    "RoleName": "Read Azure Cosmos DB Metadata",
    "Type": "CustomRole",
    "AssignableScopes": ["/"],
    "Permissions": [{
        "DataActions": [
            "Microsoft.DocumentDB/databaseAccounts/readMetadata"
        ]
    }]
}

Tip

You can create a file in the Azure Cli using either touch <filename> or the built-in editor (code .).

Use az cosmosdb sql role definition create to create a new role definition named Read Azure Cosmos DB Metadata using the custom JSON object.
```
az cosmosdb sql role definition create \
    --resource-group $resourceGroupName \
    --account-name $cosmosName \
    --body @definition.json
```
Note

In this example, the role definition is defined in a file named definition.json.

Use az role assignment create to assign the Read Azure Cosmos DB Metadata role to the system-assigned managed identity.

az cosmosdb sql role assignment create \
    --resource-group $resourceGroupName \
    --account-name $cosmosName \
    --role-definition-name "Read Azure Cosmos DB Metadata" \
    --principal-id $principal \
    --scope $scope

Programmatically access the Azure Cosmos DB keys

We now have a function app that has a system-assigned managed identity with the custom role. The following function app will query the Azure Cosmos DB account for a list of databases.

Create a local function project with the --dotnet parameter in a folder named csmsfunc. Change your shell's directory
```
func init csmsfunc --dotnet

cd csmsfunc
```
Create a new function with the template parameter set to httptrigger and the name set to readdatabases.
```
func new --template httptrigger --name readdatabases
```
Add the Azure.Identity and Microsoft.Azure.Cosmos NuGet package to the .NET project. Build the project using dotnet build.
```
dotnet add package Azure.Identity

dotnet add package Microsoft.Azure.Cosmos

dotnet build
```
Open the function code in an integrated developer environment (IDE).
Tip

If you are using the Azure CLI locally, you can open Visual Studio Code.
```
code .
```

Replace the code in the readdatabases.cs file with this sample function implementation. Save the updated file.

using System;
using System.Collections.Generic;
using System.Threading.Tasks;
using Azure.Identity;
using Microsoft.AspNetCore.Mvc;
using Microsoft.Azure.Cosmos;
using Microsoft.Azure.WebJobs;
using Microsoft.Azure.WebJobs.Extensions.Http;
using Microsoft.AspNetCore.Http;
using Microsoft.Extensions.Logging;

namespace csmsfunc
{
    public static class readdatabases
    {
        [FunctionName("readdatabases")]
        public static async Task<IActionResult> Run(
            [HttpTrigger(AuthorizationLevel.Anonymous, "get")] HttpRequest req,
            ILogger log)
        {
            log.LogTrace("Start function");

            CosmosClient client = new CosmosClient(
                accountEndpoint: Environment.GetEnvironmentVariable("COSMOS_ENDPOINT", EnvironmentVariableTarget.Process),
                new DefaultAzureCredential()
            );

            using FeedIterator<DatabaseProperties> iterator = client.GetDatabaseQueryIterator<DatabaseProperties>();

            List<(string name, string uri)> databases = new();
            while(iterator.HasMoreResults)
            {
                foreach(DatabaseProperties database in await iterator.ReadNextAsync())
                {
                    log.LogTrace($"[Database Found]\t{database.Id}");
                    databases.Add((database.Id, database.SelfLink));
                }
            }

            return new OkObjectResult(databases);
        }
    }
}

(Optional) Run the function locally

In a local environment, the DefaultAzureCredential class uses various local credentials to determine the current identity. While running locally isn't required for the how-to, you can develop locally using your own identity or a service principal.

Get your local account's principal identifier using az ad signed-in-user show.
```
az ad signed-in-user show --query "id"
```
Assign your local account role-based access control access to the Azure Cosmos DB account using az cosmosdb sql role assignment create command. Use the built-in "Cosmos DB Data Contributor" role with an id of 00000000-0000-0000-0000-000000000002.
```
az cosmosdb sql role assignment create \
    --resource-group $resourceGroupName \
    --account-name $cosmosName \
    --role-definition-id "00000000-0000-0000-0000-000000000002" \
    --principal-id "<your-principal-id>" \
    --scope "/"
```
In the local.settings.json file, add a new setting named COSMOS_ENDPOINT in the Values object. The value of the setting should be the document endpoint you recorded earlier in this how-to guide.
```
...
"Values": {
    ...
    "COSMOS_ENDPOINT": "https://msdocs-cosmos-app.documents.azure.com:443/",
    ...
}
...
```
Note

This JSON object has been shortened for brevity. This JSON object also includes a sample value that assumes your account name is msdocs-cosmos-app.
Run the function app
```
func start
```

Deploy to Azure

Once published, the DefaultAzureCredential class uses credentials from the environment or a managed identity. For this guide, the system-assigned managed identity will be used as a credential for the CosmosClient constructor.

Set the COSMOS_ENDPOINT setting on the function app already deployed in Azure.

az functionapp config appsettings set \
    --resource-group $resourceGroupName \
    --name $functionName \
    --settings "COSMOS_ENDPOINT=$cosmosEndpoint"

Deploy your function app to Azure by reusing the functionName shell variable:
```
func azure functionapp publish $functionName
```
Test your function in the Azure portal.