After having developed the first version of the podcast feed, I wanted to improve it. My idea was to cache the data already got from the http endpoint, to achieve two goals:

  • improve performances;
  • avoid that with an increasing number of users using my podcast feed, I (my podcast) would do many requests to the http endpoint.


As a caching mechanism I've chosen to use CosmosDB, a NoSQL database that has the big advantage of paying for use, completely in line with the Azure Functions mindset.


To use CosmosDB in a .NET Core application, first of all you need to include the Microsoft.Azure.DocumentDB.Core NuGet package.

The data is organized in Databases, Collections (equivalent to a SQL tables) and Documents (equivalent to SQL records). Documents are json files, serialized/deserialized to/from POCO entities with Json.Net.

This is reflected about how to create a Collection (if not existing) and return a DocumentClient:

public static async Task<DocumentClient> GetCosmosDBClientAsync(IConfiguration config)
	Uri cosmosDBEndpointUri = new Uri(config["CosmosDBEndpointUrl"]);
	string cosmosDBPrimaryKey = config["CosmosDBPrimaryKey"];

	DocumentClient cosmosDBClient = new DocumentClient(cosmosDBEndpointUri, cosmosDBPrimaryKey);

	await cosmosDBClient.CreateDatabaseIfNotExistsAsync(new Database { Id = dbName });
	await cosmosDBClient.CreateDocumentCollectionIfNotExistsAsync(UriFactory.CreateDatabaseUri(dbName), new DocumentCollection { Id = collectionName });

	return cosmosDBClient;


After you have a DocumentClient, you can call its methods to get,

var tempResult = await cosmosDBClient.ReadDocumentAsync(UriFactory.CreateDocumentUri(dbName, collectionName, podcastId));
PodcastEpisode result = (dynamic)tempResult.Resource;


Uri documentCollectionUri = UriFactory.CreateDocumentCollectionUri(dbName, collectionName);
await cosmosDBClient.CreateDocumentAsync(documentCollectionUri, episode);

and update documents:

Uri documentUri = UriFactory.CreateDocumentUri(dbName, collectionName, episode.Id);
await cosmosDBClient.ReplaceDocumentAsync(documentUri, episode);


About the POCO entities: if you do't want that CosmosDB assigns them a GUID id, then they need to have an "id" property (all in lower case). If you want to use the usual C# naming conventions, you can use tag your property with JsonProperty:

public class PodcastEpisode
	// Id of the podcast, in format yyyyMMdd
	[JsonProperty(PropertyName = "id")]
	public string Id { get; set; }
	public DateTime DateUtc { get; set; }
	public string FileName { get; set; }
	public string CompleteUri { get; set; }
	public long FileLength { get; set; }


When deploying the solution in Azure, you need to create the CosmosDB instance with API: SQL



The source code for this step is available in the dedicated GitHub repository, under the tag CosmosDB.