Azure Function Parameter from Settings - visual-studio

Question

Referring to the following example:
public static void Run([CosmosDBTrigger(
databaseName: "ToDoItems",
collectionName: "Items",
ConnectionStringSetting = "CosmosDBConnection",
LeaseCollectionName = "leases",
CreateLeaseCollectionIfNotExists = true)]IReadOnlyList<Document> documents,
ILogger log)
I understand, the connectionStringSetting isn't the connection string to use, rather it's name of the setting to look up containing the ConnectionString.
Will this also work for CollectionName and databasename as well? I understand I can experiment and figure out, but I am confused as to how this is even resolved at build time/deployment time?
I see several properties being assigned values while others are taking them from configuration? Is it the underlying constructor for CosmosDBTrigger which takes care of using appropriate value?

Answer 1

Binding to a function is a way of declaratively connecting another resource to the function; bindings may be connected as input bindings, output bindings, or both. Data from bindings is provided to the function as parameters.
here is small sample of Azure function using CosmosDB trigger that is invoked when there are inserts or updates in the specified database and collection.
using Microsoft.Azure.Documents;
using Microsoft.Azure.WebJobs;
using Microsoft.Azure.WebJobs.Host;
using System.Collections.Generic;
using Microsoft.Extensions.Logging;
namespace CosmosDBSamplesV2
{
public static class CosmosTrigger
{
[FunctionName("CosmosTrigger")]
public static void Run([CosmosDBTrigger(
databaseName: "ToDoItems",
collectionName: "Items",
ConnectionStringSetting = "CosmosDBConnection",
LeaseCollectionName = "leases",
CreateLeaseCollectionIfNotExists = true)]IReadOnlyList<Document> documents,
ILogger log)
{
if (documents != null && documents.Count > 0)
{
log.LogInformation($"Documents modified: {documents.Count}");
log.LogInformation($"First document Id: {documents[0].Id}");
}
}
}
}
and here is the binding information of same azure function which is used to pass the param value to function
Cosmos DB trigger binding in a function.json file
{
"type": "cosmosDBTrigger",
"name": "documents",
"direction": "in",
"leaseCollectionName": "leases",
"connectionStringSetting": "<connection-app-setting>",
"databaseName": "Tasks",
"collectionName": "Items",
"createLeaseCollectionIfNotExists": true
}
To answer your question how this is even resolved at build time/deployment time" :- To use it locally we pass the same binding information in host.json file and local.settings.json file.
That's how it bind the information internally by checking param name.
Hope it helps.