Initial commit carried over from private repo. This is V2.

code/MessengerApi.Configuration/Sources/Environment/Constants.EnvironmentVariables.cs

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+namespace MessengerApi.Configuration
+{
+    public static partial class Constants
+    {
+        public static class EnvironmentVariables
+        {
+            public const string SQL_CONNECTIONSTRING = nameof(SQL_CONNECTIONSTRING);
+            public const string NPG_CONNECTIONSTRING = nameof(NPG_CONNECTIONSTRING);
+            public const string PERSISTENCE_TYPE = nameof(PERSISTENCE_TYPE);
+            public const string CORS_ORIGINS = nameof(CORS_ORIGINS);
+            public const string PROXIES = nameof(PROXIES);
+            public const string QUERY_RATE_PER_MINUTE = nameof(QUERY_RATE_PER_MINUTE);
+            public const string DEFAULT_MESSAGE_LIFETIME_IN_MINUTES = nameof(DEFAULT_MESSAGE_LIFETIME_IN_MINUTES);
+            public const string HOUSEKEEPING_ENABLED = nameof(HOUSEKEEPING_ENABLED);
+            public const string HOUSEKEEPING_MESSAGE_AGE_IN_MINUTES = nameof(HOUSEKEEPING_MESSAGE_AGE_IN_MINUTES);
+            public const string HOUSEKEEPING_MESSAGE_STATE = nameof(HOUSEKEEPING_MESSAGE_STATE);
+            public const string LOGGING_VERBOSITY = nameof(LOGGING_VERBOSITY);
+        }
+    }
+}

code/MessengerApi.Configuration/Sources/Environment/EnvironmentConfigurationSource.cs

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+namespace MessengerApi.Configuration.Sources.Environment
+{
+    public class EnvironmentConfigurationSource : IEnvironmentConfigurationSource
+    {
+        public bool HasKey(string key)
+        {
+            return !string.IsNullOrWhiteSpace(System.Environment.GetEnvironmentVariable(key));
+        }
+
+        public T GetValue<T>(string key)
+        {
+            return (T)Convert.ChangeType(System.Environment.GetEnvironmentVariable(key), typeof(T));
+        }
+    }
+}

code/MessengerApi.Configuration/Sources/Environment/IEnvironmentConfigurationSource.cs

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+namespace MessengerApi.Configuration.Sources.Environment
+{
+    public interface IEnvironmentConfigurationSource : IConfigurationSource
+    {
+    }
+}