External Configuration Store Pattern

Move configuration information out of the application deployment package to a centralized location. This pattern can provide opportunities for easier management and control of configuration data, and for sharing configuration data across applications and application instances.

Context and Problem

The majority of application runtime environments include configuration information that is held in files deployed with the application, located within the application folders. In some cases it is possible to edit these files to change the behavior of the application after it has been deployed. However, in many cases, changes to the configuration require the application to be redeployed, resulting in unacceptable downtime and additional administrative overhead.

Local configuration files also limit the configuration to a single application, whereas in some scenarios it would be useful to share configuration settings across multiple applications. Examples include database connection strings, UI theme information, or the URLs of queues and storage used by a related set of applications.

Managing changes to local configurations across multiple running instances of the application, especially in a cloud-hosted scenario, may also be challenging. It may result in instances using different configuration settings while the update is being deployed.

In addition, updates to applications and components may require changes to configuration schemas. Many configuration systems do not support different versions of configuration information.

Solution

Store the configuration information in external storage, and provide an interface that can be used to quickly and efficiently read and update configuration settings. The type of external store depends on the hosting and runtime environment of the application. In a cloud-hosted scenario it is typically a cloud-based storage service, but could be a hosted database or other system.

The backing store chosen for configuration information should be fronted by a suitable interface that provides consistent and easy to use access in a controlled way that enables reuse. Ideally, it should expose the information in a correctly typed and structured format. The implementation may also need to authorize users’ access in order to protect configuration data, and be flexible enough to allow multiple versions of the configuration (such as development, staging, or production, and multiple release versions of each one) to be stored.

Figure 1 shows an overview of this pattern.

Figure 1 - An overview of the External Configuration Store pattern with optional local cache

Issues and Considerations

Consider the following points when deciding how to implement this pattern:

• Choose a backing store that offers acceptable performance, high availability, robustness, and can be backed up as part of the application maintenance and administration process. In a cloud-hosted application, using a cloud storage mechanism is usually a good choice to meet these requirements.
• Design the schema of the backing store to allow flexibility in the types of information it can hold. Ensure that it provides for all configuration requirements such as typed data, collections of settings, multiple versions of settings, and any other features that the applications using it may require. The schema should be easy to extend as requirements change in order to support additional settings.
• Consider the physical capabilities of the backing store, how it relates to the way that configuration information is stored, and the effects on performance. For example, storing an XML document containing configuration information will require either the configuration interface or the application to parse the document in order to read individual settings, and will make updating a setting more complicated, though caching the settings can help to offset slower read performance.
• Consider how the configuration interface will permit control of the scope and inheritance of configuration settings. For example, it may be a requirement to scope configuration settings at the organization, application, and the machine level; to support delegation of control over access to different scopes; and to prevent or allow individual applications to override settings.
• Ensure that the configuration interface can expose the configuration data in the required formats such as typed values, collections, key/value pairs, or property bags. However, consider the balance between capabilities and complexity of the API in order to make it useful and yet as easy to use as possible.
• Consider how the configuration store interface will behave when settings contain errors, or do not exist in the backing store. It may be appropriate to return default settings and log errors. Also consider aspects such as the case sensitivity of configuration setting keys or names, the storage and handling of binary data, and the ways that null or empty values are handled.
• Consider how you will protect the configuration data to allow access only to the appropriate users and applications. This is likely to be a feature of the configuration store interface, but it is also necessary to ensure that the data in the backing store cannot be accessed directly without the appropriate permission. Ensure strict separation between the permissions required to read and to write configuration data. Also consider whether you need to encrypt some or all of the configuration settings, and how this will be implemented within the configuration store interface.
• Keep in mind that centrally stored configurations, which change application behavior during runtime, are critically important and should be deployed, updated, and managed using the same mechanisms as deploying application code. For example, changes that can affect more than one application must be carried out using a full test and staged deployment approach to ensure that the change is appropriate for all applications that use this configuration. If an administrator simply edits a setting to update one application, it could adversely impact other applications that use the same setting.
• If an application caches configuration information, the application may need to be alerted if the configuration changes. It may be possible to implement an expiration policy over cached configuration data so that this information is automatically refreshed periodically and any changes picked up (and actioned). The Runtime Reconfiguration pattern described elsewhere in this guide may be relevant to your scenario.

When to Use this Pattern

This pattern is ideally suited for:

• Configuration settings that are shared between multiple applications and application instances, or where a standard configuration must be enforced across multiple applications and application instances.
• Where the standard configuration system does not support all of the required configuration settings, such as storing images or complex data types.
• As a complementary store for some of the settings for applications, perhaps allowing applications to override some or all of the centrally-stored settings.
• As a mechanism for simplifying administration of multiple applications, and optionally for monitoring use of configuration settings by logging some or all types of access to the configuration store.

Example

In a Microsoft Azure hosted application, a typical choice for storing configuration information externally is to use Azure storage. This is resilient, offers high performance, and is replicated three times with automatic failover to offer high availability. Azure tables provide a key/value store with the capability to use a flexible schema for the values. Azure blob storage provides a hierarchical container-based store that can hold any type of data in individually named blobs.

The following example shows how a configuration store can be implemented over Azure blob storage to store and expose configuration information. The BlobSettingsStore class abstracts blob storage for holding configuration information, and implements the ISettingsStore interface shown in the following code.

public interface IsettingsStore{  string Version { get; }  Dictionary<string, string> FindAll();  void Update(string key, string value);}

This interface defines methods for retrieving and updating configuration settings held in the configuration store, and includes a version number that can be used to detect whether any configuration settings have been modified recently. When a configuration setting is updated, the version number changes. The BlobSettingsStore class uses the ETag property of the blob to implement versioning. The ETag property of a blob is updated automatically each time the blob is written.

The ExternalConfigurationManager class provides a wrapper around a BlobSettingsStore object. An application can use this class to store and retrieve configuration information. This class uses the Microsoft Reactive Extensions library to expose any changes made to the configuration through an implementation of the IObservable interface. If a setting is modified by calling the SetAppSetting method, the Changed event is raised and all subscribers to this event will be notified.

Note that all settings are also cached in a Dictionary object inside the ExternalConfigurationManager class for fast access. The SetAppSetting method updates this cache, and the GetSetting method that an application can use to retrieve a configuration setting reads the data from the cache (if the setting is not found in the cache, it is retrieved from the BlobSettingsStore object instead).

The GetSettings method invokes the CheckForConfigurationChanges method to detect whether the configuration information in blob storage has changed by examining the version number and comparing it with the current version number held by the ExternalConfigurationManager object. If one or more changes have occurred, the Changed event is raised and the configuration settings cached in the Dictionary object are refreshed. This is an application of the Cache-Aside pattern.

The following code sample shows how the Changed event, the SetAppSettings method, the GetSettings method, and the CheckForConfigurationChanges method are implemented

public class ExternalConfigurationManager : IDisposable{  // An abstraction of the configuration store.  private readonly ISettingsStore settings;  private readonly ISubject<KeyValuePair<string, string>> changed;  ...  private Dictionary<string, string> settingsCache;  private string currentVersion;  ...  public ExternalConfigurationManager(ISettingsStore settings, ...)  {    this.settings = settings;    ...  }  ...  public IObservable<KeyValuePair<string, string>> Changed  {    get { return this.changed.AsObservable(); }  }  ...  public void SetAppSetting(string key, string value)  {    ...    // Update the setting in the store.    this.settings.Update(key, value);    // Publish the event.    this.Changed.OnNext(         new KeyValuePair<string, string>(key, value));    // Refresh the settings cache.    this.CheckForConfigurationChanges();  }  public string GetAppSetting(string key)  {    ...    // Try to get the value from the settings cache.      // If there is a miss, get the setting from the settings store.    string value;    if (this.settingsCache.TryGetValue(key, out value))    {      return value;    }    // Check for changes and refresh the cache.    this.CheckForConfigurationChanges();    return this.settingsCache[key];  }  ...  private void CheckForConfigurationChanges()  {    try    {      // Assume that updates are infrequent. Lock to avoid      // race conditions when refreshing the cache.      lock (this.settingsSyncObject)      {          {        var latestVersion = this.settings.Version;        // If the versions differ, the configuration has changed.        if (this.currentVersion != latestVersion)        {          // Get the latest settings from the settings store and publish the changes.          var latestSettings = this.settings.FindAll();          latestSettings.Except(this.settingsCache).ToList().ForEach(                                kv => this.changed.OnNext(kv));          // Update the current version.          this.currentVersion = latestVersion;          // Refresh settings cache.          this.settingsCache = latestSettings;        }      }    }    catch (Exception ex)    {      this.changed.OnError(ex);    }  }}

An ExternalConfigurationManager object can also query the BlobSettingsStore object periodically for any changes (by using a timer). The StartMonitor and StopMonitor methods illustrated in the code sample below start and stop the timer. The OnTimerElapsed method runs when the timer expires and invokes the CheckForConfigurationChanges method to detect any changes and raise the Changed event, as described earlier.

public class ExternalConfigurationManager : IDisposable{  ...  private readonly ISubject<KeyValuePair<string, string>> changed;  private readonly Timer timer;  private ISettingsStore settings;  ...  public ExternalConfigurationManager(ISettingsStore settings,                                       TimeSpan interval, ...)  {    ...    // Set up the timer.    this.timer = new Timer(interval.TotalMilliseconds)    {      AutoReset = false;    };    this.timer.Elapsed += this.OnTimerElapsed;    this.changed = new Subject<KeyValuePair<string, string>>();    ...      }  ...  public void StartMonitor()  {    if (this.timer.Enabled)    {      return;    }    lock (this.timerSyncObject)    {      if (this.timer.Enabled)      {        return;      }      this.keepMonitoring = true;      // Load the local settings cache.      this.CheckForConfigurationChanges();      this.timer.Start();    }  }  public void StopMonitor()  {    lock (this.timerSyncObject)    {      this.keepMonitoring = false;      this.timer.Stop();    }  }  private void OnTimerElapsed(object sender, EventArgs e)  {    Trace.TraceInformation(          "Configuration Manager: checking for configuration changes.");    try    {      this.CheckForConfigurationChanges();    }    finally    {      ...      // Restart the timer after each interval.      this.timer.Start();      ...    }      }    ...}

The ExternalConfigurationManager class is instantiated as a singleton instance by the ExternalConfiguration class shown below.

public static class ExternalConfiguration{  private static readonly Lazy<ExternalConfigurationManager> configuredInstance                             = new Lazy<ExternalConfigurationManager>(    () =>    {      var environment = CloudConfigurationManager.GetSetting("environment");      return new ExternalConfigurationManager(environment);    });  public static ExternalConfigurationManager Instance  {    get { return configuredInstance.Value; }  }}

The following code is taken from the WorkerRole class in the ExternalConfigurationStore.Cloud project. It shows how the application uses the ExternalConfiguration class to read and update a setting.

public override void Run(){  // Start monitoring for configuration changes.  ExternalConfiguration.Instance.StartMonitor();  // Get a setting.  var setting = ExternalConfiguration.Instance.GetAppSetting("setting1");  Trace.TraceInformation("Worker Role: Get setting1, value: " + setting);  Thread.Sleep(TimeSpan.FromSeconds(10));  // Update a setting.  Trace.TraceInformation("Worker Role: Updating configuration");  ExternalConfiguration.Instance.SetAppSetting("setting1", "new value");  this.completeEvent.WaitOne();}

The following code, also from the WorkerRole class, shows how the application subscribes to configuration events.

public override bool OnStart(){   ...  // Subscribe to the event.  ExternalConfiguration.Instance.Changed.Subscribe(     m => Trace.TraceInformation("Configuration has changed. Key:{0} Value:{1}",           m.Key, m.Value),     ex => Trace.TraceError("Error detected: " + ex.Message));
  ...
}

Related Patterns and Guidance

The following pattern may also be relevant when implementing this pattern:

• Runtime Reconfiguration Pattern. In addition to storing configuration externally, it is useful to be able to update configuration settings and have the changes applied without restarting the application. The Runtime Reconfiguration pattern describes how to design an application so that it can be reconfigured without requiring redeployment or restarting.

This pattern has a sample application associated with it. You can download the "Cloud Design Patterns – Sample Code" from the Microsoft Download Center at https://aka.ms/cloud-design-patterns-sample.

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