Work with shared instances
Starting with 10.8.1, enabling SOEs or SOIs with map services that are set to use the shared instance pool is supported. Since shared instances manage service resources and service Initialization differently from dedicated instances, not all extensions are safe to use with shared instances. You, as the developer, should follow the best coding practice to ensure that the SOE or SOI meets all the requirements and is safe to use with shared instances.
To distinguish the extensions that meet those criteria, a new extension property, named
SupportsSharedInstances, is introduced to allow the developer to flag if the extension can be used for shared instances. The following is an example of the
[ServerObjectInterceptor("MapServer", Description = "Simple REST SOE for map services published from ArcGIS Pro", DisplayName = "Simple REST SOE (Map Service - Pro)", Properties = "", SupportsSharedInstances = true)]
The code sets
true, which allows this SOI to be enabled with a map service set to use the shared instance pool. This property is an optional property with the default value
false. Unless you explicitly set it to
true as shown in the code example, the SOE or SOI can't be enabled with map services set for shared instances. You should only set this property to
true to allow the extension for shared instances after ensuring your extension follows the best coding practice.
If your extension will be enabled with multiple services that are set to use shared instances, it's important to ensure that the resources accessed by the extension of each service are handled properly.
First, you should not use global or static variables in your SOE or SOI, because these variables can be accessed by different services sharing the same instance, which can cause unexpected values to be assigned to those variables. It's safe to use local variables, because their scope is within the current extension or service and they can't be altered by other services.
Second, be cautious when you write your own singletons in extensions. Your singletons should be stateless. A singleton with a state (or parameters) in extensions can cause its state to be interfered with by other services using the same extension in the shared instance pool, therefore stateful singleton should be avoided. The singletons in ArcGIS Enterprise SDK, such as
SpatialReferenceEnvironment, are all stateless and safe to use with shared instances.
Last, SOEs and SOIs should be single threaded. Multi-threading is not supported in SOEs or SOIs for both dedicated instances and shared instances. If your SOE involves long-time processing, you should consider developing a custom geoprocessing tool to be published as a geoprocessing service instead of using an SOE.
In SOEs and SOIs, the
Init() method handles the extension's Initialization. When the extension is enabled with dedicated instances, this
Init() method is called after the service is started using either ArcGIS Server Manager or ArcGIS REST API. With shared instances, starting the service does not trigger the extension's
Init() method immediately. This method is only called after the service is accessed or receives a request. For example, the REST SOE's
Init() method will not be called until you visit the REST endpoint of the SOE or send a REST request to the SOE, and the SOI's
Init() method will not be called until you send a request to the service with which the SOI is enabled.
Since all the shared instances are managed by the DynamicMappingHost service, the life cycle of SOEs and SOIs are also subject to the status of the DynamicMappingHost service, which can be managed via ArcGIS REST API:
Starting the DynamicMappingHost service will launch the underlying DynamicMappingHost ArcSOC processes. The number of shared instances per machine setting for shared instance controls the number of the DynamicMappingHost ArcSOC processes that are running. Once the DynamicMappingHost service is started, you can access a running extension to trigger its
Init() method. Stopping the DynamicMappingHost service will first trigger the extension's
Shutdown() method, and then stop all the DynamicMappingHost ArcSOC processes.
Since each instance in the shared pool caches information about the services that have received requests, restarting the DynamicMappingHost service will clear the cache.