How the GIS server works

ArcGIS Server is a distributed system consisting of several components that can be distributed across multiple machines. Each component in the ArcGIS Server system plays a specific role in the process of managing, activating, deactivating, and load balancing the resources that are allocated to a given service or set of services.

The components of ArcGIS Server can be summarized as:

An ArcGIS Server system also includes a set of services, Web applications, ArcGIS Explorer Maps, and KML network links that have been published on the server, as well as a Manager application for creating and organizing them. This group of services and applications, with its associated Web server and GIS server, is called an ArcGIS Server instance.

The ArcGIS Server system architecture

The GIS server

The GIS server is composed of a server object manager (SOM) and server object containers (SOCs). The SOM manages the set of services that are distributed across one or more SOCs. When an application makes a direct connection to a GIS server over a LAN or WAN, it is making a connection to the SOM.

Server object containers (SOCs) host the services that are managed by the SOM. All services run on all container machines, so it's important that all container machines have access to the resources and data necessary to run each service. You can set the capacity value of a SOC machine to limit the number of running services it can host at one time.

Each container machine is capable of hosting multiple container processes, which are processes in which one or more services are running. The SOM starts and shuts down the container processes. The objects hosted within the container processes are ArcObjects components that are installed on the container machine as part of the installation of ArcGIS Server.

The SOM and SOC are processes that run on a machine; therefore, a single machine can act as both a SOM and a SOC in an ArcGIS Server configuration. If desired, the Web server and the ADF can coexist with the SOM and the SOC, allowing for a deployment of ArcGIS Server on just one machine.

Server directories

The server manages several types of directories, which are used to store files that the server needs for its work.

• Output directories are for temporary files needed by the server. Sometimes, these files will be returned to the user as output, such as map images. Some service types, such as geodata services, require output directories. For other services, an output directory is optional or not needed.
• Cache directories store caches of pre-rendered map tiles that map services can use for faster display. You can use ArcCatalog to create a cache.
• The jobs directory stores files needed by geoprocessing services. Often, geoprocessing tasks require a space to write temporary files and store information about ongoing jobs. These items are stored in the jobs directory.

Processes started by the GIS server

The Windows service "ArcGIS Server Object Manager" represents the GIS server. This service starts the following processes which will always be running on a healthy GIS server, even when all GIS services have been stopped:

ArcSOM.exe - 1 instance

• Server Object Manager process - Acts as a broker for requests to the various services

ArcSOC.exe - 2 instances

• Server Log Process - Records log messages generated from services.
• Server Directory Manager - Cleans ArcGIS Server directories.

The above-mentioned ArcSOC.exe processes can be created on any SOC machine are indistinguishable from other ArcSOC.exe processes except by size. The logging and directory processes are generally smaller than ArcSOC.exe processes that represent GIS services.

The Web server

The Web server hosts server applications and Web services written using the ArcGIS Server API. These server applications use the ArcGIS Server API to connect to a SOM, make use of services, and create other ArcObjects for use in their applications. These Web services and Web applications can be written using the ArcGIS Server Web Application Developer Framework (Web ADF). Examples of Web applications include mapping applications, disconnected editing applications, and any other application that makes use of ArcObjects and is appropriate for Web browsers.

Web services can expose, for example, map and geocode services that desktop GIS users can connect to and consume over the Internet. It is possible to create your own native Web services whose parameters are not ArcObjects types, but do perform a specific GIS function. For example, you could write a Web service called FindNearestHospital that accepts x,y coordinates as input and returns an application-defined Hospital object that has properties such as the address, name, and number of beds.

Web applications connect to GIS servers within their organization over the LAN. In this sense, the Web application or Web service is a client of the GIS server. Users connect to Web applications and Web services over the Internet or Intranet, but all of the Web application's logic runs in the Web server and sends hypertext markup language (HTML) to the browser client. The Web application itself makes use of objects and functionality running within the GIS server. This allows the development of Web applications to make use of ArcObjects in the server as would a desktop application connecting to the GIS server in client/server mode over the LAN or WAN.

As users interact with their browser, it makes requests to the Web application, which in turn makes requests to the SOM. The SOM hands back a proxy to a server object or server objects that are running within the GIS server. The Web application uses the proxy to work with the object as if it existed in the Web application's process, but all execution happens on the GIS server.

Web applications running in the Web server connect to the SOM and work with proxies to objects running within containers in the GIS server.

Clients

Clients of an ArcGIS Server system can include any of the following:

Web browsers

Anyone with a web browser and an Internet connection can perform GIS tasks on services using an appropriately designed web application. The ADF provides tools for creating web applications that make use of services. Since all the work is done on the server, end users of these web applications don't need to have any GIS software or ArcObjects installed on their machines.

ArcGIS Explorer

ArcGIS Explorer is a free lightweight desktop client for ArcGIS Server that can display data in two or three dimensions. You can add your ArcGIS Server services as data in ArcGIS Explorer for a rich navigation and viewing experience. For advanced functionality, you can use the ArcGIS Explorer SDK to develop custom tasks that work with the services.

ArcGIS Desktop

You can connect to ArcGIS Server using ArcGIS Desktop applications to make use of services running in the server. ArcGIS Desktop applications that can access services include ArcMap, ArcCatalog, ArcGlobe, and ArcReader.

With ArcCatalog, you can connect to a GIS server directly on the LAN or WAN. You can also specify the URL of a GIS server or a specific Web service running on that GIS server to indirectly connect to a GIS server over the Internet.

The GIS server administrator can also use ArcCatalog to administer the set of services and their properties. Administrators can connect to the GIS server over the LAN/WAN and use ArcCatalog to add and remove services, as well as configure how services should be run. Administrators also designate the set of container machines that are available for the server and the directories the server can use to write any output.

ArcGIS Engine applications

ArcGIS Engine applications can utilize the GIS server in a variety of ways. The simplest is by working with services in the form of data inside map or globe documents. An ArcGIS Engine developer could also design an application that sends data off to the GIS server for advanced tasks such as spatial analysis that require extensions only licensed on the server machine. Conversely, the application might request data from the server to edit or analyze within the rich user interface of the ArcGIS Engine application.