ArcGIS Server is Esri software that allows you to expose your GIS as a set of web services. It is just one component in a larger software suite called ArcGIS Enterprise that enables organizations to deploy their GIS onto the web. In Lesson 5 we'll talk more about the different parts of ArcGIS Enterprise.
Web services are software code or components that run on a specialized machine called a server. Web services receive requests from other apps and machines, called clients. The request might be to send some information or process some data. GIS web services do things related to sending and processing geographic information. Here are some examples of types of web services offered by ArcGIS Server.
- A map service exposes your map as a web service. It originates with an ArcMap document (MXD) that you "publish" on the server. Clients can use the map service to request a map image at any scale or extent in a variety of image formats. Depending on how the service is configured, the server either draws the map on the fly, or sends back some pre-drawn map images (tiles) that are retrieved from a cache. Clients can also request the geometry (vertex locations) and attributes of any feature in the map.
- A geoprocessing service allows clients to request that the server perform some GIS action or analysis and send back the result. For example, the client might send the vertices of a parcel to the server and request that the server run a model to buffer the parcel and select all other parcels that fall within the buffer. A geoprocessing service works with tools and models that you have run in ArcMap.
- A geometry service calculates lengths, areas, and spatial relationships.
- A number of other service types allow for synchronizing geodatabases over the Internet, analyzing raster images, and so on. Additionally, ArcGIS Server services can expose further, optional subsets of actions through a concept called “capabilities.” The services available through ArcGIS Server and their capabilities are listed at the ArcGIS Resources site.
ArcGIS Server components
ArcGIS Server works through the concept of distributed computing, in which you can increase the power of your server by adding more physical machines. For this reason, ArcGIS Server is made up of several different components that you can either install all on one machine or spread out among many machines. We won’t examine these components in much detail in this course because you will have ArcGIS Server installed for you, and you will only be using one machine. However, below is a brief introduction of the most common components.
Note that the ArcGIS Server architecture fundamentally changed between versions 10.0 and 10.1, and the information below describes the architecture from 10.1 onward. If you run across references to components like the "SOM" and the "SOC" in other literature, you are reading about the "old" architecture from versions 10.0 and previous.
- GIS server – The GIS server is the component where ArcGIS Server does its work. You can install the GIS server on one or many machines (although you are required to have paid the applicable Esri licensing fees for each machine). If you want to add more computing power to your ArcGIS Server system, the easiest way is by installing the GIS server on more machines and connecting them to your system.
The default setting is for all GIS servers in your site to make available the same set of web services, meaning you could make a request to any GIS server for any service. The GIS servers communicate with each other and distribute the load evenly. For advanced workflows, you can optionally organize the GIS servers in "clusters" that run dedicated subsets of services. For example, you might have three normal sized machines in Cluster A that run all your map web services, and one big machine in Cluster B that runs your geoprocessing web services.
- Web server and Web Adaptor – The GIS server component mentioned above has some rudimentary ability to serve out web services, but in most deployments of ArcGIS Server you'll want to connect it to your organization's existing web server. The web server receives the web service requests when they first come in from client applications. It then forwards the requests to the GIS server. Esri makes available a supplemental installation called the Web Adaptor, whose purpose is to connect ArcGIS Server with your web server.
Including a web server in your architecture allows you to monitor and control the URL or web address that people use for your server. Some organizations also perform authentication (challenging the user for a name and password) at the web server level of the architecture. Web servers can further be configured to block traffic from specific addresses, providing a layer of protection for your site.
A web server is also useful for hosting web applications. Once you design a nifty app to show off your web services, you need a machine to host it on. This machine should be a secured server that can handle the amount of traffic coming into your app. In this course, you'll use the same EC2 instance for your web server and GIS server, which is a common practice in small deployments. The web server software you'll use is Microsoft Internet Information Services (IIS). It comes pre-installed on your instance.
- Database – Most servers sit between a client who wants some specific piece of information, and massive collections of raw data sitting in a database. Web services provide structured access points to the data. The location of your GIS database is important to consider when building a cloud architecture for ArcGIS Server.
In ArcGIS Server deployments, the data can either reside in a collection of files (such as Esri shapefiles or file geodatabases) or it can reside in a larger relational database management system (RDBMS) such as Oracle or Microsoft SQL Server. Certain "middleware" components of the ArcGIS Server software facilitate communication between ArcGIS and the RDBMS (in former days these components were called ArcSDE).
The database can either reside directly on the ArcGIS Server machine or it can reside on its own dedicated machine. Putting the data and ArcGIS Server on the same machine can save money, but the database and the server compete for resources and this architecture ultimately has problems with scaling.
No matter where you put the data, you need to register your data locations with ArcGIS Server. This provides the server with a list of known data sources. If you attempt to publish a web service to ArcGIS Server and the data sources are not registered with the server, the data is copied to the server at the time that you publish. This may be a convenient way to copy small datasets to the cloud, but it is not a scalable way to transfer and organize large amounts of data.
Because migrating a large database "across the wire" to the cloud is cumbersome, many individuals wonder if they can deploy a server in the cloud and leave their database "on premises". Such an architecture results in multiple "hops" between geographically dispersed servers just to satisfy one request, and is not nearly as efficient as situating the database in the cloud.
ArcGIS Server accounts and security
When you run a program on the Windows operating system, it runs as a specific user account and can only do things that the account can do. This is why you sometimes see Windows popping up messages that the program needs Administrator permissions to continue. That type of message means that the account running the program is not an administrator, so you need to manually confirm that it should temporarily be allowed to do something that only an administrator would ordinarily be allowed to do.
ArcGIS Server uses an account to run the GIS server, called the ArcGIS Server account. This account is specified during the ArcGIS Server installation. You won't do much with the ArcGIS Server account in this course because it comes preconfigured when you run ArcGIS Server on Amazon EC2.
If you run ArcGIS Server in your own organization, you need to remember to give the ArcGIS Server account permission to read any GIS data used by the server. The account also needs permission to write to any datasets you will edit.
When you work with ArcGIS Server on Amazon EC2, a folder called ‘GIS Data’ is configured on the D: drive. The ArcGIS Server account has already been set up with permissions to this folder. If you put all your data in ‘GIS Data’ you don’t have to worry about manually applying permissions for the ArcGIS Server account.
How you will work with ArcGIS Server in this class
In this class you’ll work with your own ArcGIS Server that runs on Amazon EC2. It has a GIS server and the ArcGIS Server account already configured. After logging in to your server you’ll publish some map services and use them in a web app that you create. You’ll also learn techniques for speeding up your map services, using a tile cache, and how to use a map service for web editing.
This lesson gets you to the point of setting up a server, publishing a service, and making a simple web map on ArcGIS.com.