Choosing a Service Type
A service can be a socket,
pool, or launch service.
The features, advantages, and disadvantages of each of these
service types is discussed below.
Socket services consist of one or more Application Servers that run
continuously, servicing client requests. Socket services are generally
started whenever a machine is restarted (either manually or by an
operating system mechanism for starting processes at boot or login
time). The service usually runs until the machine is shut down.
Socket services are relatively simple to configure and manage and are
adequate for most applications.
Advantages
- Socket services are supported on all SAS/IntrNet platforms. Other
service types are not supported everywhere.
- The server is already running by the time a client request appears, so
clients do not have to wait for a server to start.
- The administrator has explicit control of resources allocated to
the service: the administrator can control how many servers are run
on each system and which resources are allocated to each server.
- Increasing load can be handled by adding more servers to the
service.
Disadvantages
- Servers must be started and stopped manually or by the operating
system. No automated start-up and shutdown service is provided by SAS/IntrNet
software.
- No dynamic scaling to meet increasing loads is provided. A fixed
number of servers are available to handle all client requests. A few
long-running requests can slow the entire service for all clients.
Pool services consist of a pool of Application Servers shared by clients.
Based on system loading the servers are started and stopped by the
Application Load Manager. Numerous options are provided to fine-tune the
operation of a pool service. Pool services combine some of the advantages
of socket and launch services.
Advantages
- Servers are started as needed. If all servers in the service are busy,
the Load Manager can start an additional server.
- Servers can be reused by new clients once they are started. A started
server remains in the pool until an idle timeout is reached and the server
is stopped.
- Unlike launch services, pool services can be on a different system than
the Web server and can be distributed across multiple server systems.
- Using the SAS Spawner, servers can be started under specific usernames
to control access to system resources.
Disadvantages
- Installation and configuration are more complex for pool services. The
Application Load Manager must be installed. The SAS Spawner must be
installed in most cases.
- Client requests might have to wait for a new server to start, although
this is typically no worse (and could be better) than waiting for currently
executing requests to complete in a socket service.
A launch service starts a new Application Server for each client request.
An existing server is reused only for applications that use sessions or
the _TMPCAT catalog for IDS output. Most of the features of
launch services are better provided by pool services (a new feature for
Version 8). Launch services are not generally recommended for new
installations.
Advantages
- Server start-up is automatic for each request. Once the launch
service is configured, little or no additional administration is
necessary.
- Requests run in a separate server, so a long-running request will
not block access to the service for other clients.
- Many requests can run in parallel, assuming that the system will support
the load.
- Ill-behaved applications that "crash" or "hang" a
server will not affect other client requests.
Disadvantages
- Launch services are started by the Application Broker and must
run on the same system as the Web server.
- Each new request incurs the resource overhead and delay of starting
a new server session.
- Launch services are not suitable for high user loads. There are
no settable limits on the server load. The service will attempt to start
a new server for each new client. In an extreme case, 200 simultaneous
users could cause 200 servers to be started, likely causing extreme
"memory thrashing" and very slow response for all users. Most
Web servers have limits on the number of simultaneous CGI requests that
could help to control this problem.
- Each launch service request must incur the additional time for
starting a SAS session.
- Launch services are not supported on CMS, OpenVMS, and OS/390
platforms.
- Launched servers can be difficult to shut down. A launched server
that creates a session or _TMPCAT catalog will continue
running until an idle timeout is reached. These servers cannot be
shut down other than by interrupting the server process.
