Understanding Server Load Balancing

Understanding the Load-Balancing Algorithms

The following algorithms support load balancing on SAS workspace server, stored process servers, pooled workspace servers, and OLAP servers:

Cost	(SAS Workspace Servers and SAS Stored Process Servers only) The cost algorithm assigns a cost value (determined by the administrator) to each client that connects to the server. The algorithm can also assign cost values to servers that have not started yet. When a new client requests a connection, load balancing redirects the client to the connection with the lowest cost on the machine with the lowest total cost. This is the default algorithm for stored process and standard workspace servers.
Response Time	(SAS Stored Process Servers only) Each spawner's load balancer maintains an ordered list of machines and their response times. Load balancing updates this list periodically at an interval that is specified by the administrator. When a new client requests a connection, load balancing redirects the client request to the machine at the top of the list.
Grid	(SAS Workspace Servers only) The Grid algorithm communicates with a SAS Grid Server to allow load balancing access to grid-related load information. This information is used by the object spawner to find the least loaded server machine that will accept the client request. (This algorithm is available only when the SAS Grid Server has been deployed.)
Most Recently Used	(SAS Pooled Workspace Servers only) The Most Recently Used algorithm emphasizes reusing workspace servers. This algorithm attempts to send clients into running servers before starting new servers. The goal of this algorithm is to reduce the overhead of starting new servers by using servers that are already running. This is the default algorithm for pooled workspace servers.
Least Recently Used	(SAS OLAP Servers and SAS Pooled Workspace Servers only) The Least Recently Used algorithm attempts to use the least recently used server. This algorithm provides more of a breadth-first approach to balancing the client load.

Cost Algorithm: Overview

The Cost algorithm uses a cost value to represent the work load that is assigned to each server (or server process) in the load-balancing cluster. Each time a client connects or a stored process is executed, the spawner updates the cost value for the appropriate server. When a client requests a connection to the load-balancing cluster, the spawner examines the cost values for all of the servers in the cluster, and then redirects the client to the server that has the lowest cost value.

The Cost algorithm works differently for SAS Workspace Servers and SAS Stored Process Servers:

SAS Workspace Servers. When a new client requests a connection, the load-balancing spawner redirects the client to the server that has the lowest cost value. When the client connects to the designated server, the spawner increments that server's cost by a specified value (cost per client). When that client disconnects, the spawner decrements that server's cost by the same value (cost per client).
SAS 9.2 Stored Process Servers. When a new client requests a connection, the load-balancing spawner redirects the client to the server process that has the lowest cost value on the machine with the lowest total cost. When the client connects to the designated server process, the spawner increments the cost for that process by the same value (cost per client).

The stored process server cost is determined by two values, Session Cost and Context Cost. The default values are 1 and 100 respectively. To change the default values, use the Options tab in the properties window of the logical stored process server.

Cost Algorithm: Parameters

The Cost algorithm uses the following cost parameters, which are treated as weighted values:

Cost per client	(field on the load-balancing logical server definition) specifies the default amount of weight (cost) that each client adds (when it connects) or subtracts (when it disconnects) to the total cost of the server.
Startup cost	(field on the server definition) specifies the start-up cost of the server. When a request is made to the load-balancing spawner, the spawner assigns this start-up cost value to inactive servers. A new server is not started unless it is determined that its cost (the start-up cost) is less than that of the rest of the servers in the cluster. This field enables the administrator to control the order in which servers are started. After a server is started, the cost value is 0. When a client connects to the server, the server's cost value is increased.
Maximum cost	(field on the load-balancing logical server definition) specifies the maximum cost value that each server can have. After a server reaches maximum cost, the load-balancing spawner will not redirect any more clients to the server until its cost value decreases.

Cost Algorithm: SAS Workspace Server Example

A load balancing cluster contains two workspace servers on two different machines, Machine A and Machine B. The following table displays the initial status of the cluster:

Initial Cluster Status
Parameters	Workspace Server A	Workspace Server B
Clients	3	2
Maximum Cost	500	200
Cost Per Client	100	100
Cost to Connect	300	200

At the start of the example, five clients have connected to the cluster and the client connections are balanced between the two servers. Workspace Server A has three clients and Workspace Server B has two clients. The following figure illustrates what happens when an additional client requests a connection:

New Client Connection

[New Client Connection]

The client requests a connection to Workspace Server B. The spawner on Machine B examines the cost values of all of the servers in the cluster. Workspace Server B has the least cost, but it has reached its Maximum Cost value and cannot accept any more clients. The spawner redirects the client to Workspace Server A.

The client requests a connection to Workspace Server A. The spawner on Machine A creates a server connection for the client, and then increments the cost value for Workspace Server A by the cluster's Cost Per Client value (100).

The following table displays the final status of the cluster:

Final Cluster Status
Parameters	Workspace Server A	Workspace Server B
Clients	4	2
Maximum Cost	500	200
Cost Per Client	100	100
Cost to Connect	400	200

At the end of the example, the cost to connect to Workspace Server A is 400, because there are four clients and the Cost Per Client value is 100.

Cost Algorithm: SAS Stored Process Server Example

A load-balancing cluster contains one stored process server with two server processes (MultiBridge connections), Server Process A and Server Process B. The following table displays the initial status of the cluster:

Initial Cluster Status
Parameters	Server Process A	Server Process B
Status	Running	Not Running
Clients	2	0
Stored Processes	2	0
Startup Cost	300	300
Cost to Connect	402	300
Cost Per Client	100	100

At the start of the example, Server Process A is running and has two clients. Each client on Server Process A is running one stored process, so the cost to connect for Server A is 402 (2 clients * 100 + 2 processes running * 101). Server Process B has not started yet, so the cost to connect to Server Process B is the Startup Cost (300). The following figure illustrates what happens when an additional client connects:

New Client Connection

[New Client Connection]

The client requests a connection to the stored process server. The load-balancing spawner examines the cost values of all of the servers in the cluster and determines that Server Process B has the lowest cost. The spawner redirects the client to Server Process B.

The client requests a connection to Server Process B. The spawner starts the server process and then provides a connection to the client. The spawner increments the cost value for Server Process B by the cluster's Cost Per Client value (100).

The following table displays the final status of the cluster:

Final Cluster Status
Parameters	Server Process A	Server Process B
Status	Running	Running
Clients	2	1
Stored Processes	2	0
Startup Cost	300	300
Cost to Connect	402	100
Cost Per Client	100	100

At the end of the example, the cost for Server Process B is 100, because there is one client and the Cost Per Client value is 100. There are no stored processes running, and the Startup Cost value does not apply because the server process has been started. If the client submits a stored process, the cost will increase by 101 (the standard cost per stored process).

Response Time Algorithm (SAS Stored Process Server Only)

The Response Time algorithm uses a list of server response times in order to determine which server process has the least load. For each server process in the load-balancing cluster, the load-balancing spawner maintains an ordered list of servers and their average response times. Each time the spawner receives a client request, it redirects the client to the server process at the top of the list. The spawner updates the server response times periodically. You can specify the update frequency for the response time (response refresh time) in the metadata for the load-balancing cluster.

The Response Time algorithm uses the following parameters:

Refresh rate

(field on the load-balancing logical server definition) specifies the length of the period in milliseconds that the load-balancing spawner will use the current response times. At the end of this period the spawner updates the response times for all of the servers in the cluster and then reorders the list of servers.

Note: If this field is set to 0, the load-balancing spawner does not use the response time list to redirect clients to servers. Instead, the spawner redirects clients to servers sequentially, in the order in which the servers are defined in the metadata. [cautionend]

Maximum clients

(field on the server definition) specifies the maximum number of clients that a server can have. After a server reaches its maximum number of clients, the spawner will not redirect any more clients to the server until a client disconnects.

Grid Algorithm (SAS Workspace Server Only)

If you have a SAS grid installed and configured, then you can leverage the functionality of the SAS Grid Manager to identify the SAS Workspace Server best suited to handle a SAS client's request in your cluster of workspace servers. The Grid algorithm communicates with a SAS Grid Server to allow load-balancing access to grid-related load information. This information is used by the object spawner to find the least loaded server machine that will accept the client request.

The Grid algorithm uses the following parameters:

Grid server: (field on the load-balancing logical server definition) specifies the name of the SAS Grid Server with which the object spawner gathers grid-related load information.
Grid server credentials: (field on the load-balancing logical server definition) specifies valid credentials that the object spawner uses to authenticate with the grid server.
Grid server connect timeouts: (field on the load-balancing logical server definition) specifies the amount of time (in seconds) to wait for a connection to the grid server.

Most Recently Used Algorithm (SAS Pooled Workspace Server Only)

The Most Recently Used algorithm emphasizes reusing workspace servers. This algorithm attempts to send clients into running servers before starting new servers. The goal of this algorithm is to reduce the overhead of starting new servers by using servers that are already running.

The Most Recently Used algorithm uses the following parameters:

Server process maximum: (field on the server definition) specifies the maximum number of server processes that can be created for this server definition.
Server process minimum: (field on the server definition) specifies the minimum number of server processes that are available at any time for this server definition.

Least Recently Used Algorithm (SAS Pooled Workspace Server Only)

The Least Recently Used algorithm attempts to use the least recently used server. This algorithm balances provides more of a breadth-first approach to balancing the client load.

The Least Recently Used algorithm uses the following parameters:

Server process maximum: (field on the server definition) specifies the maximum number of server processes that can be created for this server definition.
Server process minimum: (field on the server definition) specifies the minimum number of server processes that are available at any time for this server definition.

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