About Pools

A server pool is a pool containing one or more server nodes that process application traffic. The most common type of server pool contains web servers.

One of the properties of a server pool is a load balancing method. A load balancing method is an algorithm that the BIG-IP^® system uses to select a pool member for processing a request. For example, the default load balancing method is Round Robin, which causes the BIG-IP system to send each incoming request to the next available member of the pool, thereby distributing requests evenly across the servers in the pool.

One type of pool that you can create is a gateway pool. A gateway pool is a pool of routers.

You use a clone pool when you want to configure the BIG-IP system to send traffic to a pool of intrusion detection systems (IDSs). An intrusion detection system (IDS) is a device that monitors inbound and outbound network traffic and identifies suspicious patterns that might indicate malicious activities or a network attack. You can use the clone pool feature of a BIG-IP system to copy traffic to a dedicated IDS or a sniffer device.

To configure a clone pool, you first create a pool of IDS or sniffer devices and then assign the pool as a clone pool to a virtual server. The clone pool feature is the recommended method for copying production traffic to IDS systems or sniffer devices. Note that when you create the clone pool, the service port that you assign to each node is irrelevant; you can choose any service port. Also, when you add a clone pool to a virtual server, the system copies only new connections; existing connections are not copied.

You can configure a virtual server to copy client-side traffic, server-side traffic, or both:

• A client-side clone pool causes the virtual server to replicate client-side traffic (prior to address translation) to the specified clone pool.
• A server-side clone pool causes the virtual server to replicate server-side traffic (after address translation) to the specified clone pool.

You can configure an unlimited number of clone pools on the BIG-IP system.

Health monitors are a key feature of the BIG-IP system. Health monitors help to ensure that a server is in an up state and able to receive traffic. When you want to associate a monitor with an entire pool of servers, you do not need to explicitly associate that monitor with each individual server. Instead, you can simply assign the monitor to the pool itself. the BIG-IP system then automatically monitors each member of the pool.

The BIG-IP system contains many different pre-configured monitors that you can associate with pools, depending on the type of traffic you want to monitor. You can also create your own custom monitors and associate them with pools. The only monitor types that are not available for associating with pools are monitors that are specifically designed to monitor nodes and not pools or pool members. That is, the destination address in the monitor specifies an IP address only, rather than an IP address and a service port. These monitor types are:

• ICMP
• TCP Echo
• Real Server
• SNMP DCA
• SNMP DCA Base
• WMI

With the BIG-IP system, you can configure your monitor associations in many useful ways:

• You can associate a health monitor with an entire pool instead of an individual server. In this case, the BIG-IP system automatically associates that monitor with all pool members, including those that you add later. Similarly, when you remove a member from a pool, the BIG-IP system no longer monitors that server.
• When a server that is designated as a pool member allows multiple processes to exist on the same IP address and port, you can check the health or status of each process. To do this, you can add the server to multiple pools, and then within each pool, associate a monitor with the that server. The monitor you associate with each server checks the health of the process running on that server.
• When associating a monitor with an entire pool, you can exclude an individual pool member from being associated with that monitor. In this case, you can associate a different monitor for that particular pool member, or you can exclude that pool member from health monitoring altogether. For example, you can associate pool members A, B, and D with the http monitor, while you associate pool member C with the https monitor.
• You can associate multiple monitors with the same pool. For instance, you can associate both the http and https monitors with the same pool.

You can specify a minimum number of health monitors. Before Local Traffic Manager™ can report the pool member as being in an up state, this number of monitors, at a minimum, must report a pool member as being available to receive traffic.

When configuring a pool, you can specifically disable any secure network address translations (SNATs) or network address translations (NATs) for any connections that use that pool. By default, these settings are enabled. You can change this setting on an existing pool by displaying the Properties screen for that pool.

One case in which you might want to configure a pool to disable SNAT or NAT connections is when you want the pool to disable SNAT or NAT connections for a specific service. In this case, you could create a separate pool to handle all connections for that service, and then disable the SNAT or NAT for that pool.

You can specify the action that you want the BIG-IP system to take when the service on a pool member becomes unavailable.

Possible actions are:

• None. This is the default action.
• The BIG-IP^® system sends an RST (TCP-only) or ICMP message.
• the BIG-IP system simply cleans up the connection.
• the BIG-IP system selects a different node.

You should configure the system to select a different node in certain cases only, such as:

• When the relevant virtual server is a Performance (Layer 4) virtual server with address translation disabled.
• When the relevant virtual server’s Protocol setting is set to UDP.
• When the pool is a gateway pool (that is, a pool or routers)

When you take a pool member offline, and then bring it back online, the pool member can become overloaded with connection requests, depending on the load balancing method for the pool. For example, if you use the Least Connections load balancing method, the system sends all new connections to the newly-enabled pool member (because, technically, that member has the least amount of connections).

With the slow ramp time feature, you can specify the number of seconds that the system waits before sending traffic to the newly-enabled pool member. The amount of traffic is based on the ratio of how long the pool member is available compared to the slow ramp time, in seconds. Once the pool member is online for a time greater than the slow ramp time, the pool member receives a full proportion of the incoming traffic.

Another pool feature is the Type of Service (ToS) level. The ToS level is one means by which network equipment can identify and treat traffic differently based on an identifier.

As traffic enters the site, the BIG-IP system can set the ToS level on a packet. Using the IP ToS to Server ToS level that you define for the pool to which the packet is sent. the BIG-IP system can apply an iRule and send the traffic to different pools of servers based on that ToS level.

The BIG-IP system can also tag outbound traffic (that is, the return packets based on an HTTP GET) based on the IP ToS to Client ToS value set in the pool. That value is then inspected by upstream devices and given appropriate priority.

For example, to configure a pool so that a ToS level is set for a packet sent to that pool, you can set both the IP ToS to Client level and the IP ToS to Server levels to 16. In this case, the ToS level is set to 16 when sending packets to the client and when sending packets to the server.

Another setting for a pool is the Quality of Service (QoS) level. In addition to the ToS level, the QoS level is a means by which network equipment can identify and treat traffic differently based on an identifier. Essentially, the QoS level specified in a packet enforces a throughput policy for that packet.

As traffic enters the site, the BIG-IP system can set the QoS level on a packet. Using the Link QoS to Server QoS level that you define for the pool to which the packet is sent, the BIG-IP system can apply an iRule that sends the traffic to different pools of servers based on that QoS level.

The BIG-IP system can also tag outbound traffic (that is, the return packets based on an HTTP GET) based on the Link QoS to Client QoS value set in the pool. That value is then inspected by upstream devices and given appropriate priority.

For example, to configure a pool so that a QoS level is set for a packet sent to that pool, you can set the Link QoS to Client level to 3 and the Link QoS to Server level to 4. In this case, the QoS level is set to 3 when sending packets to the client, and set to 4 when sending packets to the server.

You can specify the number of times that the system tries to contact a new pool member after a passive failure. A passive failure consists of a server-connect failure or a failure to receive a data response within a user-specified interval. The default value of 0 indicates no reselects.

TCP request queuing provides the ability to queue connection requests that exceed the capacity of connections for a pool, pool member, or node, as determined by the connection limit. Consequently, instead of dropping connection requests that exceed the capacity of a pool, pool member, or node, TCP request queuing enables those connection requests to reside within a queue in accordance with defined conditions until capacity becomes available.

When using session persistence, a request becomes queued when the pool member connection limit is reached.

Without session persistence, when all pool members have a specified connection limit, a request becomes queued when the total number of connection limits for all pool members is reached.

Conditions for queuing connection requests include:

• The maximum number of connection requests within the queue, which equates to the maximum number of connections within the pool, pool member, or node. Specifically, the maximum number of connection requests within the queue cannot exceed the cumulative total number of connections for each pool member or node. Any connection requests that exceed the capacity of the request queue are dropped.
• The availability of server connections for reuse. When a server connection becomes available for reuse, the next available connection request in the queue becomes dequeued, thus allowing additional connection requests to be queued.
• The expiration rate of connection requests within the queue. As queue entries expire, they are removed from the queue, thus allowing additional connection requests to be queued.

Connection requests within the queue become dequeued when:

• The connection limit of the pool is increased.
• A pool member's slow ramp time limit permits a new connection to be made.
• The number of concurrent connections to the virtual server decreases below the connection limit.
• The connection request within the queue expires.

Load balancing is an integral part of the BIG-IP^® system. Configuring load balancing on a BIG-IP system means determining your load balancing scenario, that is, which pool member should receive a connection hosted by a particular virtual server. Once you have decided on a load balancing scenario, you can specify the appropriate load balancing method for that scenario.

A load balancing method is an algorithm or formula that the BIG-IP system uses to determine the server to which traffic will be sent. Individual load balancing methods take into account one or more dynamic factors, such as current connection count. Because each application of the BIG-IP system is unique, and server performance depends on a number of different factors, we recommend that you experiment with different load balancing methods, and select the one that offers the best performance in your particular environment.

Priority-based member activation is a feature that allows you to categorize pool members into priority groups, so that pool members in higher priority groups accept traffic before pool members in lower priority groups. The priority-based member activation feature has two configuration settings:

Priority group activation

For the priority group activation setting, you specify the minimum number of members that must remain available in each priority group in order for traffic to remain confined to that group. The allowed value for this setting ranges from 0 to 65535. Setting this value to 0 disables the feature (equivalent to using the default value of Disabled).

Priority group

When you enable priority group activation, you also specify a priority group for each member when you add that member to the pool. Retaining the default priority group value of 0 for a pool member means that the pool member is in the lowest priority group and only receives traffic when all pool members in higher priority groups are unavailable.

If the number of available members assigned to the highest priority group drops below the number that you specify, the BIG-IP^® system distributes traffic to the next highest priority group, and so on.

For example, this configuration has three priority groups, 3, 2, and 1, with the priority group activation value (shown here as min active members) set to 2.

pool my_pool {
   lb_mode fastest
   min active members 2
   member 10.12.10.7:80 priority 3
   member 10.12.10.8:80 priority 3
   member 10.12.10.9:80 priority 3
   member 10.12.10.4:80 priority 2
   member 10.12.10.5:80 priority 2
   member 10.12.10.6:80 priority 2
   member 10.12.10.1:80 priority 1
   member 10.12.10.2:80 priority 1
   member 10.12.10.3:80 priority 1
   }

Connections are first distributed to all pool members with priority 3 (the highest priority group). If fewer than two priority 3 members are available, traffic is directed to the priority 2 members as well. If both the priority 3 group and the priority 2 group have fewer than two members available, traffic is directed to the priority 1 group. The BIG-IP system continuously monitors the priority groups, and whenever a higher priority group once again has the minimum number of available members, the BIG-IP system limits traffic to that group.

When using a ratio-based load balancing method for distributing traffic to servers within a pool, you can assign a ratio weight to the corresponding pool members. The ratio weight determines the amount of traffic that the pool member receives. The ratio-based load balancing methods are: Ratio (node, member, and sessions), Dynamic Ratio (node and member), and Ratio Least Connections (node and member).

Using the priority group feature, you can assign a priority number to the pool member. The BIG-IP^® system then distributes traffic in the pool according to the priority number that you assigned to the pool member.

For example, pool members assigned to group 3, instead of pool members in group 2 or group 1, normally receive all traffic. Thus, members that are assigned a high priority receive all traffic until the load reaches a certain level or some number of members in the group become unavailable. If either of these events occurs, some of the traffic goes to members assigned to the next higher priority group.

This setting is used in tandem with the pool feature known as priority group activation. You use the priority group activation feature to configure the minimum number of members that must be available before the BIG-IP system begins directing traffic to members in a lower priority group.

Connection limits

You can specify the maximum number of concurrent connections allowed for a pool member. Note that the default value of 0 (zero) means that there is no limit to the number of concurrent connections that the pool member can receive.

Connection rate limits

The maximum rate of new connections allowed for the pool member. When you specify a connection rate limit, the system controls the number of allowed new connections per second, thus providing a manageable increase in connections without compromising availability. The default value of 0 specifies that there is no limit on the number of connections allowed per second. The optimal value to specify for a pool member is between 300 and 5000 connections. The maximum valued allowed is 100000.

Explicit monitor associations

After you have associated a monitor with a pool, the BIG-IP system automatically associates that monitor with every pool member, including those members that you add to the pool later. However, in some cases you might want the monitor for a specific pool member to be different from that assigned to the pool. In this case, you must specify that you want to explicitly associate a specific monitor with the individual pool member. You can also prevent the BIG-IP system from associating any monitor with that pool member.

Explicit monitor association for a pool member

The BIG-IP system contains many different monitors that you can associate with a pool member, depending on the type of traffic you want to monitor. You can also create your own custom monitors and associate them with pool members. The only monitor types that are not available for associating with pool members are monitors that are specifically designed to monitor nodes and not pools or pool members. These monitor types are:

• ICMP
• TCP Echo
• Real Server
• SNMP DCA
• SNMP DCA Base
• WMI

Multiple monitor association for a pool member

The BIG-IP system allows you to associate more than one monitor with the same pool member. You can:

• Associate more than one monitor with a member of a single pool. For example, you can create monitors http1, http2, and http3, where each monitor is configured differently, and associate all three monitors with the same pool member. In this case, the pool member is marked as down if any of the checks is unsuccessful.
• Assign a single IP address and service to be a member of multiple pools. Then, within each pool, you can associate a different monitor with that pool member. For example, suppose you add the pool member 10.10.10.20:80 to three separate pools: my_pool1, my_pool2, and my_pool3. You can then associate all three HTTP monitors to that same pool member. The result is that the BIG-IP system uses the http1 monitor to check the health of pool member 10.10.10.20:80 in my_pool1, the http2 monitor to check the health of pool member 10.10.10.20:80 in my_pool2, and the http3 monitor to check the health of pool member 10.10.10.20:80 in my_pool3.

You can make multiple-monitor associations either at the time you add the pool member to each pool, or by modifying a pool member’s properties later.

Availability requirement

You can specify a minimum number of health monitors. Before the BIG-IP system can report the pool member as being in an up state, this number of monitors, at a minimum, must report a pool member as being available to receive traffic.

You can enable or disable individual pool members. A pool member is a logical object on the BIG-IP^® system that represents a specific server node and service. For example, a node with an IP address of 12.10.10.3 can have a corresponding pool member 12.10.10.3:80.

When you disable a pool member, the node continues to process any active connections or any connections for the current persistence session.