Overview: Pool and Pool Members

A pool is a logical set of devices, such as web servers, that you group together to receive and process traffic. Instead of sending client traffic to the destination IP address specified in the client request, the BIG-IP Next sends the request to any of the pool members in that pool.

A pool consists of pool members. A pool member is a logical object that represents a physical node on the network. Once you have assigned a pool to a virtual server, the BIG-IP Next directs traffic coming into the virtual server to a member of that pool. An individual pool member can belong to one or multiple pools, depending on how you want to manage your network traffic.

To work effectively with pools and pool members, familiarize yourself with the following concepts:

Load balancing method in a pool

Use the load balancing modes for load balancing traffic to pool members.

The following are available load balancing modes:

  • Round Robin: The BIG-IP Next passes each new connection request to the next endpoint in line, eventually distributing connections evenly across the array of endpoints that are load-balanced. This method works well in most configurations, especially if the equipment that you are load balancing is roughly equal in processing speed and memory.

  • Weighted Round Robin: The BIG-IP Next distributes connections over endpoints in a static rotation according to the ratio weights.

  • Least Connections (member): The BIG-IP Next passes a new connection to the endpoint having the least number of active connections.

  • Ratio Least Connections (member): The BIG-IP Next selects the endpoint according to the ratio of the number of active connections in each endpoint.

  • Ratio (session): The BIG-IP Next selects the endpoint according to the ratio of the number of entries in the persistence table.

  • Fastest (application): The BIG-IP Next selects an endpoint based on the least number of current sessions.

  • Predictive (member): The BIG-IP Next uses the ranking method used by the Observed method, where endpoints are rated according to the number of current connections. However, with the Predictive method, the BIG-IP Next analyzes the trend of the ranking over time, determining whether an endpoint’s performance is currently improving or declining. The endpoints with performance rankings that are currently improving, rather than declining, receive a higher proportion of the connections.

Monitors in a pool

  • You can associate a health monitor with an entire pool instead of an individual server. The BIG-IP Next automatically associates the selected monitor with all pool members, including those that you add later. Similarly, when you remove a member from a pool, the BIG-IP Next no longer monitors that server.

  • You can associate multiple monitors with the same pool. For example, you can associate both HTTP and HTTPS monitors with the same pool.

Slow ramp time for a pool member

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 BIG-IP Next sends all new connections to the newly-enabled pool member. This is because that pool member has the least number of connections.
With the slow ramp time feature, you can specify the number of seconds that the BIG-IP Next 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.

Action on service down when pool members are unavailable

The Action On Service Down specifies the BIG-IP Next to choose another pool member and rebind the client connection to a new server connection if the target pool member becomes unavailable.

The Action On Service Down field specifies the response to an already-established connection when the target pool member becomes unavailable. A pool member can become unavailable under any of the following conditions:

  • The pool member fails the monitor health check.

  • The pool member is manually set to Force Offline state.

The following are the available settings for this field:

  • None: No action is taken on existing connections, and the connection table entry is removed based on the associated profile’s idle timeout value. The BIG-IP Next sends a TCP Reset (RST) or ICMP Unreachable once idle timeout is reached. This is the default setting.

  • Drop: The connection table entry is removed.

  • Reselect: The BIG-IP Next manages established client connections by moving them to an alternate pool member without a connection teardown or setup.

  • Reset: The RST or ICMP messages are sent to reset active connections and remove them from the connection table. If there are available pool members for the connection, the BIG-IP Next resets and clears the active connections but sends subsequent newly arriving connections to the available pool member and does not send RST or ICMP messages.

Status of the pool members

Pool members can be disabled, re-enabled, or forced offline in response to planned maintenance or unplanned service outages. The pool member status is set to Enabled by default.

When set to Disabled, a pool member continues to process persistent and active connections. It can accept new connections only if the connections belong to an existing persistence session.

When set to Forced Offline, a pool member allows existing connections to time out, but no new connections are allowed.

You should consider changing the status of a pool member under the following conditions:

  • You need to disable a pool member for maintenance.

  • You need to force a pool member offline for maintenance.

Priority grouping of pool members within a pool

Pool members can be prioritized using the Priority Group field. The priority group number determines the order in which traffic is distributed to ‌pool members. Pool members with higher priority group number will receive traffic before pool members with lower priority group number.

The Minimum Members Active field specifies the minimum number of pool members that must remain available in each priority group for traffic to be confined to that group. If the number of available pool members in a higher priority group falls below the specified value in Minimum Members Active field, then traffic is distributed to both the current group and the next highest priority group pool members. When a sufficient number of pool members become available in the higher-priority group, ‌traffic is again directed to the higher-priority group. If the Minimum Members Active field is set to zero, then the priority group is disabled and traffic is distributed among all pool members.

If a pool member has the default priority group value of 0 (zero), it is in the lowest priority group. It only gets traffic when all pool members in higher priority groups are unavailable.

For example, the following configuration has three priority groups, 30, 20, and 10, with the Minimum Members Active field set to 2.

pool my\_pool {
   lb\_mode fastest
   min active members 2
   member 10.12.10.7:80 priority 30
   member 10.12.10.8:80 priority 30
   member 10.12.10.9:80 priority 30
   member 10.12.10.4:80 priority 20
   member 10.12.10.5:80 priority 20
   member 10.12.10.6:80 priority 20
   member 10.12.10.1:80 priority 10
   member 10.12.10.2:80 priority 10
   member 10.12.10.3:80 priority 10
   }

Connections are first distributed to all pool members with priority 30 (the highest priority group). If fewer than two priority 30 members are available, traffic is directed to the priority 20 members as well. If both the priority 30 group and the priority 20 group have fewer than two members available, traffic is directed to the priority 10 group. The BIG-IP Next continuously monitors the priority groups, and whenever a higher priority group once again has the minimum number of available pool members, the traffic is redirected to that group.

Note: To configure pool and pool members, refer to How to: Manage Pool and Pool Members.