Persistence ("Sticky Connections") Capabilities

What are Persistent Connections?

When standard loadbalancing is done on a server cluster, each incoming network connection is independent of the other connections and the real server for that connection can be chosen on the basis of the loadbalancing technique used at the moment. However, there are many applications that require connections from the same client to be directed to the same server either to ensure correct operation and performance.

FTP is one of the applications that require persistent connections. The client - server data transfer is done using two connections, one is a control connection (port 21) to exchange command information and the other is a data connection (usually port 20) that transfers the data.

Another example is connections using SSL (Secure Socket Layer). When a SSL connection is made a key for the connection must be chosen and exchanged. Until the SSL key expires, all consequent connections from the same client are granted access to the server. If the persistent connections feature is not enabled new SSL key should be issued for each new connection, which will hinder the performance of the server and increase server response time.

Persistence in E-commerce

E-commerce shopping cart applications also require sticky connections in order to function properly. For example, let’s say a customer fills a "shopping cart" of goods at a Web site, and then browses a different part of the site for some details about his selection before completing the transaction. Later, when he submits his order, SysMaster may direct the customer’s request to a different server, that may not know about the user and his shopping cart. Of course, if all servers store the user information and their selected goods in a single back-end database server, this would not be a problem. But if the site is not designed this way, the specific shopping cart data resides on just one server. In this case, SysMaster must select the same server that the user was directed to in the past, in order to seamlessly process the user’s request.

The Solution

When the persistence features are enabled, SysMaster records the IP address of the client and the real server that the client was connected to initially in a reference table. When a new request comes in from the same client, the SysMaster cross references its table and when it finds the client's record, directs the traffic to the original server.

It is easily seen that keeping persistence for great periods of time is impractical and resource consuming. That is why loadbalancers use a persistence timer that limits the extent of the persistence in time. The timer measures the interval that has passed since there was a connection from a particular IP address to a specific virtual service. The time period begins as soon as there are no longer any active connections between the client and a particular real server. The timer is set to zero whenever a new connection occurs. If the timer passes the pre-configured persistence time, new connections from that IP to the same virtual service may be redirected to another real server.

Loadbalancers use more memory and need more CPU time to manage persistent connections. The resource consumption increases as the persistent time grows. Network administrators should choose the shortest possible time for the specific applications and should now use sticky connections for applications that do not need this feature.

SysMaster offers several persistence methods that allow network administrators to build high-performance networks for e-commerce.

Source IP/Destination - The SysMaster directs all connections for a given combination of Source IP and Destination to the same server. But, if a client with the same IP connects to a different service (VIP) or application port, the connections will be loadbalanced to any available server. This method meets the requirements of most applications that need persistence connections including applications using SSL connections.

Mega Proxy Sever Persistence - Big enterprises and ISPs use loadbalancing across multiple proxy servers to connect to the Internet. In such cases the same user may be coming to the site from a different proxy server for each TCP connection. Therefore source IP is not a reliable indicator of a given user. To address this problem SysMaster can be configured to treat a range of source IP addresses as a single IP address, so that all connections from a block of IP addresses are redirected to one and the same real server for the period of the persistence timer.

As you can see, persistence is an important issue when planning and designing your Web site services. While you need to loadbalance your network traffic to optimize your site’s Quality of Service, you also may need the ability to direct traffic to a particular server. SysMaster persistency features meets this requirements providing flexible and intelligent loadbalancing solutions that accommodate your network infrastructure and supports your strategic business applications.

top of page