The choice of load balancing algorithms can directly impact – for good or ill – the performance, behavior and capacity of applications. Beware making incompatible choices in architecture and algorithms.
One of the most persistent issues encountered when deploying applications in scalable architectures involves sessions and the need for persistence-based (a.k.a. sticky) load balancing services to maintain state for the duration of an end-user’s session. It is common enough that even the rudimentary load balancing services offered by cloud computing providers such as Amazon include the option to enable persistence-based load balancing. While the use of persistence addresses the problem of maintaining session state, it introduces other operational issues that must also be addressed to ensure consistent operational behavior of load balancing services.
In particular, the use of the Round Robin load balancing algorithm in conjunction with persistence-based load balancing should be discouraged if not outright disallowed.
When scaling applications there are two primary concerns: concurrent user capacity and performance. These two concerns are interrelated in that as capacity is consumed, performance degrades. This is particularly true of applications storing state as each request requires that the application server perform a lookup to retrieve the user session. The more sessions stored, the longer it takes to find and retrieve the session. The exactly efficiency of such lookups is determined by the underlying storage data structure and algorithm used to search the structure for the appropriate session. If you remember your undergraduate classes in data structures and computing Big (O) you’ll remember that some structures scale more efficiently in terms of performance than do others. The general rule of thumb, however, is that the more data stored, the longer the lookup. Only the amount of degradation is variable based on the efficiency of the algorithms used. Therefore, the more sessions in use on an application server instance, the poorer the performance. This is one of the reasons you want to choose a load balancing algorithm that evenly distributes load across all instances and ultimately why lots of little web servers scaled out offer better performance than a few, scaled up web servers.
Now, when you apply persistence to the load balancing equation it essentially interrupts the normal operation of the algorithm, ignoring it. That’s the way it’s supposed to work: the algorithm essentially applies only to requests until a server-side session (state) is established and thereafter (when the session has been created) you want the end-user to interact with the same server to ensure consistent and expected application behavior. For example, consider this solution note for BIG-IP. Note that this is true of all load balancing services:
A persistence profile allows a returning client to connect directly to the server to which it last connected. In some cases, assigning a persistence profile to a virtual server can create the appearance that the BIG-IP system is incorrectly distributing more requests to a particular server. However, when you enable a persistence profile for a virtual server, a returning client is allowed to bypass the load balancing method and connect directly to the pool member. As a result, the traffic load across pool members may be uneven, especially if the persistence profile is configured with a high timeout value.
So far so good. The problem with round robin- – and reason I’m picking on Round Robin specifically - is that round robin is pretty, well, dumb in its decision making. It doesn’t factor anything into its decision regarding which instance gets the next request. It’s as simple as “next in line", period. Depending on the number of users and at what point a session is created, this can lead to scenarios in which the majority of sessions are created on just a few instances. The result is a couple of overwhelmed instances (with performance degradations commensurate with the reduction in available resources) and a bunch of barely touched instances. The smaller the pool of instances, the more likely it is that a small number of servers will be disproportionately burdened. Again, lots of little (virtual) web servers scales out more evenly and efficiently than a few big (virtual) web servers.
Assuming a pool of similarly-capable instances (RAM and CPU about equal on all) there are other load balancing algorithms that should be considered more appropriate for use in conjunction with persistence-based load balancing configurations. Least connections should provide better distribution, although the assumption that an active connection is equivalent to the number of sessions currently in memory on the application server could prove to be incorrect at some point, leading to the same situation as would be the case with the choice of round robin. It is still a better option, but not an infallible one. Fastest response time is likely a better indicator of capacity as we know that responses times increase along with resource consumption, thus a faster responding instance is likely (but not guaranteed) to have more capacity available. Again, this algorithm in conjunction with persistence is not a panacea.
Better options for a load balancing algorithm include those that are application aware; that is, algorithms that can factor into the decision making process the current load on the application instance and thus direct requests toward less burdened instances, resulting in a more even distribution of load across available instances.
There are also non-algorithmic, i.e. architectural, solutions that can address this issue.
DIVIDE and CONQUER
In cloud computing environments, where it is less likely to find available algorithms other than industry standard (none of which are application-aware), it may be necessary to approach the problem with a divide and conquer strategy, i.e. lots of little servers. Rather than choosing one or two “large” instances, choose to scale out with four or five “small” instances, thus providing a better (but not guaranteed) statistical chance of load being distributed more evenly across instances.
If the option is available, an architectural “flanking” strategy that leverages layer 7 load balancing, a.k.a. content/application switching, will also provide better consumptive rates as well as more consistent performance. An architectural strategy of this sort is in line with sharding practices at the data layer in that it separates out by some attribute different kinds of content and serves that content from separate pools. Thus, image or other static content may come from one pool of resources while session-oriented, process intensive dynamic content may come from another pool. This allows different strategies – and algorithms – to be used simultaneously without sacrificing the notion of a single point of entry through which all users interact on the client-side.
As always, test early and test often and test under high load if you want to be assured that the load balancing algorithm is suitable to meet your operational and business requirements.