Ice supports two types of load balancing:
- Locator-based load balancing
- Proxy-based load balancing
An application can use only one type or combine both to achieve the desired behavior. This page discusses proxy-based load balancing; refer to Locator Semantics for Clients for information on load balancing with a locator.
On this page:
Proxy Connection Caching
Before we discuss load balancing, it's important to understand the relationship between proxies and connections.
By default a proxy remembers its connection and uses it for all invocations until that connection is closed. You can prevent a proxy from caching its connection by calling the ice_connectionCached proxy method with an argument of false. Once connection caching is disabled, each invocation on a proxy causes the Ice run time to execute its connection establishment process.
Note that each invocation on such a proxy does not necessarily cause the Ice run time to establish a new connection. It only means that the Ice run time does not assume that it can reuse the connection of the proxy's previous invocation. Whether the Ice run time actually needs to establish a new connection for the next invocation depends on several factors.
As with any feature, you should only use it when the benefits outweigh the risks. With respect to a proxy's connection caching behavior, there is certainly a small amount of computational overhead associated with executing the connection establishment process for each invocation, as well as the risk of significant overhead each time a new connection is actually created.
Proxies with Multiple Endpoints
A proxy can contain zero or more endpoints. A proxy with no endpoints typically means it's an indirect proxy that requires a location service to convert the proxy's symbolic information into endpoints at run time. Otherwise, a direct proxy contains at least one endpoint.
Regardless of whether endpoints are specified directly or indirectly, a proxy having multiple endpoints implies at a minimum that the target object is available via multiple network interfaces, but often also means the object is replicated to improve scalability and reliability. Such a proxy provides a client with several load balancing options even when no location service is involved. The proxy's own configuration drives the run-time behavior, depending on how the client configures the proxy's endpoint selection type and connection caching settings.
For example, suppose that a proxy contains several endpoints. In its default configuration, a proxy uses the Random endpoint selection type and caches its connection. Upon the first invocation, the Ice run time selects one of the proxy's endpoints at random and uses that connection for all subsequent invocations until the connection is closed. For some applications, this form of load balancing may be sufficient.
Suppose now that we use the Ordered endpoint selection type instead. In this case, the Ice run time always attempts to establish connections using the endpoints in the order they appear in the proxy. Normally an application uses this configuration when there is a preferred order to the servers. Again, once connected, the Ice run time uses whichever connection was chosen indefinitely.
Per-Request Load Balancing
When we disable the connection caching behavior of a proxy with multiple endpoints, its semantics undergo a significant change. Using the Random endpoint selection type, the Ice run time selects one of the endpoints at random and establishes a connection to it if one is not already established, and this process is repeated prior to each subsequent invocation. This is called per-request load balancing because each request can potentially be directed to a different server.
Using the Ordered endpoint selection type is not as common in this scenario; its main purpose would be to fall back on a secondary server if the primary server is not available, but it causes the Ice run time to attempt to contact the primary server during each request.
Here's some code that shows how to configure the proxy:
auto proxy = communicator->stringToProxy("hello:tcp -h 10.0.0.1 -p 2000:tcp -h 10.0.0.2 -p 2001");
proxy = proxy->ice_connectionCached(false);
proxy = proxy->ice_endpointSelection(Ice::Random);
// If also using a locator:
proxy = proxy->ice_locatorCacheTimeout(...);
Ice::ObjectPrx proxy = communicator->stringToProxy("hello:tcp -h 10.0.0.1 -p 2000:tcp -h 10.0.0.2 -p 2001");
proxy = proxy->ice_connectionCached(false);
proxy = proxy->ice_endpointSelection(Ice::Random);
// If also using a locator:
proxy = proxy->ice_locatorCacheTimeout(...);
We create a proxy with two endpoints, then use the factory methods to disable connection caching and set the endpoint selection type. The Random setting means that, after we've made a few invocations with the proxy, the Ice run time will have established connections to both endpoints. After incurring the initial expense of opening the connections, each subsequent invocation will randomly use one of the existing connections.
If you're also using a location service, you may want to modify the proxy's locator cache timeout to force the Ice run time in the client to query the locator more frequently.