Collocated Invocation and Dispatch
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Overview
One of the useful features of the Ice run time is that it is location transparent: the client does not need to know where the implementation of an Ice object resides; an invocation on an object automatically is directed to the correct target, whether the object is implemented in the local address space, in another address space on the same machine, or in another address space on a remote machine. Location transparency is important because it allows us to change the location of an object implementation without breaking client programs and, by using a location service such as IceDiscovery or IceGrid, addressing information such as host names and port numbers can be externalized so they do not appear in stringified proxies.
Collocated Invocations
For invocations that cross address space boundaries (or more accurately, cross communicator boundaries), the Ice run time dispatches requests via the appropriate transport. However, for a proxy invocation in which the proxy and the servant that processes the invocation share the same communicator (so-called collocated invocations), the Ice run time, by default, does not send the invocation via the transport specified in the proxy. Instead, collocated invocations take a short-cut inside the Ice run time and are dispatched more efficiently.
Note that if the proxy and the servant do not use the same communicator, the invocation is not collocated, even though caller and callee are in the same address space.
The reason for this is if collocated invocations were sent via TCP/IP, for example, invocations would still be sent via the operating system kernel (using the back plane instead of a network) and would incur the full cost of creating TCP/IP connections, trapping in and out of the kernel, and so on. By optimizing collocated requests, much of this overhead can be avoided.
For efficiency reasons, collocated invocations are not completely location transparent, that is, a collocated call has semantics that differ in some ways from calls that cross address-space boundaries. Specifically, collocated invocations differ from ordinary invocations in the following respects:
- Most collocated invocations are dispatched in the server-side thread pool just like regular invocations; the only exceptions are synchronous twoway collocated invocations with no invocation timeout, which are dispatched in the calling thread.
- The object adapter holding state is ignored: collocated invocations proceed normally even if the target object's adapter is in the holding state.
- AMI callbacks for asynchronous collocated invocations are dispatched from the servant's calling thread and not from the client-side thread pool unless AMD is used for the servant dispatch. In this case, the AMI callback is called from the client-side thread pool.
- Invocation timeouts work as usual, but connection timeouts are ignored.
In practice, these differences rarely matter. The most likely cause of surprises with collocated invocations is dispatch in the calling thread, that is, a collocated invocation behaves like a local, synchronous procedure call. This can cause problems if, for example, the calling thread acquires a lock that an operation implementation tries to acquire as well: unless you use recursive mutexes, this will cause deadlock.
The Ice run time uses the following semantics to determine whether a proxy is eligible for the collocated optimization:
- For an indirect proxy, collocation optimization is used if the proxy's adapter ID matches the adapter ID or replica group ID of an object adapter in the same communicator.
- For a well-known proxy, the Ice run time queries each object adapter Active Servant Map to determine if the servant is local.
- For a direct proxy, the Ice run time performs an endpoint search using the proxy's endpoints.
When an endpoint search is required, the Ice run time compares each of the proxy's endpoints against the endpoints of the communicator's object adapters. Only the transport, address and port are considered; other attributes of an endpoint, such as timeout settings, are not considered during this search. If a match is found, the invocation is dispatched using collocation optimization. Normally this search is executed only once, during the proxy's first invocation, although the proxy's connection caching setting influences this behavior.
Collocation optimization is enabled by default, but you can disable it for all proxies by setting the property Ice.Default.CollocationOptimized=0. You can also disable the optimization for an individual proxy using the factory method ice_collocationOptimized(false). Finally, for proxies created from a property using propertyToProxy, the property name.CollocationOptimized configures the default setting for the proxy.
Creating an Object Adapter for Collocated Invocations
An object adapter requires no endpoints if its only purpose is to dispatch collocated requests. You can create a "collocated-only" object adapter by calling createObjectAdapter with an empty name:
Ice::ObjectAdapterPtr collocAdapter = communicator->createObjectAdapter("");
Normally Ice requires an object adapter to either have endpoints or be configured with a router, but Ice relaxes this restriction when the name is empty. Note however that this means collocated-only object adapters cannot be configured using properties.
Proxies created by a collocated-only object adapter contain no endpoints, which essentially makes them well-known proxies. When the program makes its initial invocation on such a proxy, Ice searches every local object adapter for a servant with an identity matching that of the proxy. For Ice to successfully find the collocated servant, it must be present in the object adapter's active servant map. Servant locators and default servants are not queried in this situation.