Asynchronous Method Invocation (AMI) is the term used to describe the client-side support for the asynchronous programming model. AMI supports both oneway and twoway requests, but unlike their synchronous counterparts, AMI requests never block the calling thread. When a client issues an AMI request, the Ice run time hands the message off to the local transport buffer or, if the buffer is currently full, queues the request for later delivery. The application can then continue its activities and poll or wait for completion of the invocation, or receive a callback when the invocation completes.
AMI is transparent to the server: there is no way for the server to tell whether a client sent a request synchronously or asynchronously.
On this page:
Basic Asynchronous API in C++
Consider the following simple Slice definition:
Asynchronous Proxy Methods in C++
Besides the synchronous proxy methods,
slice2cpp generates the following asynchronous proxy methods:
As you can see, the single
getName operation results in
end_getName methods. (The
begin_ method is overloaded so you can pass a per-invocation context.)
begin_getNamemethod sends (or queues) an invocation of
getName. This method does not block the calling thread.
end_getNamemethod collects the result of the asynchronous invocation. If, at the time the calling thread calls
end_getName, the result is not yet available, the calling thread blocks until the invocation completes. Otherwise, if the invocation completed some time before the call to
end_getName, the method returns immediately with the result.
A client could call these methods as follows:
begin_getName does not block, the calling thread can do other things while the operation is in progress.
begin_getName returns a value of type
AsyncResult associated with this smart pointer contains the state that the Ice run time requires to keep track of the asynchronous invocation. You must pass the
AsyncResultPtr that is returned by the
begin_ method to the corresponding
begin_ method has one parameter for each in-parameter of the corresponding Slice operation. Similarly, the
end_ method has one out-parameter for each out-parameter of the corresponding Slice operation (plus the
AsyncResultPtr parameter). For example, consider the following operation:
end_op methods have the following signature:
Asynchronous Exception Semantics in C++
If an invocation raises an exception, the exception is thrown by the
end_ method, even if the actual error condition for the exception was encountered during the
begin_ method ("on the way out"). The advantage of this behavior is that all exception handling is located with the code that calls the
end_ method (instead of being present twice, once where the
begin_ method is called, and again where the
end_ method is called).
There is one exception to the above rule: if you destroy the communicator and then make an asynchronous invocation, the
begin_ method throws
CommunicatorDestroyedException. This is necessary because, once the run time is finalized, it can no longer throw an exception from the
The only other exception that is thrown by the
end_ methods is
IceUtil::IllegalArgumentException. This exception indicates that you have used the API incorrectly. For example, the
begin_ method throws this exception if you call an operation that has a return value or out-parameters on a oneway proxy. Similarly, the
end_ method throws this exception if you use a different proxy to call the
end_ method than the proxy you used to call the
begin_ method, or if the
AsyncResult you pass to the
end_ method was obtained by calling the
begin_ method for a different operation.
AsyncResult Class in C++
AsyncResult that is returned by the
begin_ method encapsulates the state of the asynchronous invocation:
The methods have the following semantics:
bool operator==(const AsyncResult&) const
bool operator<(const AsyncResult&) const
Int getHash() const
These methods allow you to create ordered or hashed collections of pending asynchronous invocations. This is useful, for example, if you can have a number of outstanding requests, and need to pass state between the
end_methods. In this case, you can use the returned
AsyncResultobjects as keys into a map that stores the state for each call.
This method prevents a queued invocation from being sent or, if the invocation has already been sent, ignores a reply if the server sends one.
cancelis a local operation and has no effect on the server. A canceled invocation is considered to be completed, meaning
isCompletedreturns true, and the result of the invocation is an
CommunicatorPtr getCommunicator() const
This method returns the communicator that sent the invocation.
virtual ConnectionPtr getConnection() const
This method returns the connection that was used for the invocation. Note that, for typical asynchronous proxy invocations, this method returns a nil value because the possibility of automatic retries means the connection that is currently in use could change unexpectedly. The
getConnectionmethod only returns a non-nil value when the
AsyncResultobject is obtained by calling
virtual ObjectPrx getProxy() const
This method returns the proxy that was used to call the
begin_method, or nil if the
AsyncResultobject was not obtained via an asynchronous proxy invocation.
const string& getOperation() const
This method returns the name of the operation.
LocalObjectPtr getCookie() const
This method returns the cookie that was passed to the
begin_method. If you did not pass a cookie to the
begin_method, the return value is null.
bool isCompleted() const
This method returns true if, at the time it is called, the result of an invocation is available, indicating that a call to the
end_method will not block the caller. Otherwise, if the result is not yet available, the method returns false.
This method blocks the caller until the result of an invocation becomes available.
bool isSent() const
When you call the
begin_method, the Ice run time attempts to write the corresponding request to the client-side transport. If the transport cannot accept the request, the Ice run time queues the request for later transmission.
isSentreturns true if, at the time it is called, the request has been written to the local transport (whether it was initially queued or not). Otherwise, if the request is still queued or an exception occurred before the request could be sent,
This method blocks the calling thread until a request has been written to the client-side transport, or an exception occurs. After
isSentreturns true if the request was successfully written to the client-side transport, or false if an exception occurred. In the case of a failure, you can call the corresponding
throwLocalExceptionto obtain the exception.
void throwLocalException() const
This method throws the local exception that caused the invocation to fail. If no exception has occurred yet,
bool sentSynchronously() const
This method returns true if a request was written to the client-side transport without first being queued. If the request was initially queued,
sentSynchronouslyreturns false (independent of whether the request is still in the queue or has since been written to the client-side transport).
Polling for Completion in C++
AsyncResult methods allow you to poll for call completion. Polling is useful in a variety of cases. As an example, consider the following simple interface to transfer files from client to server:
The client repeatedly calls
send to send a chunk of the file, indicating at which offset in the file the chunk belongs. A naïve way to transmit a file would be along the following lines:
This works, but not very well: because the client makes synchronous calls, it writes each chunk on the wire and then waits for the server to receive the data, process it, and return a reply before writing the next chunk. This means that both client and server spend much of their time doing nothing — the client does nothing while the server processes the data, and the server does nothing while it waits for the client to send the next chunk.
Using asynchronous calls, we can improve on this considerably:
With this code, the client sends up to
numRequests + 1 chunks before it waits for the least recent one of these requests to complete. In other words, the client sends the next request without waiting for the preceding request to complete, up to the limit set by
numRequests. In effect, this allows the client to "keep the pipe to the server full of data": the client keeps sending data, so both client and server continuously do work.
Obviously, the correct chunk size and value of
numRequests depend on the bandwidth of the network as well as the amount of time taken by the server to process each request. However, with a little testing, you can quickly zoom in on the point where making the requests larger or queuing more requests no longer improves performance. With this technique, you can realize the full bandwidth of the link to within a percent or two of the theoretical bandwidth limit of a native socket connection.
Generic Completion Callbacks in C++
begin_ method is overloaded to allow you to provide completion callbacks. Here are the corresponding methods for the
The second version of
begin_getName lets you override the default context. (We discuss the purpose of the
cookie parameter in the next section.) Following the in-parameters, the
begin_ method accepts a parameter of type
Ice::CallbackPtr. This is a smart pointer to a callback class that is provided by the Ice run time. This class stores an instance of a callback class that you implement. The Ice run time invokes a method on your callback instance when an asynchronous operation completes. Your callback class must provide a method that returns
void and accepts a single parameter of type
const AsyncResultPtr&, for example:
Note that your callback class must derive from
IceUtil::Shared. The callback method can have any name you prefer but its signature must match the preceding example.
The implementation of your callback method must call the
end_ method. The proxy for the call is available via the
getProxy method on the
AsyncResult that is passed by the Ice run time. The return type of
Ice::ObjectPrx, so you must down-cast the proxy to its correct type. (You should always use an
uncheckedCast to do this, otherwise you will send an additional message to the server to verify the proxy type.)
Your callback method should catch and handle any exceptions that may be thrown by the
end_ method. If you allow an exception to escape from the callback method, the Ice run time produces a log entry by default and ignores the exception. (You can disable the log message by setting the property
Ice.Warn.AMICallback to zero.)
To inform the Ice run time that you want to receive a callback for the completion of the asynchronous call, you pass the callback instance to the
Note the call to
Ice::newCallback in this example. This helper function expects a smart pointer to your callback instance and a member function pointer that specifies your callback method.
Using Cookies for Generic Completion Callbacks in C++
It is common for the
end_ method to require access to some state that is established by the code that calls the
begin_ method. As an example, consider an application that asynchronously starts a number of operations and, as each operation completes, needs to update different user interface elements with the results. In this case, the
begin_ method knows which user interface element should receive the update, and the
end_ method needs access to that element.
The API allows you to pass such state by providing a cookie. A cookie is an instance of a class that you write; the class can contain whatever data you want to pass, as well as any methods you may want to add to manipulate that data.
The only requirement on the cookie class is that it must derive from
Ice::LocalObject. Here is an example implementation that stores a
WidgetHandle. (We assume that this class provides whatever methods are needed by the
end_ method to update the display.)
When you call the
begin_ method, you pass the appropriate cookie instance to inform the
end_ method how to update the display:
end_ method can retrieve the cookie from the
AsyncResult by calling
getCookie. For this example, we assume that widgets have a
writeString method that updates the relevant UI element:
The cookie provides a simple and effective way for you to pass state between the point where an operation is invoked and the point where its results are processed. Moreover, if you have a number of operations that share common state, you can pass the same cookie instance to multiple invocations.
Type-Safe Completion Callbacks in C++
The generic callback API is not entirely type-safe:
- You must down-cast the return value of
getProxyto the correct proxy type before you can call the
- You must call the correct
end_method to match the operation called by the
- If you use a cookie, you must down-cast the cookie to the correct type before you can access the data inside the cookie.
- You must remember to catch exceptions when you call the
end_method; if you forget to do this, you will not know that the operation failed.
slice2cpp generates an additional type-safe API that takes care of these chores for you. The type-safe API is provided as a template that works much like the
Ice::Callback class of the generic API, but requires strongly-typed method signatures.
To use type-safe callbacks, you must implement a callback class that provides two callback methods:
- A success callback that is called if the operation succeeds
- A failure callback that is called if the operation raises an exception
As for the generic API, your callback class must derive from
IceUtil::Shared. Here is a callback class for an invocation of the
The callback methods can have any name you prefer and must have
void return type. The failure callback always has a single parameter of type
const Ice::Exception&. The success callback parameters depend on the operation signature. If the operation has non-
void return type, the first parameter of the success callback is the return value. The return value (if any) is followed by a parameter for each out-parameter of the corresponding Slice operation, in the order of declaration.
To receive these callbacks, you instantiate your callback instance and specify the methods you have defined before passing a smart pointer to a callback wrapper instance to the
Note how this code creates instances of two smart pointer types generated by
Callback_Employees_getNumberPtr. Each smart pointer points to a template instance that encapsulates your callback instance and two member function pointers for the callback methods. The name of this smart pointer type is formed as follows:
Also note that the code uses helper functions to initialize the smart pointers. The first argument to the helper function is your callback instance, and the two following arguments are the success and failure member function pointers, respectively. The name of this helper function is formed as follows:
It is legal to pass a null pointer as the success or failure callback. For the success callback, this is legal only for operations that have
void return type and no out-parameters. This is useful if you do not care when the operation completes but want to know if the call failed. If you pass a null exception callback, the Ice run time will ignore any exception that is raised by the invocation.
The type of the success and exception member function pointers is provided as
Exception typedefs by the callback template. For example, you can ignore exceptions for an invocation of
getName as follows:
Using Cookies for Type-Safe Completion Callbacks in C++
begin_ method optionally accepts a cookie as a trailing parameter. As for the generic API, you can use the cookie to share state between the
end_ methods. However, with the type-safe API, there is no need to down-cast the cookie. Instead, the cookie parameter that is passed to the
end_ method is strongly typed. Assuming that you have defined a
Cookie class and
CookiePtr smart pointer, you can pass a cookie to the
begin_ method as follows:
The callback methods of your callback class simply add the cookie parameter:
Lambda Completion Callbacks in C++
slice2cpp generates overloaded versions of the
begin_ methods that accept lambda functions, eliminating the need to create a separate callback object. The Slice compiler generates the following methods for the
The overloaded versions of
begin_getName allow you to use the type-safe and generic styles, with and without a context argument. Notice that default values are provided for the
sent arguments, meaning you can omit these arguments if you wish to ignore their events. The first two functions above correspond to the type-safe API and the second two have generic API semantics.
begin_ methods are generated by default, but can only be used with a compiler that supports C++11 lambda functions, and with an Ice installation that was built with C++11 features enabled.
We can invoke
begin_getName as shown below:
For the sake of brevity we have omitted the error handling that would normally be necessary.
Asynchronous Oneway Invocations in C++
You can invoke operations via oneway proxies asynchronously, provided the operation has
void return type, does not have any out-parameters, and does not raise user exceptions. If you call the
begin_ method on a oneway proxy for an operation that returns values or raises a user exception, the
begin_ method throws an
For the generic API, the callback method looks exactly as for a twoway invocation. However, for oneway invocations, the Ice run time does not call the callback method unless the invocation raised an exception during the
begin_ method ("on the way out").
For the type-safe API, the
newCallback helper for
void operations is overloaded so you can omit the success callback. For example, here is how you could call
Flow Control in C++
Asynchronous method invocations never block the thread that calls the
begin_ method: the Ice run time checks to see whether it can write the request to the local transport. If it can, it does so immediately in the caller's thread. (In that case,
AsyncResult::sentSynchronously returns true.) Alternatively, if the local transport does not have sufficient buffer space to accept the request, the Ice run time queues the request internally for later transmission in the background. (In that case,
AsyncResult::sentSynchronously returns false.)
This creates a potential problem: if a client sends many asynchronous requests at the time the server is too busy to keep up with them, the requests pile up in the client-side run time until, eventually, the client runs out of memory.
The API provides a way for you to implement flow control by counting the number of requests that are queued so, if that number exceeds some threshold, the client stops invoking more operations until some of the queued operations have drained out of the local transport.
For the generic API, you can create an additional callback method:
As with any other callback method, you are free to choose any name you like. For this example, the name of the callback method is
sent. You inform the Ice run time that you want to be informed when a call has been passed to the local transport by specifying the
sent method as an additional parameter when you create the
If the Ice run time can immediately pass the request to the local transport, it does so and invokes the
sent method from the thread that calls the
begin_ method. On the other hand, if the run time has to queue the request, it calls the
sent method from a different thread once it has written the request to the local transport. In addition, you can find out from the
AsyncResult that is returned by the
begin_ method whether the request was sent synchronously or was queued, by calling
For the generic API, the
sent method has the following signature:
For the type-safe API, there are two versions, one without and one with a cookie:
For the version with a cookie,
<CookiePtr> is replaced with the actual type of the cookie smart pointer you passed to the
sent methods allow you to limit the number of queued requests by counting the number of requests that are queued and decrementing the count when the Ice run time passes a request to the local transport.
Asynchronous Batch Requests in C++
Applications that send batched requests can either flush a batch explicitly or allow the Ice run time to flush automatically. The proxy method
ice_flushBatchRequests performs an immediate flush using the synchronous invocation model and may block the calling thread until the entire message can be sent. Ice also provides asynchronous versions of this method so you can flush batch requests asynchronously.
end_ice_flushBatchRequests are proxy methods that flush any batch requests queued by that proxy.
In addition, similar methods are available on the communicator and the
Connection object that is returned by
AsyncResult::getConnection. These methods flush batch requests sent via the same communicator and via the same connection, respectively.
Concurrency Semantics for AMI in C++
The Ice run time always invokes your callback methods from a separate thread. This means that you can safely use a non-recursive mutex without risking deadlock. There is one exception to this rule: the run time calls the
sent callback from the thread calling the
begin_ method if the request could be sent synchronously. In the
sent callback, you know which thread is calling the callback by looking at the
sentSynchronously member or parameter, so you can take appropriate action to avoid a self-deadlock.