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 and invocations never block. 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 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.

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Basic Asynchronous API in JavaScript

Consider the following simple Slice definition:

module Demo
{ 
    interface Employees
    {
        string getName(int number);
        string getAddress(int number);
    }
}

Asynchronous Proxy Methods in JavaScript

slice2js generates the following asynchronous proxy methods:

EmployeesPrx = class extends Ice.ObjectPrx
{
    getName(number, context) { ... }
    getAddress(number, context) { ... }
}

As you can see, a Slice operation maps to a method of the same name. (Each function accepts an optional trailing per-invocation context.)

The getName function, for example, sends an invocation of getName. Because proxy invocations do not block, the application can do other things while the operation is in progress. The application receives notification about the completion of the request by registering callbacks for success and failure.

The getName function, as with all functions corresponding to Slice operations, returns a value of type Ice.AsyncResult. This value contains the state that the Ice run time requires to keep track of the asynchronous invocation.

A proxy function has one parameter for each in-parameter of the corresponding Slice operation. For example, consider the following operation:

double op(int inp1, string inp2, out bool outp1, out long outp2);

The op member function has the following signature:

function op(inp1, inp2, context); // Returns Ice.AsyncResult

The operation's return value, as well as the values of its two out-parameters, are delivered to the success callback upon completion of the request.

Passing Parameters in JavaScript

The parameter passing rules for the JavaScript mapping are very simple: parameters are passed either by value (for simple types) or by reference (for complex types). Semantically, the two ways of passing parameters are identical: it is guaranteed that the value of a parameter will not be changed by the invocation.

Here is an interface with operations that pass parameters of various types from client to server:

struct NumberAndString
{
    int x;
    string str;
}

sequence<string> StringSeq;

dictionary<long, StringSeq> StringTable;

interface ClientToServer
{
    void op1(int i, float f, bool b, string s);
    void op2(NumberAndString ns, StringSeq ss, StringTable st);
    void op3(ClientToServer* proxy);
}

The Slice compiler generates the following proxy methods for these definitions:

ClientToServerPrx = class extends Ice.ObjectPrx
{
    op1(i, f, b, s, context) { ... }
    op2(ns, ss, st, context) { ... }
    op3(proxy, context) { ... }
}

Given a proxy to a ClientToServer interface, the client code can pass parameters as in the following example:

let p = ...;          // Get ClientToServerPrx proxy...

p.op1(42, 3.14, true, "Hello world!");  // Pass simple literals

let i = 42;
let f = 3.14;
let b = true;
let s = "Hello world!";
p.op1(i, f, b, s);                      // Pass simple variables

let ns = new NumberAndString();
ns.x = 42;
ns.str = "The Answer";
let ss = [];
ss.push("Hello world!");
let st = new StringTable();
st.set(0, ss);
p.op2(ns, ss, st);                      // Pass complex variables

p.op3(p);                               // Pass proxy

Null Parameters in JavaScript

Some Slice types naturally have "empty" or "not there" semantics. Specifically, sequences, dictionaries, and strings all can be null, but the corresponding Slice types do not have the concept of a null value. To make life with these types easier, whenever you pass null as a parameter or return value of type sequence, dictionary, or string, the Ice run time automatically sends an empty sequence, dictionary, or string to the receiver.

This behavior is useful as a convenience feature: especially for deeply-nested data types, members that are sequences, dictionaries, or strings automatically arrive as an empty value at the receiving end. This saves you having to explicitly initialize, for example, every string element in a large sequence before sending the sequence in order to avoid a run-time error. Note that using null parameters in this way does not create null semantics for Slice sequences, dictionaries, or strings. As far as the object model is concerned, these do not exist (only empty sequences, dictionaries, and strings do). For example, whether you send a string as null or as an empty string makes no difference to the receiver: either way, the receiver sees an empty string.

Optional Parameters in JavaScript

Optional parameters use the same mapping as required parameters. The only difference is that undefined can be passed as the value of an optional parameter or return value to indicate an "unset" condition. Consider the following operation:

optional(1) int execute(optional(2) string params, out optional(3) float value);

A client can invoke this operation as shown below:

proxy.execute("--file log.txt").then(
    r => {
		let [retval, value] = r;
        if(value !== undefined)
        {
            console.log("value =", value);
        }
    });

A well-behaved program must always compare an optional parameter to undefined prior to using its value.

For Slice types that support null semantics, such as proxies and objects by value, passing null for an optional parameter has a different meaning than passing undefined:

• null means a value was supplied for the parameter, and that value happens to be null
• undefined means no value was supplied for the parameter

undefined is not a legal value for required parameters.

Asynchronous Exception Semantics in JavaScript

If an invocation raises an exception, the exception is delivered to the failure callback, even if the actual error condition for the exception was encountered during the proxy function ("on the way out"). The advantage of this behavior is that all exception handling is located in the failure callback (instead of being present twice, once where the proxy function is called, and again in the failure callback).

There are two exceptions to this rule:

• if you destroy the communicator and then make an asynchronous invocation, the opAsync method throws CommunicatorDestroyedException directly.
• a call to an Async function can throw TwowayOnlyException. An Async function throws this exception if you call an operation that has a return value or out-parameters on a oneway proxy.

AsyncResult Type in JavaScript

The AsyncResult object returned by an Ice API call encapsulates the state of the asynchronous invocation.

An instance of AsyncResult defines the following properties:

• communicator
  The communicator that sent the invocation.

• connection
  This method returns the connection that was used for the invocation. Note that, for typical 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 connection property only returns a non-nil value when the AsyncResult object is obtained by calling flushBatchRequests on a Connection object.

• proxy
  The proxy that was used for the invocation, or nil if the AsyncResult object was not obtained via a proxy invocation.

• adapter
  The object adapter that was used for the invocation, or nil if the AsyncResult object was not obtained via an object adapter invocation.

• operation
  The name of the operation being invoked.

AsyncResult objects also support the following methods:

• cancel()
  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. cancel is a local operation and has no effect on the server. A canceled invocation is considered to be completed, meaning isCompleted returns true, and the result of the invocation is an Ice.InvocationCanceledException.

• isCompleted()
  This method returns true if, at the time it is called, the result of an invocation is available. Otherwise, if the result is not yet available, the method returns false.

• isSent()
  When you make a proxy invocation, 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. isSent returns 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, isSent returns false.

• sentSynchronously()
  This method returns true if a request was written to the client-side transport without first being queued. If the request was initially queued, sentSynchronously returns false (independent of whether the request is still in the queue or has since been written to the client-side transport).

• throwLocalException()
  This method throws the local exception that caused the invocation to fail. If no exception has occurred yet, throwLocalException does nothing.

As described in the next section, an AsyncResult object provides additional functionality, including support for success and failure callbacks, that it inherits from a base type.

Promise Type in JavaScript

The Ice.AsyncResult type actually derives from Ice.Promise, which in turn derives from the standard JavaScript Promise. As you'll see below, one especially useful feature of a promise is its support for chaining, which allows you to make a series of asynchronous calls while still maintaining code readability.

Basic Promise Concepts

A promise represents a value that might be available in the future. The object always starts out in the pending state, and then transitions to a fulfilled or rejected state via calls to its resolve and reject methods, respectively. This state transition occurs only once; any subsequent calls to resolve or reject are ignored. The initial caller of resolve or reject can supply an argument that the promise object will pass along to its success or failure callbacks. Any number of callbacks can be registered on a single promise, and they can be registered before or after the promise has been resolved. The resolve and reject methods are helper methods added in Ice.Promise and are just references to the resolve and reject callback functions passed to the promise executor.

Ice.Promise = class extends Promise
{
    constructor(cb)
    {
        let res, rej;
        super((resolve, reject) => {
                res = resolve;
                rej = reject;

                if(cb !== undefined)
                {
                    cb(resolve, reject);
                }
            });

        this.resolve = res;
        this.reject = rej;
     }
...
}

A promise invokes any registered callbacks at the time the promise is resolved or rejected. Callbacks registered after a promise has been settled are invoked immediately.

Getting Started with Promises

The Promise method you'll use most often is then, inherited from JavaScript's standard Promise object. It accepts two arguments representing the success and failure callbacks:

function then(success, failure) { ... }

Both arguments are optional. Here is a simple example:

let p = new Ice.Promise();
p.then(
    r => {
        let [str, i] = r;
        console.log("received " + str + " and " + i);
    },
    ex => {
        console.log("failed: " + ex);
    });
 
try
{ 
    // do something...
    p.resolve(["string arg", 5]);
}
catch(ex)
{
    p.reject(ex);
} 

The call to resolve resolves the promise and supplies an argument that the promise passes to the success callback. Similarly, the call to reject passes along the exception caught by the caller.

As you can see, the success and fail callbacks typically must know what arguments to expect when the promise is resolved.

Chaining Promises

The convenience and power offered by the promise concept becomes clear when you need to execute a sequence of asynchronous actions. Consider this example:

var p = new Ice.Promise();
p.then(    // A
    () => {
        // Step 1...
    }
).then(    // B
    () => {
        // Step 2...
    }
).then(    // C
    () => {
        // Step 3...
    }
);

// None of this happens until...
p.resolve();

Each call to then returns a new promise that is automatically chained to the preceding one. The promise returned by then in A resolves when Step 1 completes. The successful completion of A causes Step 2 to execute, and the successful completion of Step 2 resolves the promise returned by then in B, triggering Step 3 to execute. Finally, the whole chain of events won't begin until the initial promise p resolves.

Handling Errors with Promises

Let's extend our previous example to include an error handler:

var p = new Ice.Promise();
p.then(    // A
    () => {
        // Step 1...
    }
).then(    // B
    () => {
        // Step 2...
    }
).catch(
    ex => {
        // handle errors
    }
);

Here we call the catch(failure) method to establish an exception handler at the end of the promise chain, which is equivalent to calling then(undefined, failure). You'll notice that none of the intermediate steps defines its own failure callback, with the intention that all errors will be handled in one location. After a call to p.resolve triggers execution of the chain, an exception raised by any of these steps propagates through each subsequent promise in the chain until a failure callback is found. If instead the application calls p.reject, none of the steps would execute and only the exception handler would be called.

Now let's examine the behavior when an intermediate step defines a failure callback:

let p = new Ice.Promise();
p.then(
    () => {
        console.log("step 1");
    },
    () => {
        console.log("fail 1");
    }
).then(
    () => {
        console.log("step 2");
    }
).catch(
    (ex) => {
        console.log("error");
    }
);
p.reject();
 
// Outputs:
fail 1
step 2

The results might surprise you at first. If a failure callback completes without raising an exception, the promise is considered to have resolved successfully and therefore triggers the success callback of the next promise in the chain. On the other hand, if a failure callback raises an exception, it will propagate to the next failure callback in the chain. This behavior allows a failure callback to recover from error situations (if possible) and continue with the normal logic flow, or raise an exception and skip to the next error handler.

Next we'll illustrate this point by throwing an exception:

let p = new Ice.Promise();
p.then(
    () => {
        console.log("step 1");
    },
    ex => {
        console.log("fail 1");
        throw ex;
    }
).then(
    () => {
        console.log("step 2");
    }
).catch(
    ex => {
        console.log("error");
    }
);
p.fail();
 
// Outputs:
fail 1
error

Here you can see that the exception raised in the first failure callback bypasses Step 2 and triggers the final exception handler.

Passing Values between Promises

The promise implementation forwards any value returned by a success or failure callback to the next success callback in the chain, as shown below:

let p = new Ice.Promise();
p.then(
    () => {
        return "World!";
    }
).then(
    arg => {
        console.log("Hello " + arg);
    }
);
p.resolve();
 
// Outputs:
Hello World!

Chaining Asynchronous Events with Promises

Now that we've discussed promise fundamentals, let's explore more realistic use cases. The primary purpose of promises is chaining together a sequence of asynchronous events such that the next step in the chain won't execute until the previous step has completed. Common examples in Ice applications include a series of proxy invocations in which the calls must be made sequentially in a certain order, and proxy invocations in which the results of a preceding call must be obtained before the next call can be made.

Consider the following example:

let p = new Ice.Promise();
p.then(
    () => {  // Step 1
        let promise = // do something that creates a promise...
        return promise;
    }
).then(
    () => {  // Step 2
        let promise = // do something else that creates a promise...
        return promise;
    }
).then(
    () => {  // Step 3
        console.log("all done");
    }
);

p.resolve();

Steps 1 and 2 each initiate some asynchronous action that creates a promise, and the steps return the promise object as the result of the callback. Normally the result value would be passed as the argument to the next success callback in the chain, as we discussed earlier. However, when a callback returns a promise, execution of the next success or failure callback becomes dependent on the completion of the returned promise. In the example above, calling p.succeed causes Step 1 to execute, but Step 2 won't execute until the promise returned by Step 1 resolves successfully.

Promise API

Instances of Ice.Promise extend the JavaScript Promise and add the following methods to it:

• finally(fn)
  If the function throws an exception or returns a rejected promise that becomes the rejection value, otherwise the value that was used to resolve the promise is used to resolve the promise returned by finally. See examples below.

• delay(ms)
  Returns a new promise whose success or failure callback is invoked after the specified delay (in milliseconds). The results of the previous promise in the chain are forwarded to the next promise in the chain, as if the delay promise was not present.

• resolve(value)
  Resolves this promise. The value is passed to the success callback of the next promise in the chain.

• reject(value)
  Resolves this promise as failed. The value is passed to the failure callback of the next promise in the chain.

Additionally, the Promise type defines the following functions:

• try(fn)
  Returns a new promise whose execution depends on the completion of the given function. This function allows you to write code similar to the traditional try/catch/finally blocks of synchronous code, as shown below:

  Ice.Promise.try( () => { // perform asynchronous actions...   // return a promise to the last action in the chain... } ).catch( ex => { // handle errors } ).finally( () => { // clean up when the last action completes } );

  The try function makes error handling especially convenient because the chained exception handler will be called if an exception is thrown directly by the try callback, or if the callback returns a promise that later fails. Note that you could also arrange the exception and finally blocks this way:

  Ice.Promise.try( () => { // perform asynchronous actions...   // return a promise to the last action in the chain... } ).finally( () => { // clean up when the last action completes } ).catch( ex => { // handle errors } );

  If the try callback fails with an exception, the finally callback will be invoked and the failure will propagate to the exception callback. Of course, the order in which the callbacks are invoked in an error situation differs from the previous example.

• delay(ms, value)
  Similar to the prototype method described earlier, this function returns a promise whose success callback is invoked after the specified delay (in milliseconds):

  Ice.Promise.delay(100, "John").then( name => { // Invoked after 100ms console.log("Hi " + name); });

Completion Callbacks in JavaScript

The AsyncResult promise returned by a proxy invocation resolves when the remote operation completes successfully or fails. The semantics of the success and failure functions depend on the proxy's invocation mode:

• Twoway proxies
  The success or failure function executes after the Ice run time in the client receives a reply from the server.
• Oneway and datagram proxies
  The success function executes after the Ice run time passes the message off to the socket. The failure function will only be called if an error occurs while Ice is attempting to marshal or send the message.
• Batch proxies
  The success function executes after the Ice run time queues the request. The failure function will only be called if an error occurs while Ice is attempting to marshal the message.

For all asynchronous Ice APIs, the failure function receives two arguments: the exception that caused the failure, and a reference to the AsyncResult promise for the failed invocation.

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. Ice adds a reference to the AsyncResult object associated with the request as the final parameter to every success callback.  Recall the example we showed earlier:

double op(int inp1, string inp2, out bool outp1, out long outp2);

The success callback for op will receive and array with all the parameters:

function handleSuccess(r)
{
   let [returnVal, outp1, outp2] = r;
   ...
}

The failure callback is invoked if the invocation fails because of an Ice run time or user exception. This callback function must have the following signature:

function handleAnException(ex)

For example, the code below shows how to invoke the getName operation:

let proxy = ...;
let empNo = ...;
proxy.getName(empNo).then(
    name => {
        console.log("Name of employee #" + empNo + " is " + name);
    }
).catch(
    ex => {
        console.log("Failed to get name of employee #" + empNo);
        console.log(ex);
    }
);

Let's extend the example add a call to getAddress:

let proxy = ...;
let empNo = ...;
proxy.getName(empNo).then(
    name => {
        console.log("Name of employee #" + empNo + " is " + name);
        return proxy.getAddress(empNo);
    }
).then(
    addr => {
        console.log("Address of employee #" + empNo + " is " + addr);
    }
).catch(
    ex => {
        console.log("failed for employee #" + empNo);
        console.log(ex);
    }
);

Notice here that the success callback for getName returns the AsyncResult promise for getAddress, which means the second success callback won't be invoked until getAddress completes.

Asynchronous Oneway Invocations in JavaScript

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 proxy function on a oneway proxy for an operation that returns values or raises a user exception, the proxy function throws an instance of TwowayOnlyException.

The callback functions look exactly as for a twoway invocation. The failure function is only called if the invocation raised an exception during the proxy function ("on the way out"), and the success function is called as soon as the message is accepted by the local network connection. The success function takes no arguments.

Asynchronous Batch Requests in JavaScript

You can invoke operations via batch 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 proxy function on a oneway proxy for an operation that returns values or raises a user exception, the proxy function throws an instance of TwowayOnlyException.

The callback functions look exactly as for a twoway invocation. The failure function is only called if the batch invocation raised an exception before being queued. The success function takes no arguments. The returned promise for a batch oneway invocation is always completed and indicates the successful queuing of the batch invocation. It can also be marked completed if an error occurs before the request is queued.

Applications that send batched requests can either flush a batch explicitly or allow the Ice run time to flush automatically. The asynchronous proxy method ice_flushBatchRequests initiates an immediate flush of any batch requests queued by that proxy.

In addition, similar methods are available on the communicator and the Connection object that is accessible via the connection property of AsyncResult. These methods flush batch requests sent via the same communicator and via the same connection, respectively.

All versions of ice_flushBatchRequests return a promise whose success callback executes as soon as the batch request message is accepted by the local network connection. The success function takes no arguments. Ice only invokes the failure callback if an error occurs while attempting to send the message.

See Also

• Request Contexts
• Batched Invocations