Oneway and datagram invocations are normally sent as individual messages, that is, the Ice run time sends the oneway or datagram invocation to the server immediately, as soon as the client makes the call. If a client sends a number of oneway or datagram invocations in succession, the client-side run time traps into the OS kernel for each message, which is expensive. In addition, each message is sent with its own message header, that is, for N messages, the bandwidth for N message headers is consumed. In situations where a client sends a number of oneway or datagram invocations, the additional overhead can be considerable.

To avoid the overhead of sending many small messages, you can send oneway and datagram invocations in a batch: instead of being sent as a separate message, a batch invocation is placed into a client-side buffer by the Ice run time. Successive batch invocations are added to the buffer and accumulated on the client side until they are flushed, either explicitly by the client or automatically by the Ice run time.

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Proxy Methods for Batched Invocations

Several proxy methods support the use of batched invocations. In Slice, these methods would look as follows:

Object* ice_batchOneway();
Object* ice_batchDatagram();
void ice_flushBatchRequests();

The ice_batchOneway and ice_batchDatagram methods create a new proxy configured for batch invocations. Once you obtain a batch proxy, messages sent via that proxy are buffered by the proxy instead of being sent immediately. Once the client has invoked one or more operations on a batch proxy, it can call ice_flushBatchRequests to explicitly flush the batched invocations. This causes the batched messages to be sent "in bulk", preceded by a single message header. On the server side, batched messages are dispatched by a single thread, in the order in which they were written into the batch. This means that messages from a single batch cannot appear to be reordered in the server. Moreover, either all messages in a batch are delivered or none of them. (This is true even for batched datagrams.)

Asynchronous versions of ice_flushBatchRequests are also available; see the relevant language mapping for more information.

As of Ice 3.6, batched invocations are queued by the proxy on which the request was invoked (this is true for all proxies except fixed proxies). It's important to be aware of this behavior for several reasons:

• Batched invocations queued on a proxy will be lost if that proxy is deallocated prior to being flushed
• Proxy instances maintain separate queues even if they refer to the same target object

• Using proxy factory methods may (or may not) create new proxy instances, which affects how batched invocations are queued. Consider this example:

  C++

  Ice::CommunicatorPtr comm = ...;
  Ice::ObjectPrx batch = comm->stringToProxy("...")->ice_batchOneway(); // Creates a batch oneway proxy
  Ice::ObjectPrx secureBatch = batch->ice_secure(); // Might create a new proxy instance
  batch->ice_ping();
  secureBatch->ice_ping();
  batch->ice_flushBatchRequests(); // Might also flush requests on secureBatch
  

  At run time, the batch and secureBatch variables might refer to the same proxy instance or two separate proxy instances, depending on the original stringified proxy's configuration. As a result, flushing requests using one variable may or may not flush the requests of the other.

Calling ice_flushBatchRequests on a proxy behaves like a oneway invocation in that automatic retries take place and an exception is raised if an error occurs while establishing a connection or sending the batch message.

Automatically Flushing Batched Invocations

The default behavior of the Ice run time, as governed by the configuration property Ice.BatchAutoFlushSize, automatically flushes batched invocations as soon as a batched request causes the accumulated message to exceed the specified limit. When this occurs, the Ice run time immediately flushes the existing batch of requests and begins a new batch with this latest request as its first element.

For batched oneway invocations, the value of Ice.BatchAutoFlushSize specifies the maximum message size in kilobytes; the default value is 1MB. In the case of batched datagram invocations, the maximum message size is the smaller of the system's maximum size for datagram packets and the value of Ice.BatchAutoFlushSize.

Automatic flushing is enabled by default as a convenience for clients to ensure a batch never exceeds the configured limit. A client can track batch request activity, and even implement its own auto-flush logic, by installing an interceptor.

Batched Invocations for Fixed Proxies

A fixed proxy is a special form of proxy that an application explicitly creates for use with a specific bidirectional connection. Batched invocations on a fixed proxy are not queued by the proxy, as is the case for regular proxies, but rather by the connection associated with the fixed proxy. Automatic flushing continues to work as usual for batched invocations on fixed proxies, and you have three options for manually flushing:

• Calling ice_flushBatchRequests on a fixed proxy flushes all batched requests queued by its connection; this includes batched requests from other fixed proxies that share the same connection
• Calling Connection::flushBatchRequests flushes all batched requests queued by the target connection
• Calling Communicator::flushBatchRequests flushes all batched requests on all connections associated with the target communicator

The synchronous versions of flushBatchRequests block the calling thread until the batched requests have been successfully written to the local transport. To avoid the risk of blocking, you must use the asynchronous versions instead (assuming they are supported by your chosen language mapping).

Note the following limitations in case a connection error occurs:

• Any requests queued by that connection are lost
• Automatic retries are not attempted
• The proxy method ice_flushBatchRequests and Connection::flushBatchRequests will raise an exception, but Communicator::flushBatchRequests ignores any errors

Connection::flushBatchRequests and Communicator::flushBatchRequests always send the batched requests without compression. To send a batch of fixed-proxies request with compression, you need to call ice_flushBatchRequests on a fixed proxy created with ice_compress(true), for example:

fixedPrx->ice_compress(true)->ice_flushBatchRequests(); 

Considerations for Batched Datagrams

For batched datagram invocations, you need to keep in mind that, if the data for the invocations in a batch substantially exceeds the PDU size of the network, it becomes increasingly likely for an individual UDP packet to get lost due to fragmentation. In turn, loss of even a single packet causes the entire batch to be lost. For this reason, batched datagram invocations are most suitable for simple interfaces with a number of operations that each set an attribute of the target object (or interfaces with similar semantics). Batched oneway invocations do not suffer from this risk because they are sent over stream-oriented transports, so individual packets cannot be lost.

If automatic flushing is enabled, Ice's default behavior uses the smaller of Ice.BatchAutoFlushSize and Ice.UDP.SndSize to determine the maximum size for a batch datagram message.

Compressing Batched Invocations

Batched invocations are more efficient if you also enable compression for the transport: many isolated and small messages are unlikely to compress well, whereas batched messages are likely to provide better compression because the compression algorithm has more data to work with.

Active Connection Management and Batched Invocations

As for oneway invocations, server-side Active Connection Management (ACM) can interfere with batched invocations over TCP or TCP-based transports (SSL, WebSocket, etc.). With server-side ACM enabled, it's possible for a server to close the connection at the wrong moment and not process a batch – with no indication being returned to the client that the batch was lost. We recommend that you either disable ACM for the server side, or enable ACM heartbeats in the client to ensure the connection remains active.

Batched Invocation Interceptors

Ice 3.6 added a batch interceptor API that allows you to implement your own auto-flush algorithm or receive notification when an auto-flush fails. We present the C++ API below (the API for other languages is very similar):

namespace Ice {
class BatchRequest {
public:
    virtual void enqueue() const = 0;
    virtual int getSize() const = 0;
    virtual const std::string& getOperation() const = 0;
    virtual const Ice::ObjectPrx& getProxy() const = 0;
};
 
class BatchRequestInterceptor : public IceUtil::Shared {
public:
    virtual void enqueue(const BatchRequest& req, int count, int size) = 0;
};
typedef IceUtil::Handle<BatchRequestInterceptor> BatchRequestInterceptorPtr;

// C++11 only:
BatchRequestInterceptorPtr
newBatchRequestInterceptor(const std::function<void (const BatchRequest&, int, int)>&);
}

You install an interceptor by setting the batchRequestInterceptor member of the InitializationData object that the application constructs when initializing a new communicator:

namespace Ice {
struct InitializationData {
    ...
    BatchRequestInterceptorPtr batchRequestInterceptor;
};
}

The Ice run time invokes the enqueue method on the interceptor for each batch request, passing the following arguments:

• req - An object representing the batch request being queued
• count - The number of requests currently in the queue
• size - The number of bytes consumed by the requests currently in the queue

The request represented by req is not included in the count and size figures.

A batch request is not queued until the interceptor calls BatchRequest::enqueue. The minimal interceptor implementation is therefore:

class BatchRequestInterceptorI : public Ice::BatchRequestInterceptor {
public:
    virtual void enqueue(const BatchRequest& req, int count, int size)
    {
        req.enqueue();
    }
};

A more sophisticated implementation might use its own logic for automatically flushing queued requests:

class BatchRequestInterceptorI : public Ice::BatchRequestInterceptor {
public:
    BatchRequestInterceptorI(const Ice::CommunicatorPtr& communicator)
    {
        _limit = communicator->getProperties()->getPropertyAsIntWithDefault("Ice.BatchAutoFlushSize", 1024);
        _limit *= 1024; // Convert to bytes
    }
 
    virtual void enqueue(const BatchRequest& req, int count, int size)
    {
        if(size + req.getSize() > _limit)
            req.getProxy()->begin_ice_flushBatchRequests(
                [=](const Ice::Exception& ex) { /* Handle error */ },
                [=](bool sentSynchronously) { /* Sent callback */ });
 
        req.enqueue();
    }
 
private:
    int _limit;
};

In this example, the implementation consults the existing Ice property Ice.BatchAutoFlushSize to determine the limit that triggers an automatic flush. If a flush is necessary, the interceptor can obtain the relevant proxy by calling getProxy on the BatchRequest object.

Specifying your own exception handler when calling begin_ice_flushBatchRequests gives you the ability to take action if a failure occurs (Ice's default automatic flushing implementation ignores any errors). Aside from logging a message, your options are somewhat limited because it's not possible for the interceptor to force a retry.

See Also

• Oneway Invocations
• Datagram Invocations
• Communicators
• Using Connections
• Ice Protocol and Encoding
• Protocol Compression
• Active Connection Management