at:tutorial:distribution
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at:tutorial:distribution [2008/09/16 17:26] – * tvcutsem | at:tutorial:distribution [2009/01/29 10:30] – elisag | ||
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Building on the actor-based concurrency model explained in the [[actors|previous chapter]], this chapter discusses the distribution provisions of AmbientTalk. For actors to communicate across the boundaries of a single device, actors need to be capable of discovering one another' | Building on the actor-based concurrency model explained in the [[actors|previous chapter]], this chapter discusses the distribution provisions of AmbientTalk. For actors to communicate across the boundaries of a single device, actors need to be capable of discovering one another' | ||
- | These requirements correspond to the cornerstones of the Ambient-Oriented Programming paradigm. The seamless integration of language support for dealing with partial failures and performing service discovery, hinge on AmbientTalk' | + | These requirements correspond to the cornerstones of the Ambient-Oriented Programming paradigm. The seamless integration of language support for dealing with partial failures and performing service discovery, hinge on AmbientTalk' |
Before delving in these topics, we illustrate how to activate the network facilities of AmbientTalk in the next section. | Before delving in these topics, we illustrate how to activate the network facilities of AmbientTalk in the next section. | ||
Line 63: | Line 63: | ||
As '' | As '' | ||
- | ===== Partial Failure Handling | + | ===== Dealing with Transient Failures |
- | Let us consider again the example instant messenger application described in previous section to further explain the semantics of AmbientTalk' | + | Let us consider again the example instant messenger application described in previous section to further explain the semantics of AmbientTalk' |
- | When an object discovers a service type, the '' | + | When an object discovers a service type, the '' |
- Objects are always passed //by far reference//, | - Objects are always passed //by far reference//, | ||
Line 90: | Line 90: | ||
This code illustrate how the instant messenger application notifies when a buddy goes online or offline. In the above code, '' | This code illustrate how the instant messenger application notifies when a buddy goes online or offline. In the above code, '' | ||
- | In order to cope with partial failures, AmbientTalk also allows developers to retract all currently unsent messages from the far reference outbox by means of the '' | + | In order to cope with partial failures, AmbientTalk also allows developers to retract all currently unsent messages from the remote |
The '' | The '' | ||
Line 105: | Line 105: | ||
The function '' | The function '' | ||
+ | |||
+ | ===== Dealing with Permanent Failures ===== | ||
+ | |||
+ | As explained in the previous section, remote far references have been designed to be resilient to intermittent disconnections by default. This behaviour is desirable because it can be expected that many partial failures in mobile ad hoc networks are the result of transient network partitions. However, not all network partitions are transient. For example, a remote device has crashed or has moved out of the wireless communication range and does not return. Such permanent failures should also be dealt by means of compensating actions, e.g. application-level failure handling code. | ||
+ | |||
+ | To deal with permanent failures, AmbientTalk uses the concept of leasing. A lease denotes the right to access a resource for a specific duration that is negotiated by the owner of a resource and a resource claimant (called the lease grantor and lease holder, respectively) when the access is first requested. | ||
+ | |||
+ | ====Leased Object References==== | ||
+ | |||
+ | A leased object reference is a remote far reference that grants access to a remote object for a limited period of time. When the time period has elapsed, the access to the remote object is terminated and the leased reference is said to //expire//. Similarly to remote far references, a leased reference abstracts client objects from the actual network connection state. Client objects can send a message to the remote object even if a leased references is disconnected at that time. Message are accumulated in order to be transmitted when the reference becomes reconnected. When the leased reference expires it, messages are discarded since an expired leased reference behaves as a // | ||
+ | |||
+ | {{ : | ||
+ | |||
+ | ====Working with leased object references==== | ||
+ | |||
+ | The code snippet below illustrates a leased far reference in the context of an online shopping application. In the example, a client object can ask a server to start a shopping session by sending it the '' | ||
+ | |||
+ | < | ||
+ | def openSession() { | ||
+ | def shoppingCart := Cart.new(); // to store purchased items | ||
+ | def session := object: { | ||
+ | def addItemToCart(anItem) { ... } | ||
+ | def checkOutCart() { ... } | ||
+ | }; | ||
+ | def leasedSession := lease: minutes(5) for: session; | ||
+ | leasedSession; | ||
+ | }; | ||
+ | </ | ||
+ | |||
+ | The '' | ||
+ | |||
+ | < | ||
+ | We assume the use of futures to get the return value of the '' | ||
+ | </ | ||
+ | |||
+ | At client side, a customer can ask a server to open a shopping session as follows: | ||
+ | |||
+ | < | ||
+ | def mySession := server< | ||
+ | ... | ||
+ | mySession< | ||
+ | </ | ||
+ | |||
+ | The future attached to the '' | ||
+ | |||
+ | < | ||
+ | renew: mySession for: minutes(5); | ||
+ | revoke: mySession; | ||
+ | </ | ||
+ | |||
+ | The '' | ||
+ | |||
+ | When no renewal is performed due to a network partition outlasting the lease time period or in absence of utilization, | ||
+ | |||
+ | < | ||
+ | when: mySession expired: { | ||
+ | ... // free up resources used by this session e.g. the cart | ||
+ | } | ||
+ | </ | ||
+ | |||
+ | The construct takes as parameters a leased reference and a block of code that is asynchronously triggered upon the lease expiration. This allows client and service objects to treat a failure as permanent (i.e. to detect when the reference is permanently broken) and to perform application-level failure handling. At server side, this has important benefits for memory management. Once all leased references to a service object have expired, the object becomes subject to garbage collection once it is no longer locally referenced. | ||
+ | |||
+ | ====Leasing patterns==== | ||
+ | As is the case in other leasing mechanisms, determining the proper lease renewal period is not straightforward and may even depend on system parameters such as the number of clients. In AmbientTalk, | ||
+ | |||
+ | The first variant is a // | ||
+ | by the remote object. In other words, as long as the client uses the remote object, the leased reference is transparently renewed by the interpreter. | ||
+ | |||
+ | < | ||
+ | def openSession() { | ||
+ | def shoppingCart := Cart.new(); // to store purchased items | ||
+ | def session := object: { | ||
+ | def addItemToCart(anItem) { ... } | ||
+ | def checkOutCart() { ... } | ||
+ | }; | ||
+ | def leasedSession := renewOnCallLease: | ||
+ | leasedSession; | ||
+ | }; | ||
+ | </ | ||
+ | |||
+ | Similar to '' | ||
+ | |||
+ | The second variant is a // | ||
+ | |||
+ | < | ||
+ | def myObject: = object:{ | ||
+ | ... | ||
+ | }; | ||
+ | def leasedObject := singleCallLease: | ||
+ | </ | ||
+ | |||
+ | Similar to the other two constructs, '' | ||
+ | |||
+ | ====Integrating leasing with future-type message passing==== | ||
+ | |||
+ | Single-call leases are useful for objects adhering to a single call pattern, such as callback objects. Callback objects are often used in asynchronous message passing schemes in order for remote object to be able to return values. These callback objects are typically remotely accessed only once by remote objects with the computed return value. In AmbientTalk, | ||
+ | |||
+ | We have integrated leasing into futures by parameter-passing a future attached to an asynchronous message via a singe-call lease which either expires due to a timeout or upon the reception of the computed return value. The timeout for the implicit single-call lease on a future can be set by annotating the asynchronous message with a @Due annotation as follows: | ||
+ | |||
+ | < | ||
+ | def sessionFuture := server< | ||
+ | when: sessionFuture becomes: { |session| | ||
+ | // open session with server | ||
+ | }catch: TimeoutException using: { |e| | ||
+ | // unable to open a session, do some clean-up if necessary | ||
+ | } | ||
+ | </ | ||
+ | |||
+ | If the future is resolved, the session variable stores a leased object reference to the remote session object. | ||
+ | |||
+ | < | ||
+ | Note that specifying a catch: block for the TimeoutException is equivalent to installing a when: | ||
+ | </ | ||
+ | |||
+ | ====Importing leased object references==== | ||
+ | |||
+ | Similar to futures, leased object references have been built reflectively on top of AmbientTalk. | ||
+ | |||
+ | To use the language constructs for leased references, you should import the leasedref module as follows: | ||
+ | import / | ||
+ | |||
+ | < | ||
+ | leasedref module exports support primitives to manipulate time intervals (i.e. minutes, seconds, millisecs) so that you do not need to explicitly import the timer module. Remember to exclude those methods from the leasedref import statement if some other module has already imported them, e.g. if futures are enabled. | ||
+ | </ | ||
+ | |||
+ | More information pertaining to the API of the leased references language module can be found in the appendix. | ||
===== Garbage collecting remote references ===== | ===== Garbage collecting remote references ===== | ||
+ | |||
+ | < | ||
+ | Under Construction: | ||
+ | Update this section to explain takeOffline in the contest of unitesting to unexport remotely accessible objects. | ||
+ | </ | ||
As explained in the previous section, AmbientTalk' | As explained in the previous section, AmbientTalk' |
at/tutorial/distribution.txt · Last modified: 2009/01/30 16:13 by tvcutsem