Synchronous vs. asynchronous links?

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Synchronous vs. asynchronous links?

When distributed applications are linked by ALE business processes, the question often arises as to how tight the link should be. Synchronous and asynchronous links have both advantages and disadvantages.

Synchronous links have the advantage that the sub-process on the server can return values to the sub-process on the client that has started the link. Problems with synchronous links occur if the communication line or the server is temporarily not available. If this happens, the sub-process on the client can not be finished (otherwise there would be data inconsistencies).

(Example: There is a logistics system and a financial system. Every stock movement in logistics has to be posted in the general ledger of the financial system. If the link between logistics and finance is synchronous, no stock movement can be recorded in the logistics system if the communication line to the financial system is down.)

Because of this, synchronous links are usually used if the client only wants to get some data from the server and the sub-processes on the server do not have to write any data to the database.

With asynchronous links the sub-process on the client can be finished even if the communication line or the server is not available. In this case the message is stored in the database and the communication can be done later. The disadvantage of asynchronous links is that the sub-process on the server can not return information to the calling sub-process on the client. A special way for sending information back to the client is required. In addition, a special error handling mechanism is required to handle errors on the receiving side.

Asynchronous links are used if a synchronous link is not applicable. For the problems with sending return information to the client and with error handling there is some support from the ALE services.

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