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Compendium of "synchronous" definitions

From: David Booth <dbooth@w3.org>
Date: Thu, 27 Feb 2003 19:58:27 -0500
Message-Id: <>
To: www-ws-arch@w3.org

Here are the proposed definitions of "synchronous" that I've collected from 
the list.  If I've missed any, it was only due to volume of messages I was 
trying to sort through -- not due to any desire to slight anyone -- so 
please accept my apology.  If that happened, and you do feel that (yet) 
another definition should be considered, please re-submit it to the list by 
reply message and give it a unique name as I've done with the others below, 
so that we can be clear when in referring to them.


Definition dbooth-2
[This is my attempt at combining the main concepts I've seen in others.]
Synchronous interaction
An interaction is synchronous if the parties are involved in the 
interaction at the same time and the interaction carries an expectation of 
immediate processing.  A one-way interaction is synchronous if successful 
message delivery implies that the message either has been processed by the 
receiver or is actively being processed.  A round-trip or more complex 
interaction is synchronous if the initiator pauses some of its processing 
to wait for the interaction's constituent messages to be processed.  In a 
round-trip interaction, the request and response are often sent over the 
same communication channel.

Definition dbooth-1
Property of an interaction whose results are directly following the 
interaction. An interaction between an initiator and a respondent is 
synchronous if the initiator blocks some further processing while it waits 
for a corresponding action, response or acknowledgement from the respondent.

Definition ugo-2c
Asynchronous: A request/response interaction is said to be asynchronous 
when the request and response are chronologically decoupled. In other 
words, the client agent does not have to "wait" for the response once it 
issues the initial request. The exact meaning of "not having to wait" 
depends on the characteristics of the client agent (including the transfer 
protocol it uses). Examples include receiving the response on a different 
thread, on a different socket, on a different end-point, by polling the 
server, etc.

Synchronous: The opposite of asynchronous.

Definition daveo-1
a programmatic flow of control on the sender effectively does nothing but 
wait for a response after sending it's request

Definition daveo-2
the request and response flow forwards and backwards over the same virtual 
connection between the sender and receiver.

Definition moberg-1
A web service response is said to be synchronous iff it is returned using 
the same network connection used in sending the request to which it is a 
response. (This implies that only one URL would be needed for 
request-response MEP when the response is synchronous and uses a transfer 
protocol that has URLs. It also implies that the request and response occur 
within the interval of time that the network connection exists. Also, there 
is overhead in setting up only one TCP connection when TCP is used, and 
since we are talking about IP _connections_ that will be almost always the 

A web service response is said to asynchronous iff it is returned using a 
network connection that is distinct from that used for sending the request 
to which it is a response. (Implications: Two URLs are needed to configure 
a request-response MEP when the response is ==>asynchronous. The response 
connection may occur while the request connection is still open or after it 
is closed. For TCP-based transfer protocols, two connection setups will be 

Definition assaf-3
A transport is synchronous iff the request is returned using the same 
network connection.

Definition cutler-1
Synchronous: a request/response exchange that is correlated by virtue of a 
serialized, sequenced exchange of messages between requestor and 
respondant, typically over the same socket or stream.

Asynchronous: a request/response exchange that is not synchronous, 
typically relying on some mechanism such as Message-ID within the messages 
to correlate the request and response messages.

Definition ferris-1
synchronous message exchange (applies to oneway as well as 
request/response) requires that both sender and receiver, or initiator and 
respondant, processes are running/active at the same time as the exchange 
takes place. In the case of request/response, the exchange is synchronous 
if both sender and receiver remain in the running/active state for both the 
request and response.

asynchronous message exchange (also applies to oneway or request response) 
does not require, but does not preclude, that both sender and receiver, or 
initiator and respondant, processes are running/active at the same time as 
the exchange takes place. It typcally requires some form of mediation 
between the sender and receiver such as a message queue.

Definition mikec-1
(Taken from 
http://whatis.techtarget.com/definition/0,,sid9_gci213080,00.html ?)
In program-to-program communication, synchronous communication requires 
that each end of an exchange of communication respond in turn without 
initiating a new communication. A typical activity that might use a 
synchronous protocol would be a transmission of files from one point to 
another. As each transmission is received, a response is returned 
indicating success or the need to resend. Each successive transmission of 
data requires a response to the previous transmission before a new one can 
be initiated. Synchronous program communication is contrasted with 
asynchronous program communication.

Definition walden-1
Synchronous, then, places a constraint on a response such that the response 
must be received within a strictly or loosely defined time quantum (strict 
vs lax synchrony), or else the exchange fails.
Asynchronous differs in that no amount of elapsed time signals the failure 
of the exchange.

Definition assaf-1
An operation is synchronous if both service requester and service provider
engage will always engage in that operation at the same time.

Definition assaf-2
An interaction is synchronous if activities demarcated by that interaction
will always be performed at the same time.

David Booth
W3C Fellow / Hewlett-Packard
Telephone: +1.617.253.1273
Received on Thursday, 27 February 2003 19:59:49 UTC

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