I'm trying to figure the cleanest way to deal with a scenario where you
have a long chain of callbacks that are executing, and then something in
your environment/program changes in such a way that negates the need for
that work, and could even lead to invalid results if it is allowed to
progress.
An example would probably help. Say I have an app that takes a connection
from client, and then starts some work on their behalf. The work entails
marshalling the resources of a bunch of network resources on their behalf,
so that work gets kicked off. Then the client disconnects or otherwise
signals he wants to cancel the original request. It might look something
like this:
var app = require('my-app-thingy')() ;
app.onRequest( function( req, res ){
//1: we just got a request from a client
writeTransactionStartTimeToDatabase() ; //write a record to database to
record start time
req.on('canceled', function() {
//3: we just got notified that the client has canceled the request
writeTransactionEndTimeToDatabase()
}) ;
//2: start some long running activity
acquireNetworkResource( function(err, resource) {
//4: we got the requested resource
doSomeOtherAsynchronousActivity( function( err ) {
//5: all done
writeTransactionEndTimeToDatabase() ; //just wrote wrong time to database
}) ;
}) ;
})
At time 1, I get a request from a client, and immediately (at time 2)
launch some work on their behalf. At time 3, however, the request is
canceled. Now I need to shut down that work. At time 4, part of the long
running task returns, and since I am not checking the status of the request
I launch another activity -- at time 5, that returns and I now write a
second record to the database recording the incorrect end time -- the
proper end time was written earlier (time 2) and is now overwritten.
Obviously, I could just check the status of the request at every callback
stage, but that quickly gets ugly (what if there are multiple conditions
that needed to be checked).
Is there a pattern for writing code to handle this type of scenario simply
and cleanly?
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