[SalesForce] How to synchronize asynchronous Apex

I've used this pattern in a number of projects to achieve this, by calling this from the finish method of the Batch Apex class:

private void reSubmitJob( )
{
        // try again in a minute
        Datetime sysTime = System.now().addSeconds( 60 );

        String chronExpression = '' + sysTime.second() + ' ' + sysTime.minute() + ' ' + sysTime.hour() + ' ' + sysTime.day() + ' ' + sysTime.month() + ' ? ' + sysTime.year();

        System.schedule( 'Scheduled Apex ' + sysTime, chronExpression, this );
}

Then, in your Schedule execute call Database.executeBatch() and abort the scheduled job System.abortJob(SC.getJobTriggerId()) to remove the 'used' job from the Scheduled Job list.

Note, Summer '13 gives you the System.scheduleBatch method which may also be useful for what you are trying to achieve. Though I have noticed some potential issues with that myself.

EDIT: one of the things to be wary of is the number of seconds you specify for your re-schedule. Sometimes, if this is too low (e.g. 30 seconds) the system takes longer than that to schedule it and the time passes and your job never runs. I've found 1 minute to be generally acceptable but have also been caught out with that on some occasions when I guess there is a lot of activity.

EDIT 2: this is not deemed best practice, see post Is there a simple way to serialize execution of Batchable class?. It's called 'suicidal apex' :D and should be used with caution.

See this blog post for more info but you can lock sObject records. To lock records, simply use the FOR UPDATE keywords in your SOQL statements. You do not have to manually commit the records so if your Apex script finishes successfully the changes are automatically committed to the database and the locks are released. If your Apex script fails, any database changes are rolled back and the locks are also released.

for (List<opportunity> ops : [select id from Opportunity
   where stagename = 'Closed Lost' for update]) {
    // process the records and issue DML
}

The Apex runtime engine locks not only the parent sObject record but all child records as well. So if you lock, for instance, an Opportunity sObject all of its Opportunity Line Items will be locked as well. Other users will be able to read these records but not make changes to them while the lock is in place.

If your record is locked and another thread tries to commit changes, the platform will retry for roughly 5 -10 seconds before failing with a “Resource Unavailable” error. For end users, I believe if they try to save a locked record from the Salesforce.com UI, they will receive an error message stating that the record has been changed and that they should reload the page. I can’t confirm but I’ve seen this in the past.