Best way to schedule a batch job repeating in short frequency

Question

Requirement

Run a batch in every other 15mins in the clock (i.e. clock time 00:15, 00:30, 00:45 01:00....).

Method 1 - create a schedulable to run the batch every 15mins.

• Pros: it allows me, in execute() method of schedulable, to check if the batch is already scheduled or running so it can abort if needed.

• Cons: I need to create 4 separate schedulable instances.

Method 2 - use System.scheduleBatch() to run the batch every 15mins.

• Pros: no need to create 4 instances
• Cons: it seems no place/hook to allow me to check if already scheduled or running right before the batch starts.

Summary/Issue

No ideal solution, either way has opposite pros&cons.

Answer 1

1. When using System.scheduleBatch, if you always use the same name every time for scheduling this batch, an attempt to schedule the batch whilst it is already scheduled will result in an exception and the batch won't get double scheduled.
2. You can use the Async Apex Jobs table to provide info about whether the specified batch is already executing and you might do this before scheduling the batch.
3. By adding this "already executing" test also into the batch's start method (see this answer for more info) you can ensure that if a second instance does manage to start (due to race conditions, for example) you prevent this from actually doing anything by throwing an exception, returning a query locator with LIMIT 0 or by returning null - the middle option is cleaner and errors are not reported to admins in this case, but this will consume an additional async call when finish is invoked).
4. The batch finish can look to re-schedule itself either with a fixed 15 minute delay or by aligning with the real time clock (on the hour, quarter past, half past or quarter to) as needed.

NB: If something causes the batch to exit abnormally before it re-schedules itself there's nothing here to get the batch going again of course...