macOS Sierra’s background dispatching covers far more than just automatic Time Machine backups. When those become infrequent or irregular, or forgotten altogether, it means that all the other services which are run by the same scheduling and dispatching systems (DAS and CTS) also become unreliable. This article lists those which I know about so far.

Browse CTS log entries, and you’ll see that these scheduling and dispatch systems are kept busy. On average, over a twelve hour period of intermittent use, a Mac running Sierra 10.12.5 (without macOS Server) runs a new activity every two minutes or so. When these systems fail, they may not dispatch any new activities, or they may only dispatch one every half hour or so. Leaving your Mac running despite the failure of these systems leaves it short of some very important services.

1. Key services which you probably can’t do without for long

Time Machine backups – these are the most obvious to most users, and are run hourly when Time Machine is turned on. They can be rescheduled using a LaunchAgent or similar, but there may come a time when you need to rely on a backup which may not exist.

The metadata server mds is run in several different tasks more frequently than every hour. This not only supports Spotlight searches, but other search-related features, and is essential for Time Machine backups too. There is no other way to run those services, so your metadata indexes will become progressively more out of date and less useful. You cannot run mds directly.

If you use apps which take advantage of document versioning, then revisiond, which is the storage manager for document versioning, is very important to their maintenance. This is normally run every couple of hours, but could become more frequent when using versions extensively. There is no other way to run this service.

Software Update runs automatic checks for updates every 6 hours or so. You can run these manually, but they are likely to get overlooked unless you are very disciplined. This should not affect security updates which are pushed out silently, though.

2. Important synchronisation services

Address Book (Contacts) synchronisation is dispatched on a fairly regular hourly basis. When normal dispatching is not functioning, your address book will be left unsynchronised.

A Calendar alarm check is made every hour. This is again lost when dispatching services fail.

iCal Calendar synchronisation normally occurs every 15 minutes, but is lost when dispatching services fail. Your Calendars will therefore become progressively incorrect.

3. iCloud

Several iCloud services depend on normal dispatching, and will start to fail when that is lost. These appear to include:

• bird, which handles housekeeping and synchronisation of documents in iCloud; this is normally run hourly;
• CloudKit and iCloud services housekeeping are run more frequently than each hour;
• syncdefaultsd, an unknown service thought to be related to iCloud synchronisation, runs infrequently.

4. Safari

Safari is now dependent on several services for its normal function. Those which are managed by dispatching services appear to include:

• Safari Safe Browsing update service, which is normally run every 30 minutes or so;
• Safari ParsecD, which is thought to be the autofill service update, and runs every 15 minutes or so;
• parsecd itself is an unknown network service (possibly related to security) which is run every 10 minutes approximately.

5. Miscellaneous

There are a lot of other services which only run infrequently, or whose function is unknown. Although losing those services may not be as significant as those above, some could easily result in odd effects.

These include:
awdd, thought to be Apple Wireless Diagnostic Data, which runs infrequently;
the Core Duet service, which is probably involved in energy management heuristics, and runs infrequently;
Apple’s new Differential Privacy system requires periodic housekeeping tasks, which run infrequently;
diskspaced is most probably a daily storage management check;
Geolocation Services run every hour;
Help Book registration checks are performed daily, to ensure that they remain in kilter;
Apple Maps runs an infrequent check for any announcements;
mediaanalysisd is an unknown service which runs infrequently;
mobileassetd is another unknown service which runs infrequently;
photoanalysisd is a Photos support service which runs every couple of hours;
Siri analyses its metrics infrequently;
suggestd is the Core Suggestions service, and runs every 10 minutes or so;
PDCardFileManager may be related to Apple Pay or similar services, and performs a daily revocation check.

Together, these account for the majority of the 70-80 activities which DAS keeps in its queue at any time. I suspect that there are another 20 to 30 which I don’t know about, and other Macs, particularly laptop models, may have additional hardware-specific background services which are also managed by DAS and CTS. macOS Server could install its own too.

When DAS and CTS stop dispatching activities properly, there is much more at stake than just Time Machine backups. Reinstating normal dispatching of these background services is important, and needs to be performed promptly.