Persistence

PPS maintains its objects in memory while it is running. It will, as required:

• save its objects to persistent storage, either on demand while it is running, or at shutdown
• restore its objects on startup, either immediately, or on first access (deferred loading)

---

PPS may be used to create objects which are rarely (or never) published or subscribed to, but for which persistence is required. “Shutdown” means the orderly exit of the PPS server process. An orderly exit can be triggered by SIGTERM, SIGINT, or SIGQUIT. Other signals (such as SIGKILL, SIGSEGV and SIGABRT) do not result in an orderly exit, and do not, therefore, constitute a “shutdown” for the purposes of persistence.

---

Persistent storage

The underlying persistent storage used by PPS relies on a reliable filesystem, such as:

• disk — QNX 6 filesystem
• NAND Flash — ETFS filesystem
• Nor Flash — FFS3 filesystem
• other — customer generated filesystem

If you need to persist an object to specialized hardware, such as a small NVRAM, which does not support a file system, you can create your own client which subscribes to the a PPS object to be saved. On each object change, PPS will notify your client, allowing the client to update the NVRAM in real time.

Persistence and filesystem limitations

The persistence directory where PPS objects are stored uses exactly the same directory hierarchy as the PPS root directory. Object persistence is, therefore, limited by the capabilities of the underlying persistence filesystem: its support for path and filename lengths, and directory nesting limits.

For example, the QNX Neutrino NFS server supports a maximum nesting depth of 15 levels. This limit also applies to PPS using this service.

Saving objects

On shutdown, PPS always saves any modified objects to a persistent filesystem. You can also force PPS to save an object at any time by calling fsync() on the object.

When PPS saves to a persistent filesystem, it saves every object to its own file, in a directory hierarchy that reproduces the hierarchy of the PPS object tree. For example, with a default configuration, the PPS object /pps/foo/bar is stored at /var>pps/foo/bar.

The default location for the PPS directory is /var/pps. You can use the PPS -p option to change this location.

---

You can set object and attribute qualifiers to have PPS not save specific objects or attributes. For more information, see the chapter Options and Qualifiers. To ensure multiple language support, all strings should use UTF-8 to encode extended character sets.

---

Loading objects

When PPS starts up, it immediately builds the directory hierarchy from the encoded filenames on the persistent filesystem. In its default configuration, PPS defers loading the objects in the directories until first access to one of the files. This access could be an open() call on a PPS object, or a readdir() call on the PPS directory.

You can use the -l option on startup to have PPS load directory and object names but not the object contents, or load directories, objects and object contents. For more information, see “Running PPS” in the chapter QNX PPS Service.