REGISTRY(4) Device Drivers Manual REGISTRY(4)

registry, regquery - registration of services dynamically

ndb/registry [ -f dbfile ]

ndb/regquery [ -m mntpt ] [ -n ] [ attr value ] ...

Registry is a file server that provides registration and location of services by their attributes where services come and go. It was originally intended to register Grid resources but has much wider use. Registry serves 9P on its standard input. That can be mounted in the local name space (see below for an example), or served over a connection (eg, via listen (1)). The conventional local mount point is /mnt/registry, which is where registries (2) expects to find the local registry by default. There can, however, be any number of registries active, each supporting a different group of users and services, in different authentication domains.

Registry serves a one-level directory containing a few control files and one file per registered service. All the files are text files with similar format: zero or more lines of one or more words, separated by white space (blank or tab). A word can be quoted, using single quotes in the style of sh (1), allowing it to contain white space or represent the empty string ''. Within a quoted word, a single quote character is represented by two adjacent ones. (In programs, use String->quoted from string (2), or more simply the %q string format of sys-print (2) to quote strings appropriately.)

The control files are:

A text file that is opened and written to register a new service. Opening new returns a file descriptor to a new but anonymous service file. It must be written to set its name and initial description. The first write contains one or more words:
svcname [ attr value ]..."

The first word, svcname, is the proposed service name. The write returns an error if the name duplicates any existing name. Otherwise, a new file svcname will appear in the directory. The optional attribute value pairs form the initial service description. The service is removed from the register by default when the file is closed. To advertise a service without having to hold open a file descriptor, include an attr named persist with non-zero value; the service will then remain registered until explicitly removed (see below).

A read-only text file containing a list of all the currently-registered services, one per line. Each line contains a sequence of words. The first word is the service name and the remaining ones are paired: attribute name then value.
A text file which should be opened for reading and writing. Each write provides a set of attribute/value pairs that acts as a filter, selecting only those services that possess all the given attributes with the given values; the value "*" matches any value. Subsequent reads return a list of services, in the same format as index, but listing only the selected services. The selection is made from those available at the time of the write. There is a separate filter for each open instance of find, so that several processes can set different filters simultaneously. (Note that after the write, the file offset must be set to zero using sys-seek (2) before reading.)
A read-only text file that can be used to detect when changes are made to the registry. Reading from it blocks until a change is made. At that point, the read completes and returns a string with a newline terminated decimal version number representing the number of changes made to the registry, which matches Qid.vers for the registry directory. Multiple changes are coalesced into one report.

A service file created by new may be read by anyone, but may be written only by its owner. Each write must contain a set of attribute/value pairs, formatted as above, which adds new attributes to the service description and changes the value of existing attributes. The file remains until the service is struck off the register. The owner of a service can remove its registration at any time by removing the corresponding service file (eg, using rm (1) or sys-remove (2)).

Typically, most services register themselves dynamically, perhaps using their locations (eg, network address) as their service names. (Registry itself does not interpret the name.) The -f option causes registry to load initially a set of static service descriptions from dbfile, which is in attrdb (6) format. The database entry for each service contains the pair service=svcname, causing registry to make an entry for the given svcname with attribute/value pairs initialised from the rest of that database entry. For example:

service=net!click!1234
	description='snapshot service' auth=none

Static entries are regarded as persistent but can be removed explicitly by the registry owner.

Regquery looks for services in the registry mounted at mntpt (default: /mnt/registry) that match all the given attribute/value pairs. It prints the resulting list of services, one per line. If the -n option is given, regquery prints only the service name(s); otherwise it prints the service description as well. If no attributes are given on the command line, regquery prompts for successive queries, one per line, and prints each result.

Start a registry that appears at the standard location for local registries, /mnt/registry:

mount {ndb/registry} /mnt/registry

Make that registry available to the network on authenticated connections:

listen -v 'tcp!*!registry' {export /mnt/registry&}

/appl/cmd/ndb/registry.b

attrdb (2), registries (2)

It is not currently possible to have two values with the same attribute name in the same service description.