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| -rw-r--r-- | hurd/translator.mdwn | 1 | ||||
| -rw-r--r-- | hurd/translator/writing/nsmux.mdwn | 106 |
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diff --git a/hurd/translator.mdwn b/hurd/translator.mdwn index ea420a72..cfb1f7db 100644 --- a/hurd/translator.mdwn +++ b/hurd/translator.mdwn @@ -73,6 +73,7 @@ Read about translator [[short-circuiting]]. * [[cvsfs]] * [[tmpfs]] * [[procfs]] +* [[nsmux|translator/writing/nsmux]] * ... diff --git a/hurd/translator/writing/nsmux.mdwn b/hurd/translator/writing/nsmux.mdwn new file mode 100644 index 00000000..735c1fde --- /dev/null +++ b/hurd/translator/writing/nsmux.mdwn @@ -0,0 +1,106 @@ +[[!meta copyright="Copyright © 2009 Free Software Foundation, Inc."]] + +[[!meta license="""[[!toggle id="license" text="GFDL 1.2+"]][[!toggleable +id="license" text="Permission is granted to copy, distribute and/or modify this +document under the terms of the GNU Free Documentation License, Version 1.2 or +any later version published by the Free Software Foundation; with no Invariant +Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license +is included in the section entitled +[[GNU Free Documentation License|/fdl]]."]]"""]] + +# nsmux + +`nsmux` implements the simplest use-case of namespace-based translator +selection (see below). + +## Source + +`nsmux` translator can be obtained with the following command: + + $ git clone git://github.com/scolobb/nsmux.git + +`filter` translator can be obtained with the following command: + + $ git clone git://github.com/scolobb/filter.git + +The filter is not yet working. + +It is highly probable that soon the code will be moved to Savannah. + +## Namespace-based Translator Selection + +Namespace-based translator selection is the special technique of using +"magic" filenames for both accessing the file and setting translators +on it. + +A "magic" filename is a filename which contains an unescaped sequence +of two commas: ",,". This sequence can be escaped by adding another +comma: ",,,". In the magic filename the part up to the first double +commas is interpreted as the filename itself; the remaining segments +into which the string is split by occurrences of ",," are treated as +names of translators located under `/hurd/`. + +The simplest advantage before traditional way of setting +translators is shown in the following examples. Compare this + + $ settrans -a file translator1 + $ settrans -a file translator2 + $ cat file + +to this: + + $ cat file,,translator1,,translator2 + +One simple command versus three more lengthy ones is an obvious +improvement. However, this advantage is not the only one and, +probably, not even the most important. + +What is a good candidate for the most important advantage is that +translators requested via "magic" filenames are session-bound. In +other words, by running `cat file,,translator` we set a translator +visible *only* to `cat`, while the original file remains untranslated. +Such session-specific translators are called **dynamic** and there is +no (theoretical) way for a client to get a port to a dynamic +translator requested by another client. + +Obviously, dynamic translators can be stacked, similarly to static +translators. Also, dynamic translator stacks may reside on top of +static translator stacks. + +An important operation of namespace-based translator selection is +*filtering*. Filtering basically consists in looking up a translator +by name in the stack and ignoring translators located on top of it. +Note that filtering does not mean dropping some translators: in the +current implementation a filter is expected to be a normal dynamic +translator, included in the dynamic translator stack similarly to +other translators. + +An important detail is that filtering is not limited to dynamic +translator stacks: a filter should be able to descend into static +translator stacks as well. + +Although the concept of filtering may seem purely abstract in the +simplest use-case of setting dynamic translators on top of files, the +situation changes greatly when dynamic translator stacks on top of +directories are considered. In this case, the implementation of +namespace-based translator selection is expected to be able to +propagate the dynamic translators associated with the directory down +the directory structure. That is, all files located under a directory +opened with magic syntax, are expected to be translated by the same +set of translators. In this case having the possibility to +specifically discard some of the translators set up on top of certain +files is very useful. + +Note that the implementation of propagation of dynamic translators +down directories is not fully conceived at the moment. The +fundamental problem is distinguishing between situations when the +dynamic translators are to be set on the underlying files of the +directory or on the directory itself. + +## Currently Implemented + +Currently there a working (though not heavily tested) implementation +of the simplest use-case of namespace-based translator selection in +the form of translator `nsmux`. The filter is partially implemented +and this is the immediate goal. Propagating translators down +directories is the next objective. |