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diff --git a/doc/info/mds.texinfo b/doc/info/mds.texinfo index 50e9b3a..d198cbf 100644 --- a/doc/info/mds.texinfo +++ b/doc/info/mds.texinfo @@ -12,7 +12,7 @@ @dircategory Graphics environment @direntry -* mds: (mds). XXX +* mds: (mds). The micro-display server @end direntry @@ -31,17 +31,17 @@ Texts. A copy of the license is included in the section entitled @ifnottex @node Top -@top mds -- XXX +@top mds -- The micro-display server @insertcopying @end ifnottex @titlepage @title mds -@c @subtitle XXX +@subtitle The micro-display server @author by Mattias Andrée (maandree) @page -@c @center `The possibilities are... like endless.' -- Scootaloo @c@c cannot really reuse this quote +@c @center `' @vskip 0pt plus 1filll @insertcopying @end titlepage @@ -52,6 +52,7 @@ Texts. A copy of the license is included in the section entitled @menu * Overview:: Brief overview of @command{mds}. +* Architecture:: Architectural overview of @command{mds}. * GNU Free Documentation License:: Copying and sharing this manual. @end menu @@ -60,8 +61,182 @@ Texts. A copy of the license is included in the section entitled @node Overview @chapter Overview -Lorem ipsum +@command{mds}@footnote{mds stands for micro-display server} +is a display server protocol and an implementation of said +protocol. What makes @command{mds} stand out is its core +design choice: it is desigend just like a microkernel. +Rather than one, possibly modular, process --- a monolithic +process --- mds is comprised of many small servers, each +exchangable and responsible for one thing. +@command{mds} goal is neither security, performance nor +a perfect graphical experience. @command{mds} is all +about flexibility and freedom 0@footnote{The freedom to run +the program as you wish, for any purpose}. + + + +@node Architecture +@chapter Architecture + +@menu +* Layers:: The layers of the display server. +* Interprocess Communication:: How servers and clients communicate. +@end menu + +@node Layers +@section Layers + +The @command{mds} display server in architectured in +three layers. The first layer is called the kernel. +The kernel is responsible for acquiring a display +server index@footnote{As with any display server, +the system can have multiple instances of +@command{mds} running at the same time.}, set up +environment variables to indicate which display +server and display server instance is being used, +create a domain socket for the display server and +start the master server and restart it if it crashes, +and then clean up the system when the display server +closes. The kernel only responsible for creating +the domain socket for communication with the display +server, it is not responsible for using it, that +mission falls to the master server. + +The second layer is the master server. The master +server has two responsibilities: coordinating +message passing between other servers and clients +@footnote{In @command{mds} their is no functional +distinction between servers and clients, the +distinction is purely semantic.} and starting +other servers. + +The third layer is the other servers and clients. +protocolwise there is no specification on how +they are started. But in the reference +implementation of the master server, this is +done by starting a shell script with the +pathname @file{$@{XDG_CONFIG_HOME@}/mdsinitrc} +and the user is responsible for providing the +logic in that shell script.@footnote{Moonstruck +users are allowed to implement this in C +or any other language of their choosing.} +@c Which is better: cray-cray users, lunatic users, +@c moonstruck users, insane users, ballers, madmen, +@c loony tunes? +These servers implements the actual functionality +of the display server. + + + +@node Interprocess Communication +@section Interprocess Communication + +Intrinsic to @command{mds} is a powerful +interprocess communication mechanism. Servers +and clients connect to the display server by +connecting to a domain socket served by the +master server. A server or client that has +connected to the display server can do three +things: + +@itemize +@item +Request assignment of a unique ID. + +@item +Multicast a message. + +@item +Join or leave a multicast groups. +@end itemize + +Upon assignment of an ID the master server +will automatically place the client in a +multicast group for that specific client. +This automatically multicast group assignment +is done by the master server simply so you +as a debugger do not forget to do so. When +a client is disconnected it will and out a +message to a specific multicast group that +the client, refered to by it's ID, have closed. + +A message in the @command{mds} protocol is +comprised of two parts: headers and a payload. +When a client joins a multicast group it is +actually say that it is interested and receiving +broadcasts containing a specific header or a +specific header--value pair, or that it is +interesting in all messages@footnote{This +could be used for logging, possibly spying and +networking.}. Thus a message is automatically +multicasted to groups indicated by its headers. + +The multicast groups and receiving of groups +is called interceptions. The interesting +property of interceptions is that they may +be modifying. When a server registers for +message interception it can say that it wants +to be able to modify messages. If this is done +and the server receives a message for which it +has said it want to be able to modify it, +the master server will wait for that server +to respond before it send the message to +the next server in the interception list. +The server can choose to do three things +with a message that it has opted in for +modification of: leave the message as-is, +modify the message, or consume the message. +A message consumption is done by modify +the message to make it empty. A consumed +message will not be send to any further +clients or servers in the interception list. + +To make this mechanism sensible, a server or +client can set a priority when it registers +for interception (does not need to be +modifying.) When a message is broadcasted it +will be received by all servers in the +interception except the original sender, +unless it gets consumes. The order in which +the master server sends the message to the +recipients is determined by priority the +servers registed with. The message first sent +to the recipients with highest priority and +last to the recipients with lowestr priority, +and orderd by the priority between those +priorities. Of two or more servers have the +same priority the order in which they will +receive the message, of those recipients, +is arbitrary. + +An interesting property of this machanism +is demonstrated in the @command{mds-vt} +server. Unlike most servers @command{mds-vt} +maintains two concurrent connections to +the display. Once @command{mds-vt} receives +a signal from the OS kernel requesting to +switch virtual terminal, @command{mds-vt} +will from one of its connections send +out a message and wait for it to be +received in its other connection and the +let the OS kernel switch virtual terminal. +The secondary connection to the display +has registered interception with lower +priority of the message that the primary +connection broadcasts. This message will +be received by other servers that will +let the message continue to the next +server in the interception list once that +server is ready for the OS kernel to switch +virtual terminal. All of these server has +registered modifying interception of the +message but none will actually modify or +consume the message; it is only used a +mechanism for letting @command{mds-vt} know +when all servers are ready for the switch +without having to know how many they are +and wait for a reply from all of them. @node GNU Free Documentation License |