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In this section we give an overview of how the LASH system operates,
describing the server and client objects and operations that make it
work. The lashd
server must be running in order for clients
to participate in the system; clients cannot interoperate soley between
themselves. The server maintains a list of connected clients and a list
of projects with which these clients are associated.
The server and clients exchange events and configs over their connections. There is one, and only one, bi-directional connection between a client and the server. The transport for this connection is currently TCP.
An event is a very simple object having two relevant properties:
a type and an optional arbitrary character string. The type defines
what the event means to the recipient, and the string allows additional
information to be included with it. For example, if a client wishes
the server to save the current project, it sends a LASH_Save
event to the server. While saving the project, the server may wish to
tell a client to save its data in a certain directory. To so, it sends
a LASH_Save_File
event to the client with a string containing
the name of a directory into which the client should save its data files.
Clients can save data on the server if they wish. To do this, the
client declares that it wants to save data on the server when it
initialises the server connection and then later sends one or more
configs to the server. A config is also a very simple object.
It has a client-unique character string key, and a value of arbitrary
size and type (well, almost arbitrary; its size must be able to be
described by a uint32_t
integer due to byte-order conversions
done when sending data over the network.)
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In this section we will examine a typical session in some detail, describing the server and client operations that take place. In the session, the server is started, a number of clients connect, the session is saved and then restored.
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Before all else, the user starts the server. It starts up and begins listening for connections from clients. It doesn't do much else.
To keep track of what is happening with LASH, the user can run the
lash_panel
program (though this is not necessary, and it can
be started later at any time).
The environment variable LASH_START_SERVER
can be set to have any
LASH client automatically start the server if one isn't already running.
Doing this you can simply put lash_panel
(or any other LASH
client) in your applications menu and have LASH automatically work without
having to remember to start the server manually.
If you're using a Bourne compatible shell like bash
(if you don't
know, you probably are) you can enable auto-start with the following command:
export LASH_START_SERVER=1
If you would like to always have the server started, put that command in the file ~/.bashrc (or in /etc/profile if you are root and want it set system wide) to have it run whenever a new shell is started.
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The user then starts a JACK client program. It opens a connection to the server and provides it with all information that the server will need to run the application again. This information includes: the current directory that the user was in when they ran the program, the command line that started the application and the class of the client (a character string that the client application provides the initialisation routine that will never change over all initialisations.)
With this information is included a set of flags that describe the client
to the server. This particular client saves data to files and wants
the server to tell it where to save files when the project is saved,
so it has the LASH_Config_File
flag set.
The client library starts two threads for communication with the server, one for sending data and the other for recieving. It also sends, along with the client supplied data, a number of parameters that were extracted from the client's command line options before it checked them. This optionally includes the name of the project that the client should initially be associated with and a 128-bit, world-unique identifier for this particular client instance (the LASH ID.)
Server-side, the server wakes up to the fact that a new connection has arrived and immediately adds it to a list of open connections and then goes back to waiting. When the client sends the requisite information, the server looks at it and decides what to do with the client. This client has not requested a specific project to which it should be connected. However, there are no existing projects so the server creates a new project with the name `project-1' in the directory `/home/user/audio-projects/project-1' (assuming the user didn't specify a different default directory when running configure.) It also generates a new LASH ID for the client. It then adds the client to the new project and goes back to listening.
If the user has the lash_panel
client running, the new project
will appear as a tab with the title `project-1', and the new client
will appear in the client list for that project.
The client then connects up to the JACK server and, after having done
this, sends a LASH_Jack_Client_Name
event to the server with the
name that it registered to JACK with as the string. This notifies the
server that it is a JACK client and needs its JACK port connections saved
and restored. The server will now pay attention to any activity regarding
the client (ie, port creation and destruction and port connection and
disconnection.)
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The user then starts a second client that uses the ALSA sequencer
interface and wishes to save data on the server. It connects to
the server with a different class to the JACK client and with the
LASH_Config_Data_Set
flag set.
The server sees that this client also didn't specify a project, and so adds it to the first available project; the same one as the previous project, `project-1'. It also sees that the client wants to store data on the server, and so it creates a directory within the project directory for this data to be stored in and creates a database-style object to manage the client's data.
If the user has the lash_panel
client running, both clients
will now be visible in the clients list for `project-1'.
The client then connects to the ALSA sequencer and sends its
client ID to the server in the first character of the string of a
LASH_Alsa_Client_Name
event. The server regards this similarly
to the other client's JACK client name.
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After the user has done some work in the two clients, they want to
save their work. They click the Save button in lash_panel
(or use the save
command in lash_control
), and a
LASH_Save
event is sent to the server. The server recieves this
and then iterates through each client in the project and checks its flags.
The JACK client saves data by itself (it has the LASH_Config_File
flag set,) so the server creates a directory under the project directory
for it to save in and then sends a LASH_Save_File
event to the
client with a string containing the name of the directory it made.
The client recieves the event and saves its data into the specified
directory.
Next, the server examines the ALSA client. It wishes to save data on the
server, so the server sends a LASH_Save_Data_Set
to the client.
With all of the clients iterated through, it now saves all the information
it needs to be able to restore them; their working directory, command
line options, etc. In order to do this, it asks the JACK server to
find the connections for the JACK client, and asks the ALSA sequencer to
find the connections for the ALSA client. It uses the client name and
ID that both clients sent to the server after opening their connections
to the respective systems. All of this information is stored in a file
under the project's directory. When this is done, the server goes back
to listening for events and configs.
The client, meanwhile, has recieved the LASH_Save_Data_Set
event
and sends back a number of configs to the server. When it has sent all
the data it wishes to be saved, it sends back a LASH_Save_Data_Set
event. The server passes all of the configs to the object managing
the data store for the ALSA client. When the server recieves the
LASH_Save_Data_Set
event from the client, it tells the data store
to write the data to disk. The save is now complete.
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Unfortunately for the user, the ALSA client crashes. The server detects
that the client has disconnected, and puts the client on a list of lost
clients for the project. The user then starts another copy of the client,
which connects to the server in the same way it did before. This time,
however, the server checks through the list of lost clients and finds
that the class of the new client matches the class of the lost client
and so it resumes the lost client using the new one. It gives it the
128-bit ID of the lost client, adds it to the project, and then sends
a LASH_Restore_Data_Set
event to the client. The client then
cleans itself up, ready to recieve the data set. The server sends the
client the configs, and then another LASH_Restore_Data_Set
event.
The client recieves this data and its state has been restored that of
the client that crashed.
The user can stop this behaviour by specifying the `--lash-no-autoresume' option on the client's command line.
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The user has to go off and do other things, and so they close down the
clients and the server. Some time later, the user comes back and wants
to start working again so first, as always, they start up the server.
They then start the lash_panel
program. Using the File->Open
menu item, the user selects the directory (not file!) where they saved
the project (by default `~/audio-projects/project-1', but you
can save to a more descriptive name). The lash_panel client sends a
LASH_Restore
event to the server with the specified directory as
the string. The server opens the file that it saved before, and reads
in all the information about the project and its clients. It creates
a new project with this information. The clients are created as lost
clients, however.
The server then iterates through each client and starts a new copy of it using the information provided when the original client connected. It also adds some command line options that are extracted by the client library. These specify the LASH ID of the client, the project name that it should be connecting to and the server's hostname and port. It then goes back to waiting.
The new JACK client then connects to the server as normal. When the
server recieves it connection, it checks the client against the project's
list of lost clients. This time, however, it has its ID specified, so
the server will only resume a client with a matching ID. Lo and behold,
such a client exists. The server resumes the old JACK client, telling it
to load its state from the files in the project directory that the client
previously stored. It does so with a LASH_Restore_File
event with
the string as the directory name. The ALSA client does exactly the same,
except having its data restored through LASH_Restore_Data_Set
as described above.
Only one thing remains for the clients to be fully restored: the JACK and
ALSA sequencer connections. This happens when the clients send their
LASH_Jack_Client_Name
and LASH_Alsa_Client_ID
events.
The connections are stored with the LASH ID rather than the JACK client
name or ALSA client ID. When the client registers its name or ID, the
connections are converted from the LASH ID to the JACK client name or ALSA
client ID, and the connections are restored. It also pays attention to
connections to other clients within the same project, converting between
JACK client names, ALSA client IDs and LASH IDs as appropriate.
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