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\section*{Storage Daemon Design}
\label{_ChapterStart3}
\index{Storage Daemon Design }
\index{Design!Storage Daemon }
\addcontentsline{toc}{section}{Storage Daemon Design}

This chapter is intended to be a technical discussion of the Storage daemon
services and as such is not targeted at end users but rather at developers and
system administrators that want or need to know more of the working details of
{\bf Bacula}. 

\subsection*{SD Design Introduction}
\index{Introduction!SD Design }
\index{SD Design Introduction }
\addcontentsline{toc}{subsection}{SD Design Introduction}

The Bacula Storage daemon provides storage resources to a Bacula installation.
An individual Storage daemon is associated with a physical permanent storage
device (for example, a tape drive, CD writer, tape changer or jukebox, etc.),
and may employ auxiliary storage resources (such as space on a hard disk file
system) to increase performance and/or optimize use of the permanent storage
medium. 

Any number of storage daemons may be run on a given machine; each associated
with an individual storage device connected to it, and BACULA operations may
employ storage daemons on any number of hosts connected by a network, local or
remote. The ability to employ remote storage daemons (with appropriate
security measures) permits automatic off-site backup, possibly to publicly
available backup repositories. 

\subsection*{SD Development Outline}
\index{Outline!SD Development }
\index{SD Development Outline }
\addcontentsline{toc}{subsection}{SD Development Outline}

In order to provide a high performance backup and restore solution that scales
to very large capacity devices and networks, the storage daemon must be able
to extract as much performance from the storage device and network with which
it interacts. In order to accomplish this, storage daemons will eventually
have to sacrifice simplicity and painless portability in favor of techniques
which improve performance. My goal in designing the storage daemon protocol
and developing the initial prototype storage daemon is to provide for these
additions in the future, while implementing an initial storage daemon which is
very simple and portable to almost any POSIX-like environment. This original
storage daemon (and its evolved descendants) can serve as a portable solution
for non-demanding backup requirements (such as single servers of modest size,
individual machines, or small local networks), while serving as the starting
point for development of higher performance configurable derivatives which use
techniques such as POSIX threads, shared memory, asynchronous I/O, buffering
to high-speed intermediate media, and support for tape changers and jukeboxes.


\subsection*{SD Connections and Sessions}
\index{Sessions!SD Connections and }
\index{SD Connections and Sessions }
\addcontentsline{toc}{subsection}{SD Connections and Sessions}

A client connects to a storage server by initiating a conventional TCP
connection. The storage server accepts the connection unless its maximum
number of connections has been reached or the specified host is not granted
access to the storage server. Once a connection has been opened, the client
may make any number of Query requests, and/or initiate (if permitted), one or
more Append sessions (which transmit data to be stored by the storage daemon)
and/or Read sessions (which retrieve data from the storage daemon). 

Most requests and replies sent across the connection are simple ASCII strings,
with status replies prefixed by a four digit status code for easier parsing.
Binary data appear in blocks stored and retrieved from the storage. Any
request may result in a single-line status reply of ``{\tt 3201\ Notification\
pending}'', which indicates the client must send a ``Query notification''
request to retrieve one or more notifications posted to it. Once the
notifications have been returned, the client may then resubmit the request
which resulted in the 3201 status. 

The following descriptions omit common error codes, yet to be defined, which
can occur from most or many requests due to events like media errors,
restarting of the storage daemon, etc. These details will be filled in, along
with a comprehensive list of status codes along with which requests can
produce them in an update to this document. 

\subsubsection*{SD Append Requests}
\index{Requests!SD Append }
\index{SD Append Requests }
\addcontentsline{toc}{subsubsection}{SD Append Requests}

\begin{description}

\item [{append open session = \lt{}JobId\gt{} [  \lt{}Password\gt{} ]  }]
   \index{SPAN class }
   A data append session is opened with the Job ID given by  {\it JobId} with
client password (if required) given by {\it Password}.  If the session is
successfully opened, a status of {\tt 3000\ OK} is  returned with a ``{\tt
ticket\ =\ }{\it number}'' reply used to  identify subsequent messages in the
session. If too many sessions are open, or  a conflicting session (for
example, a read in progress when simultaneous read  and append sessions are
not permitted), a status of  ``{\tt 3502\ Volume\ busy}'' is returned. If no
volume is mounted, or  the volume mounted cannot be appended to, a status of 
``{\tt 3503\ Volume\ not\ mounted}'' is returned.  

\item [append data = \lt{}ticket-number\gt{}  ]
   \index{SPAN class }
   If the append data is accepted, a  status of {\tt 3000\ OK data address =
\lt{}IPaddress\gt{} port = \lt{}port\gt{}} is returned,  where the {\tt
IPaddress} and {\tt port} specify the IP address and  port number of the data
channel. Error status codes are  {\tt 3504\ Invalid\ ticket\ number} and  {\tt
3505\ Session\ aborted}, the latter of which indicates the  entire append
session has failed due to a daemon or media error.  

Once the File daemon has established the connection to the data channel 
opened by the Storage daemon, it will transfer a header packet followed  by
any number of data packets. The header packet is of the form:  

{\tt \lt{}file-index\gt{} \lt{}stream-id\gt{} \lt{}info\gt{}}  

The details are specified in the 
\ilink{Daemon Protocol}{_ChapterStart2}  section of this
document.  

\item [*append abort session = \lt{}ticket-number\gt{}  ]
   \index{SPAN class }
   The open append session with ticket {\it ticket-number} is aborted; any blocks
not yet written to permanent media are discarded. Subsequent attempts to 
append data to the session will receive an error status of  {\tt 3505\
Session\ aborted}.  

\item [append end session = \lt{}ticket-number\gt{}  ]
   \index{SPAN class }
   The open append session with ticket {\it ticket-number} is marked complete; no
further blocks may be appended. The storage daemon will give priority to
saving  any buffered blocks from this session to permanent media as soon as
possible.  

\item [append close session = \lt{}ticket-number\gt{}  ]
   \index{SPAN class }
   The append session with ticket {\it ticket} is closed. This message  does not
receive an {\tt 3000\ OK} reply until all of the content of the  session are
stored on permanent media, at which time said reply is given,  followed by a
list of volumes, from first to last, which contain blocks from  the session,
along with the first and last file and block on each containing  session data
and the volume session key identifying data from that session in  lines with
the following format:  

{\tt {\tt Volume = }\lt{}Volume-id\gt{} \lt{}start-file\gt{}
\lt{}start-block\gt{}  \lt{}end-file\gt{} \lt{}end-block\gt{}
\lt{}volume-session-id\gt{}}where {\it Volume-id} is the volume label,  {\it
start-file} and {\it start-block} are the file and block containing the  first
data from that session on the volume, {\it end-file} and  {\it end-block} are
the file and block with the last data from the session on  the volume and {\it
volume-session-id} is the volume session ID for blocks from the  session
stored on that volume. 
\end{description}

\subsubsection*{SD Read Requests}
\index{SD Read Requests }
\index{Requests!SD Read }
\addcontentsline{toc}{subsubsection}{SD Read Requests}

\begin{description}

\item [Read open session = \lt{}JobId\gt{} \lt{}Volume-id\gt{}
   \lt{}start-file\gt{} \lt{}start-block\gt{}  \lt{}end-file\gt{}
   \lt{}end-block\gt{} \lt{}volume-session-id\gt{} \lt{}password\gt{}  ]
\index{SPAN class }
where {\it Volume-id} is the volume label,  {\it start-file} and {\it
start-block} are the file and block containing the  first data from that
session on the volume, {\it end-file} and  {\it end-block} are the file and
block with the last data from the session on  the volume and {\it
volume-session-id} is the volume session ID for blocks from the  session
stored on that volume.  

If the session is successfully opened, a status of  

{\tt {\tt 3100\ OK Ticket\ =\ }{\it number}``}  

is returned with a reply used to identify  subsequent messages in the session.
If too many sessions are open, or a  conflicting session (for example, an
append in progress when simultaneous read  and append sessions are not
permitted), a status of  ''{\tt 3502\ Volume\ busy}`` is returned. If no
volume is mounted, or  the volume mounted cannot be appended to, a status of 
''{\tt 3503\ Volume\ not\ mounted}`` is returned. If no block with  the given
volume session ID and the correct client ID number appears in the  given first
file and block for the volume, a status of  ''{\tt 3505\ Session\ not\
found}`` is returned.  

\item [Read data = \lt{}Ticket\gt{} \gt{} \lt{}Block\gt{}  ]
   \index{SPAN class }
   The specified Block of data from open read session with the specified Ticket
number  is returned, with a status of {\tt 3000\ OK} followed  by a ''{\tt
Length\ =\ }{\it size}`` line giving the length in  bytes of the block data
which immediately follows. Blocks must be retrieved in  ascending order, but
blocks may be skipped. If a block number greater than the  largest stored on
the volume is requested, a status of  ''{\tt 3201\ End\ of\ volume}`` is
returned. If a block number  greater than the largest in the file is
requested, a status of  ''{\tt 3401\ End\ of\ file}`` is returned.  

\item [Read close session = \lt{}Ticket\gt{}  ]
   \index{SPAN class }
   The read session with Ticket number is closed. A read session  may be closed
at any time; you needn't read all its blocks before closing it.  
\end{description}

{\it by 
\elink{John Walker}{http://www.fourmilab.ch/}
January 30th, MM } 
