m4_comment([$Id: client.so,v 1.12 2003/10/18 19:16:07 bostic Exp $])

m4_ref_title(RPC Client/Server,
    Client program, @RPC client, rpc/intro, rpc/server)

m4_p([dnl
Changing a m4_db application to remotely call a server program requires
only a few changes on the client side:])

m4_nlistbegin
m4_nlist([dnl
The client application must create and use a m4_db environment; that
is, it cannot simply call the m4_ref(dbh_create) function, but must
first call the m4_ref(dbenv_create) function to create an environment
in which the database will live.])

m4_nlist([dnl
The client application must call m4_ref(dbenv_create) using the
m4_ref(DB_RPCCLIENT) flag.])

m4_nlist([dnl
The client application must call the additional m4_ref(DbEnv) method
m4_ref(dbenv_set_rpc_server) to specify the database server.  This call must
be made before opening the environment with the m4_ref(dbenv_open)
call.])
m4_nlistend

m4_p([dnl
The client application provides a few pieces of information to m4_db as
part of the m4_ref(dbenv_set_rpc_server) call:])

m4_nlistbegin
m4_nlist([dnl
A client structure.  Applications wanting to control their own client
structures can pass one in, and m4_db will use it to connect to the
server.  Most applications will not make use of this argument and should pass
in NULL.  If this argument is used, the hostname and client timeout
arguments are ignored.  Applications using this mechanism must
create their client structures using DB_RPC_SERVERPROG as the program
number and DB_RPC_SERVERVERS as the version number.])
m4_nlist([dnl
The hostname of the server.  The hostname format is not specified by
m4_db, but must be in a format acceptable to the local network support
-- specifically, the RPC clnt_create interface.])
m4_nlist([The client
timeout.  This is the number of seconds the client will wait for the
server to respond to its requests.  A default is used if this value is
zero.])
m4_nlist([dnl
The server timeout.  This is the number of seconds the server will allow
client resources to remain idle before releasing those resources.  The
resources this applies to are transactions and cursors because those
objects hold locks; and if a client dies, the server needs to release
those resources in a timely manner.  This value is really a hint to the
server because the server may choose to override this value with its
own.])
m4_nlistend

m4_p([dnl
The only other item of interest to the client is the home directory
that is given to the m4_ref(dbenv_open) call.
The server is started with a list of allowed home directories.
The client must use one of those names (where a name is the last
component of the home directory).  This allows the pathname structure
on the server to change without client applications needing to be
aware of it.])

m4_p([dnl
Once the m4_ref(dbenv_set_rpc_server) call has been made, the client is
connected to the server, and all subsequent m4_db
operations will be forwarded to the server.  The client does not need to
be otherwise aware that it is using a database server rather than
accessing the database locally.])

m4_p([dnl
It is important to realize that the client portion of the m4_db library
acts as a simple conduit, forwarding m4_db interface arguments to the
server without interpretation.  This has two important implications.
First, all pathnames must be specified relative to the server.  For
example, the home directory and other configuration information passed
by the application when creating its environment or databases must be
pathnames for the server, not the client system.  In addition, because
there is no logical bundling of operations at the server, performance
is usually significantly less than when m4_db is embedded within the
client's address space, even if the RPC is to a local address.])

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