FirebirdDB
/
firebird
peilaus alkaen https://github.com/FirebirdSQL/firebird.git


			
				
					
						
						
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<P STYLE="text-indent: 0.39in; margin-bottom: 0in"><FONT SIZE=4>Providers.</FONT></P>
<P STYLE="text-indent: 0.39in; margin-bottom: 0in"><BR>
</P>
<P STYLE="text-indent: 0.39in; margin-bottom: 0in"><FONT SIZE=4>Providers
architecture is definitely one of key features of Firebird 3. But to
be precise this is far not new feature - providers existed in
Firebird's predecessors long ago, and in 'deeply hidden' form are
present in any Firebird version. Initially providers were introduced
to deal with a task which is currently well known to be performed by
ODBC, ADO, BDE, etc. They were needed to make it possible to access
different database engines using single external interface. But later
providers architecture (known that time as OSRI - Open Systems
Relational Interface) also showed itself as very efficient for
supporting mix of old and new database formats (different major ODS)
at single server and having mixed connections to local and remote
databases. Implemented in Firebird 3 providers make it possible to
support all this modes (remote connections, different ODS databases,
foreign engines) and also providers chaining (when some provider is
using callback to standard API when performig an operation on
database).</FONT></P>
<P STYLE="text-indent: 0.39in; margin-bottom: 0in"><FONT SIZE=4>Main
element of providers architecture is YValve. On initial attach (or
create) database call it scans the list of known providers and calls
them one by one until tried provider completes requested operation
successfully. For already established connection appropriate provider
is called at once with almost zero overhead. Lets take a look at some
samples of YValve operation when it selects appropriate provider at
attach stage.</FONT></P>
<P STYLE="text-indent: 0.39in; margin-bottom: 0in"><BR>
</P>
<P STYLE="text-indent: 0.39in; margin-bottom: 0in"><FONT SIZE=4>Next
samples are with default configuration, which contains 3 providers:
<B>Remote</B> (establish network connection), <B>Engine14</B> (main
database engine) and <B>Loopback</B> (force network connection to
local server for database name without explicitly given network
protocol).</FONT></P>
<P STYLE="text-indent: 0.39in; margin-bottom: 0in"><FONT SIZE=4>When
one attaches to database called <I>RemoteHost:dbname</I> (TCP syntax)
or <I>\\RemoteHost\dbname</I> (NetBios) <B>Remote</B> provider
detects explicit network protocol syntax and (being the first
provider in the list) at once redirects such call to <I>RemoteHost</I>.
That's how typical client configuration works.</FONT></P>
<P STYLE="text-indent: 0.39in; margin-bottom: 0in"><FONT SIZE=4>When
database name does not contain network protocol (just <I>dbname</I>)
<B>Remote</B> provider rejects it and <B>Engine14</B> provider comes
to stage. It tries to open <I>dbname</I> &ndash; and in case of
success we get embedded connection to the database. Pay attention &ndash;
we do not need special embedded library to have embedded connection,
standard client loads appropriate provider and becomes embedded
server.</FONT></P>
<P STYLE="text-indent: 0.39in; margin-bottom: 0in"><FONT SIZE=4>But
what happens if engine returned an error on an attempt to attach to
database? Certainly, if file for database to be attached does not
exist there is no interest at all, but embedded connection may also
be impossible when user, attaching to it, does not have enough rights
to open database file. This is normal case if database was not
created by that user in embedded mode or if he was not explicitly
given OS rights for embedded access to databases on given box.
Moreover, setting access rights in such a manner is a requirement for
correct superserver operation. So after failure of <B>Engine14</B> to
access database <B>Loopback</B> provider is attempted for an attach.
It does not differ much from <B>Remote</B>, but tries to access
database <I>dbname</I> on a server running on local host. On windows
XNET protocol (also known as local connection) is used for it, posix
systems prepend <I>dbname</I> with <I>localhost:</I> and use TCP
connection. In case of success remote-like connection is established
with database no matter that it's located on a local machine.</FONT></P>
<P STYLE="text-indent: 0.39in; margin-bottom: 0in"><BR>
</P>
<P STYLE="text-indent: 0.39in; margin-bottom: 0in"><FONT SIZE=4>Certainly
use of providers is not limited with this 3 standard ones. Firebird 3
does not support pre-ODS 12 databases. But in FB 3 &ndash; not in
alpha1 &ndash; we plan to have additional provider to access old (ODS
from 8 to 11) format databases. Removing old ODS support from engine
helps to make it's code simpler and a bit faster. Taking into an
account that this faster sometimes takes place in performance
critical places (like search of a key in an index block) avoiding old
code and related branches makes Firebird work really faster.
Providers architecture at the same time makes it possible to access
old databases when changing Firebird version.</FONT></P>
<P STYLE="text-indent: 0.39in; margin-bottom: 0in"><FONT SIZE=4>The
strong feature of providers architecture is ability for user to add
his own providers to server and/or client. You may be surprised &ndash;
what else except remote connection is needed on client? But do not
forget about providers chaining. Imagine a case when database is
accessed via very slow network connection &ndash; something like 3G
or even worse GPRS. A strong desire to cache rarely changed but
rather big tables on a client is first that comes on my mind to make
it work faster. Such systems were really implemented, but to do it
one had to rename fbclient to something arbitrary and load it into
own library called fbclient. This makes it possible to use standard
tools to access the database at the same time caching required
tables. Works, but solution is obviously far from ideal. With
providers architecture instead libraries renaming one just adds local
caching provider which can use any method to detect connections to it
(something like <I>cache@</I> prefix in the beginning of database
name or whatever else you choose). In this sample when database name
<I>cache@RemoteHost:dbname </I><SPAN STYLE="font-style: normal">is
used caching provider accepts such connection and invokes YValve once
again with traditional database name </SPAN><I>RemoteHost:dbname</I><SPAN STYLE="font-style: normal">.
But when user later performs any call to his database caching
provider gets control on it before </SPAN><SPAN STYLE="font-style: normal"><B>Remote</B></SPAN>
<SPAN STYLE="font-style: normal">one and for locally cached table can
avoid calls to remote server.</SPAN></FONT></P>
<P STYLE="text-indent: 0.39in; margin-bottom: 0in; font-style: normal">
<FONT SIZE=4>Using chaining one can implement a lot of other useful
things like database replication without need in triggers - just
repeat same calls for replication host when (for example) transaction
is commited. In this case chaining provider is installed on server,
not on client, and no modification of command line is needed at all.
To avoid cycling when performing callback to YValve at attach time
such provider can modify list of providers using isc_dpb_config
parameter in DPB &ndash; for details please see README.configuration.
BTW, same technique may be used at client too.</FONT></P>
<P STYLE="text-indent: 0.39in; margin-bottom: 0in; font-style: normal">
<FONT SIZE=4>And certainly we should not forget about ability to
access foreign DB engines using providers. This looks strange at the
first glance when a lot of other tools performing this task already
exist. But let's take into an account ability to access other
Firebird databases using EXECUTE STATEMENT. With provider to ODBC or
other common tool to access various data sources it's getting
possible to directly access from procedures and triggers (using
mentioned EXECUTE STATEMENT) data from almost any database, at least
any, having a driver in chosen access tool. Certainly it's possible
to have a provider to access some particular type of foreign database
engine if one wants to avoid ODBC layer for some reason. </FONT>
</P>
<P STYLE="text-indent: 0.39in; margin-bottom: 0in; font-style: normal">
<FONT SIZE=4>Description of how to access databases using providers
API is present in the interfaces part of Firebird examples and
therefore there is no need repeating it here. Moreover, except
IStatement (interface used for execution of SQL operators) and
IEvents (works with Firebird events) all functions in interfaces are
similar to old API.</FONT></P>
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</P>
<P STYLE="margin-bottom: 0in"><FONT SIZE=4>Questions and answers.</FONT></P>
<P STYLE="text-indent: 0.39in; margin-bottom: 0in; font-style: normal">
<FONT SIZE=4>Q. Interfaces and providers are probably very good, but
I have old task written using plain functions API and for a lot of
reasons I can't rewrite it in the nearest future. Does it mean I have
problem migrating to Firebird 3?</FONT></P>
<P STYLE="text-indent: 0.39in; margin-bottom: 0in; font-style: normal">
<FONT SIZE=4>A. Definitely no problems. Old API is supported for
backward compatibility in Firebird 3 and will be supported in future
versions as long as people need it. Moreover, since Firebird 3 one
can access from XSQLDA API records longer than 64Kbytes.</FONT></P>
<P STYLE="text-indent: 0.39in; margin-bottom: 0in; font-style: normal">
<FONT SIZE=4>Q. And what about performance when using old API?</FONT></P>
<P STYLE="text-indent: 0.39in; margin-bottom: 0in; font-style: normal">
<FONT SIZE=4>A. Functional API is implemented as a very thin layer
over interfaces. Code in most case is trivial &ndash; convert passed
handles to pointers to interfaces (this step was always present but
called 'handles validation') and invoke appropriate function from
interface. The only a bit more complex place are functions that
execute SQL operator and fetch data from it. But SQLDA and related to
it data moves has never been the most fast place of functional API,
it was one the reasons to have new API and logic between new and old
API does not add much to that old overhead.</FONT></P>
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