2013-06-11 14:29:14 +02:00
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2014-11-13 11:30:41 +01:00
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2013-06-11 14:29:14 +02:00
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2014-11-13 11:30:41 +01:00
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2013-06-11 14:29:14 +02:00
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<P STYLE="text-indent: 0.39in; margin-bottom: 0in"><FONT SIZE=4>Providers.</FONT></P>
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<P STYLE="text-indent: 0.39in; margin-bottom: 0in"><BR>
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</P>
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<P STYLE="text-indent: 0.39in; margin-bottom: 0in"><FONT SIZE=4>Providers
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2013-06-17 03:32:09 +02:00
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architecture is definitely one of key features of Firebird 3. But to
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be precise this is far not new feature - providers existed in
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Firebird's predecessors long ago, and in 'deeply hidden' form are
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present in any Firebird version. Initially providers were introduced
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to deal with a task which is currently well known to be performed by
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ODBC, ADO, BDE, etc. They were needed to make it possible to access
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different database engines using single external interface. But later
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providers architecture (known that time as OSRI - Open Systems
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Relational Interface) also showed itself as very efficient for
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supporting mix of old and new database formats (different major ODS)
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at single server and having mixed connections to local and remote
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databases. Implemented in Firebird 3 providers make it possible to
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support all this modes (remote connections, different ODS databases,
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foreign engines) and also providers chaining (when some provider is
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using callback to standard API when performig an operation on
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database).</FONT></P>
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<P STYLE="text-indent: 0.39in; margin-bottom: 0in"><FONT SIZE=4>Main
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element of providers architecture is YValve. On initial attach (or
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create) database call it scans the list of known providers and calls
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them one by one until tried provider completes requested operation
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successfully. For already established connection appropriate provider
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is called at once with almost zero overhead. Lets take a look at some
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samples of YValve operation when it selects appropriate provider at
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attach stage.</FONT></P>
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<P STYLE="text-indent: 0.39in; margin-bottom: 0in"><BR>
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</P>
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<P STYLE="text-indent: 0.39in; margin-bottom: 0in"><FONT SIZE=4>Next
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samples are with default configuration, which contains 3 providers:
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<B>Remote</B> (establish network connection), <B>Engine13</B> (main
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database engine) and <B>Loopback</B> (force network connection to
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local server for database name without explicitly given network
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protocol).</FONT></P>
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<P STYLE="text-indent: 0.39in; margin-bottom: 0in"><FONT SIZE=4>When
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one attaches to database called <I>RemoteHost:dbname</I> (TCP syntax)
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or <I>\\RemoteHost\dbname</I> (NetBios) <B>Remote</B> provider
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detects explicit network protocol syntax and (being the first
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provider in the list) at once redirects such call to <I>RemoteHost</I>.
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That's how typical client configuration works.</FONT></P>
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<P STYLE="text-indent: 0.39in; margin-bottom: 0in"><FONT SIZE=4>When
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database name does not contain network protocol (just <I>dbname</I>)
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<B>Remote</B> provider rejects it and <B>Engine13</B> provider comes
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to stage. It tries to open <I>dbname</I> – and in case of
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success we get embedded connection to the database. Pay attention –
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we do not need special embedded library to have embedded connection,
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standard client loads appropriate provider and becomes embedded
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server.</FONT></P>
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<P STYLE="text-indent: 0.39in; margin-bottom: 0in"><FONT SIZE=4>But
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what happens if engine returned an error on an attempt to attach to
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database? Certainly, if file for database to be attached does not
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exist there is no interest at all, but embedded connection may also
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be impossible when user, attaching to it, does not have enough rights
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to open database file. This is normal case if database was not
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created by that user in embedded mode or if he was not explicitly
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given OS rights for embedded access to databases on given box.
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Moreover, setting access rights in such a manner is a requirement for
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correct superserver operation. So after failure of <B>Engine13</B> to
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access database <B>Loopback</B> provider is attempted for an attach.
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It does not differ much from <B>Remote</B>, but tries to access
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database <I>dbname</I> on a server running on local host. On windows
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XNET protocol (also known as local connection) is used for it, posix
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systems prepend <I>dbname</I> with <I>localhost:</I> and use TCP
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connection. In case of success remote-like connection is established
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with database no matter that it's located on a local machine.</FONT></P>
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<P STYLE="text-indent: 0.39in; margin-bottom: 0in"><BR>
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</P>
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<P STYLE="text-indent: 0.39in; margin-bottom: 0in"><FONT SIZE=4>Certainly
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use of providers is not limited with this 3 standard ones. Firebird 3
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does not support pre-ODS 12 databases. But in FB 3 – not in
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alpha1 – we plan to have additional provider to access old (ODS
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from 8 to 11) format databases. Removing old ODS support from engine
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helps to make it's code simpler and a bit faster. Taking into an
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account that this faster sometimes takes place in performance
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critical places (like search of a key in an index block) avoiding old
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code and related branches makes Firebird work really faster.
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Providers architecture at the same time makes it possible to access
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old databases when changing Firebird version.</FONT></P>
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<P STYLE="text-indent: 0.39in; margin-bottom: 0in"><FONT SIZE=4>The
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strong feature of providers architecture is ability for user to add
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his own providers to server and/or client. You may be surprised –
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what else except remote connection is needed on client? But do not
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forget about providers chaining. Imagine a case when database is
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accessed via very slow network connection – something like 3G
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or even worse GPRS. A strong desire to cache rarely changed but
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rather big tables on a client is first that comes on my mind to make
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it work faster. Such systems were really implemented, but to do it
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one had to rename fbclient to something arbitrary and load it into
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own library called fbclient. This makes it possible to use standard
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tools to access the database at the same time caching required
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tables. Works, but solution is obviously far from ideal. With
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providers architecture instead libraries renaming one just adds local
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caching provider which can use any method to detect connections to it
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(something like <I>cache@</I> prefix in the beginning of database
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name or whatever else you choose). In this sample when database name
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<I>cache@RemoteHost:dbname </I><SPAN STYLE="font-style: normal">is
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used caching provider accepts such connection and invokes YValve once
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again with traditional database name </SPAN><I>RemoteHost:dbname</I><SPAN STYLE="font-style: normal">.
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But when user later performs any call to his database caching
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provider gets control on it before </SPAN><SPAN STYLE="font-style: normal"><B>Remote</B></SPAN>
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<SPAN STYLE="font-style: normal">one and for locally cached table can
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avoid calls to remote server.</SPAN></FONT></P>
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<P STYLE="text-indent: 0.39in; margin-bottom: 0in; font-style: normal">
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<FONT SIZE=4>Using chaining one can implement a lot of other useful
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things like database replication without need in triggers - just
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repeat same calls for replication host when (for example) transaction
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is commited. In this case chaining provider is installed on server,
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not on client, and no modification of command line is needed at all.
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To avoid cycling when performing callback to YValve at attach time
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such provider can modify list of providers using isc_dpb_config
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parameter in DPB – for details please see README.configuration.
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BTW, same technique may be used at client too.</FONT></P>
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<P STYLE="text-indent: 0.39in; margin-bottom: 0in; font-style: normal">
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<FONT SIZE=4>And certainly we should not forget about ability to
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access foreign DB engines using providers. This looks strange at the
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first glance when a lot of other tools performing this task already
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exist. But let's take into an account ability to access other
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Firebird databases using EXECUTE STATEMENT. With provider to ODBC or
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other common tool to access various data sources it's getting
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possible to directly access from procedures and triggers (using
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mentioned EXECUTE STATEMENT) data from almost any database, at least
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any, having a driver in chosen access tool. Certainly it's possible
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to have a provider to access some particular type of foreign database
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engine if one wants to avoid ODBC layer for some reason. </FONT>
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</P>
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<P STYLE="text-indent: 0.39in; margin-bottom: 0in; font-style: normal">
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<FONT SIZE=4>Description of how to access databases using providers
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API is present in the interfaces part of Firebird examples and
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therefore there is no need repeating it here. Moreover, except
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IStatement (interface used for execution of SQL operators) and
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IEvents (works with Firebird events) all functions in interfaces are
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similar to old API.</FONT></P>
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<P STYLE="text-indent: 0.39in; margin-bottom: 0in"><BR>
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</P>
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<P STYLE="margin-bottom: 0in"><FONT SIZE=4>Questions and answers.</FONT></P>
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<P STYLE="text-indent: 0.39in; margin-bottom: 0in; font-style: normal">
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<FONT SIZE=4>Q. Interfaces and providers are probably very good, but
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I have old task written using plain functions API and for a lot of
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reasons I can't rewrite it in the nearest future. Does it mean I have
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problem migrating to Firebird 3?</FONT></P>
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<P STYLE="text-indent: 0.39in; margin-bottom: 0in; font-style: normal">
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<FONT SIZE=4>A. Definitely no problems. Old API is supported for
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backward compatibility in Firebird 3 and will be supported in future
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versions as long as people need it. Moreover, since Firebird 3 one
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can access from XSQLDA API records longer than 64Kbytes.</FONT></P>
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<P STYLE="text-indent: 0.39in; margin-bottom: 0in; font-style: normal">
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<FONT SIZE=4>Q. And what about performance when using old API?</FONT></P>
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<P STYLE="text-indent: 0.39in; margin-bottom: 0in; font-style: normal">
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<FONT SIZE=4>A. Functional API is implemented as a very thin layer
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over interfaces. Code in most case is trivial – convert passed
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handles to pointers to interfaces (this step was always present but
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called 'handles validation') and invoke appropriate function from
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interface. The only a bit more complex place are functions that
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execute SQL operator and fetch data from it. But SQLDA and related to
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it data moves has never been the most fast place of functional API,
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it was one the reasons to have new API and logic between new and old
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API does not add much to that old overhead.</FONT></P>
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<P STYLE="text-indent: 0.39in; margin-bottom: 0in"><BR>
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