Providers.

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).

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.

Next samples are with default configuration, which contains 3 providers: Remote (establish network connection), Engine13 (main database engine) and Loopback (force network connection to local server for database name without explicitly given network protocol).

When one attaches to database called RemoteHost:dbname (TCP syntax) or \\RemoteHost\dbname (NetBios) Remote provider detects explicit network protocol syntax and (being the first provider in the list) at once redirects such call to RemoteHost. That's how typical client configuration works.

When database name does not contain network protocol (just dbname) Remote provider rejects it and Engine13 provider comes to stage. It tries to open dbname – and in case of success we get embedded connection to the database. Pay attention – we do not need special embedded library to have embedded connection, standard client loads appropriate provider and becomes embedded server.

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 Engine13 to access database Loopback provider is attempted for an attach. It does not differ much from Remote, but tries to access database dbname on a server running on local host. On windows XNET protocol (also known as local connection) is used for it, posix systems prepend dbname with localhost: 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.

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 – not in alpha1 – 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.

The strong feature of providers architecture is ability for user to add his own providers to server and/or client. You may be surprised – 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 – 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 cache@ prefix in the beginning of database name or whatever else you choose). In this sample when database name cache@RemoteHost:dbname is used caching provider accepts such connection and invokes YValve once again with traditional database name RemoteHost:dbname. But when user later performs any call to his database caching provider gets control on it before Remote one and for locally cached table can avoid calls to remote server.

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 – for details please see README.configuration. BTW, same technique may be used at client too.

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.

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.

Questions and answers.

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?

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.

Q. And what about performance when using old API?

A. Functional API is implemented as a very thin layer over interfaces. Code in most case is trivial – 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.