使用 TFDConnection 的 pooled 連接池

使用 TFDConnection 的 pooled 連接池

從開始看到這個屬性，就一直認爲他可以提供一個連接池管理功能， 苦於文檔資料太少， 甚至在幫助中對該屬性的使用都沒有任何介紹，如果你搜索百度，也會發現基本沒資料。

最後終於在其官方網站看到了其完整相關的英文資料，雖然沒有正面介紹該屬性，但卻是要啓用該屬性的詳細方法：

Defining Connection (FireDAC)

General

A connection definition is a set of parameters that defines how to connect an application to a DBMS using a specific FireDAC driver. It is the equivalent of a BDE alias, ADO UDL (stored OLEDB connection string), or ODBC Data Source Name (DSN). For the list of supported database management systems and corresponding parameters, see FireDAC Database Connectivity.

FireDAC supports 3 connection definition kinds:

Type	Description	Pros	Cons
Persistent	Has a unique name, is managed by the FDManager, and is stored in a connection definition file.	May be defined once and reused across many applications. May be pooled.	The parameters (server address, DB name, and so on) are publicly visible and may be changed incidentally. FDManager has to be reactivated or the Delphi IDE has to be restarted to make a newly added definition visible at design time.
Private	Has a unique name, is managed by the FDManager, but is NOT stored in a connection definition file.	Connection definition parameters are not visible "outside" the application. May be pooled.	The application needs to create a private connection definition after each program restarts and cannot share it with the other programs. Cannot be created at design time.
Temporary	Has no name, is not stored in a connection definition file, and is not managed by the FDManager.	The simplest way to create a connection definition is to fill in the TFDConnection.Params property. Can be created at design time using the TFDConnection component editor.	Similar to private. Also cannot be referenced by name and cannot be pooled.

Connection Definition File

The persistent connection definitions are stored in an external file - the connection definition file. This file has the standard INI text file format. It can be edited by FDExplorer or FDAdministrator utilities at first, manually, or by code. By default the file is C:\Users\Public\Documents\Embarcadero\Studio\14.0\FireDAC\FDConnectionDefs.ini.

Note: If you add a new persistent connection definition using FDExplorer or FDAdministrator while the RAD Studio IDE is running, it is not visible to the FireDAC design time code. To refresh the persistent connection definitions list, you need to reactivate FDManager or restart the RAD Studio IDE.

Sample content of this file:

[Oracle_Demo]
DriverID=Ora
Database=ORA_920_APP
User_Name=ADDemo
Password=a
MetaDefSchema=ADDemo
;MonitorBy=Remote
 
[MSSQL_Demo]
DriverID=MSSQL
Server=127.0.0.1
Database=Northwind
User_Name=sa
Password=
MetaDefSchema=dbo
MetaDefCatalog=Northwind
MonitorBy=Remote

An application can specify a connection definition file name in the FDManager.ConnectionDefFileName property. FireDAC searches for a connection definition file in the following places:

• If ConnectionDefFileName is specified:
  • search for a file name without a path, then look for it in an application EXE folder.
  • otherwise just use a specified file name.
• If ConnectionDefFileName is not specified:
  • Look for FDConnectionDefs.ini in an application EXE folder.
  • If the file above is not found, look for a file specified in the registry key HKCU\Software\Embarcadero\FireDAC\ConnectionDefFile. By default it is C:\Users\Public\Documents\Embarcadero\Studio\14.0\FireDAC\FDConnectionDefs.ini.

Note: At design time, FireDAC ignores the value of the FDManager.ConnectionDefFileName, and looks for a file in a RAD Studio Bin folder or as specified in the registry. If the file is not found, an exception is raised.

If FDManager.ConnectionDefFileAutoLoad is True, a connection definition file loads automatically. Otherwise, it must be loaded explicitly by calling the FDManager.LoadConnectionDefFile method before the first usage of the connection definitions. For example, before setting TFDConnection.Connected to True.

Creating a Persistent Connection Definition

A persistent connection definition can be created using FDExplorer or FDAdministrator. Here is how you can do that in code. Also see the demo FireDAC\Samples\Comp Layer\TFDConnection\ConnectionDefs.

The following code snippet creates a connection definition named "MSSQL_Connection", which has all parameters required to connect to the Microsoft SQL Server running locally, using the OS authentication (SSPI):

uses
  FireDAC.Comp.Client, FireDAC.Stan.Intf;
var
  oDef: IFDStanConnectionDef;
begin
  oDef := FDManager.ConnectionDefs.AddConnectionDef;
  oDef.Name := 'MSSQL_Connection';
  oDef.DriverID := 'MSSQL';
  oDef.Server := '127.0.0.1';
  oDef.Database := 'Northwind';
  oDef.OSAuthent := True;
  oDef.MarkPersistent;
  oDef.Apply;
  .....................
  FDConnection1.ConnectionDefName := 'MSSQL_Connection';
  FDConnection1.Connected := True;

FDManager is a global instance of the FireDAC connection manager. Its property FDManager.ConnectionDefs: IFDStanConnectionDefs is a collection of the persistent and private connection definitions. The AddConnectionDef method adds a new connection definition. The MarkPersistent method marks a connection definition as persistent. The Apply method saves a connection definition to a connection definition file. Without the MarkPersistent call, the connection definition is private.

Creating a Private Connection Definition

A private connection definition can be created only in code. The code is similar to the one above, but without the MarkPersistent call.

Also, you can use a technique similar to BDE:

var
  oParams: TStrings;
begin
  oParams := TStringList.Create;
  oParams.Add('Server=127.0.0.1');
  oParams.Add('Database=Northwind');
  oParams.Add('OSAuthent=Yes');
  FDManager.AddConnectionDef('MSSQL_Connection', 'MSSQL', oParams);
  .....................
  FDConnection1.ConnectionDefName := 'MSSQL_Connection';
  FDConnection1.Connected := True;

Creating a Temporary Connection Definition

A temporary connection definition can be created at design time using the FireDAC Connection Editor. In order to do this, double-click a TFDConnection to invoke the editor:

Or at run time in code by filling the TFDConnection.Params property. This is the simplest way to create a connection definition.

FDConnection1.DriverName := 'MSSQL';
FDConnection1.Params.Add('Server=127.0.0.1');
FDConnection1.Params.Add('Database=Northwind');
FDConnection1.Params.Add('User_name=sa');
FDConnection1.Connected := True;

Another option is to specify a connection string at run time by filling the TFDConnection.ConnectionString property. A connection string may be a convenient way to specify connection definition parameters for certain types of applications. For example:

FDConnection1.ConnectionString := 'DriverID=MSSQL;Server=127.0.0.1;Database=Northwind;User_name=sa';
FDConnection1.Connected := True;

Editing a Connection Definition

An application may need an ability to create and edit a connection definition at run time using standard FireDAC Connection Editor dialog. To edit a temporary connection definition stored in TFDConnection, use the code:

uses
  FireDAC.VCLUI.ConnEdit;
...
if TfrmFDGUIxFormsConnEdit.Execute(FDConnection1, '') then
  FDConnection1.Connected := True;

To edit a connection definition represented as a FireDAC connection string, use the code:

uses
  FireDAC.VCLUI.ConnEdit;
...
var
  sConnStr: String;
...
sConnStr := FDConnection1.ResultConnectionDef.BuildString();
if TfrmFDGUIxFormsConnEdit.Execute(sConnStr, '') then begin
  FDConnection1.ResultConnectionDef.ParseString(sConnStr);
  FDConnection1.Connected := True;
end;