Two-tier Client Server

• Delphi:
  • BDE without middleware
    • standard 2 tier approach
    • using TClientDataSet (requires C/S version or Delphi 6 Pro)
• see also:
  • Delphi Programming Main Index
  • Borland database engine
  • Networked database solutions
  • Client Server middleware tools
  • Multi-tiered client server
  • Powerbuilder Enterprise 6.5
  • Relational database management systems

Delphi:

• BDE without middleware (ie. no MIDAS/ENTERA/etc):
  • standard 2 tier approach:
    • all levels of Delphi
    • TDatabase to point to remote database
  • using TClientDataSet:
    • Delphi 3 or higher
    • ref: DI 1998 April p30-35 DI9804DM.zip
    • TClientDataset gets data from a DBDataset on same machine via its IProvider interface:
      • assign at runtime:
        • Uses: BdeProv (else runtime message "No provider available")
        • Form.create:
          • ClientDataset1.Provider := Table1.provider;
          • ClientDataset1.Data := Table1.provider.data;
      • assign at design time:
        • ClientDataSet command Assign Local Data
          • NB. DFM size will grow as it stores all the physical table data assoc. with DBDataset if it is made active
            • trim this by executing the Clear Data command
        • optionally can assign provider at design time if Delphi 3.02 or higher:
          • assign a component which has IProvider interface (eg. TTable, Query) to TClientDataset.DataProvider property.
          • NB. this allows you to add fields from the table & create calculated fields - these must be done on TClientDataSet as the IProvider interface does not pass calculated fields from DBDataSet to ClientDataSet!
            • think of the DBdataset component as a separate application that can't be manipulated by you with code
      • NB. leave RemoteServer properties blank as this is only used in 3-tier!
    • Advantages:
      • dramatically reduce network traffic in several instances:
        • reading an entire table
        • static lookup tables
        • sorting a table
      • TClientDataSet:
        • stores all of its data in memory & thus is very fast
        • you can control the content of the data packets retrieved in 3 ways:
          • number of records retrieved at one time via the PacketRecords property
          • using Fields editor to create persistent field objects determines which fields will be retrieved (but must include the primary key field(s)).
          • TProvider.Options lets you control whether BLObs & detail records will be fetched automatically or must be specifically requested in code
        • has the ability to read & write its contents to local files via LoadFromFile & SaveToFile:
          • store metadata, data & change log for that table
          • => you can retrieve data from server database, edit it, save edits to local CDS file & later, when you are disconnected from the database, you can load the data from the CDS file & still have the ability to undo changes using standard ClientDataset methods
          • => can save lookup tables locally to save continual network traffic of rarely changed lookups (see lookup.dpr in DI9804DM.zip)
        • AddIndex & DeleteIndex methods allow you to change whether index is ascending or descending & this sorting is done on the data in memory without having to re-run a query to sort on a different field=> very fast, no network traffic, can sort on calculated fields!
          • NB. to sort on calculated fields, the field must be specified as being fkInternalCalc, however, doing so means its data will be stored in memory too just like a normal field rather than having its values derived only when needed, thus if there is no need to sort on the calculated field leave its FieldKind property as calculated field.
        • Delphi 4: can define aggregates (eg. sum, min, max, avg, count) via a property inspector at design time with ability to change:
          • displayed name, groupingLevel (number of index fields to group on), indexname
        • refreshRecords method avoids clearing the change log & losing unapplied changes as would occur with the refresh method, thus retrieving a new set of records from the server & merges them with the changes in the change log as though the changes were originally made to the new records. If any conflicts occur, it triggers an OnReconcileError event.
        • is the official way to handle cached updates:
          • no longer have to apply updates when you move from a master record as was the case in Delphi 2.
          • can delay applying updates via ClientDataset.Provider.ApplyUpdates
        • setting parameters for a query or stored procedure is easier in D4 than D3, as it now has the Params property
        • allows creation, reading & writing of single-user flat files without need for the BDE!
      • readies your application for transitioning to 3-tier model
    • Disadvantages:
      • need to deploy 2 additional COM-based files:
        • DBClient.dll & STDVCL32.dll which need to be copied to the Windows System directory & registered as for OCX's (eg. with Regsvr32.exe)
      • delaying ApplyUpdates may cause inconsistencies when using Paradox files but works well with SQL back-ends such as Sybase SQL Anywhere
      • to ensure TDataset's events are activated, you need to set TProvider.ResolveToDataSet to true although doing so will be slower as updates must 1st be processed by the dataset component rather than applied directly to the database using SQL.