Android Tutorial Remote Database Connector (RDC) - Connect to any remote DB

jRDC version 2 is available here: https://www.b4x.com/android/forum/t...ation-of-rdc-remote-database-connector.61801/

Spoiler: Old and shouldn't be open

This tutorial covers a new framework named Remote Database Connector (RDC). The purpose of RDC is to make it simple to develop Android applications that interact with remote database servers.

There are two components in this framework: a lightweight Java web server (the middleware) and a B4A class named DBRequestManager that interacts with this web server.

The Java web server can connect to any database platform that provides a JDBC driver.

This includes: MySQL, SQL Server, Oracle, Sybase, DB2, postgreSQL, Firebird and many others.

You can also use the web server without a database server by connecting to a local SQLite database file.

The Java web-server is a simple server that connects to the database server and to the Android clients.
As this is a Java app you can run it on Linux or Windows computers. It requires Java JRE 6 or 7.

This solution is much more powerful than the PHP (MySQL) and ASP.Net (MS SQL) solutions that are already available.

Main advantages over previous solutions:

• Support for many types of database platforms.
• Significantly better performance.
• SQL statements are configured in the server (safer).
• Support for all types of statements, including batch statements.
• Support for BLOBs.

Server Configuration

JDBC is a Java API that provides a standard method to access any database. A database driver (jar file) is required for each type of database. You will need to download the driver for your database and put it in the jdbc_driver folder.

The Java server configuration is saved in a file named config.properties.
This file includes two sections: general configuration and a list of SQL commands.

For example:

Note that the configuration fields are case sensitive.

DriverClass / JdbcUrl - The values of these two fields are specific to each database platform. You will usually find the specification together with the required driver jar file.

User / Password - Database user and password.

ServerPort - The Java server will listen to this provided port.

Debug - If Debug is set to true then the SQL commands list will be loaded on every request. This is useful during development as it allows you to modify the commands without restarting the server process.

SQL Commands - A list of commands. Each command starts with 'sql.'. The commands can include question marks (parameterised queries). Question marks will be replaced with the values provided by the Android clients. Note that the command name is case sensitive and it doesn't include the 'sql.' prefix.

Client Code
The client sends requests to the server. There are two types of requests: query requests (usually SELECT statements) and batch requests (any other statement).

Note that both the client and the server can manage multiple requests in parallel.
Usually you will only need a single DBRequestManager object.

Each request is made of a command (or a list of commands) and a tag. The tag can be any object you like. You can use it later when the result is ready. The tag is not sent to the server.

A command is an object of type DBCommand:

Type DBCommand (Name As String, Parameters() As Object)

Name - The case sensitive command name as configured in the server configuration (without sql.).
Parameters - An array of objects that will be sent to the server and will replace the question marks in the command.

For example to send a SELECT request:

Dim cmd As DBCommand
cmd.Initialize
cmd.Name = "select_animal"
cmd.Parameters = Array As Object("cat 1")
reqManager.ExecuteQuery(cmd, 0, "cat 1")

ExecuteQuery expects three parameters: the command, maximum number of rows to return or 0 if there is no limit and the tag value.

Under the hood DBRequestManager creates a HttpJob for each request. The job name is always "DBRequest".

You should handle the JobDone event in your activity or service:

Sub JobDone(Job As HttpJob)
   If Job.Success = False Then
     Log("Error: " & Job.ErrorMessage)
   Else
     If Job.JobName = "DBRequest" Then
       Dim result As DBResult = reqManager.HandleJob(Job)
       If result.Tag = "cat 1" Then 'query tag
         For Each records() As Object In result.Rows
           Dim name As String = records(0) 'or records(result.Columns.Get("name"))
           Log(name)
         Next
       End If
     End If
   End If
   Job.Release
End Sub

As you can see in the above code, DBRequestManager.HandleJob method takes the Job and returns a DBResult object:

Type DBResult (Tag As Object, Columns As Map, Rows As List)

Tag - The request tag.
Columns - A Map with the columns names as keys and the columns ordinals as values.
Rows - A list that holds an array of objects for each row in the result set.

Non-select commands are sent with ExecuteCommand (single command) or ExecuteBatch (any number of commands). It is significantly faster to send one request with multiple commands over multiple requests. Batch statements are executed in a single transaction. If one of the commands fail all the batch commands will be cancelled (roll-backed).

Note that for non-select requests, Rows field in the results will hold an array with a single int for each command. The int value is the number of affected rows (for each command separately).

Initializing DBRequestManager:

Sub Activity_Create(FirstTime As Boolean)
   If FirstTime Then
     reqManager.Initialize(Me, "http://192.168.0.100:17178")
   End If
End Sub

The first parameter is the module that will handle the JobDone event.
The second parameter is the link to the Java server (with the port number).

DBRequestManager provides the following helper methods for common tasks related to BLOB fields: FileToBytes, ImageToBytes and BytesToImage.It also includes a method named PrintTable that prints DBTable objects to the logs.

Framework Setup

1. Unpack the server zip file.
2. You will need to download the driver for your database platform and copy the jar file to jdbc_driver folder.
3. Edit config.properties as discussed above.
4. Edit RunRLC.bat and set the path to java.exe. If you are running it on Linux then you need to create a similar script with the path to java. On Linux you should change the ';' separator to ':'.
5. Run the server
   You should see something like:
   
   
   
   
   
   Note that the path to config.properties is printed in the second line.
6. Add an exception for the server port in your firewall.
7. Try to access the server from the browser:
   
   
   
   
   
   You will probably not see the above message on the first run . Instead you will need to check the server output and read the error message.
   If you are able to call the server from the local computer and not from other devices then it is a firewall issue.

Tips

• MySQL driver is available here: http://dev.mysql.com/downloads/connector/j/
• Google for <your database> JDBC Driver to find the required driver. For SQL Server you can use this open source project: http://jtds.sourceforge.net/
• The server uses an open source project named c3p0 to manage the database connection pool. It can be configured if needed by modifying the c3p0.properties file.
• Use a text editor such as Notepad++ to edit the properties file.
• If you are running the server on Linux then you can run it with nohup to prevent the OS from killing the process when you log out.
• Java doesn't need to be installed. You can just unpack the files.

The server is based on the two following open source projects:
Jetty - http://www.eclipse.org/jetty/
c3p0 - http://www.mchange.com/projects/c3p0/index.html

http://www.mchange.com/projects/c3p0/index.html

 

[Erel]

Please use [ code ] [ /code ] tags (without spaces) when posting code.

Which database gets corrupted? The local one or the remote one?

 

[slydog43]

The local one. Seems only to happen when I add records to a table in a loop. I have also tried a batch nonquery, but similar results. I must be missing something simple. Any samples with many batches operations?

Im trying to fill many tables locally from a remote database. it seems to fill the first table, maybe the second one, but then corrupts the db after I add more tables to be filled.

I also can't seem to populate a single table with a few thousand records from a MS-SQL database to sqlite on the phone. It gets corrupt everytime.

When I say corrupt I mean I can't copy the db file off the phone onto my PC and open with
the program SQLite Expert Personal 3.4.76.2306 which I can without populating it will many records. I can still seem to work with the database on the phone though. Very strange.

 

[slydog43]

I can get my program to populate 10 tables (with only 10-100 records) without any issues, but when I try to populate a table with over 1000 records, I get a corrupt db. I have tried with MSSql and SQLite as the backend without changes. Can anyone successfully populate a sqlite db file on a phone from a remote database with more than 1000 records (with about 10 fields, text and numeric). Very frustrating, I'm so close I know, but I think I found a problem here.

 

[slydog43]

I'm on to something I think, I can add 780 records, but when I get to 781 it corrupts the DB.

using this code

Sub btnAddRecs_Click
  
    Dim iTempID As Int
    Dim iTempSEQ As Int
    Dim sTempDesc As String
    Dim sTempAbbrev As String
  
    mySQL1.BeginTransaction
    Try
        For i = 1 To 781 '500, 600, 750, 760, 770, 780  works, but 5000, 1000, 900, 800, 790, 785, 783, 781 does NOT  these number are from a blank body table
            iTempID = i
            iTempSEQ = i
            sTempDesc = "test" & i
            sTempAbbrev = i
            mySQL1.ExecNonQuery("INSERT INTO Body (BodyID, BodySEQ, BodyDesc, BodyAbbrev) VALUES (" & iTempID & ", " & iTempSEQ & ", '" & sTempDesc & "', '" & sTempAbbrev & "'" & ")")
        Next
        mySQL1.TransactionSuccessful
    Catch
        Log(LastException.Message)
    End Try
    mySQL1.EndTransaction
    Msgbox("Done","")
End Sub

erel, Did I find a bug?? I also tried without using transactions, but same trouble at the exact same number.

 

[Erel]

Please start a new thread for this question. It has nothing to do with RDC. If possible also upload a full example that demonstrates this issue.

 

[Endomi]

I don't understand how to return a query result.
After this line, how do I get the result of this query:

reqManager.ExecuteQuery(cmd, 0, "cat 1")

Or is a result returned in this code?

Sub JobDone(Job As HttpJob)
  If Job.Success = False Then
    Log("Error: " & Job.ErrorMessage)
  Else
    If Job.JobName = "DBRequest" Then
      Dim result As DBResult = reqManager.HandleJob(Job)
      If result.Tag = "cat 1" Then 'query tag
        For Each records() As Object In result.Rows
          Dim name As String = records(0) 'or records(result.Columns.Get("name"))
          Log(name)
        Next
      End If
    End If
  End If
  Job.Release
End Sub

If so, must I create a Job parameter, or does it exist in some other module so I can pass it as a parameter?
Thanks

 

[Erel]

The Job is created by DBManager. You get the results in JobDone.

 

[Endomi]

I really struggle with this, but anyway.
Here is the code in my Log-In button, for the sake of being as simple as possible, I just query whether the user-name entered
already exists or not.

The "selectuserpwd" query just returns a count of records that match the entered user name.
Do you know how/where I can read the returned count of the query?

Sub btnLogIn_Click
    Dim cmd As DBCommand
    cmd.Initialize
    cmd.Name = "selectuserpwd"
    cmd.Parameters = Array As Object(txtUser.Text)
    Dim sResult As String
    reqManager.ExecuteQuery(cmd, 1,txtUser.Text  )
End Sub

 

[Erel]

You should handle the JobDone event as described here: http://www.b4x.com/android/forum/threads/remote-database-connector-connect-to-any-remote-db.31540/

Result.Rows is a List with the records returned. Result.Rows.Size will return the number or records found.

 

[Endomi]

Thanks my problem is solved, I now see that the JobDone gets called implicitly.

 

[somed3v3loper]

I can not find the folder jdbc_driver .
Do free web host providers hide such folder?
Or should I create it?
I want to test it online not on my PC.
Thanks in advance

 

[Endomi]

The jdbc_driver folder is in the RDC-Server zip folder .

 

[Carlos Bernardino]

Hi there!

Is it possible to use INNER JOINS on the SQL Commands included in the config.properties file?

Thanks in advance!

 

[Erel]

You can use any feature supported by the SQL engine.

 

[bashka_abdyli]

Is RDC possible to install on Goddady hosting site? If yes is there any tutorial ?

Thanks in advance,

 

[Erel]

You will need a VPS solution (virtual private server).

 

[georgelbs]

Has anyone been able to establish a connection to Oracle?

 

[Erel]

Which database engine?

 

[somed3v3loper]

I do not have VPS so I think I have to use PHP instead.

 

[Reviewnow]

Few questions.
Once RDC is started on server, How to stop? just close command window?

I have multiple databases for a single application, is there a way to set the database name dynamically? or can multiple instances of RDC run under different ports?

Restarting RDC on server reboot, automatically? Schedule Task?

Can RDC be restarted with a Remote Command

Please advise

Thanks so much...