Exploring a SQLite Database on Android

or “How to read SQLite DB from a desktop”

SQLite is the relational, embedded, ACID compliant database that comes with Android. Due to this fact it is certainly the most deployed DB engine on this planet. In case your application need to have CRUD features for local persisted data and the complexity level is beyond a simple text file, you have to consider it.

A challenge is to look into the (raw) DB from your desktop (if you dont want to build and integrate a DB viewer into your app). As Android apps store databases into their respective /data subfolder and if you don’t have a rooted phone, you cant look inside this folder.

I am not aware of any tool that can open a connection to the DB remotely, so the best way is to copy the DB file into the accessible SD card (or whatever the phone and its manufacturer considers as SD card, even the internal memory mounted as SD card), download it to your desktop and open it with a tool like the SQLite DB Browser.

Let’s put some sourcecode here as reference

Create a simple demo DB

No bells and whistles, no helper classes, etc. just the most simple way to create DB and a table.

 
    private void createDB() {
        SQLiteDatabase sampleDB = this.openOrCreateDatabase("MYDEMODB", MODE_PRIVATE, null);
        sampleDB.execSQL("CREATE TABLE IF NOT EXISTS MYTABLE (Last VARCHAR, First VARCHAR, Role VARCHAR);");
        sampleDB.execSQL("INSERT INTO MYTABLE Values ('Smith','John','CEO');");
        sampleDB.execSQL("INSERT INTO MYTABLE Values ('Thomson','Allan','CTO');");
        sampleDB.close();
    }

Copy the DB to your SD card

 
    private void exportDB() {
        File mySd = Environment.getExternalStorageDirectory();
        File myData = Environment.getDataDirectory();

        FileChannel src = null;
        FileChannel des = null;
        String currentDBPath = "/data/" + getApplicationContext().getPackageName() + "/databases/MYDEMODB";
        String exportDBPath = "MYDEMODB";

        File currentDB = new File(myData, currentDBPath);
        File backupDB = new File(mySd, exportDBPath);
        try {
            src = new FileInputStream(currentDB).getChannel();
            des = new FileOutputStream(backupDB).getChannel();
            des.transferFrom(src, 0, src.size());
            src.close();
            des.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

After download to your local drive you can use the SQLite Browser to open the file. Very useful data debugging or for apps that collect data and you can’t implement an upload of the data to a server via the internet connection.

SQLite DB Browser

SQLite DB Browser (Structure View)

 

SQLite DB Browser

SQLite DB Browser (Data View)

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PostgreSQL Replication Express Setup

The system I work on we deploy almost solely on the Amazon AWS platform. Even I try to design the architecture in a way not to be locked-in too much into Amazon, I make use of the Amazon tools and products as much as possible (EC2, VPC, S3, SNS). PostgreSQL is our reference DB and the only DB product in production environments, still we run dedicated instances with PostgreSQL. I am quite delighted about AWS recent offering staring RDS with PostgreSQL. While is is still in BETA and I did not started yet with a conclusive test and migration plan, I need to maintain our existing instances.

There are plenty of books and tutorials about setting up PostgreSQL replication with on-board tools, without going into the details I share the express setup in this tutorial based on Streaming Replication which is part of PostgreSQL since version 9.0. I highly recommend to review the parameters and settings from the below tutorial as your project might have different requirements.

References

Tutorial

Remarks

  • The tutorial is based on PostgreSQL 9.2 running on Ubuntu Server
  • Paths and settings are all the PostgreSQL defaults.
  • This is async setup, the master will not wait for feedback from the salve and continue to work even the slave is not available

Prerequisite

  • 2 Server running the same PostgreSQL version (9.0+)
  • Backup your data or use a sandbox environment.
  • In the tutorial I refer to
    MASTER (ip: 0.0.0.1) and
    SLAVE (ip: 0.0.0.2)

Configuration

Master

  • Create a replicator user
    sudo -u postgres psql -c "CREATE USER replicator REPLICATION LOGIN ENCRYPTED PASSWORD 'mypassword';"
    
  • Add the slave ip to /etc/postgresql/9.2/main/pg_hba.conf
    host    replication     all      0.0.0.2/32         trust
    
  • Modify parameters in /etc/postgresql/9.2/main/postgresql.conf
    wal_level = hot_standby
    max_wal_senders = 3
    checkpoint_segments = 3
    wal_keep_segments = 3
    

    Review these parameters and set them up according to your requirements

  • Start the PostgreSQL instance
    service postgresql start
    

Slave

  • Modify parameters in /etc/postgresql/9.2/main/postgresql.conf
    wal_level = hot_standby
    max_wal_senders = 3
    checkpoint_segments = 3
    wal_keep_segments = 3
    hot_standby = on
    
  • Stop the PostgreSQL instance
    service postgresql stop
    
  • Clean up the old data directory
    sudo -u postgres rm -rf /var/lib/postgresql/9.2/main
    
  • Copy the database from the master with pg_basebackup
    sudo -u postgres pg_basebackup -h 0.0.0.1 -D /var/lib/postgresql/9.2/main -U replicator -v -P -x
    

    You can see the backup progress and it should result in something likes

    root@:/var/lib/postgresql/9.2/main# transaction log start point: 41/7D000020
    31524952/31524952 kB (100%), 2/2 tablespaces (/var/lib/postgresql/9.2/main/PG_9.)
    transaction log end point: 41/7D0002A8
    pg_basebackup: base backup completed
    
  • Create a recovery configuration file /var/lib/postgresql/9.2/main/recovery.conf
    standby_mode = 'on'
    primary_conninfo = 'host=0.0.0.1 port=5432 user=replicator password=mypassword sslmode=require'
    trigger_file = '/tmp/postgresql.trigger'
    
  • Start the PostgreSQL instance
    service postgresql start
    

    Check the pglogs.

Test the replication

  • Open any table with pgadmin on the master and apply a change, it should be reflected in the slave within short time.
  • Try to change data on the slave, it will fail due to the hot-standby mode

Monitor the replication

  • The master instance will not alert you when the replication is down. You can check by yourself or create a little cronjob to do it for you with this sql statement.
    sudo -u postgres psql -x -c "select * from pg_stat_replication;"
    

    You get the status back (if it is running, otherwise the statement will return ‘no rows’).

    Check replication

    Check replication

Netbeans + Visual Paradigm = EJB Tutorial (Part 4)

Based on the previous parts of this tutorial (1, 2, 3) we will modify the application and Glassfish to connect to PostgreSQL or Oracle as DB.

  • Preparing Glassfish libraries
    Download the drivers for Oracle and PostgreSQL and add them to your {GLASSFISHHOME}/domains/{YOURDOMAIN}/lib
    Restart Glassfish.

    Glassfish lib folder

    Continue reading

Netbeans + Visual Paradigm = EJB Tutorial (Part 3)

In part 1 of this tutorial we created an EJB using Netbeans and Visual Paradigm, in part 2 a little ZK application to read data using the EJB. In part 3 we will move away from the Derby DB to PostgreSQL and Oracle DB and challenge ourselves with identifier more than 30 characters, which is an issue for Oracle (yes, it is 2011). We will add columns with more than 30 characters and play with a few different column types (the ones showing up in a normal DB layout).

  • Add new fields to the ERD
    this_is_a_very_long_remark_field
    floatcol
    numbercol

    Updated ERD

  • Continue reading