PGTop for Android: Things I learned making this app

PGTop for Android
Things I learned making this app

Mark Wong
markwkm@postgresql.org
Port1and, Oregon, PostgreSQL Users Group

20 January 2011

PGTop for Android

A PostgresSQL stats monitoring app
for Android:
http://github.com/markwkm/PGTop

markwkm (PDXPUG) PGTop for Android 20 January 2011 2 / 37

How did I get started?

Once upon a time I got developer edition G1 from Google...


And it sat idle...

• Postgres JDBC driver for Android platform didn’t work
• Native C development kit too hard for me to just build just the libpq
library from the postgres source


And then some motivation...

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/ ~~~/ . o O ( PostgreSQL contest! )
,----( oo )
/ __ __/
/| ( |(
^ /___ / |
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/
/
_.-""""-._
.’ ‘.
/ O O . o O ( Free Nexus One! )
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/ +----------------+/
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New developments

Dave Cramer builds a version of the postgres JDBC jar that works on Android:
http://groups.google.com/group/pgandroid/attach/4ee44940c9f7319f/postgresql.jar?part=2


Slapped together and entered PGTop for Android:


Not a winner. :(


Oh well, continue to make a few improvement...


What did I learn along the way?


How to set up a development environment

• Requires 32-bit java development environment
• Requires Android sdk
• Android emulator
• Android jars
• Android packaging tools
• Optional to have Eclipse
• ui designer gui
• hides xml

• Optional to have Android ndk for developing native C library


How to make a launcher icon
I need to learn how to use Inkscape or Gimp better to make the ascii art icon
meet the recommended guidelines better:


About layouts

Only used scroll view and linear layouts.


About ui widgets

• TextViews (labels)
• EditViews (input text)
• Spinners (selection)
• Buttons
• custom menus


How the threading model works

• child threads don’t have access to ui objects
• use android.os.Handler to pass messages to parent thread


How PGTop uses threads
Create a Handler:
private Handler handler = new Handler() {
@Override
public void handleMessage(Message msg) {
// Update UI objects...
}
}

Child thread loops continuously to query PostgreSQL database for stats and
updates class variables with data.
Pass a message to a handler to call the method that updates the UI:
handler.sendEmptyMessage(0);

How to use SQLite3 interface
To store database connection information (oversimpliﬁed sample):
public class PGConnectionOpenHelper extends SQLiteOpenHelper

SQLiteDatabase db = openHelper.getReadableDatabase();

Cursor c = db.rawQuery(SELECT_CONNECTIONS, null);

while (c.moveToNext()) {
c.getString(c.getColumnIndex("database"));
}


How to read the stdout/stderr log
The debugger was too hard for me.


__ __
/ ~~~/ . o O ( Code review time! )
,----( oo )
/ __ __/
/| ( |(
^ /___ / |
|__| |__|-"


PostgreSQL JDBC

Now for some simple examples for connecting to a PostgreSQL database and
querying some data. Warning, code formatted to ﬁt better on these slides. . .


Open a Database Connection
Load the PostgreSQL JDBC driver and open a database connection using SSL:
Class.forName("org.postgresql.Driver");
String url;
url = "jdbc:postgresql://pghost:5432/pgdatabase" +
"?sslfactory=org.postgresql.ssl.NonValidatingFactory" +
"&ssl=true";
Connection conn = DriverManager.getConnection(url,
"pguser",
"pgpass");
Don’t forget to close the connection when you’re done.
conn.close();


Execute a Query
Building on the previous slide, select the name of all relations from pg class
and iterate through every row returned:
String sql;
sql = "SELECT relname FROM pg_class WHERE relkind = ’r’;"

Statement st = conn.createStatement();
ResultSet rs = st.executeQuery(sql);
while (rs.next()) {
Columns are enumerated starting with 1.
String relname = rs.getString(1);
}
rs.close();
st.close();

Execute a Query with a Bind Value
Building on the previous slide, select the number of all relations from pg class:
String sql = "SELECT COUNT(*) FROM pg_class WHERE relkind = ?;"

PreparedStatement ps = conn.createStatement();
// Bind variables are enumerated starting with 1;
ps.setString(1, "r");
ResultSet rs = ps.executeQuery(sql);

rs.next();
long count = rs.getLong(1);

rs.close();
ps.close();

More JDBC Code Examples

The PostgreSQL JDBC documentation has more examples at
http://jdbc.postgresql.org/documentation/head/


Using cursors


Executing INSERT, UPDATE, DELETE SQL statements


Creating and modifying database objects


Calling stored functions


Handling binary data


JDBC escapes e.g. strings, outer joins, date-time, scalar functions


PostgreSQL Extensions to the JDBC API
• Geometric data types
• Large objects
• Listen/Notify
• Server prepared statements


Connection pools


Further JDBC Reading

JDBC API Documentation and JDBC Speciﬁcation
http://jdbc.postgresql.org/documentation/head/reading.html


__ __
/ ~~~/ . o O ( Thank you! )
,----( oo )
/ __ __/
/| ( |(
^ /___ / |
|__| |__|-"


Acknowledgements

Hayley Jane Wakenshaw

__ __
/ ~~~/
,----( oo )
/ __ __/
/| ( |(
^ /___ / |
|__| |__|-"


License

This work is licensed under a Creative Commons Attribution 3.0 Unported
License. To view a copy of this license, (a) visit
http://creativecommons.org/licenses/by/3.0/us/; or, (b) send a
letter to Creative Commons, 171 2nd Street, Suite 300, San Francisco,
California, 94105, USA.


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