(This post was completely rewritten and updated on October 30, 2015)
What is TinyDB?
TinyDB is a simple “database” that stores data on your phone or tablet. Unlike program variables that go away when your app is finished running or your phone is re-set, values stored in TinyDB remain on your phone for use the next time your app is run.
About Memory on your Phone or Tablet
Your smart phone or tablet typically has two primary types of memory: RAM and FLASH memory.
RAM stands for “random access memory” – but today we mostly think of RAM as memory that can be accessed very fast (as compared to Flash or hard drive memory storage). RAM retains values as long as power is applied to the RAM circuity. Once we turn off the power, the values stored in RAM are lost. (In some applications, extra batteries are used to continuously provide power to RAM even when the “normal” power is turned off.)
Flash memory retains values when the power is turned off. But access to Flash RAM is not as fast as access to conventional RAM memory.
Why is it called “Flash”? There was an early version of memory technology where the memory was erased by literally flashing it with ultraviolet light. However the inventor of Flash RAM chose the name “Flash” for different reasons. Modern Flash RAM is read, written and erased electronically.
App Inventor variables are stored in RAM memory – and the content of RAM is erased or reset whenever the power is turned off. TinyDB, on the other hand, stores values in FLASH RAM, where the values remain even when the power is turned off.
TinyDB provides a simple way to store and retrieve data efficiently and to store the data in long-term storage. TinyDB is based on the concept of a “tag” to identify the stored data, and the data value. Think of a “tag” as like using your name as your identification to look up your address:
Tag value: Martin
Value: 123 Main St, Anytown, USA
Tag value: Alexa
Value: 321 Other St, Someplace, USA
TinyDB uses the “tag” (such as Alexa) to quickly locate the corresponding value. Even if you have 100 names and addresses stored in TinyDB, TinyDB can look up the “tag” quickly and use the tag to find the corresponding value. We do not need to know how TinyDB does its look up so fast – it just does it [see Footnote 1].
In most database programs, the “tag” is known as a “key” or “key value”. App Inventor uses the name “tag” in place of “key”. As I am used to the name “key”, I tend to use “key” were I should have used “tag” in App Inventor! You will see this is the sample program, below!
To learn how to put TinyDB in operation, we construct a very simple app, described below.