Writing your first Dapp¶

This section is a brief tutorial for dapp developers. We illustrate how to write a dapp by commenting the source code of a decentralized key-value storage service. The full source code of this dapp is available at https://github.com/contractvm/cvm-dapp-blockstore. The key-value storage dapp offers two APIs: set(key, value) and get(key). A set request saves a new immutable key-value pair, while get retrieves a previously set value. Since set changes the state of the dapp, this API requires the client to publish a suitable message in the blockchain. Nodes scan the blockchain for new messages, and when they find a set request, they save the new key-value pair in their local database. Instead, executing a get does not require to publish any message, because nodes can handle this request internally.

The dapp is split in two main parts: the first is the dapp which runs in nodes, while the second implements the library that client applications use to interact with the dapp. Note that, while we have chosen Python for developing this dapp, our framework supports arbitrary programming languages. The mechanism used to this purpose is standard: the programmer codes the core features of dapp in her preferred language, and the framework uses them through the foreign function interface.

We start by creating an empty dapp with the empty template:

dappman -w

Dapp name: myfirstdapp
Description: This is my first dapp
Authors (comma separated): Davide Gessa
Select a template:
         0 . empty (https://github.com/contractvm/cvm-dapp-empty)
         1 . blockstore (https://github.com/contractvm/cvm-dapp-blockstore)
         2 . helloworld (https://github.com/contractvm/cvm-dapp-helloworld)
         3 . fifomom (https://github.com/contractvm/cvm-dapp-fifomom)
         4 . cotst (https://github.com/contractvm/cvm-dapp-cotst)
Template: 0
Creating directory for dapp: myfirstdapp
Downloading template: empty
Extracting template
Setting up directories
String replace for dapp name
Creating manifest.json
Dapp myfirstdapp sucessfully created

This command will create a new dapp tree:

myfirstdapp/
        requirements.txt
        setup.py
        README.md
        manifest.json
        dapp/
                myfirstdapp.py
                __init__.py
        library/
                myfirstdapp/
                        __init__.py
                        EmptyManager.py
        samples/
                config.py
                data

Now every time we want to install the updated dapp, we type:

dappman -i /path/of/your/dapp/myfirstdapp

Now we can write the parts of the dapp: