1. Write a simple contract

Let's write a simple example that implements a counter. Create a file named counter.go and type in the following content

package main
​
import (
    "github.com/orbs-network/orbs-contract-sdk/go/sdk/v1"
    "github.com/orbs-network/orbs-contract-sdk/go/sdk/v1/state"
)
​
var PUBLIC = sdk.Export(add, get)
var SYSTEM = sdk.Export(_init)
​
var COUNTER_KEY = []byte("count")
​
func _init() {
    state.WriteUint64(COUNTER_KEY, 0)
}
​
func add(amount uint64) {
    count := state.ReadUint64(COUNTER_KEY)
    count += amount
    state.WriteUint64(COUNTER_KEY, count)
}
​
func get() uint64 {
    return state.ReadUint64(COUNTER_KEY)
}

Don't worry if you don't fully understand the code at this point.

2. Start Gamma server

We'll test our contract on Gamma server. Start it from terminal

gamma-cli start-local

You can keep an eye on Prism (http://localhost:3000/) to see new blocks and transactions apear as you continue with the deployment in the next steps.

3. Deploy the contract

To deploy the counter contract, run in terminal

gamma-cli deploy counter.go -name MyCounter

If the deploy is successful, you'll see a response similar to this

{
  "RequestStatus": "COMPLETED",
  "TxId": "7Y4urVmKvunYsxh7kKhUoQ72XjSJcdkBxxzBcauC9icC9gzMy8mPDcg",
  "ExecutionResult": "SUCCESS",
  "OutputArguments": [],
  "TransactionStatus": "COMMITTED",
  "BlockHeight": "1869",
  "BlockTimestamp": "2018-12-05T13:05:51.347Z"
}

4. Send a transaction to increment the counter

Write the transaction details in a JSON file named add-25.json

{
  "ContractName": "MyCounter",
  "MethodName": "add", 
  "Arguments": [
    {
      "Type": "uint64",
      "Value": "25"
    }
  ]
}

More Information about sending transactions and the Gamma-cli json files structures can be found at the gamma in depth chapter of this documentation

To increment the counter by 75, let's send this transaction 3 times from terminal

gamma-cli send-tx add-25.json -signer user1
gamma-cli send-tx add-25.json -signer user1
gamma-cli send-tx add-25.json -signer user1

Note that the transaction will be signed by user1, an example account found in orbs-test-keys.json

5. Read the counter value

Write the query details in a JSON file named get.json

{
  "ContractName": "MyCounter",
  "MethodName": "get",
  "Arguments": []
}

This query will read the counter value from the contract's state. Send it from terminal

gamma-cli run-query get.json

Note that transactions that change state require consensus by several nodes. Reading state with queries is a simpler action that doesn't require consensus.

6. Stop Gamma server

Since we're done testing, the server is no longer needed. Let's stop it from terminal

gamma-cli stop-local