# Getting Started

# First Tendermint App

As a general purpose blockchain engine, Tendermint is agnostic to the application you want to run. So, to run a complete blockchain that does something useful, you must start two programs: one is Tendermint Core, the other is your application, which can be written in any programming language. Recall from the intro to ABCI that Tendermint Core handles all the p2p and consensus stuff, and just forwards transactions to the application when they need to be validated, or when they're ready to be committed to a block.

In this guide, we show you some examples of how to run an application using Tendermint.

# Install

The first apps we will work with are written in Go. To install them, you need to install Go and put $GOPATH/bin in your $PATH; see here for more info.

Then run

go get github.com/tendermint/tendermint
cd $GOPATH/src/github.com/tendermint/tendermint
make tools
make install_abci

Now you should have the abci-cli installed; you'll see a couple of commands (counter and kvstore) that are example applications written in Go. See below for an application written in JavaScript.

Now, let's run some apps!

# KVStore - A First Example

The kvstore app is a Merkle tree that just stores all transactions. If the transaction contains an =, e.g. key=value, then the value is stored under the key in the Merkle tree. Otherwise, the full transaction bytes are stored as the key and the value.

Let's start a kvstore application.

abci-cli kvstore

In another terminal, we can start Tendermint. If you have never run Tendermint before, use:

tendermint init
tendermint node

If you have used Tendermint, you may want to reset the data for a new blockchain by running tendermint unsafe_reset_all. Then you can run tendermint node to start Tendermint, and connect to the app. For more details, see the guide on using Tendermint.

You should see Tendermint making blocks! We can get the status of our Tendermint node as follows:

curl -s localhost:26657/status

The -s just silences curl. For nicer output, pipe the result into a tool like jq or json_pp.

Now let's send some transactions to the kvstore.

curl -s 'localhost:26657/broadcast_tx_commit?tx="abcd"'

Note the single quote (') around the url, which ensures that the double quotes (") are not escaped by bash. This command sent a transaction with bytes abcd, so abcd will be stored as both the key and the value in the Merkle tree. The response should look something like:

{
  "jsonrpc": "2.0",
  "id": "",
  "result": {
    "check_tx": {},
    "deliver_tx": {
      "tags": [
        {
          "key": "YXBwLmNyZWF0b3I=",
          "value": "amFl"
        },
        {
          "key": "YXBwLmtleQ==",
          "value": "YWJjZA=="
        }
      ]
    },
    "hash": "9DF66553F98DE3C26E3C3317A3E4CED54F714E39",
    "height": 14
  }
}

We can confirm that our transaction worked and the value got stored by querying the app:

curl -s 'localhost:26657/abci_query?data="abcd"'

The result should look like:

{
  "jsonrpc": "2.0",
  "id": "",
  "result": {
    "response": {
      "log": "exists",
      "index": "-1",
      "key": "YWJjZA==",
      "value": "YWJjZA=="
    }
  }
}

Note the value in the result (YWJjZA==); this is the base64-encoding of the ASCII of abcd. You can verify this in a python 2 shell by running "YWJjZA==".decode('base64') or in python 3 shell by running import codecs; codecs.decode(b"YWJjZA==", 'base64').decode('ascii'). Stay tuned for a future release that makes this output more human-readable.

Now let's try setting a different key and value:

curl -s 'localhost:26657/broadcast_tx_commit?tx="name=satoshi"'

Now if we query for name, we should get satoshi, or c2F0b3NoaQ== in base64:

curl -s 'localhost:26657/abci_query?data="name"'

Try some other transactions and queries to make sure everything is working!

# Counter - Another Example

Now that we've got the hang of it, let's try another application, the counter app.

The counter app doesn't use a Merkle tree, it just counts how many times we've sent a transaction, or committed the state.

This application has two modes: serial=off and serial=on.

When serial=on, transactions must be a big-endian encoded incrementing integer, starting at 0.

If serial=off, there are no restrictions on transactions.

In a live blockchain, transactions collect in memory before they are committed into blocks. To avoid wasting resources on invalid transactions, ABCI provides the CheckTx message, which application developers can use to accept or reject transactions, before they are stored in memory or gossipped to other peers.

In this instance of the counter app, with serial=on, CheckTx only allows transactions whose integer is greater than the last committed one.

Let's kill the previous instance of tendermint and the kvstore application, and start the counter app. We can enable serial=on with a flag:

abci-cli counter --serial

In another window, reset then start Tendermint:

tendermint unsafe_reset_all
tendermint node

Once again, you can see the blocks streaming by. Let's send some transactions. Since we have set serial=on, the first transaction must be the number 0:

curl localhost:26657/broadcast_tx_commit?tx=0x00

Note the empty (hence successful) response. The next transaction must be the number 1. If instead, we try to send a 5, we get an error:

> curl localhost:26657/broadcast_tx_commit?tx=0x05
{
  "jsonrpc": "2.0",
  "id": "",
  "result": {
    "check_tx": {},
    "deliver_tx": {
      "code": 2,
      "log": "Invalid nonce. Expected 1, got 5"
    },
    "hash": "33B93DFF98749B0D6996A70F64071347060DC19C",
    "height": 34
  }
}

But if we send a 1, it works again:

> curl localhost:26657/broadcast_tx_commit?tx=0x01
{
  "jsonrpc": "2.0",
  "id": "",
  "result": {
    "check_tx": {},
    "deliver_tx": {},
    "hash": "F17854A977F6FA7EEA1BD758E296710B86F72F3D",
    "height": 60
  }
}

For more details on the broadcast_tx API, see the guide on using Tendermint.

# CounterJS - Example in Another Language

We also want to run applications in another language - in this case, we'll run a Javascript version of the counter. To run it, you'll need to install node.

You'll also need to fetch the relevant repository, from here, then install it:

git clone https://github.com/tendermint/js-abci.git
cd js-abci
npm install abci

Kill the previous counter and tendermint processes. Now run the app:

node example/counter.js

In another window, reset and start tendermint:

tendermint unsafe_reset_all
tendermint node

Once again, you should see blocks streaming by - but now, our application is written in Javascript! Try sending some transactions, and like before - the results should be the same:

# ok
curl localhost:26657/broadcast_tx_commit?tx=0x00
# invalid nonce
curl localhost:26657/broadcast_tx_commit?tx=0x05
# ok
curl localhost:26657/broadcast_tx_commit?tx=0x01

Neat, eh?