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 <introduction.html#ABCI-Overview>`__ 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 <https://golang.org/doc/install>`__ and put
``$GOPATH/bin`` in your
``$PATH``; see `here <https://github.com/tendermint/tendermint/wiki/Setting-GOPATH>`__ for more info.

Then run

::

    go get -u github.com/tendermint/abci/cmd/abci-cli

If there is an error, install and run the ``glide`` tool to pin the
dependencies:

::

    go get github.com/Masterminds/glide
    cd $GOPATH/src/github.com/tendermint/abci
    glide install
    go install ./cmd/abci-cli

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 <https://en.wikipedia.org/wiki/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 <./using-tendermint.html>`__.

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

::

    curl -s localhost:46657/status

The ``-s`` just silences ``curl``. For nicer output, pipe the result
into a tool like `jq <https://stedolan.github.io/jq/>`__ or
`jsonpp <https://github.com/jmhodges/jsonpp>`__.

Now let's send some transactions to the kvstore.

::

    curl -s 'localhost:46657/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": {
          "code": 0,
          "data": "",
          "log": ""
        },
        "deliver_tx": {
          "code": 0,
          "data": "",
          "log": ""
        },
        "hash": "2B8EC32BA2579B3B8606E42C06DE2F7AFA2556EF",
        "height": 154
      }
    }

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

::

    curl -s 'localhost:46657/abci_query?data="abcd"'

The result should look like:

::

    {
      "jsonrpc": "2.0",
      "id": "",
      "result": {
        "response": {
          "code": 0,
          "index": 0,
          "key": "",
          "value": "61626364",
          "proof": "",
          "height": 0,
          "log": "exists"
        }
      }
    }

Note the ``value`` in the result (``61626364``); this is the
hex-encoding of the ASCII of ``abcd``. You can verify this in
a python 2 shell by running ``"61626364".decode('hex')`` or in python 3 shell by running ``import codecs; codecs.decode("61626364", 'hex').decode('ascii')``. Stay
tuned for a future release that `makes this output more human-readable <https://github.com/tendermint/abci/issues/32>`__.

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

::

    curl -s 'localhost:46657/broadcast_tx_commit?tx="name=satoshi"'

Now if we query for ``name``, we should get ``satoshi``, or
``7361746F736869`` in hex:

::

    curl -s 'localhost:46657/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:46657/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:46657/broadcast_tx_commit?tx=0x05
    {
      "jsonrpc": "2.0",
      "id": "",
      "result": {
        "check_tx": {
          "code": 0,
          "data": "",
          "log": ""
        },
        "deliver_tx": {
          "code": 3,
          "data": "",
          "log": "Invalid nonce. Expected 1, got 5"
        },
        "hash": "33B93DFF98749B0D6996A70F64071347060DC19C",
        "height": 38
      }
    }

But if we send a ``1``, it works again:

::

    > curl localhost:46657/broadcast_tx_commit?tx=0x01
    {
      "jsonrpc": "2.0",
      "id": "",
      "result": {
        "check_tx": {
          "code": 0,
          "data": "",
          "log": ""
        },
        "deliver_tx": {
          "code": 0,
          "data": "",
          "log": ""
        },
        "hash": "F17854A977F6FA7EEA1BD758E296710B86F72F3D",
        "height": 87
      }
    }

For more details on the ``broadcast_tx`` API, see `the guide on using
Tendermint <./using-tendermint.html>`__.

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 <https://nodejs.org/en/download/>`__.

You'll also need to fetch the relevant repository, from `here <https://github.com/tendermint/js-abci>`__ then install it. As go devs, we
keep all our code under the ``$GOPATH``, so run:

::

    go get github.com/tendermint/js-abci &> /dev/null
    cd $GOPATH/src/github.com/tendermint/js-abci/example
    npm install
    cd ..

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

::

    node example/app.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:

::

    curl localhost:46657/broadcast_tx_commit?tx=0x00 # ok
    curl localhost:46657/broadcast_tx_commit?tx=0x05 # invalid nonce
    curl localhost:46657/broadcast_tx_commit?tx=0x01 # ok

Neat, eh?

Basecoin - A More Interesting Example
-------------------------------------

We saved the best for last; the `Cosmos SDK <https://github.com/cosmos/cosmos-sdk>`__ is a general purpose framework for building cryptocurrencies. Unlike the ``kvstore`` and ``counter``, which are strictly for example purposes. The reference implementation of Cosmos SDK is ``basecoin``, which demonstrates how to use the building blocks of the Cosmos SDK.

The default ``basecoin`` application is a multi-asset cryptocurrency
that supports inter-blockchain communication (IBC). For more details on how
basecoin works and how to use it, see our `basecoin
guide <http://cosmos-sdk.readthedocs.io/en/latest/basecoin-basics.html>`__

In this tutorial you learned how to run applications using Tendermint
on a single node. You saw how applications could be written in different
languages, and how to send transactions and query for the latest state.
But the true power of Tendermint comes from its ability to securely and
efficiently run an application across a distributed network of nodes,
while keeping them all in sync using its state-of-the-art consensus
protocol. Next, we show you how to deploy Tendermint testnets.
