Multi-Paxos with riak_ensemble Part 3

In the previous post I showed how to use riak_ensemble in a rebar3 project, now I will show how to create an HTTP API for the Key/Value store using Cowboy and jsone.

This post assumes that you have erlang and rebar3 installed, I'm using erlang 19.3 and rebar3 3.4.3.

The source code for this post is at https://github.com/marianoguerra/cadena check the commits for the steps.

Dependency Setup

To have an HTTP API we will need an HTTP server, in our case we will use Cowboy 2.0 RC 3, for that we need to:

  1. Add it as a dependency (we will load if from git since it's still a release candidate)
  2. Add it to our list of applications to start when our application starts
  3. Add it to the list of dependencies to include in our release
  4. Set up the HTTP listener and routes when our application starts

We setup just one route that is handled by the cadena_h_keys module, it's a plain HTTP handler, no fancy REST stuff for now, there we handle the request on the init/2 function itself, we pattern match against the method field on the request object and handle:

POST
set a key in a given ensemble to the value sent in the JSON request body
GET
get a key in a given ensemble, if not found null will be returned in the value field in the response
DELETE
delete a key in a given ensemble, returns null both if the key existed and if itdidn't

Any other method would get a 405 Method Not Allowed response.

The route has the format /keys/<ensemble>/<key>, for now we only allow the root ensemble to be set in the <ensemble> part of the path.

We also add the jsone library to encode/decode JSON and the lager library to log messages.

We add both to the list of dependencies to include in the release.

We will also need to have a way to override the HTTP port where each instance listens to so we can run a cluster on one computer and each node can listen for HTTP requests on a different port.

The dev and prod releases will listen on 8080 as specified in vars.config.

node1 will listen on port 8081 (override in vars_node1.config)

node2 will listen on port 8082 (override in vars_node2.config)

node3 will listen on port 8083 (override in vars_node3.config)

To avoid having to configure this in sys.config we will define a cuttlefish schema in config.schema that cuttlefish will use to generate a default config file and validation code for us.

We have to replace the variables from variable overrides in our config.schema file for each release before it's processed by cuttlefish itself, for that we use the template directive on an overlay section on the release config.

Build devrel:

make revrel

Check the configuration file generated for each node at:

_build/node1/rel/cadena/etc/cadena.conf
_build/node2/rel/cadena/etc/cadena.conf
_build/node3/rel/cadena/etc/cadena.conf

The first part is of interest to us, it looks like this for node1, the port number is different in node2 and node3:

## port to listen to for HTTP API
##
## Default: 8081
##
## Acceptable values:
##   - an integer
http.port = 8081

## number of acceptors to user for HTTP API
##
## Default: 100
##
## Acceptable values:
##   - an integer
http.acceptors = 100

## folder where ensemble data is stored
##
## Default: ./cadena_data
##
## Acceptable values:
##   - text
data.dir = ./cadena_data

Start 3 nodes in 3 different shells:

make node1-console
make node2-console
make node3-console

Start enseble and join nodes, I created a target called devrel-setup in the Makefile to make it easier:

make devrel-setup

Let's set key1 in ensemble root to 42 on node1 (port 8081):

curl -X POST http://localhost:8081/keys/root/key1 -d 42

Response:

{"data":{"epoch":2,"key":"key1","seq":10,"value":42},"ok":true}

Let's get key1 in ensemble root to 42 on node2 (port 8082):

curl -X GET http://localhost:8082/keys/root/key1

Response:

{"data":{"epoch":2,"key":"key1","seq":10,"value":42},"ok":true}

Same on node3:

curl -X GET http://localhost:8083/keys/root/key1

Response:

{"data":{"epoch":2,"key":"key1","seq":10,"value":42},"ok":true}

Overwrite on node1:

curl -X POST http://localhost:8081/keys/root/key1 -d '{"number": 42}'

Response:

{"data":{"epoch":2,"key":"key1","seq":400,"value":{"number":42}},"ok":true}

Get on node2:

curl -X GET http://localhost:8082/keys/root/key2
{"data":{"epoch":3,"key":"key2","seq":11,"value":null},"ok":true}

Let's set key2 in ensemble root to {"number": 42} on node1 (port 8081):

curl -X POST http://localhost:8081/keys/root/key2 -d '{"number": 42}'

Response:

{"data":{"epoch":3,"key":"key2","seq":67,"value":{"number":42}},"ok":true}

Get it on node2:

curl -X GET http://localhost:8082/keys/root/key2

Response:

{"data":{"epoch":3,"key":"key2","seq":67,"value":{"number":42}},"ok":true}

Delete key2 in ensemble root on node2:

curl -X DELETE http://localhost:8082/keys/root/key2

Response:

{"data":{"epoch":3,"key":"key2","seq":137,"value":null},"ok":true}

Check that it was removed by trying to get it again on node2:

curl -X GET http://localhost:8082/keys/root/key2

Response:

{"data":{"epoch":3,"key":"key2","seq":137,"value":null},"ok":true}

There you go, now you have a Consistent Key Value Store with an HTTP API.

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