Dike
Dike is a framework based on paxos for implementing a distributed, data-oriented application logic which enforces consistency given some requirements.
Installation
For building Dike at least the following erlang applications and their dependencies are required:
- lager
- regine
- tetrapak
Compilation of dike:
rebar get-deps && rebar compile
Unit-tests
For running common tests, you need tetrapak
tetrapak test
Using dike
When using Dike you are basically implementing the paxos_server behaviour. It is similar to gen_server or gen_fsm.
However, the return value of the message handling callback is a bit different from the gen_ modules. Instead of returning a reply message, an anonymous function with no argument is to be returned. Dike will call this function once per request that is processed by the application logic.
The following two points must be considered when implementing a paxos_server:
- The changes done by the Paxos server implementation to its state must only depend on the request and the current state of the application logic. External data like timestamps must be included in the requests.
- Sideeffects, like sending a message, must be wrapped in the returned function to make sure they are executed only once.
export_state/1 and init/2 are used for serializing/desirializing of the paxos server's state. An example implementation can be found in src/arithmetic_paxos.erl.
When the paxos server should be started, first run:
dike_test:local_init(), dike:start().
Starting multinode setup on a one machine:
dike_test:nodes_dike_init("my_node_namespace", 5).
You will need to make sure 5 nodes are started with the same master list (containing these 5 nodes) which is configured via the masters key in the dike application. Dike supports a single node mode now, for using only put the single node in the masters list.
Check if Dike works and which groups are currently running in it by executing:
dike_dispatcher:get_routing_table().
dike_dispatcher:refresh_routing_table().
adding groups (sets up paxos_server module under a specific name):
dike_master:add_group(GroupName, PaxosServerModule).
adding nodes (node is registered at dike_master which may start group instances on the added node):
dike_master:join(Node).
Sending a message to a started paxos_server (for this to work dike needs to be started on the requesting node but the node may not have joined):
dike_dispatcher:request(GroupName, MSG).
Failure tolerance on 5 node setup
Dike guarantees for the system to be operational with up to two failing nodes, each group's state is replicated five times.
To achieve this Dike uses Paxos.