Discnt, in-memory, distributed counters

discnt.io

Discnt provides in memory distributed eventually consistent counters.

Setup

To play with Discnt please do the following:

1. Compile Discnt, if you can compile Redis, you can compile Discnt, it's the usual no external deps thing. Just type make. Binaries (discnt-cli and discnt-server) will end up in the src directory.
2. Run a few Discnt nodes in different ports. Create different discnt.conf files following the example discnt.conf in the source distribution.
3. After you have them running, you need to join the cluster. Just select a random node among the nodes you are running, and send the command CLUSTER MEET <ip> <port> for every other node in the cluster.

To run a node, just call ./discnt-server.

For example if you are running three Discnt servers in port 7711, 7712, 7713 in order to join the cluster you should use the discnt-cli command line tool and run the following commands:

./discnt-cli -p 7711 cluster meet 127.0.0.1 7712
./discnt-cli -p 7711 cluster meet 127.0.0.1 7713

Your cluster should now be ready. You can try to add a job and fetch it back in order to test if everything is working:

./discnt-cli -p 7711
127.0.0.1:7711> INCRBY test 0.1
0.1
127.0.0.1:7711> GET test
0.1

Remember that you can increment counters on different nodes as Discnt is multi master.

API

Discnt API is composed of a small set of commands, since the system solves a single very specific problem. The main commands are:

INCRBY counter_name increment

Increments the counter by the specified increment. If the counter does not exist, it is set to 0 before performing the operation. If the command is successful the new incremented value is stored and returned to the caller as a string. The precision of the output is fixed at 17 digits after the decimal point regardless of the actual internal precision of the computation.

SET counter_name value

Set a counter to the specified value.

A SET also resets the local prediction to 0 and broadcasts it.

GET counter_name [STATE]

Get the value of the counter. If the counter does not exist 0 is retured. The precision of the output is fixed at 17 digits after the decimal point regardless of the actual internal value. If the optional STATE is specified GET returns a multi reply containing the value and either the string CONSISTENT or INCONSISTENT depending on the cluster state.

PRECISION counter_name [value]

Set or get the precision for the local counter. See Predictions.

For more see discnt.io/api.html

Client libraries

Discnt uses the same protocol as Redis itself. To adapt Redis clients, or to use it directly, should be pretty easy. However note that Discnt default port is 5262 and not 6379.

FAQ

Is Discnt part of Redis?

No, it is a standalone project, however a big part of the Redis networking source code, nodes message bus, libraries, and the client protocol, were reused in this new project.

However while it is a separated project, conceptually Discnt is related to Redis, since it tries to solve a Redis use case in a vertical, ad-hoc way.

Who created Discnt?

Discnt is a side project of Erik Dubbelboer.

What does Discnt mean?

DIStributed CouNTers.