Clojure Micro-Service framework based on Slacker RPC
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Slacker Cluster


This is the cluster module for slacker, a clojure RPC framework.

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Base on service discovery by ZooKeeper, slacker has a solution for high availability and load balancing. You can have several slacker servers in a cluster serving namespaces of functions. The cluster-enabled slacker client will choose one (or several, based on your grouping function) of these servers to call. Once a server is added to or removed from the cluster, the client will receive a notification from zookeeper and establish/destroy the connection to the server.

To create such a slacker cluster, you have to deploy at least one zookeeper instance in your topology.


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Cluster Enabled Slacker Server

On the server side, using the server starter function from slacker.server.cluster, add an option :cluster and provide your cluster information.

(use 'slacker.server.cluster)
(start-slacker-server ...
                      :cluster {:name "cluster-name"
                                :zk ""})

Cluster information here:

  • :name the cluster name (required)
  • :zk zookeeper server address (required)
  • :node server IP (optional, if you don't provide the server IP here, slacker will try to detect server IP by connecting to zk, on which we assume that your slacker server are bound on the same network with zookeeper)
  • :server-data allows you to set some small amount, server specific data that will be available to client grouping function to choose server.

Cluster Enabled Slacker Client

On the client side, you have to specify the zookeeper address instead of a particular slacker server. Use the clustered-slackerc:

(use 'slacker.client.cluster)
(def sc (clustered-slackerc "cluster-name" ""))
(use-remote 'sc 'slapi)

Important: You should make sure to use the use-remote and defn-remote from slacker.client.cluster instead of slacker.client.

Client Grouping

New in 0.10.1. By default, slacker cluster client randomly pick a server to run your invocation. But at some situation, you may like to override this behavior, for example, run a function on all servers.

The grouping option gives you total control of this behavior.

(defn my-grouping [ns-name fn-name params slacker-client servers]

(clustered-slackerc "cluster-name" "" :grouping my-grouping)

The first three arguments represent your current invocation. And you can return one or more servers based on the information and your business logic. Server specific data is available from (server-data client "server-addr").

You can also return constant value:

  • :all call on all servers available
  • :random pick a random server to call
  • :first always call the first server in client cached server list
  • :leader always pick the leader node in cluster (new in 0.12)
  • :least-in-flight call the server with least pending requests

Grouping results

When you are using :grouping function, you might get multiple return values from different servers. A new option :grouping-results is to define how many values to return for the call. Possible values are:

  • :single default, only return a single value, behavior same as the function call
  • :vector returns values from different servers as a vector
  • :map returns values from different servers as a map, with server's host:port as key
  • :nil always ignore the result and return nil
  • (fn [ns-name fn-name params]) a function that returns values above. You can use different grouping-results policy according to function you call.

Note that :vector and :map would break default behavior of the function call, and change return type.

Grouping exceptions

You can decide whether the client will throw an exception when some of remote functions throwing exceptions.

  • :all default, only throw exception when all remote functions return error
  • :any will throw exception when any remote server caused an error

If you choose :all but some of servers failed to return, we will only return results from servers succeeded.

Grouping at function level

You can also define :grouping and :grouping-results function at defn-remote level.

(defn-remote sc slacker.example.api/timestamp
  :grouping :all
  :grouping-results :single)

If there is no server in the cluster provides the function, you can add :unavailable-value option on defn-remote for return value on this situation.

Server Manager

When the server started with option :manager true, a management API is exposed as a namespace: slacker.cluster.manager.

There are functions under this namespace:

  • (offline) put this server to offline mode then no more requests will be sent to it. The server is still running and you can call it from address.
  • (offline-ns nsname) put a namespace on this server to offline mode.
  • (online) put the server to online mode.
  • (online-ns nsname) put a namespace on this server to online mode.
  • (set-server-data! data) set the data associated with this server.
  • (server-data) get the data with this server.
  • (shutdown) shutdown the server completely.


There is a cluster example in the source code. To run the server, start a zookeeper on your machine ( We have an example zookeeper server you can start with:

lein run-example-zk

Start server instance:

lein run-example-server 2104

Open a new terminal, start another server instance:

lein run-example-server 2105

On another terminal, you can run the example client:

lein run-example-client

By checking logs, you can trace the calls on each server instance.

API doc

API doc is generated against latest master branch, with lein-codox with Travis CI.


Copyright (C) 2011-2016 Sun Ning

Distributed under the Eclipse Public License, the same as Clojure.