The document describes the JActor Cluster Platform, which allows distributed, robust Java applications across multiple nodes. It discovers nodes, prevents duplicate clusters, manages servers across nodes, and provides tools for monitoring and configuration. Developers can port applications by writing an adapter class. The platform runs by setting the CLASSPATH and executing the main class, and includes commands for management.

1. Distributed, Robust Software
Made Easy
Introducing the
JActor Cluster Platform
Bill La Forge, CTO
JActor Consulting Pvt. Ltd
laforge49@gmail.com
http://jactorconsulting.com

2. The Jactor Cluster Platform
● Discovers new nodes (JVMs) running in the cluster
and detects when a node is no longer active.
● Prevents more than one cluster from being active due
to a network failure by requiring a quorum of hosts to
be present.
● Creates and manages Server objects across multiple
nodes.
● Determines the least busy node in the cluster or on a
given host for running a Server.
● Provides basic tools for monitoring and configuring the
cluster.

3. The Jactor Cluster is a
Platform
● To port a Java application to the JActor Cluster
Platform, you need to write an adapter class, a
subclass of Server, for starting and stopping
your application and which provides access, as
needed, to other servers running in the cluster.
● The Apache SSHD Project has already been
ported to the JActor Cluster Platform to provide
remote console access.
● The next port will likely be OSGi.

4. Running the JActor Cluster
Platform
● Just set the CLASSPATH and run the main function of
agilewiki.jaconfig.JACNode on all the hosts of the
cluster.
● There is a single, optional argument: the port number used
to communicate with other nodes in the cluster. It has a
default value of 8880.
● If no port number is specified, an operator console is also
started up.
● To run a server, its class file or a jar containing it must be
included in the CLASSPATH.
● To change the basic configuration, simply subclass
JACNode and override the appropriate methods.

5. The Config Database
● The config database holds key/value pairs of strings.
● This is a fully replicated, eventually consistent
database (NOSQL). Every node in the cluster has its
own copy.
● There is no log file, and no operational complications.
If a database copy gets corrupted, it gets rebuilt when
the node is restarted.
● The database holds the cluster configuration, as well
as encrypted operator passwords.
● Application servers may subscribe to database change
notifications.

6. Cluster Commands (1)
● help lists the cluster commands.
● halt exits the local node.
● who lists the operators that are logged in
across the cluster.
● broadcast sends a message to all users.
● write {name} {msg} sends the msg to an
operator.

7. Cluster Commands (2)
● localServers lists all the local servers.
● startup {server class name} {args string}
starts a local server.
● server {server name} {command line} sends
a command to a local server.
● server {server name} help lists the commands
of a server.

8. Cluster Commands (3)
● channels lists the addresses of the other
nodes in the cluster.
● heap lists the memory usage of all the nodes in
the cluster.
● servers lists all the servers running in the
cluster.
● to {node address} {command line} sends a
command to another node.

9. Config Commands
● server config values lists all the assignments.
● server config assign {name} {value} assigns a value
to a name.
● server config assign {name} removes an
assignment.
● server config changePassword changes your
password.
● server config setPassword {operator name}
assigns the operator's password.
● server config clearPassword {operator name}
removes the operator's password.

10. Quorum
● A cluster is not active unless it has a quorum of hosts.
This prevents multiple clusters from running with
inconsistent state due to a networking failure.
● The value quorum.totalHostCount must be set in the
config database. The quorum is calculated as
totalHostCount / 2 + 1.
● Example: server config assign
quorum.totalHostCount 5 requires a quorum of 3.
● With a totalHostCount of 5, the cluster will remain
active even if 2 hosts are down.

11. Cluster Manager Server
● The clusterManager server runs on only one node in
the cluster, and only when the server is active.
● The clusterManager is used to startup servers which
subclass ClusterServer.
● The config database is used to drive clusterManager.
It looks for assignments of the form clusterManager.
{server name} = {server class name} {args string}
● Servers started by clusterManager are restarted when
the node where they are running goes down.

12. Host Manager Server
● A hostManager server runs on one node on
every host in the cluster, even if the cluster is
not active.
● The hostManager is used to startup servers
which subclass HostServer. These servers are
run on every host in the cluster.
● The config database is used to drive the
hostManager servers, which look for
assignments of the form hostManager.{server
name} = {server class name} {args string}

13. SSH Server
● The ssh server integrates Apache SSHD and supports
operator login via an ssh client like PuTTY.
● To have the ssh run on every host in the cluster, just
enter this command: server config assign
hostManager.ssh
org.agilewiki.config.SSHHostServer {ssh port
number}
● To list all the ssh servers in the cluster, enter this
command: server hostManager.ssh all
● The all command lists the port number and the
number of operators using each ssh server.