mlk

from peak.events.trellis import *

##
## Example #2: Observing and managing server instances in a cluster
##

class ServerInstance(Component):
    started = attr(False)
    cpu_load = attr(0.0) # 100% is 1.0


class ServerCluster(Component):
    servers = make(Set)

    @compute
    def avg_load(self):
        if not self.servers:
            return 1.0
        return sum(s.cpu_load for s in self.servers) / len(self.servers)


    max_load = attr(0.9)

    @compute
    def need_more_servers(self):
        # we use `not servers.added` to avoid CircularityError due to avg_load recalculation
        # after pending server has started
        return (self.avg_load > self.max_load) and not self.servers.added

    @maintain
    def autostart_servers(self):
        if self.need_more_servers and self.pending_server is None:
            self.start_new_server()


    pending_server = attr()

    @modifier
    def start_new_server(self):
        assert not self.pending_server
        self.pending_server = ServerInstance()

    @maintain
    def on_started(self):
        if self.pending_server and self.pending_server.started:
            self.servers.add(self.pending_server)
            self.pending_server =  None


class ClusterReporter(Component):
    cluster = attr()

    def __init__(self, cluster):
        self.cluster = cluster

    @perform
    def watch_server_count(self):
        print 'Servers: %d; Average load: %d%%; Pending: %s' % (
                                        len(self.cluster.servers),
                                        100 * self.cluster.avg_load,
                                        'YES' if self.cluster.pending_server else 'NO')


##########################################
##########################################


cl = ServerCluster()
rep = ClusterReporter(cl)
# Servers: 0; Average load: 100%; Pending: YES

s1 = cl.pending_server
s1.started = True
# Servers: 1; Average load: 0%; Pending: NO

s1.cpu_load = 1.0
# Servers: 1; Average load: 100%; Pending: YES

s2 = cl.pending_server
s2.started = True
# Servers: 2; Average load: 50%; Pending: NO

s2.cpu_load = 0.5
# Servers: 2; Average load: 75%; Pending: NO

s2.cpu_load = 0.9
# Servers: 2; Average load: 95%; Pending: YES