Redis sentinel monitor assignments

redis_sentinel.py

# A proposed method for choosing which slaves to monitor within the
# Redis HA sentinel monitors

def select_slaves_to_monitor(slave_info, all_sentinels, my_id, my_slaves):
    '''
    slave_info = [
        ('ip_or_host:port', ['sentinel-id, ...]),
        ...
    ]
    
    all_sentinels = ['sentinel-id', ...]
    '''

    # sorting used as an precondition for the sentinel choice algorithm
    slave_info.sort()
    all_sentinels.sort()
    slaves = 2 * zip(*slave_info)[0]

    # each sentinel will monitor 'count' slaves, and the first 'extra'
    # sentinels in the all_sentinels list will monitor 1 additional slave
    count, extra = divmod(len(slaves), len(all_sentinels))

    # we don't need to generate the full assignment list, but we do here for
    # the sake of understanding what is going on
    assignments = {}
    index = 0
    for spos, sentinel in enumerate(all_sentinels):
        scount = count + (spos < extra)
        assignments[sentinel] = set(slaves[index:index+scount])
        index += scount

    # a few small adjustments to the data to make access fast
    my_assignment = assignments[my_id]
    current_slaves = dict(slave_info)
    my_slaves = set(my_slaves)

    # find slaves that we need to stop monitoring
    to_disconnect = []
    for slave in my_slaves - my_assignment:
        # we need to wait to disconnect until there are at least 2 monitoring
        # sentinels
        if len(current_slaves[slave]) > 2:
            to_disconnect.append(slave)

    # find slaves that we are supposed to monitor, but have not yet monitored
    to_connect = list(my_assignment - my_slaves)

    return to_connect, to_disconnect