考虑使用redis来保持多个casbin执行器实例之间的一致性。
可以直接借助redis的消息发布订阅机制，实现各个节点之间策略数据的增量同步
使用数据库，表名为casbin_rule
代码实现的是WatcherEx接口，与watch接口相比，可以区分更新的动作类型，如：更新策略，删除策略
// WatcherEx is the strengthened Casbin watchers.
type WatcherEx interface {
Watcher
// UpdateForAddPolicy calls the update callback of other instances to synchronize their policy.
// It is called after Enforcer.AddPolicy()
UpdateForAddPolicy(sec, ptype string, params ...string) error
// UpdateForRemovePolicy calls the update callback of other instances to synchronize their policy.
// It is called after Enforcer.RemovePolicy()
UpdateForRemovePolicy(sec, ptype string, params ...string) error
// UpdateForRemoveFilteredPolicy calls the update callback of other instances to synchronize their policy.
// It is called after Enforcer.RemoveFilteredNamedGroupingPolicy()
UpdateForRemoveFilteredPolicy(sec, ptype string, fieldIndex int, fieldValues ...string) error
// UpdateForSavePolicy calls the update callback of other instances to synchronize their policy.
// It is called after Enforcer.RemoveFilteredNamedGroupingPolicy()
UpdateForSavePolicy(model model.Model) error
// UpdateForAddPolicies calls the update callback of other instances to synchronize their policy.
// It is called after Enforcer.AddPolicies()
UpdateForAddPolicies(sec string, ptype string, rules ...[]string) error
// UpdateForRemovePolicies calls the update callback of other instances to synchronize their policy.
// It is called after Enforcer.RemovePolicies()
UpdateForRemovePolicies(sec string, ptype string, rules ...[]string) error
}
Redis 的 SUBSCRIBE 命令可以让客户端订阅任意数量的频道， 每当有新信息发送到被订阅的频道时， 信息就会被发送给所有订阅指定频道的客户端。
三个客户端client1、client2、client5订阅了channel1频道，到有新消息是通过Publish命令发送给频道channel1
