P2P Service Registries for Apache Slider¶
Alongside the centralized service registries, there's much prior work on P2P discovery systems, especially for mobile and consumer devices.
They perform some multicast- or distributed hash table-based lookup, and tend to have common limitations:
the bootstrapping problem
security: can you trust the results to be honest?
consistency: can you trust the results to be complete and current?
Bootstrapping is usually done via multicast, possibly then switching to unicast for better scale. As multicasting doesn't work in cloud infrastructures, none of the services work unmodified in public clouds. There's multiple anecdotes of Amazon's SimpleDB service being used as a registry for in-EC2 applications. At the very least, this service and its equivalents in other cloud providers could be used to bootstrap ZK client bindings in cloud environments.
Service Location Protocol¶
Service Location Protocol is a protocol for discovery services that came out of Sun, Novell and others -it is still available for printer discovery and suchlike
It supports both a multicast discovery mechanism, and a unicast protocol to talk to a Directory Agent -an agent that is itself discovered by multicast requests, or by listening for the agent's intermittent multicast announcements.
There's an extension to DHCP, RFC2610, which added the ability for DHCP to advertise Directory Agents -this was designed to solve the bootstrap problem (though not necessarily security or in-cloud deployment). Apart from a few mentions in Windows Server technical notes, it does not appear to exist.
[RFC2608] Service Location Protocol, Version 2 , IEEE, 1999
[RFC3224] Vendor Extensions for Service Location Protocol, Version 2, IETF, 2003
[RFC2610] DHCP Options for Service Location Protocol, IETF, 1999
The multicast discovery service implemented in Apple's Bonjour system --multicasting DNS lookups to all peers in the subnet.
This allows for URLs and hostnames to be dynamically positioned, with DNS domain searches allowing for enumeration of service groups.
This protocol scales very badly; the load on every client in the
subnet is is O(DNS-queries-across-subnet), hence implicitly
The special domains
_udp. and their subdomains can also be
served up via a normal DNS server.
Attribute-driven service enumeration, which drives the, Java-client-only model of downloading client-side code. There's no requirement for the remote services to be in Java, only that drivers are.
This is a library that implements the SWIM protocol to propagate information around a cluster. Apparently works in virtualized clusters too. It's already been used in a Flume-on-Hoya provider.
An HP Labs-built High Availability tuple-space in SmartFrog; used in production in some of HP's telco products. An agent publishes facts into the T-Space, and within one heartbeat all other agents have it. One heart-beat later, unless there's been a change in the membership, the publisher knows the others have it. One heartbeat later the agents know the publisher knows it, etc.
The shared knowledge mechanism permits reasoning and mathematical proofs.
Strict ordering between heartbeats implies an ordering in receipt. This is stronger than ZK's guarantees.
Lets you share a moderate amount of data (the longer the heartbeat interval, the more data you can publish).
Provided the JVM hosting the Anubis agent is also hosting the service, liveness is implicit
Secure to the extent that it can be locked down to allow only nodes with mutual trust of HTTPS certificates to join the tuple-space.
(Currently) bootstraps via multicast discovery.
Brittle to timing, especially on virtualized clusters where clocks are unpredictable.
It proved good for workload sharing -tasks can be published to it, any agent can say "I'm working on it" and take up the work. If the process fails, the task becomes available again. We used this for distributed scheduling in a rendering farm.
This was another HP Labs project, related to the Cooltown "ubiquitous computing" work, which was a decade too early to be relevant. It was also positioned by management as a B2B platform, so ended up competing with - and losing against - WS-* and UDDI.
Carmen aimed to provide service discovery with both fixed services, and with highly mobile client services that will roam around the network -they are assumed to be wireless devices.
Services were published with and searched for by attributed, locality was considered to be a key attribute -local instances of a service prioritized. Those services with a static location and low rate of change became the stable caches of service information -becoming, as with skype, "supernodes".
Bootstrapping the cluster relied on multicast, though alternatives based on DHCP and DNS were proposed.