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Basic Services: Architecture Options

Basic Services: Architecture Options. Vance Maverick ADAPT Bologna Feb. 13, 2003. Adaptability goals for BS. Adaptability to failure Fault tolerance Transparent failover Exactly-once semantics Adaptability to load Load distribution Support for CS load adaptability. Overview. J2EE

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Basic Services: Architecture Options

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  1. Basic Services:Architecture Options Vance MaverickADAPT BolognaFeb. 13, 2003

  2. Adaptability goals for BS • Adaptability to failure • Fault tolerance • Transparent failover • Exactly-once semantics • Adaptability to load • Load distribution • Support for CS load adaptability

  3. Overview • J2EE • Replication based on group communication • JBoss • Replication strategies • Maximalist and minimalist examples • Commonalities • Conclusion

  4. HTTPrequest SessionBean EntityBean EJBContainer WebContainer Servlet DatabaseLayer J2EE • Three layers: • web • business • database • Designed for distribution, not replication

  5. Containers and Components • J2EE developers create “components” • Web and EJB tiers are “containers” running components • Component: class or group of related classes • Container: “engine” or environment that instantiates and invokes the component

  6. Component state • EJB specifies serialization of bean state (for entity beans, only with CMP) • For HttpSession, apply standard Java serialization to contents

  7. J2EE: Transactions • Transaction manager: 2-phase coordinator • Governs database transaction through JDBC driver • One logical transaction associated with current thread • Association propagated with remote calls

  8. Transactional attributes • EJB “deployment descriptor”: transactional attributes per method • “Required”: if not present, EJB container must create one • “Mandatory”: association must be present in client (i.e. servlet)

  9. JBoss • Mature open-source implementation of J2EE • Replication since v3.0 • Passive • Component state transmitted in GC messages • HttpSession • SFSB instance data • Entity bean data • Transaction manager unaware of replication

  10. “Maximalist” proposal • Every request and response “logged” by multicast to all replicas • Not only HTTP req/resp but also any other component • Database component (above JDBC) • Periodic state updates • On failure, new replica replays from last state point • Transactions? Must be replicated

  11. “Minimalist” proposal • Use GC sparingly • All processing for a request in one transaction, on one replica • Multicast all state changes in a single message to all replicas • For exactly-once, includes request ID , full HTTP response • On failure, let client retry

  12. “Minimalist”: commit • J2EE transaction not replicated • Custom TX participant on local node sends message • First committer wins • Loser aborts transaction • Database-level locking useful but not required

  13. Common framework? • Intervenes in all request/response pairs, at both ends • including HTTP and database • Listens to transactions • Invokes “replication manager” embodying the replication policy • RM performs group communication, invokes components, sets sate • Dummy implementation: standard non-replicated behavior

  14. Conclusion • Research opportunities • Replication + transactions • Group communication support • Database (separate Madrid project) • Open questions • Choose replication strategy now,or explicitly allow for alternatives • Discussion

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