Connectivity Overview

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Integration

Ref: Enterprise Integration Patterns

Typical ways to integrate:

  1. File transfer - bulk transfer - infrequent changes
  2. Shared DB - data sharing
  3. RPC - functionality sharing
  4. Messaging - frequent small data exchange

What to use when

  • JDBC - connector for relational databases
  • Asynchronous messaging - loosely coupled integration - JMS or JCA
    • JMS also supports quality of service, higher throughput, guaranteed delivery
    • Asynchronous model more complex
  • Synchronous - tightly coupled integrations - JCA/RPC - SOAP/CORBA/RMI
    • JCA for high quality request processing and error handling
  • Legacy communication - JCA

Loosely coupled requirements

  • data encapsulation/encoding
  • allow transformations inside channel
  • queue requests - don't depend on availability
  • location - logical address - configurable outside code

RPC

Advantages

  • synchronous
  • data encapsulated

Disadvanates

  • tight coupling - parameters, availability
  • latency

Shared DB

Advantages of a shared DB

  • data always current
  • no need to resolve inconsistent data from different sources
  • enforced data quality - DB constraints

Disadvantages of a shared DB

  • tightly coupled
  • difficult to define a shared schema that works for all systems
  • semantic dissonance
  • performance bottleneck - all applications fight for one resource
  • distribution/location

JMS

Advantages of JMS

  • better encapsulation than shared DB
  • more reliable than RPC
    • automatic retries for sending and receiving messages
  • background processing - scalability
  • allows both sender and receiver to operate at maximum pace- variable timing
    • control over rate of processing independent on external load
  • ideal for disconnected systems - PDAs and laptops
  • mediator pattern - messaging system can provide high availability, load balancing, QoS, performance tuning
  • may be asynchronous or synchronous - request-reply
  • asynchronous - reduces impact of latency - remote calls
  • supports different topologies: broadcast/point to point/routing

Disadvantages of JMS

  • Difficult programming model to understand and debug
    • logic spread around event handlers
    • concurrency
    • may need to track context on callbacks
    • Out of sequence issues
  • Many systems require synchronous responses
  • Performance - overhead of sending large messages or lots of small messages
  • Availability
  • Vendor specific


Synchronous JMS

Typically done with a temporary queue

  • see QueueRequester/TopicRequester
  • problems
    • queue deleted when context goes - lost on crash (temporary queues and messages deleted on connection closing)
  • consider asynchronous callback instead

Role of the Resource Manager

http://java.sun.com/blueprints/guidelines/designing_enterprise_applications_2e/transactions/transactions9.html

Three types:

JDBC

  • Manage transactions across multiple components (possibly different tiers)
  • Multiple JDBC resources may not be portable

JMS

  • Manage transactions across multiple components (possibly different tiers)
  • Multiple JMS providers may not be portable
  • Messages not transmitted until transaction committed - transaction rollback = message discarded
  • Messages not consumed until transaction committed - transactions roll back = put back on queue
  • Sending and waiting for an acknowledgment can not be done within the same transcation

JCA

  • RA can choose level of transaction support
  1. NoTransaction
  2. LocalTransaction - only one resource can participate in transaction
  3. XATransaction ( must also support LocalTransaction) - Two phase commit - multiple resources can participate.