Is High Scalability SOA an Oxymoron?

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All too many Service Oriented Architecture (SOA) practitioners seem to have a belief, that because SOA deals with distributed computing, that scalability is a given. The reality however is that conventional SOA practices tend to work against the development of high scalability applications. This article shows the properties of a system that can achieve high scalability and then contrasts it with conventional SOA practices.

The patterns found in a system that exhibits high scalability are the following:

  • State Routing
  • Behavior Routing
  • Behavior Partitioning
  • State Partitioning
  • State Replication
  • State Coordination
  • Behavior Coordination
  • Messaging

This has been discussed in a previous blog entry “A Design Pattern for High Scalability“. SOA based systems conventionally cover Routing, Coordination and Messaging. However, the patterns of Partitioning and Replication are inadequately addressed by SOA systems. For reference, one can refer to the SOA Patterns book that I’ve covered in this review. The words “Partitioning” and “Replication” unsurprisingly can’t be found in the book’s index. Scalability apparently isn’t a concern to be addressed by SOA patterns.

What then are the patterns that we can introduce to SOA to ensure scalability? Here are a couple of suggested patterns from the previous article:

  • Behavior Partitioning
    • Loop Parallelism
    • Fork/Join
    • Map/Reduce
    • Round Robin Allocation
    • Random Allocation
    • Weighted Allocation
  • State Partitioning (Favors Latency)
    • Distributed Caching
    • HTTP Caching
    • Sharding
  • State Replication (Favors Availability in Partition Failure)
    • Synchronous Replication with Distributed Locks and Local Transactions
    • Synchronous Replication with Local Locks and Distributed Transactions
    • Synchronous Replication with Local Locks and Local Transactions
    • Asynchronous Replication with Update Anywhere
    • Asynchronous Replication with Update at the Master Site only

How can these patterns be manifested in a SOA system?

To achieve Behavioral Partitioning, the construct of a Command Pattern (see: Command Pattern) and the Functor Pattern can be used. In the conventional SOA architecture, behavior (as in executable code) needs to be propagated through the network, to be executed by receiving services. In lieu of a commonly agreed standard, one may either employ XQuery as a stand in for this capability. One should therefore can define services to accept XQuery in a way analogous to how SemanticWeb systems accept SPARQL. A key to achieving scalability is that behavior be allowed to be move close to the data that it will act on. Behavior that works on data through remote invocations is a guarantee to kill scalability. See “Hot Trend: Move Behavior To Data For A New Interactive Application Architecture“.

To achieve State Partitioning, SOA based system need to adopt the notion of persistent identifiers of data. WS-I has the notion of WS-Addressing which typically are used to reference endpoints as opposed to actual entities. What is needed is that this addressing or persistent identifiers act analogous to Consistent Hashing so that entities may be partitioned and accessible using multiple endpoints. Identifier based services would need to be stood up to perform the redirection to the endpoints.

Finally, there is the issue of Replication to support availability and fail-over. The Identifier based services described early may function as routers to handle the fail-over. Alternative, one may employ proxy servers in the manner described in A New Kind of Tiered Architecture. The replication capability however will require the exposure of new kinds of services that support a replication protocol. The most basic of which would be to provide a Publish and Subscribe interface.

To conclude, high scalability in SOA may indeed be possible. It is a bass-ackwards way of achieving high scalability, but if your only option is to use SOA, then there may just be a possibility to achieve it.

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