High Level Architecture
SCSB is designed to enable efficient, real-time transactions between the partner integrated library systems (ILS), their web-accessible discovery environments, and the inventory management system (IMS) or storage facility’s system.
One side SCSB talks to multiple institution library systems using SIP2 or NCIP or REST APIs and on the other side it talks to inventory management systems using REST APIs.
SCSB is designed to provide
Real-time availability of all shared collection items in OPAC systems
Validate and process the requests in real-time
Provide real-time item status
Integration with Partner ILS to support requesting and circulation.
Eliminate cross loading bibliographic and item records in ILS (Voyager, Sierra, Alma)
The following diagram depicts the the environment (of the collaborating systems) where the solution fits in. SCSB is the middleware solution.
Layered Architecture
The architecture includes four distinct layers:
Presentation Layer
Services Layer
Data Services Layer
Data Layer
Presentation Layer
The presentation layer deals with user interface aspects of the system.
Services Layer
The services layer encapsulates specific business rules, which are made available to the presentation layer. The presentation layer requests enterprise services, which are then fulfilled by this layer. The architecture envisages providing a seamless enterprise service layer communicating with internal data stores and 3rd party services. The data access layer supports the enterprise service layer by serving the data required.
Data Services Layer
The data services layer provides fundamental services to fulfill the business needs (fulfilled through enterprise services) such as Search, Request Item, etc. The data services layer serves data required by enterprise services. Data services support both relational database and Solr.
Services implementing data access to relational database will leverage Java Persistence Architecture (JPA), providing separation of object persistence and data access logic from a particular persistence mechanism (relational database) in data layer. This approach provides the flexibility to change the applications persistence mechanism without the need to re-engineer application logic that interacts with the data layer. Persistence classes are developed following the object-oriented idiom including association, inheritance, polymorphism, composition, and collections. This framework provides the flexibility to express queries in its own portable SQL extension, as well as in native SQL, or with object-oriented criteria.
Services implementing data access to Solr / Lucene search will wrap the Solr RESTFul API's to provide features such as search, filter, sort and navigation.
Data Layer
The data layer serves as the data store for all persistent information in the system including the relational database and search engine indexes.
RDBMS data layer will comprise of MySQL cluster. RDBMS data layer will be accessed only from the data access layer via Data Access Objects (DAOs). RDBMS cluster architecture allows a single physical database to be accessed by concurrent instances running across several different CPUs. The proposed data layer will be composed of a group of independent servers or nodes that operate as a single system. These nodes have a single view of the distributed cache memory for the entire database system providing applications access to more horsepower when needed while allowing computing resources to be used for other applications when database resources are not as heavily required. In the event of a sudden increase in traffic, proposed system can distribute the load over many nodes, a feature referred to as load balancing. In addition to this, proposed system can protect against failures caused by unexpected hardware, operating system or server crashes, as well as processing loss caused by planned maintenance. When a node failure occurs, connection attempts can fail over to other nodes in the cluster, which assumes the work of the failed node. When connection failover occurs and a service connection is redirected to another node, users can continue to access the service, unaware that it is now provided from a different node.
A single Solr instance can support more than one index using Solr cores (single index per core). A single large index can be a performance overhead. SolrCloud distributes a single index on different machines, commonly referred as shards. All shards of the same index making one large index are referred as collection. While collection supports index scaling, it does not provide redundancy. Replication of shards provides redundancy and fault tolerance.
Zookeeper maintains the SolrCloud, by distributing the index across shards and federating the search through the collection. SolrCloud uses leaders and an overseer. In the event of leader or the cluster overseer failure, automatic fail over will choose new leaders or a new overseer transparently to the user and they will seamlessly takeover their respective jobs. Any Solr instance can be promoted to one of these roles.