[JENA-2049] Binding Improvements

• Type: Improvement
• Status: Closed
• Priority: Major
• Resolution: Done
• Fix Version/s: Jena 4.0.0
• Component/s: ARQ
• Assignee: Andy Seaborne
• Reporter: Andy Seaborne
• Created: 16/Feb/21 17:46
• Updated: 17/Apr/21 16:50
• Resolved: 18/Feb/21 11:24

Description

Binding is a map from variables to nodes used in query processing. It is central to execution of SPARQL.

Context

One feature is that binding objects form a tree. To add variables to an existing binding, whether a join or assignment (BIND), a new binding object is created with the original as parent. This avoid copy costs. The parent can be shared across solutions.

This design leads to large numbers of Binding objects, the majority of which are quite small (1 or 2 variables).

Current Situation

For general building, the binding implementation is the general BindingMap. This is used a lot because the number of variables is not easy to determine at the time the Binding object is allocated.

Binding1 is the special case of one var/node pair. This "one slot" binding requires the application to know there will be only one slot binding so it has limited use.

BindingMap is not immutable, although as a Binding the mutable
operations need to be accessed with a cast.

BindingMap is implemented using a Map (HashMap) with a special case of a
binding of one slot.

Maps are several java object including a array. It is quite large and Java objects require initialization. For streaming queries, the total footprint is quite low; the GC will reclaim space as results rows are finished with. For queries that accumulate results (e.g general sort), bindings are held for a period of time.

Proposal

• A builder pattern to accumulate "(var,node)" pairs during a constriction phase before a build step that can then choose the most appropriate immutable Binding implementation.
  This takes the work of selecting the right binding implementation off the
  rest of the code.
• Expand the number fixed length bindings: Special cases for 0, 1, 2, 3, and 4 pairs.
• The builder can be reused (same thread) so the cost of object creation for the builder
  can be amortized.
• Truly immutable bindings for long term stability.

Compatibility

Retain, marked deprecated, the BindingFactory methods to create a BindingMap so existing code continues as before, with deprecation to indicate it would be a good idea to switch to the new way.

Non-goal

This is not a speed up (nor a slow down).

Issue Links

• duplicates: JENA-121 Improvements to Bindings