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About read strategies
Last updated at 7:49 pm UTC on 14 September 2006
Magma provides programs with transparent access to an arbitrarily large, persistent object model. Memory consumption is minimized by way of the well-known "Proxy" design pattern. Parts of the object-model that are not being used by your program are "truncated" in your local image with a lightweight proxy object. Should the program happen to send a message to the proxy, the object it represents is materialized from the repository automatically. The proxy then "becomes" the real object.

There is a cost to this materialization process, so doing too many of them may cause performance issues. When a proxy is materialized, it is usually assumed that you will send a message to it that will require access to at least one of its instance variables. For this reason, it is generally desirable to bring back the proxy and several of the objects it references.

But bringing back more objects creates more work for the server, network and your client. For ideal performance, you want each materialization to bring back just the objects that will be needed, but not too many that won't be.

Magma's default ReadStrategy will read one level deep by default, which means the proxy object and the objects each of its instance variables refers to. As a programmer, you can change this default 0, 1, 2 or 3 (or maybe even 4, but I wouldn't go beyond that) by supplying a ReadStrategy.

minimumDepth, plus delta-depth

The minimumDepth is how far it reads for any objects, all objects, every time, all the time. Therefore, you want to keep it very small, probably 0 or 1. Then, you can use a ReadStrategy to specify how much deeper to read, below the minimumDepth, by class and instance-variable. So what this means exactly is, whenever the server is reading a graph and encounters a buffer for the specified class, it will be sure to also grab n-levels deeper, regardless of how deep it already is, for whatever variable name specified.

Let's say you are about to display a list of Employees and their department names in a list. The Employee class defines a 'name' attribute, which is a String, and a 'department' object, an instance of class Department. Departments also have a name. If the list was to include each employee name and their department name, the default ReadStrategy would retrieve each Employee with its Department as a proxy. So the list would populate slowly because each Department would be materialized one at a time. To improve this performance, you could specify the ReadStrategy to read one additional level on the 'department' object.

	| employeeListReadStrategy |
	employeeListReadStrategy :=
		(MaReadStrategy minimumDepth: 1)
			forVariableNamed: 'department'
			onAny: Employee
			readToDepth: 1
	mySession readStrategy: employeeListReadStrategy. "returns instantly, no server call"
	"now enumerate the collection"
	myEmployees do: [ :each | ".. display name and department name in your list.." ]