pal: Product Access Layer

Product Access Layer allows data stored logical “pools” to be accessed with light weight product refernces by data processers, data storage, and data consumers. A data product can include a context built with references of relevant data. A ProductStorage interface is provided to handle saving/retrieving/querying data in registered pools.

Rationale

In a data processing pipeline or network of processing nodes, data products are generated within a context which may include input data, reference data, and auxiliary data of many kind. It is often needed to have relevant context recorded with a product. However the context could have a large size so including their actual data as metadata of the product is often impractical.

Once FDI data are generated they can have a reference through which they can be accessed. The size of such references are typically less than a few hundred bytes, like a URL. In the product context only data references are recorded.

This package provides MapContext, ProductRef, Urn, ProductStorage, ProductPool, and Query classes (simplified but mostly API-compatible with Herschel Common Science System v15.0) for the storing, retrieving, tagging, and context creating of data product modeled in the dataset package.

Definitions

URN

The Universial Resource Name (URN) string has this format:

urn:poolname:resourceclass:serialnumber

where

resourceclass

full class name of the resource (product)

poolname

scheme + :// + place + directory

scheme

file, mem, http … etc

place

192.168.5.6:8080, c:, an empty string … etc

directory

A label for the pool that is by default used as the full path where the pool is stored. ProductPool.transformpath() can used to change the directory here to other meaning.

• for file scheme: / + name + / + name + … + / + name

• for mem scheme: / + name + /

serialnumber

internal index. str(int).

ProductRef

This class not only holds the URN of the product it references to, but also records who ( the _parents_) are keeping this reference.

Context and MapContext

Context is a Product that holds a set of productRef s that accessible by keys. The keys are strings for MapContext which usually maps names to product references.

ProductStorage

A centralized access place for saving/loading/querying/deleting data organized in conceptual pools. One gets a ProductRef when saving data.

ProductPool

An place where products can be saved, with a reference for the saved product generated. The product can be retrieved with the reference. Pools based on different media or networking mechanism can be implemented. Multiple pools can be registered in a ProductStorage front-end where users can do the saving, loading, querying etc. so that the pools are collectively form a larger logical storage.

Query

One can make queries to a ProductStorage and get back a list of references to products that satisfy search chriteria. Queries can be constructed using Python predicate expressions about a product and its metadata, or a function that returns True or False.

run tests

in the same directory:

make test2

Design

Packages

Classes