What are the required functionnalities of ETL frameworks ?

Question

I am writing an ETL (in python with a mongodb backend) and was wondering : what kind of standard functions and tools an ETL should have to be called an ETL ?

This ETL will be as general purpose as possible, with a scriptable and modular approach. Mostly it will be used to keep different databases in sync, and to import/export datasets in different formats (xml and csv) I don't need any multidimensional tools, but it is a possibility that it'll needed later.

Answer 1

Here's a random list, in no particular order:

1. Connect to a wide range of sources, including all the major relational databases.
2. Handle non-relational data sources like text files, Excel, XML, etc.
3. Allow multiple sources to be mapped into a single target.
4. Provide a tool to help map from source to target fields.
5. Offer a framework for injecting transformations at will.
6. Programmable API for writing complex transformations.
7. Optimize load process for speed.

Answer 2

Let's think of the ETL use cases for a moment.

1. Extract.
   • Read databases through a generic DB-API adapter.
   • Read flat files through a similar adapter.
   • Read spreadsheets through a similar adapter.
2. Cleanse.
   • Arbitrary rules
   • Filter and reject
   • Replace
   • Add columns of data
3. Profile Data.
   • Statistical frequency tables.
4. Transform (see cleanse, they're two use cases with the same implementation)
5. Do dimensional conformance lookups.
   • Replace values, or add values.
6. Aggregate.
   • At any point in the pipeline
7. Load.
   • Or prepare a flat-file and run the DB product's loader.

Further, there are some additional requirements that aren't single use cases.

• Each individual operation has to be a separate process that can be connected in a Unix pipeline, with individual records flowing from process to process. This uses all the CPU resources.

• You need some kind of time-based scheduler for places that have trouble reasoning out their ETL preconditions.

• You need an event-based schedule for places that can figure out the preconditions for ETL processing steps.

Note. Since ETL is I/O bound, multiple threads does you little good. Since each process runs for a long time -- especially if you have thousands of rows of data to process -- the overhead of "heavyweight" processes doesn't hurt.

Answer 3

Automatic / heuristic mapping of column names. E.g simple string mappings:

DB1: customerId

DB2: customer_id

I find a lot of the work I (have) done in DTS / SSIS could've been automatically generated.

• not necessarily "required functionality", but would keep a lot of your users very happy indeed.