===
ETL
===

ETL framework is most commonly used in [staged sync](../../eth/stagedsync).

It implements a pattern where we extract some data from a database, transform it,
then put it into temp files and insert back to the database in sorted order.

Inserting entries into our KV storage sorted by keys helps to minimize write
amplification, hence it is much faster, even considering additional I/O that
is generated by storing files.

It behaves similarly to enterprise [Extract, Tranform, Load] frameworks, hence the name.
We use temporary files because that helps keep RAM usage predictable and allows
using ETL on large amounts of data.

Example
=======

.. code-block:: go

    func keyTransformExtractFunc(transformKey func([]byte) ([]byte, error)) etl.ExtractFunc {
        return func(k, v []byte, next etl.ExtractNextFunc) error {
            newK, err := transformKey(k)
            if err != nil {
                return err
            }
            return next(k, newK, v)
        }
    }

    err := etl.Transform(
            db,                                              // database
            dbutils.PlainStateBucket,                        // "from" bucket
            dbutils.CurrentStateBucket,                      // "to" bucket
            datadir,                                         // where to store temp files
            keyTransformExtractFunc(transformPlainStateKey), // transformFunc on extraction
            etl.IdentityLoadFunc,                            // transform on load
            etl.TransformArgs{                               // additional arguments
                Quit: quit,
            },
        )
    if err != nil {
        return err
    }

Data Transformation
===================

Data could be transformed in two places along the pipeline:

* transform on extraction

* transform on loading

Transform On Extraction
=======================

.. code-block:: go

    type ExtractFunc func(k []byte, v []byte, next ExtractNextFunc) error

Transform on extraction function receives the currenk key and value from the
source bucket.

Transform On Loading
====================

.. code-block:: go

    type LoadFunc func(k []byte, value []byte, state State, next LoadNextFunc) error

As well as the current key and value, the transform on loading function
receives the `State` object that can receive data from the destination bucket.

That is used in index generation where we want to extend index entries with new
data instead of just adding new ones.

Buffer Types
============

Before the data is being flushed into temp files, it is getting collected into
a buffer until if overflows (`etl.ExtractArgs.BufferSize`).

There are different types of buffers available with different behaviour.

* `SortableSliceBuffer` -- just append (k, v1), (k, v2) onto a slice. Duplicate keys
    will lead to duplicate entries: [(k, v1) (k, v2)].

* `SortableAppendBuffer` -- on duplicate keys: merge. (k, v1), (k, v2)
    will lead to k: [v1 v2]

* `SortableOldestAppearedBuffer` -- on duplicate keys: keep the oldest. (k,
    v1), (k v2) will lead to k: v1

Loading Into Database
=====================

We load data from the temp files into a database in batches, limited by
`IdealBatchSize()` of an `ethdb.Mutation`.

Saving & Restoring State
========================

Interrupting in the middle of loading can lead to inconsistent state in the
database.

To avoid that, the ETL framework allows storing progress by setting `OnLoadCommit` in `etl.TransformArgs`.

Then we can use this data to know the progress the ETL transformation made.

You can also specify `ExtractStartKey` and `ExtractEndKey` to limit the number
of items transformed.


`etl.Transform` function
========================

The vast majority of use-cases is when we extract data from one bucket and in
the end, load it into another bucket. That is the use-case for `etl.Transform`
function.

`etl.Collector` struct
======================

If you want a more modular behaviour instead of just reading from the DB (like
generating intermediate hashes in https://github.com/ledgerwatch/erigon/blob/master/core/chain_makers.go, you can use
`etl.Collector` struct directly.

It has a `.Collect()` method that you can provide your data to.


Optimizations
=============

* if all data fits into a single file, we don't write anything to disk and just
    use in-memory storage.