Topics covered:

  • Temporal table joins


Currency conversion is a great example of a problem that cannot be solved without considering the time dimension. As exchange rates vary significantly every day using an average rate for a certain period is a gross oversimplification of a problem that can give very inaccurate results. This example shows how kamu and streaming SQL can be used to join your overseas trading account data with an exchange rate stream to bring your calculations to the common currency denominator.


Getting Started

To follow this example checkout kamu-cli repository and navigate into examples/currency_conversion sub-directory.

Create a temporary kamu workspace in that folder using:

kamu init

You can either follow the example steps below or fast-track through it by running:

kamu add --recursive .
kamu pull --all

Root Datasets

We will be using two root datasets:

  • - contains daily exchange rates between USD and CAD.
  • - contains a transaction ledger from a fake trading account similar to what you’d get in data export from your trading platform.

The is sourcing its data from a file located in data/ sub-directory, while the exchange rates are obtained from an external source - the Bank of Canada historical exchange rates dataset.

Let’s add the root datasets and ingest data:

kamu add
kamu pull --all

You can verify the result using tail command or the SQL shell:

kamu tail

Or explore the dataset via SQL shell:

kamu sql
select * from "" limit 5;
| system_time |    date    | currency_base | currency_target |     rate     |
|     ...     | 1999-01-04 |      USD      |       CAD       | 1.5271863602 |
|     ...     | 1999-01-05 |      USD      |       CAD       | 1.5237489398 |
|     ...     | 1999-01-06 |      USD      |       CAD       | 1.5082176616 |
|     ...     | 1999-01-07 |      USD      |       CAD       | 1.5132393398 |
|     ...     | 1999-01-08 |      USD      |       CAD       | 1.5132515653 |

Remember, that after adding a dataset from a yaml file kamu creates an internal representation of it in the workspace .kamu directory, so to make any changes to the dataset you will need to re-add the dataset again after changing the yaml file.

kamu delete
# <Make changes to the yaml file>
kamu add

# Alternatively
kamu add --replace

Converting transaction values into CAD

If you look at dataset - it contains the trading activity of a US account, so all prices and settlement amounts there are listed in US Dollars (USD). But let’s imagine that you live in Canada and all of your other accounts and your credit cards are all in Canadian Dollars (CAD). It would be a lot easier for you to monitor your financials if they were in one currency.

If you analyze your financials on a quarterly or yearly basis you might’ve dealt with this problem by looking up an average exchange rate for a certain time period and used it in your calculations. This is of course an oversimplification that may give misleading results - your transactions are most likely not distributed uniformly throughout the time period, and if you live in a country with volatile economy or trading crypto - the exchange rate may fluctuate significantly.

Ideally we would like to take every single transaction and convert the price using the exchange rate for the corresponding date. Thankfully, kamu can make this process very easy using the temporal table joins.

Temporal table joins (see this great explanation in Apache Flink’s blog) take one of the streams and represent it as a three-dimensional table. When joining to such a table you need to pass in the time argument to tell join to consider the version of the table that would exist at that point in time.

Let’s have a look at the dataset:

kind: DatasetSnapshot
version: 1
  kind: Derivative
    - kind: SetTransform
        - datasetRef:
        - datasetRef:
        kind: Sql
        engine: flink
          - name:
              - currency_base
        query: |
            tr.`price` as `price_usd`,
            tr.`price` * exc.`rate` as `price_cad`,
            tr.`settlement` as `settlement_usd`,
            tr.`settlement` * exc.`rate` as `settlement_cad`
          FROM `` as tr
          LEFT JOIN `` FOR SYSTEM_TIME AS OF tr.`event_time` as exc
          ON tr.`currency` = exc.`currency_base` AND exc.`currency_target` = 'CAD'          
The excessive use of back ticks is currently caused by the SQL parser used by Apache Flink which is overly sensitive to reserved words - this should improve in future versions.

Using the temporalTables section we instruct the Flink engine to use event stream to create a temporal table of the same name.

Here’s how this three-dimensional table would look like when sliced at different points in time:

@ 1999-01-04
| currency_base | currency_target |     rate     |
|      USD      |       CAD       | 1.5271863602 |

@ 1999-01-05
| currency_base | currency_target |     rate     |
|      USD      |       CAD       | 1.5237489398 |

@ 1999-01-06
| currency_base | currency_target |     rate     |
|      USD      |       CAD       | 1.5082176616 |

It basically remembers the last observed value of every column grouped by the provided primaryKey (currency_base in our case).

The LEFT JOIN FOR SYSTEM_TIME AS OF tr.event_time part can be interpreted as us taking every transaction event from, indexing the temporal table at this event’s event_time and then joining the same event with the resulting (now ordinary two-dimensional) table.

Note that SYSTEM_TIME AS OF syntax is a relic of SQL:2011 standard and should not be confused with Kamu’s system_time column. The actual join is performed in the event time space. You can learn more about temporal event time joins in Flink documentation.

With theory out of the way, it’s time to give this a try:

kamu add
kamu pull

The results should be:

kamu tail
| system_time | event_time | symbol | quantity | price_usd |      price_cad      |
|     ...     | 2016-01-04 | SPY    | 1        | 201.0200  | 279.85643605283600  |
|     ...     | 2016-02-01 | SPY    | 1        | 193.6500  | 271.38399945165000  |
|     ...     | 2016-03-01 | SPY    | 1        | 198.1100  | 267.84574042498800  |
|     ...     | 2016-04-01 | SPY    | 1        | 206.9200  | 269.58244227661600  |
|     ...     | 2016-05-02 | SPY    | 1        | 207.9700  | 260.35607413671100  |

The best thing about this is that you may never have to touch this SQL query again. Each time you run kamu pull in the future the latest transaction data will be ingested along with latest exchange rates, producing new transactions with converted prices. This is the “write once - run forever” philosophy of kamu that combines best propagation times with the accuracy of solving temporal problems without taking any shortcuts.

Once you master this example - make sure to check out the Stock Market Trading example that introduces another really important mechanism related to temporal joins - the watermark.