Meltano gives you the flexibility to alter data passing through your EL pipelines to do precisely what you need for your use case. Although commonly users want to replicate their data in the most raw format, using ELT vs ETL, there are many use cases that require minor alterations to data on the fly. This is where mappers, also referred to as inline stream maps, shine!
The mapper features can be viewed in 2 categories: stream level or property (record) level. Stream level features allow you to modify the stream itself, whereas the property level features allow you to modify contents of the stream.
There are lots of use cases for mappers and part of their benefit is allowing you to add custom behavior into your pipelines even when using out of the box connectors. Here are a few examples of common use cases for mappers.
Some users have strict requirements to not store any raw PII data in their destination system. In this case you can use mappers to either hash the PII data on the fly so it arrives in your destination but not in plain text, or you can simply remove it. If you need to remove PII you can consider using selection criteria to avoid replicating it in the first place but mappers give you more precision if needed.
Sometimes users will install a tap from MeltanoHub and realize that its providing the data they want but maybe not in a custom format they need. There are at least 2 options:
Using mappers in option 2 allows you to get the behavior that you want without the burden of making code changes and maintaining a custom variant moving forward.
Mappers let you make stream level alterations like renaming, duplicating, or splitting a stream. This can be used to enforce consistent naming conventions for tables in your warehouse or duplicating a stream to add mapper alterations while also preserving the original stream. Another example is for reverse ETL use cases where the target usually expects the data to arrive in streams and fields named in a particular way, you can achieve this using mappers.
There are two ways to use mapper functionality within Meltano.
You can install a standalone mapper plugin that runs between a tap and a target i.e.
meltano run tap-csv <MAPPER> target-jsonl or if either your tap or target is based on the Meltano SDK then you can configure the
stream_map_config settings in the plugin configuration, also referred to as inline stream maps.
If either your tap or your target is built on the Meltano SDK then it automatically has the mapper features, or also referred to as inline stream maps, built in.
This is the easiest way to get mapper functionality into your pipeline by avoiding some of the limitations of standalone mappers (only works with
meltano run, requires installing another plugin, etc.).
The limitation is that the mapper functionality is baked into the SDK so if you ever need to make customizations its a bit less flexible relative to standalone mappers where you can create your own forks.
A few example configurations using inline stream maps are:
This example shows the SDK based meltanolabs variant of tap-github configured to lowercase all repo names in the issues stream.
- name: tap-github variant: meltanolabs pip_url: git+https://github.com/MeltanoLabs/tap-github.git config: stream_maps: issues: repo: record['repo'].lower()
For more details and examples refer to the SDK documentation.
The other way of getting mapper functionality is to use a standalone mapper plugin that is inserted between your tap and target to do the translation
meltano run tap-csv <MAPPER> target-jsonl.
These can be used even when your your tap and/or target are not SDK based.
The limitation with this approach is that currently you can only run these using the
meltano run command and not using
meltano elt or
The advantage relative to the SDK inline stream maps is that you can fork the mapper plugin or build your own to do whatever you need.
The two most common mapper plugins are:
These can be added like any other plugin:
meltano discover mappers meltano add mapper <mapper name> # Example meltano add mapper meltano-map-transformer
An example taken from the meltano-map-transformer repository shows the mapper altering casing of the data. Given this input CSV and mapper configurations:
plugins mapper: - name: meltano-map-transformer variant: meltano pip_url: git+https://github.com/MeltanoLabs/meltano-map-transform.git mappings: - name: lower config: stream_maps: customers: __alias__: customers_v5 first_name: first_name.lower() # three different ways of accessing the variable last_name: record['last_name'].lower() email: _['email'].upper() count_t: str(last_name.count("t")) # need to cast to str because it could be NULL! ip_address: __NULL__
meltano run tap-csv hash_email target-sqlite the result would be:
To see more examples check out the plugin repo.
meltano eltcommand? #
No, currently only
meltano run support standalone mapper plugins.
No, BATCH message type does not currently support mapping functionality. Follow along with this discussion where we talk about how stream maps could support BATCH messages.
It’s mostly preference but here are some trade offs to consider:
No, you only need one of your plugins to be SDK based in order to get this functionality.