Data gets into Fennel datasets via Sources - in fact, sources are the only mechanism for data to reach a Fennel dataset.

Fennel ships with data connectors to all common datastores so that you can 'source' your Fennel datasets from your external datasets. In addition to the pull based sources that read from external data sources, Fennel also ships with a push based source called Webhook for you to manually push data into Fennel datasets.

Let's see an example with postgres connector:

1from fennel.connectors import source, Postgres
2from fennel.datasets import dataset
4postgres = Postgres(
5    name="my-postgres",
6    host=os.environ["POSTGRES_HOST"],
7    db_name=os.environ["POSTGRES_DB_NAME"],
8    port=5432,
9    username=os.environ["POSTGRES_USERNAME"],
10    password=os.environ["POSTGRES_PASSWORD"],
13table = postgres.table("user", cursor="update_time")
15@source(table, disorder="14d", cdc="append", every="1m")
17class UserLocation:
18    uid: int
19    city: str
20    country: str
21    update_time: datetime


In this example, first an object is created that knows how to connect with your Postgres database. Then we specify the table within this database (i.e. user) that we intend to source from. And finally, a decorator is added on top of the dataset which should be sourcing from this table.

And that's it - once this is written, UserLocation dataset will start mirroring your postgres table userand will update as the underlying Postgres table updates.

Schema Matching

Fennel has a strong typing system and all ingested data is evaluated against the dataset schema. See Fennel types to see the full list of types supported by Fennel.

Schema As Subset of Source

All the fields defined in Fennel dataset must be present in your source dataset though it's okay if your external data source has additional fields - they are simply ignored. In this example, it is expected that the user table in Postgres will have at least four columns -uid, city, country, and update_time with appropriate types.

Not only those fields must be present, they must be of the right types (or be convertible into right types via some parsing/casting). If ingested data doesn't match with the schema of the Fennel dataset, the data is discarded and not admitted to the dataset. Fennel maintains logs of how often it happens and it's possible to set alerts on that.

Due to this, often no changes are needed to Fennel when the schema of your external data changes. For instance, if you add new fields to your Postgres, those new fields are simply ignored by Fennel. And if you remove fields that aren't referenced by Fennel datasets, nothing needs to change either.

Type Matching/Casting Rules

Here is how various types are matched from the sourced data (note that type matching/casting only happens during sourcing - once data has been parsed in the given type, from then onwards, there is no type casting/coercion anywhere else in Fennel)

  • int, float, str, bool respectively match with any integer types, float types, string types and boolean types. For instance, Fennel's int type matches with INT8 or UINT32 from Postgres.
  • List[T] matches a list of data of type T.
  • Dict[T] matches any dictionary from strings to values of type T.
  • Optional[T] matches if either the value is null or if its non-null value matches type T. Note that null is an invalid value for any non-Optional types.
  • struct is similar to dataclass in Python or struct in compiled languages and matches JSON data with appropriate types recursively.
  • datetime matching is a bit more flexible to support multiple common data formats to avoid bugs due to incompatible formats. Fennel is able to safely parse datetime from all the following formats.

Datetime Formats

  • Integers that describe timestamp as interval from Unix epoch e.g. 1682099757 Fennel is smart enough to automatically deduce if an integer is describing timestamp as seconds, milliseconds, microseconds or nanoseconds
  • Strings that are decimal representation of an interval from Unix epoch e.g."1682099757"
  • Strings describing timestamp in RFC 3339 format e.g. '2002-10-02T10:00:00-05:00' or '2002-10-02T15:00:00Z' or '2002-10-02T15:00:00.05Z'
  • Strings describing timestamp in RFC2822 format e.g. 'Sun, 23 Jan 2000 01:23:45 JST'
  • When data is being read from arrow representations (say parquet files), date32 and date64 types are also converted to datetime (with time being UTC midnight)

Stacking Multiple Sources

Stacking two (or more) sources on top of a dataset simply leads to the dataset getting hydrated from all of them. A rather common scenario where this is useful is when realtime data is say in Kafka but with a short retention and the archive is stored, say in S3. Here is one (but not the only) way of doing this:

1cutoff = datetime(2024, 1, 1, 0, 0, 0)
2bucket = s3.bucket("data", path="orders")
4@source(bucket, disorder="1w", cdc="append", until=cutoff)
5@source(kafka.topic("order"), disorder="1d", cdc="append", since=cutoff)
7class Order:
8    uid: int
9    skuid: int
10    at: datetime
Using s3 for dev and Kafka for prod


Managing Different Dev / Prod Sources

Fennel supports stacking multiple sources but selectively choosing only a subset to be used in a given environment. With this, you can have the same logical dataset get hydrated from different physical external stores in production and dev environments.

1@source(s3.bucket("data", path="*"), disorder="1w", cdc="append", env="dev")
2@source(kafka.topic("order"), disorder="1d", cdc="append", env="prod")
4class Order:
5    uid: int
6    skuid: int
7    at: datetime
10    message="some commit msg",
11    datasets=[Order],
12    env="prod",  # use only the sources with env="prod"


In above example, sources are given an env kwarg and during the commit operation, env is specified to be 'prod' to select only the 'prod' sources. The effect of above is same as having a single prod Kafka source. If with the same definition, the commit operation was modified to select for "dev" env, the other source would have been selected instead.

This way, sources in all environments can be defined inline with the dataset definition while still selectively specifying different sources in different environments.

Load Impact of Sources

Fennel sources have negligible load impact on the external data sources.

For instance, in the above example, as long as indices are put on the cursor field, Fennel will make a single SELECT query on Postgres every minute. And once data reaches Fennel datasets, all subsequent operations are done using the copy of the data stored on Fennel servers, not the underlying data sources. This ensures that external data sources don't need to be over-provisioned (or changed in any way) just for Fennel to be able to read the data.

Configuring Sources

There is tremendous variety and complexity just within data ingestion. Fennel aims to abstract much of that via the following carefully chosen set of knobs that can be used to configure data sources:


Fennel natively supports change data capture (aka CDC) as well as other modes of ingestion (e.g. append). You can configure the cdc parameter to specify how should your data be interpreted and converted to valid change log data.

You'd typically use append for sourcing keyless datasets, for instance, event streams from Kafka that only have inserts but no deletes/updates. If you want to process both inserts/updates (but no deletes) for keyed datasets, upsert is a good option. Other options are debezium if you want Fennel to ingest existing CDC stream in debezium format (say from Kafka or S3) or native for data stores that offer a native CDC export.


Fennel, like many other streaming systems, is designed to robustly handle out of order data. If there are no bounds on how out of order data can get, internal state maintained by Fennel can become very large. Unlike many other streaming systems (like Flink or Spark streaming), Fennel keeps this state on disk, not RAM, which eliminates OOM issues. But even then, it's desirable to garbage collect this state when all data before a timestamp has been seen.

This is usually handled by a technique called Watermarking where max out of order delay is specified. This max out of order delay of a source is called disorder in Fennel, and once specified at source level, is respected automatically by each downstream pipeline.


The frequency with which you want Fennel to check the external data source for new data. Fennel is built with continuous ingestion in mind so you could set it to small values and still be fine.


While using append CDC strategy, some (but not all) sources use a cursor to do incremental ingestion of data. It does so by remembering the last value of the cursor column (in this case update_time) and issuing a query of the form SELECT * FROM user WHERE update_time > {last_seen_update_time - disorder}.

Clearly, this works only when the cursor field is monotonically increasing with row updates - which Fennel expects you to ensure. It is also advised to have an index of the cursor column so that this query is efficient.

Many data sources don't need an explicit cursor and instead use other implicit mechanisms to track and save ingestion progress.


The preproc field in the source provides a way to ingest a column that doesn't exist. Instead, it is either given a default value or to base the value of that column on another column.


The since parameter (of type datetime) in the source provides a way to ingest data from a specific time onwards from the source. When set, only the rows having the value of their timestamp field >= the since value are ingested.


Analogous to since, until, when set restricts the ingestion to only those rows where the value of the timestamp field is < the until value.

since and until are very useful when building datasets that are union of batch & realtime sources. For example, read from s3 until a timestamp, and from that same timestamp onwards, read from kafka using since.


By default, Fennel assumes all data sources are unbounded and will keep getting new data. However, a source can be marked as finite by setting bounded to True. In such cases, if Fennel is unable to obtain any new data from source despite trying for at least idleness period, Fennel assumes that the source has been exhausted. This allows Fennel to free up any state that isn't needed any more, thereby saving costs and improving performance.


The idleness field, when non-None, signifies that a bounded source is expected to be marked closed after a specified duration of idleness.

On This Page

Edit this Page on Github