Defer
[β] Beta Feature
This is net-new functionality in v0.18.0, with iterative improvements to come. If you encounter unexpected behavior, please post in Slack or open an issue.
Defer is a powerful feature that makes it possible to run a subset of models or tests in a sandbox environment, without having to first build their upstream parents. This can save time and computational resources when you want to test a small number of models in a large project.
Defer requires that a manifest from a previous dbt invocation be passed to the --state
flag or env var. Together with the state:
selection method, these features enable "Slim CI". Read more about state.
Usage
$ dbt run --models [...] --defer --state path/to/artifacts$ dbt test --models [...] --defer --state path/to/artifacts
When the --defer
flag is provided, dbt will resolve ref
calls differently depending on two criteria:
- Is the referenced node included in the model selection criteria of the current run?
- Does the reference node exist as a database object in the current environment?
If the answer to both is no—a node is not included and it does not exist as a database object in the current environment—references to it will use the other namespace instead, provided by the state manifest.
Ephemeral models are never deferred, since they serve as "passthroughs" for other ref
calls.
When using defer, you may be selecting from production datasets, development datasets, or a mix of both. Note that this can yield unexpected results
- if you apply env-specific limits in dev but not prod, as you may end up selecting more data than you expect
- when executing tests that depend on multiple parents (e.g.
relationships
), since you're testing "across" environments
Deferral requires both --defer
and --state
to be set, either by passing flags explicitly or by setting environment variables (DBT_DEFER_TO_STATE
and DBT_ARTIFACT_STATE_PATH
). If you use dbt Cloud, read about how to set up CI jobs.
Example
In my local development environment, I create all models in my target schema, dev_alice
. In production, the same models are created in a schema named prod
.
I access the dbt-generated artifacts (namely manifest.json
) from a production run, and copy them into a local directory called prod-run-artifacts
.
run
I've been working on model_b
:
selectid,count(*)from {{ ref('model_a') }}group by 1
I want to test my changes. Nothing exists in my development schema, dev_alice
.
- Standard run
- Deferred run
$ dbt run --models model_b
create or replace view dev_me.model_b as (selectid,count(*)from dev_alice.model_agroup by 1)
Unless I had previously run model_a
into this development environment, dev_alice.model_a
will not exist, thereby causing a database error.
test
I also have a relationships
test that establishes referential integrity between model_a
and model_b
:
version: 2models:- name: model_bcolumns:- name: idtests:- relationships:to: ref('model_a')field: id
(A bit silly, since all the data in model_b
had to come from model_a
, but suspend your disbelief.)
- Without defer
- With defer
dbt test --models model_b
select count(*) as validation_errorsfrom (select id as id from dev_alice.model_b) as childleft join (select id as id from dev_alice.model_a) as parent on parent.id = child.idwhere child.id is not nulland parent.id is null
The relationships
test requires both model_a
and model_b
. Because I did not build model_a
in my previous dbt run
, dev_alice.model_a
does not exist and this test query fails.