Blog post: to defer or to clone (Publish Tues 10/31)

website/blog/2023-10-31-to-defer-or-to-clone.md

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+---
+
+title: To defer or to clone, that is the question
+description: "In dbt v1.6, we introduce support for zero-copy cloning via the new dbt clone command. In this blog post, Kshitij will cover what clone is, how it is different from deferral, and when to use each."
+slug: to-defer-or-to-clone
+
+image: /img/blog/2023-10-31-to-defer-or-to-clone/preview.png
+
+authors: [kshitij_aranke, doug_beatty]
+
+tags: [analytics craft]
+hide_table_of_contents: false
+
+date: 2023-10-31
+is_featured: true
+
+---
+
+Hi all, I’m Kshitij, a senior software engineer on the Core team at dbt Labs.
+One of the coolest moments of my career here thus far has been shipping the new `dbt clone` command as part of the dbt-core v1.6 release.
+
+However, one of the questions I’ve received most frequently is guidance around “when” to clone that goes beyond [the documentation on “how” to clone](https://docs.getdbt.com/reference/commands/clone).
+In this blog post, I’ll attempt to provide this guidance by answering these FAQs:
+
+1. What is `dbt clone`?
+2. How is it different from deferral?
+3. Should I defer or should I clone?
+<!--truncate-->
+## What is `dbt clone`?
+
+`dbt clone` is a new command in dbt 1.6 that leverages native zero-copy clone functionality on supported warehouses to **copy entire schemas for free, almost instantly**.
+
+### How is this possible?
+
+Well, the warehouse “cheats” by only copying metadata from the `source` schema to the `target` schema; the underlying data remains at rest during this operation. 
+This metadata includes materialized objects like tables and views, which is why you see a **clone** of these objects in the target schema.
+
+In computer science jargon, `clone` makes a copy of the pointer from the `source` schema to the underlying data; after the operation there are now two pointers (`source` and `target` schemas) that each point to the same underlying data.
+
+## How is cloning different from deferral?
+
+On the surface, cloning and deferral seem similar – **they’re both ways to save costs in the data warehouse.**
+They do this by bypassing expensive model re-computations – clone by [eagerly copying](https://en.wikipedia.org/wiki/Evaluation_strategy#Eager_evaluation) an entire schema into the target schema, and defer by [lazily referencing](https://en.wikipedia.org/wiki/Lazy_evaluation) pre-built models in the source schema.
+
+Let’s unpack this sentence and explore its first-order effects:
+
+|                           | defer                                                                                                                                                              | clone                                                                                                                                    |
+|---------------------------|--------------------------------------------------------------------------------------------------------------------------------------------------------------------|------------------------------------------------------------------------------------------------------------------------------------------|
+| **How do I use it?**      | Implicit via the `--defer` flag                                                                                                                                    | Explicit via the `dbt clone` command                                                                                                     |
+| **What are its outputs?** | Doesn't create any objects itself, but dbt might create objects in the target schema if they’ve changed from those in the source schema.                           | Copies objects from source schema to target schema in the data warehouse, which are persisted after operation is finished.               |
+| **How does it work?**     | Compares manifests between source and target dbt runs and overrides ref to resolve models not built in the target run to point to objects built in the source run. | Uses zero-copy cloning if available to copy objects from source to target schemas, else creates pointer views (`select * from my_model`) |
+
+These first-order effects lead to the following second-order effects that truly distinguish clone and defer from each other:
+
+|                                                                          | defer                                                                                                                              | clone                                                                                  |
+|--------------------------------------------------------------------------|------------------------------------------------------------------------------------------------------------------------------------|----------------------------------------------------------------------------------------|
+| **Where can I use objects built in the target schema?**                  | Only within the context of dbt                                                                                                     | Any downstream tool (e.g. BI)                                                          |
+| **Can I safely modify objects built in the target schema?**              | No, since this would modify production data                                                                                        | Yes, cloning is a cheap way to create a sandbox of production data for experimentation |
+| **Will data in the target schema drift from data in the source schema?** | No, since deferral will always point to the latest version of the source schema                                                    | Yes, since clone is a point-in-time operation                                          |
+| **Can I use multiple source schemas at once?**                           | Yes, defer can dynamically switch between source schemas e.g. ref unchanged models from production and changed models from staging | No, clone copies objects from one source schema to one target schema                   |
+
+## Should I defer or should I clone?
+
+Putting together all the points above, here’s a handy cheat sheet for when to defer and when to clone:
+
+|                                                                           | defer | clone |
+|---------------------------------------------------------------------------|-------|-------|
+| **Save time & cost by avoiding re-computation**                           | ✅     | ✅     |
+| **Create database objects to be available in downstream tools (e.g. BI)** | ❌     | ✅     |
+| **Safely modify objects in the target schema**                            | ❌     | ✅     |
+| **Avoid creating new database objects**                                   | ✅     | ❌     |
+| **Avoid data drift**                                                      | ✅     | ❌     |
+| **Support multiple dynamic sources**                                      | ✅     | ❌     |
+
+To absolutely drive this point home:
+
+1. If you send someone this cheatsheet by linking to this page, you are deferring to this page
+2. If you print out this page and write notes in the margins, you have cloned this page
+
+## Putting it in practice
+
+Using the cheat sheet above, let’s explore a few common scenarios and explore whether we should use defer or clone for each:
+
+1. **Testing staging datasets in BI**
+
+    In this scenario, we want to:
+    1. Make a copy of our production dataset available in our downstream BI tool
+    2. To safely iterate on this copy without breaking production datasets
+
+    Therefore, we should use **clone** in this scenario
+
+2. **[Slim CI](https://discourse.getdbt.com/t/how-we-sped-up-our-ci-runs-by-10x-using-slim-ci/2603)**
+
+    In this scenario, we want to:
+    1. Refer to production models wherever possible to speed up continuous integration (CI) runs
+    2. Only run and test models in the CI staging environment that have changed from the production environment
+    3. Reference models from different environments – prod for unchanged models, and staging for modified models
+
+    Therefore, we should use **defer** in this scenario
+
+3. **[Blue/Green Deployments](https://discourse.getdbt.com/t/performing-a-blue-green-deploy-of-your-dbt-project-on-snowflake/1349)**
+
+    In this scenario, we want to:
+    1. Ensure that all tests are always passing on the production dataset, even if that dataset is slightly stale
+    2. Atomically rollback a promotion to production if tests aren’t passing across the entire staging dataset
+
+    In this scenario, we can use **clone** to implement a deployment strategy known as blue-green deployments where we build the entire staging dataset and then run tests against it, and only clone it over to production if all tests pass.
+
+
+As a rule of thumb, deferral lends itself better to continuous integration (CI) use cases whereas cloning lends itself better to continuous deployment (CD) use cases.
+
+## Wrapping Up
+
+In this post, we covered what `dbt clone` is, how it is different from deferral, and when to use each. Often, they can be used together within the same project in different parts of the deployment lifecycle. 
+
+Thanks for reading, and I look forward to seeing what you build with `dbt clone`.
+
+*Thanks to [Jason Ganz](https://docs.getdbt.com/author/jason_ganz) and [Gwen Windflower](https://www.linkedin.com/in/gwenwindflower/) for reviewing drafts of this article*

website/blog/authors.yml

@@ -306,6 +306,15 @@ kira_furuichi:
   name: Kira Furuichi
   organization: dbt Labs
 
+kshitij_aranke:
+  image_url: /img/blog/authors/kshitij-aranke.jpg
+  job_title: Senior Software Engineer
+  links:
+    - icon: fa-linkedin
+      url: https://www.linkedin.com/in/aranke/
+  name: Kshitij Aranke
+  organization: dbt Labs
+
 lauren_benezra:
   image_url: /img/blog/authors/lauren-benezra.jpeg
   job_title: Analytics Engineer