Prefect blocks store typed configuration that can be used across workflows and deployments. The most common use case for blocks is storing credentials used to access external systems such as AWS or GCP. Prefect supports a large number of common blocks and provides a Python SDK for creating your own.
Blocks and parametersBlocks are useful for sharing configuration across flow runs and between flows.For configuration that will change between flow runs, we recommend using parameters.

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How blocks work

There are three layers to a block: its type, a document, and a Python class.

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Block type

A block type is essentially a schema registered with the Prefect API. This schema can be inspected and discovered in the UI on the Blocks page. To see block types available for configuration, use prefect block type ls from the CLI or navigate to the Blocks page in the UI and click +. The block catalogue in the UI These types separate blocks from Prefect variables, which are unstructured JSON documents. In addition, block schemas allow for fields of SecretStr type which are stored with additional encryption and not displayed by default in the UI. Block types are identified by a slug that is not configurable. 
from prefect.blocks.core import Block


class Cube(Block):
    edge_length_inches: float


# register the block type under the slug 'cube'
Cube.register_type_and_schema()
Users should rarely need to register types in this way - saving a block document will also automatically register its type.

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Block document

A block document is an instantiation of the schema, or block type. A document contains specific values for each field defined in the schema. All block types allow for the creation of as many documents as you wish. Building on our example above: 
from prefect.blocks.core import Block


class Cube(Block):
    edge_length_inches: float


# instantiate the type with specific values
rubiks_cube = Cube(edge_length_inches=2.25)

# store those values in a block document 
# on the server for future use
rubiks_cube.save("rubiks-cube")


# instantiate and save another block document
tiny_cube = Cube(edge_length_inches=0.001)
tiny_cube.save("tiny")
Block documents can also be created and updated in the UI or API for easy change management. This allows you to work with slowly changing configuration without having to redeploy all workflows that rely on it; for example, you may use this to rotate credentials on a regular basis without touching your deployments.

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Block class

A block class is the primary user-facing object; it is a Python class whose attributes are loaded from a block document. Most Prefect blocks encapsulate additional functionality built on top of the block document. For example, an S3Bucket block contains methods for downloading data from, or upload data to, an S3 bucket; a SnowflakeConnector block contains methods for querying Snowflake databases. Returning to our Cube example from above: 
from prefect.blocks.core import Block

class Cube(Block):
    edge_length_inches: float

    def get_volume(self):
        return self.edge_length_inches ** 3

    def get_surface_area(self):
        return 6 * self.edge_length_inches ** 2


rubiks_cube = Cube.load("rubiks-cube")
rubiks_cube.get_volume() # 11.390625
The class itself is not stored server-side when registering block types and block documents. For this reason, we highly recommend loading block documents by first importing the block class and then calling its load method with the relevant document name.

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Pre-registered blocks

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Built-in blocks

Commonly used block types come built-in with Prefect. You can create and use these block types through the UI without installing any additional packages.
BlockSlugDescription
Custom Webhookcustom-webhookCall custom webhooks.
Discord Webhookdiscord-webhookCall Discord webhooks.
Local File Systemlocal-file-systemStore data as a file on a local file system.
Mattermost Webhookmattermost-webhookSend notifications through a provided Mattermost webhook.
Microsoft Teams Webhookms-teams-webhookSend notifications through a provided Microsoft Teams webhook.
Opsgenie Webhookopsgenie-webhookSend notifications through a provided Opsgenie webhook.
Pager Duty Webhookpager-duty-webhookSend notifications through a provided PagerDuty webhook.
Remote File Systemremote-file-systemAccess files on a remote file system.
SecretsecretStore a secret value. The value will be obfuscated when this block is logged or shown in the UI.
Sendgrid Emailsendgrid-emailSend notifications through Sendgrid email.
Slack Webhookslack-webhookSend notifications through a provided Slack webhook.
SMBsmbStore data as a file on a SMB share.
Twilio SMStwilio-smsSend notifications through Twilio SMS.
Built-in blocks should be registered the first time you start a Prefect server. If the auto-registration fails, you can manually register the blocks using prefect blocks register.For example, to register all built-in notification blocks, run prefect block register -m prefect.blocks.notifications.

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Blocks in Prefect integration libraries

Some block types that appear in the UI can be created immediately, with the corresponding integration library installed for use. For example, an AWS Secret block can be created, but not used until the prefect-aws library is installed. Find available block types in many of the published Prefect integrations libraries. If a block type is not available in the UI, you can register it through the CLI.
BlockSlugIntegration
ECS Taskecs-taskprefect-aws
MinIO Credentialsminio-credentialsprefect-aws
S3 Buckets3-bucketprefect-aws
Azure Blob Storage Credentialsazure-blob-storage-credentialsprefect-azure
Azure Container Instance Credentialsazure-container-instance-credentialsprefect-azure
Azure Container Instance Jobazure-container-instance-jobprefect-azure
Azure Cosmos DB Credentialsazure-cosmos-db-credentialsprefect-azure
AzureML Credentialsazureml-credentialsprefect-azure
BitBucket Credentialsbitbucket-credentialsprefect-bitbucket
BitBucket Repositorybitbucket-repositoryprefect-bitbucket
Databricks Credentialsdatabricks-credentialsprefect-databricks
dbt CLI BigQuery Target Configsdbt-cli-bigquery-target-configsprefect-dbt
dbt CLI Profiledbt-cli-profileprefect-dbt
dbt Cloud Credentialsdbt-cloud-credentialsprefect-dbt
dbt CLI Global Configsdbt-cli-global-configsprefect-dbt
dbt CLI Postgres Target Configsdbt-cli-postgres-target-configsprefect-dbt
dbt CLI Snowflake Target Configsdbt-cli-snowflake-target-configsprefect-dbt
dbt CLI Target Configsdbt-cli-target-configsprefect-dbt
Docker Containerdocker-containerprefect-docker
Docker Hostdocker-hostprefect-docker
Docker Registry Credentialsdocker-registry-credentialsprefect-docker
Email Server Credentialsemail-server-credentialsprefect-email
BigQuery Warehousebigquery-warehouseprefect-gcp
GCP Cloud Run Jobcloud-run-jobprefect-gcp
GCP Credentialsgcp-credentialsprefect-gcp
GcpSecretgcpsecretprefect-gcp
GCS Bucketgcs-bucketprefect-gcp
Vertex AI Custom Training Jobvertex-ai-custom-training-jobprefect-gcp
GitHub Credentialsgithub-credentialsprefect-github
GitHub Repositorygithub-repositoryprefect-github
GitLab Credentialsgitlab-credentialsprefect-gitlab
GitLab Repositorygitlab-repositoryprefect-gitlab
Kubernetes Cluster Configkubernetes-cluster-configprefect-kubernetes
Kubernetes Credentialskubernetes-credentialsprefect-kubernetes
Kubernetes Jobkubernetes-jobprefect-kubernetes
Shell Operationshell-operationprefect-shell
Slack Credentialsslack-credentialsprefect-slack
Slack Incoming Webhookslack-incoming-webhookprefect-slack
Snowflake Connectorsnowflake-connectorprefect-snowflake
Snowflake Credentialssnowflake-credentialsprefect-snowflake
Database Credentialsdatabase-credentialsprefect-sqlalchemy
SQLAlchemy Connectorsqlalchemy-connectorprefect-sqlalchemy
Anyone can create a custom block type and, optionally, share it with the community.

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Additional resources

You can manage blocks with the Prefect CLI. You can manage blocks with the Terraform provider for Prefect.