Skip to main content
Version: 1.0 (alpha)

Defining Applications

The wasmCloud Application Deployment Manager uses the Open Application Model to define application specifications. Because this specification is extensible and platform agnostic, it makes for an ideal way to represent applications with metadata specific to wasmCloud. Don't worry if OAM seems overwhelming, you don't need to know much about it. We're using it as a way of defining application components in a flexible way that's familiar to a lot of people who have been working in the cloud space.

In this model, an application specification is a set of metadata about the app, as well as a list of components. Each component within an application is decorated with various traits. These core building blocks allow us to make it very easy to define incredibly powerful deployments. wasmCloud defines a number of traits that are specific to our hosts, but let's go through the model from top to bottom.

Application​

The application is the top-most definition in an OAM specification. The metadata about the application that we're mostly concerned with are the name and version:

yaml
apiVersion: core.oam.dev/v1beta1
kind: Application
metadata:
  name: my-example-app
  annotations:
    version: v0.0.1
    description: "This is my app"
spec: ...

wadm considers the name field to be a globally unique value, and the version number to follow semantic versioning conventions, though we only enforce that it looks like a semantic version - we have no way to detect if you've produced a breaking change in a patch release.

When application specifications are stored, they are keyed by name and a history of all versions is maintained, with the most recently pushed version being considered the newest. We do not make assumptions about whether one version string (e.g. is v1.10 is newer than v1.1?) is newer or older than any other. Version history is based purely on time of storage.

Components​

While OAM allows us to define any component in a specification, there are only a few components with which wadm is concerned:

  • component - represents a specification of a component
  • capability - represents a specification of a capability provider

Within the components field of a specification, you define an application component as follows:

yaml
spec:
  components:
    - name: echo
      type: component
      properties:
        image: wasmcloud.azurecr.io/echo:0.3.8
      traits: ...

The image property of the component component contains a file reference, an OCI image reference URL, or a Bindle image reference. When you attempt to store this model, wadm will reach out to the artifact repository and attempt to pull some metadata about that image (such as its primary key, embedded security information, etc). This means that storing application specifications with invalid/unreachable OCI references is not allowed.

To launch a component from a local file, you should prefix the path with file://, as follows:

yaml
spec:
  components:
    - name: echo
      type: component
      properties:
        image: file:///Users/wasmcloud/echo/build/echo_s.wasm
      traits: ...
info

When launching a component from a local file, ensure that the environment variable WASMCLOUD_ALLOW_FILE_LOAD=true is set when you launch wasmCloud. This is the default for hosts running with wash up. Only absolute paths are supported, since clients cannot reliably assume which directory the target host was started from. When running hosts locally with wash up for development, however, it is possible to use relative paths (which the host then converts to an absolute path) for convenience.

To define a capability provider, we include a capability component, as follows:

yaml
- name: keyvalue
      type: capability
      properties:
        image: wasmcloud.azurecr.io/kvredis:0.22.0

Just like when manipulating a lattice imperatively, the things that differentiate one capability provider from another are its contract and its public key (which we obtain by looking up the image).

Traits​

Traits are, as their name applies, metadata associated with a component. The OAM trait system is completely extensible, so as wadm gains more functionality, it can support more traits. Right now, the following traits are supported:

  • spreadscaler
  • daemonscaler
  • link

Spread Scaler​

The spreadscaler trait contains a specification for how you would like to scale a set number of instances of a component. We call it a spread scaler because you declare how you would like the instances of that component spread across the hosts within your lattice by specifying targets with host labels. You can think of this like affinity and anti-affinity rules combined with a scale specification.

Take a look at the following sample spreadscaler spec:

yaml
traits:
- type: spreadscaler
    properties:
    replicas: 4
    spread:
        - name: eastcoast
        requirements:
            zone: us-east-1
        weight: 80
        - name: westcoast
        requirements:
            zone: us-west-1
        weight: 20

This definition states that, for this component (a spread scaler can apply to a component or capability), you want a total of 4 instances, with 80% of them going to hosts with the zone label set to us-east-1 and 20% of them going to hosts with the zone label set to us-west-1. Because this system uses labels as selectors, and you can set any arbitrary label on your hosts, you can define practically any conditions for the spread rules.

If you leave the requirements section blank then all hosts will be considered possible targets for that component. You can also leave the spread definition off so you can simply state that you would like n replicas and you don't care where or how you get them:

yaml
traits:
- type: spreadscaler
    properties:
    replicas: 4

⚠️ NOTE: if you define a label/value pair requirement and wadm is unable to find hosts that match this constraint, it will consider this a deployment failure and will not fall back to arbitrary placement.

Daemon Scaler​

The daemonscaler trait is an alternative to the spreadscaler trait. It is a trait that deploys a certain number of instances of a component on every host in your lattice that matches specified labels. Take a look at the following sample daemonscaler spec:

yaml
traits:
- type: daemonscaler
    properties:
      replicas: 4
      spread:
          - name: eastcoast
          requirements:
              zone: us-east-1
          - name: westcoast
          requirements:
              zone: us-west-1

Note that this looks similar to the above spreadscaler spec, but the daemonscaler is responsible for running a certain number of instances of a component on every host that matches the label requirements. So, instead of running 4 total instances, it will run 4 instances on every host that either has the zone label set to us-east-1 or us-west-1. If you leave off the spread key entirely, it will run the specified number of instances on every host in your lattice.

For the Kubernetes developer

The daemonscaler works just like a Kubernetes DaemonSet, spreading components across all hosts that match the label requirements.

The link trait links two components together with a set of configuration values.

yaml
# Link to KVredis with local connection
- type: link
  properties:
    target: keyvalue
    namespace: wasi
    package: keyvalue
    interfaces:
      - atomic
      - eventual
    target_config:
      - name: redis-connect-local
        properties:
          URL: redis://127.0.0.1:6379

The value of the target field is a component whose name field matches that. The values is a simple key-value map that will be passed as link definition configuration data at deployment time. Note that the value here must be a string, so if you're passing a value like "false" or "125" ensure that you wrap it in single or double quotes.

Putting it All Together​

So far we've seen bits and pieces of the application specification YAML. The following is an example of a complete manifest:

yaml
apiVersion: core.oam.dev/v1beta1
kind: Application
metadata:
  name: kvcounter-rust
  annotations:
    version: v0.0.1
    description: "Kvcounter demo"
    experimental: true
spec:
  components:
    - name: kvcounter
      type: actor
      properties:
        image: file://./build/http_hello_world_s.wasm
      traits:
        # Govern the spread/scheduling of the actor
        - type: spreadscaler
          properties:
            replicas: 1
        # Link to KVredis with local connection
        - type: link
          properties:
            target: keyvalue
            namespace: wasi
            package: keyvalue
            interfaces:
              - atomic
              - eventual
            target_config:
              - name: redis-connect-local
                properties:
                  URL: redis://127.0.0.1:6379

    # Add a capability provider that mediates HTTP access
    - name: httpserver
      type: capability
      properties:
        image: wasmcloud.azurecr.io/httpserver:0.19.1
      traits:
        # Link the HTTP server, and inform it to listen on port 8080
        # on the local machine
        - type: link
          properties:
            target: http-hello-world
            namespace: wasi
            package: http
            interfaces:
              - incoming-handler
            source_config:
              - name: listen-config
                properties:
                  ADDRESS: 127.0.0.1:8080
    # Add a capability provider that interfaces with the Redis key-value store
    - name: keyvalue
      type: capability
      properties:
        image: wasmcloud.azurecr.io/kvredis:0.22.0

⚠️ NOTE: while wadm can "claim" resources like components and providers and differentiate between wadm-managed versus unmanaged, this is not so with link definitions. If you manually push new link definition information that overrides or conflicts with the link definitions in your wadm spec, you could experience unexpected behavior until wadm corrects for it. You must take extreme care that wadm-managed link definitions don't conflict with external link definitions.