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Version: 1.x

Linking at Runtime

Overview​

A runtime link is a declared connection between a component and another entity. Runtime links are typically defined in Wadm deployment manifests and resolved on instantiation, regardless of where the linked entities are running.

Link definitions have both a source and a target, corresponding to the component imports and exports, and specify the relevant WASI interfaces.

  • In a link declaration, the source is the importer. You can think of it as the source of the requirement that needs to be fulfilled.
  • The target is the exporter. Specifically, it is the function exposed in an export that the source will call.

Link definitions are unidirectional: the source (importer) can call the target (exporter), but not the other way around.From a component's point of view, code only refers to an interface, e.g. wasi-keyvalue. Components do not know which specific entity is on the other side of a link.

Linking at runtime makes sense when...

  • You want components to be able to scale and failover independently
  • You want to be able to update components independently
  • Data sources for each component are distributed, and you want each component to run as close to its data source as possible to reduce data transmitted over the wire

Runtime links are typically defined in Wadm deployment manifests. Here is an example of a definition for a link from a component (importing on the keyvalue interface) to a provider (exporting on the same interface):

yaml
components:
  - name: http-component
    type: component
    properties:
      image: file://./build/http_hello_world_s.wasm
    traits:
      - type: link
        properties:
          name: foo
          target: kvredis
          namespace: wasi
          package: keyvalue
          interfaces: [store]
          target_config:
            - name: redis-url
              properties:
                url: redis://127.0.0.1:6379

A link is always defined as a trait of the source—that is, the importer. In addition to a target, the link definition has properties including...

  • name, an optional unique identifier for this link which can be used to distinguish between different usages of the same interface. If not specified, this value is default.
  • target, which specifies the name of the exporting entity to be called
  • namespace, which in the example above refers to wasi
  • package, which specifies the particular interface group used
  • interfaces, which lists specific store interface from wasi-keyvalue
  • target_config, defining configuration data to pass to a provider

When a provider acts as a source rather than a target, it can instead take a source_config field. You can find configuration options for first-party providers in the repositories for the specific providers in GitHub.

You can use the wit2wadm tool to automatically generate a Wadm deployment manifest for a given component.

In wasmCloud, components may be linked to...

Built-in providers​

The wasmCloud host provides several functions available for import by components through built-in providers:

For example, the wasmCloud host's built-in logging function could satisfy a component's logging import. Built-in providers form wasmCloud's trusted compute base—they represent highly-vetted code that provides basic, commonly-used functionalities.

To use a built-in provider, you need only include the import in a component's world.wit and call it using the WASI interface in question. It is not necessary to include built-in providers in a deployment manifest—since they are already present on any wasmCloud host, the host will fulfill the imports automatically.

Example: Logging

Generate a new Rust project:

shell
wash new component hello --template-name hello-world-rust

Add logging to the project's world.wit file in the wit directory:

wit
package wasmcloud:hello;

world hello {
  import wasi:logging/logging; 

  export wasi:http/incoming-handler@0.2.0;
}

Update the Rust code to add a logging message:

rust
wit_bindgen::generate!();

use exports::wasi::http::incoming_handler::Guest;
use wasi::http::types::*;

struct HttpServer;

impl Guest for HttpServer {
    fn handle(_request: IncomingRequest, response_out: ResponseOutparam) {
        // Add logging
        wasi::logging::logging::log(
            wasi::logging::logging::Level::Info,
            "",
            &format!("Hello to your logs from Rust"),
        );
        let response = OutgoingResponse::new(Fields::new());
        response.set_status_code(200).unwrap();
        let response_body = response.body().unwrap();
        response_body
            .write()
            .unwrap()
            .blocking_write_and_flush(b"Hello from Rust!\n")
            .unwrap();
        OutgoingBody::finish(response_body, None).expect("failed to finish response body");
        ResponseOutparam::set(response_out, Ok(response));
    }
}

export!(HttpServer);

Remember: it is not necessary to include built-in providers in a deployment manifest.

Launch a local wasmCloud host with wash up and deploy:

shell
wash app deploy wadm.yaml

When you invoke the component with curl localhost:8080, in addition to an HTTP response you will see a log message:

shell
INFO handle_invocation{params=IncomingHttpHandle component_id="http_hello_world-http_component" component_ref="file:///Users/user/wasmcloud/wip/linking/logtest/build/http_hello_world_s.wasm"}:log: wasmcloud_host::wasmbus::handler: Hello to your logs from Rust component_id="http_hello_world-http_component" level=Level::Info context=""

Other components​

A component may be linked to another component at runtime (over the lattice) or build (via composition).

Linking components at runtime is a two-step process:

  1. Imports and exports must be declared in the components' world.wit.
  2. Links must be defined in your Wadm deployment manifest.
Example: Two Components

Here is an example manifest for an example application comprised of two components and an http-server provider:

yaml
apiVersion: core.oam.dev/v1beta1
kind: Application
metadata:
  name: http-hello-world
  annotations:
    version: v0.0.2
    description: 'HTTP hello world demo in Rust, using the WebAssembly Component Model and WebAssembly Interfaces Types (WIT)'
spec:
  components:
    - name: http-component
      type: component
      properties:
        image: file://./build/http_hello_world_s.wasm
      traits:
        # Govern the spread/scheduling of the component
        - type: spreadscaler
          properties:
            instances: 1
        - type: link
          properties:
            target: pong-component
            namespace: example
            package: pong
            interfaces: [pingpong]

    - name: pong-component
      type: component
      properties:
        image: file://../pong/virt.wasm
      traits:
        # Govern the spread/scheduling of the component
        - type: spreadscaler
          properties:
            instances: 1

    # Add a capability provider that enables HTTP access
    - name: httpserver
      type: capability
      properties:
        image: ghcr.io/wasmcloud/http-server:0.22.0
      traits:
        # Link the httpserver to the component, and configure the HTTP server
        # to listen on port 8080 for incoming requests
        - type: link
          properties:
            target: http-component
            namespace: wasi
            package: http
            interfaces: [incoming-handler]
            source_config:
              - name: default-http
                properties:
                  address: 127.0.0.1:8080
  • The http-component is exporting on the wasi-http interface and importing on a custom "pingpong" interface. It is the importer in relation to the pong-component, so it is the source for that link.
  • The pong-component is exporting on the custom "pingpong" interface. This is its only link relationship, and it is the exporter, so no links are defined as a trait of pong-component.
  • The httpserver provider is importing on the wasi-http interface. It is the importer in relation to http-component, so it is the source for that link, and basic configuration is performed in the source_config.

If you would like to build and run the example described by this manifest, follow Steps 1 and 2 in the composition example readme.

Remember that the wit2wadm tool is available to help you automatically generate a Wadm deployment manifest for a given component. Because it extrapolates link definitions from a component's WIT interfaces, it can be very helpful for not only generating manifests but understanding linking relationships in your application.

Providers​

Providers are longer-lived host plugins which may facilitate connections to external resources. A component may be linked at runtime to a provider, which may be running either on the same wasmCloud host or another host on the lattice.

A provider only ever dispatches messages to—or receives messages from—components. When a link is established for a particular component, the provider can remember that component's identity and use it for subsequent dispatches. As a result, the only information a linked provider needs is the component's identity. This identity is typically used for managing specific resources on behalf of the component, such as database connections, open TCP sockets, etc.

Example: Unpacking the Quickstart

The wasmCloud Quickstart (running through the "Adding capabilities" section) provides a great example of a component using two different providers. The component you build in the tutorial exports on the wasi-http interface (linking with the http-server provider) and imports on the wasi-keyvalue interface (linking with the kvredis provider).

Here's the manifest for the completed Quickstart example:

yaml
apiVersion: core.oam.dev/v1beta1
kind: Application
metadata:
  name: hello-world
  annotations:
    version: v0.0.1
    description: 'HTTP hello world demo'
spec:
  components:
    # The component with our business logic
    - name: http-component
      type: component
      properties:
        image: ghcr.io/wasmcloud/components/http-hello-world-rust:0.1.0
      traits:
        - type: spreadscaler
          properties:
            instances: 1
        # Link definition for http-component -> kvredis
        - type: link
          properties:
            target: kvredis
            namespace: wasi
            package: keyvalue
            interfaces: [atomics, store]
            target_config:
              - name: redis-url
                properties:
                  url: redis://127.0.0.1:6379
    # The kvredis provider
    - name: kvredis
      type: capability
      properties:
        image: ghcr.io/wasmcloud/keyvalue-redis:0.27.0
    # The httpserver provider
    - name: httpserver
      type: capability
      properties:
        image: ghcr.io/wasmcloud/http-server:0.22.0
      traits:
        # Link definition for httpserver -> http-component
        - type: link
          properties:
            target: http-component
            namespace: wasi
            package: http
            interfaces: [incoming-handler]
            source_config:
              - name: default-http
                properties:
                  address: 127.0.0.1:8080

  • The http-component is importing functions on the keyvalue interface. It is the importer in relation to kvredis, meaning that it is the source for the link definition: the link is defined as a trait of http-component.

  • The http-component is exposing a function on the http interface. It is the exporter in relation to the http-server provider—in this case, the http-server provider is the source and the http-component is the target.

  • Both the kvredis and httpserver providers are configured in the link definition, using a target_config field (if the provider is the target) or a source_config field (if the provider is the source).

Remember that the wit2wadm tool is available to help you automatically generate a Wadm deployment manifest for a given component. Because it extrapolates link definitions from a component's WIT interfaces, it can be very helpful for not only generating manifests but understanding linking relationships in your application.

In some cases, you may wish to assign names to links. This can be useful when a source component needs to use the same interface with different configurations under different circumstances.

  • For example, a component may use the wasi:keyvalue interface to link to both a local key-value cache for ephemeral storage and a cloud-based store for more resilient storage.
  • In this example, you're using the same key-value interface and the same operations, but the interface needs to be configured differently. These different configurations of the interface can be differentiated with a link name.

Link names can be set imperatively with the wash link put command or declaratively via Wadm manifest, but are typically easiest to manage from a manifest. Link names are an optional field in a link definition and default to the value default if not specified. Below is an excerpt of a manifest defining names for two differently configured links:

yaml
- type: link
  properties:
    name: foo
    target: your_provider_or_component
    namespace: wasi
    package: keyvalue
    interfaces: [store]
    target_config:
      # foo configuration
- type: link
  properties:
    name: bar
    target: your_provider_or_component
    namespace: wasi
    package: keyvalue
    interfaces: [store]
    target_config:
      # bar configuration

Once a link name is defined, in your component or provider's code, you can use wasmCloud's wasmcloud:bus interface to make a call to a target by link name. You can include the wasmcloud:bus interface by importing the interface in your top-level world:

wit
world component {
  import wasmcloud:bus/lattice@1.0.0;
}

Below are snippets of sample code that use different link names across languages supported by wasmCloud:

rust
let yourinterface = wasmcloud::bus::lattice::CallTargetInterface::new(
    "wasi",
    "keyvalue",
    "store",
);

// Sets the operative link for interface to the named link foo
wasmcloud::bus::lattice::set_link_name("foo", vec![yourinterface]);
// Calls over link foo to perform a keyvalue operation
let x = wasi::keyvalue::store::function(args);

// Sets the operative link for interface to the named link bar
wasmcloud::bus::lattice::set_link_name("bar", vec![yourinterface]);
// Calls over link bar to perform a keyvalue operation
let y = wasi::keyvalue::store::function(args);

Designed for flexibility at runtime​

Links are first-class citizens of a lattice. A link can be declared (or removed) before or after any of the parties of that link are running. Each time a provider is started, it is provided with a list of pre-existing links. Additionally, the provider is notified whenever a new link is declared, or an existing link is removed.

The ability to update links at runtime is a powerful feature of wasmCloud. There are several scenarios where this is useful, including:

  • Swapping to an alternate provider implementation, such as an in-memory cache vs. an external data store.
  • Upgrading a provider, or failing over to a backup provider.
  • Routing requests to a provider running with specific characteristics, such as locality or configuration.

Runtime links can be created imperatively using the wash CLI, which can be helpful for development workflows and use-cases such as those above. See the wash link subcommand documentation for more information.

note

"Order of operations" doesn't matter in runtime linking: while configuring an application, low-level commands to set links and start resources can arrive in any order. Additionally, all providers must treat messages to set/remove links as idempotent.

Keep reading​