Server Downlinks

In the Downlinks guide, we discuss downlinks - bidirectionally-streaming, persistent subscriptions to lanes - and went through an example of a client-side downlink. This guide will showcase downlinks between two Swim servers.

Server-side downlinks provide a simple and efficient way to integrate Swim applications allowing them to be linked or chained together with ease.

There are two big things to manage when dealing with downlinks: data and connections. This guide will focus heavily on the former; we will expand on connection management in a future, more advanced article.



All downlink classes can be imported from package swim.api.downlink.


Downlinks must be instantiated against Swim refs, i.e. specific server-side or client-side objects. Although several permutations exist, the builder pattern is the same each time:

  1. Invoke downlink() against your ref for an event downlink, or downlinkFoo() for a foo downlink (e.g. downlinkMap() for a map downlink)
  2. Build the downlink’s hostUri using hostUri() (this step can only be omitted if your Swim ref is server-side, and you are targeting a lane within the same server), the downlink’s nodeUri using nodeUri(), and the downlink’s laneUri using laneUri()
  3. Override any lifecycle callback functions, which default to no-ops
  4. In strongly-typed languages (Java, Typescript), optionally parametrize the downlink
  5. Optionally set the keepSynced (pull all existing data from a lane before processing new updates; defaults to false) and keepLinked (enable consistent reads from the downlink (unnecessary for write-only downlinks); defaults to true) flags
  6. Invoke open() on the downlink to initiate data flow
  7. When finished, invoke close() on the downlink to stop data flow

Lifecycle callbacks and updating lanes

Every event downlink has a customizable onEvent(V event) callback function that specifies the action to take upon every event received by the target lane.

For all other (i.e. lane-specific) downlinks, recall that every data-storing lane can be acted upon by methods specific to that lane type (e.g. set for value lanes; put, remove, drop, take, and clear for map lanes). These options also exist on correctly-configured lane-specific downlinks. Furthermore, for every such method foo, each downlink has a didFoo(Object... args) method that follows similar lifecycle semantics to onEvent(), but with more useful callback parameters. For example, every MapDownlink<K, V> has access to didUpdate(K key, V newValue, V oldValue), didRemove(K key, V oldValue), didDrop(int dropCount), didTake(int keepCount), and didClear() methods.


Unlike with lanes, which additionally offer parametrized methods, downlink parametrization requires providing ``swim.structure.Forms` through a builder pattern.


Server-side, downlinks can be issued either against plane contexts:

// swim/basic/SupplierPlane
package swim.basic;

import swim.api.SwimRoute;
import swim.api.agent.AgentRoute;
import swim.api.plane.AbstractPlane;
import swim.kernel.Kernel;
import swim.server.ServerLoader;
import swim.structure.Form;

public class SupplierPlane extends AbstractPlane {

  public final static String WAREHOUSE_HOST_URI = "warp://localhost:9001";

  AgentRoute<SupplierAgent> supplierAgentType;

  AgentRoute<CustomerAgent> customerAgentType;

  public static void main(String[] args) {
    final Kernel kernel = ServerLoader.loadServerStack();
    final ActorSpace plane = (ActorSpace) kernel.getSpace("supplier");


    //Create a value downlink to a different Swim server directly from this plane
        .didSet((newValue, oldValue) -> {
          logMessage("latest supply id received at warehouse: " + newValue);

or against Agent contexts using the AbstractAgent.downlinkFoo() methods, both as fields within Agents, and as local variables within methods (latter demonstrated here):

// swim/basic/
package swim.basic;

import swim.api.SwimLane;
import swim.api.agent.AbstractAgent;
import swim.api.lane.CommandLane;
import swim.structure.Form;
import swim.structure.Text;

import java.util.Objects;

public class CustomerAgent extends AbstractAgent {

  public CommandLane<String> register = this.<String>commandLane()
      .onCommand(location -> {

  private void addStockNotificationDownlink(final String location) {
    final String warehouseNodeUri = "/warehouse/" + location;

    //Create a value downlink to a different Swim server from an agent
        .didSet((newValue, oldValue) -> {
          if (!Objects.equals(newValue, oldValue))
            logMessage("customer received new stock notification, resupply: " + newValue);

Client-side downlinks have been showcased in the Downlinks guide.

Try It Yourself

A standalone project that combines all of these snippets and handles any remaining boilerplate is available here.