Callable message flows

A message flow can call another flow directly. You can deploy both flows to IBM® App Connect Enterprise or IBM App Connect on IBM Cloud®, or you can deploy one flow to IBM App Connect Enterprise and one flow to IBM App Connect on IBM Cloud.

A message flow can complete many different actions. If any of those actions are labor-intensive, you can split them from the main flow and complete them somewhere else. Callable flows also facilitate reuse because they can be called by multiple message flows. You can split your flows between different applications in the same integration server, or between different integrations servers, which can also be in different integration nodes. Your callable flows must be in applications. You cannot deploy callable flows in libraries or integration projects.

Some parts of a message flow might logically belong in a specific location. For example, if your flow queries an on-premises database, performance is better if that part of the flow remains on premises. But if part of your flow queries a website multiple times, performance might be improved by running that part of the flow in the cloud. You can call the cloud-based flow from your on-premises flow, and the flow in the cloud does not use any of your on-premises resources.

If your callable flows are both deployed to IBM App Connect on IBM Cloud, they can communicate with each other as soon as you deploy them. You do not need to set up communication between them. You also do not need to set up communication if both flows are deployed to the same integration server on IBM App Connect Enterprise.

If you are splitting processing between different integration servers, or between IBM App Connect Enterprise and IBM App Connect on IBM Cloud, your flows communicate by using a Switch server and connectivity agents. The Switch server is a special type of integration server that routes data. You cannot deploy anything to the Switch server. The connectivity agents contain the certificates that your flows require to communicate securely with the Switch server. The connectivity agents must be running in the integration servers where you have deployed your on-premises message flows.

You can split processing synchronously between message flows by using the CallableFlowInvoke node in the calling flow, and CallableInput and CallableReply nodes in the callable flow. Alternatively, you can split processing asynchronously between message flows, by using the CallableFlowAsyncInvoke node in the calling flow, CallableInput and CallableReply nodes in the callable flow, and the CallableFlowAsyncResponse node in the response flow. You can also choose to share data between the flows that contain these asynchronous nodes (the calling flow and the response flow) by storing and retrieving data in the UserContext folder in the environment tree. For more information about sharing data between flows, see Sharing data between a calling flow and a response flow.

Note: This topic is about using the callable flow technique between App Connect Enterprise message flows. You can also use the callable flows technique with on-cloud event-driven flows and API flows in IBM App Connect on IBM Cloud. Such on-cloud flows can be called by an App Connect Enterprise CallableFlowInvoke node, and such on-cloud flows can use the Invoke action node to call other on-cloud event-driven flows (that have the Input event node) or App Connect Enterprise message flows (that have the CallableInput node) either on cloud or on premises. For example, see the tutorial: Sharing data and processing from on-premises activities with on-cloud SaaS applications, by using an on-cloud callable flow.

You can split flows into multiple callable flows. However, for simplicity, the following scenarios assume that you are using two App Connect Enterprise message flows only: a calling flow and a callable flow.

Scenarios

Splitting flows synchronously between applications on the same integration server

A message flow in an IBM App Connect Enterprise integration server calls a message flow in the same integration server, but in a different application. For this scenario, you do not need to set up a connection agent. This scenario is supported on all operating systems.
The diagram shows two applications in the same integration server.  The CallableFlowInvoke node in a flow in application one calls the CallableInput node of a flow in application 2.

Splitting flows synchronously between applications on different integration servers

A message flow in an IBM App Connect Enterprise integration server calls a message flow in a different integration server.
The diagram shows two applications in two different integration servers. Each integration server is in a separate integration node.  The CallableFlowInvoke node in a flow in application 1 uses a Switch server to call the CallableInput node of a flow in application 2.

If your flows are split between different integration servers in IBM App Connect Enterprise, you must run a command that creates some configuration files. You use one of the configuration files to create the Switch server, and the other file to configure connectivity agents for each integration server. The integration servers can be in the same integration node or different integration nodes. This scenario is supported on Windows and Linux® only.

Splitting flows synchronously between IBM App Connect Enterprise and IBM App Connect on IBM Cloud

A message flow in IBM App Connect Enterprise calls a message flow in IBM App Connect on IBM Cloud, or a message flow in IBM App Connect on IBM Cloud calls a message flow in IBM App Connect Enterprise.
The diagram shows an application in Integration Bus, and an application in App Connect Enterprise.  The CallableFlowInvoke node in a flow in application 1 in Integration Bus uses a Switch server in the cloud to call the CallableInput node of a flow in application 2 in App Connect Enterprise.
The diagram shows an application in App Connect Enterprise, and an application in Integration Bus.  The CallableFlowInvoke node in a flow in application 1 in the cloud uses a Switch server in the cloud to call the CallableInput node of a flow in application 2 in Integration Bus.

If your flows are split between IBM App Connect Enterprise and IBM App Connect on IBM Cloud, the Switch server is managed by IBM App Connect on IBM Cloud. You download a file from the cloud that configures an on-premises connectivity agent. This scenario is supported on Windows and Linux only.

Splitting flows asynchronously in IBM App Connect Enterprise, or between IBM App Connect Enterprise and IBM App Connect on IBM Cloud

A message flow contains a CallableFlowAsyncInvoke node, which calls a second (callable) message flow containing a CallableInput node and a CallableReply node. A third (response) flow contains a CallableFlowAsyncResponse node.

When a message is passed into the calling message flow, the CallableFlowAsyncInvoke node sends the contents of the message body and environment folders to the CallableInput node of the callable flow, and then completes immediately. When the callable flow completes processing, the CallableReply node sends the message body and environment folder data to the CallableFlowAsyncResponse node in the response flow.

If your flows are split between different integration servers, you must create a Switch server (which routes data) and connectivity agents to allow the flows to communicate securely. For more information, see Preparing the environment for callable flows.

Flow configuration

When you decide how to split your message flow processing, create your message flows. The calling flow includes a CallableFlowInvoke or CallableFlowAsyncInvoke node, which calls a CallableInput node in a second (callable) flow. When the message is processed by the callable flow, a CallableReply node returns information to either the CallableFlowInvoke node in the calling flow (when flows that are split synchronously) or the CallableFlowAsyncResponse node (when flows that are split asynchronously). The calling flow uses a combination of the application name and the endpoint name of the CallableInput node to identify which callable flow to call.
The diagram shows that a CallableFlowInvoke node in the main message flow calls a remote flow.  The remote flow completes some processing, then responds to the main flow.

In this example, the CallableFlowInvoke node parses the incoming message in full so that it is in a suitable format to send to the CallableInput node. Therefore, you should validate the message before it reaches the CallableFlowInvoke node. If the message fails validation at this point, it can be rolled back.

Multiple message flows can call the same callable flow. Therefore, your callable flow might complete a set of actions that are common to multiple flows.
The illustration shows 3 different calling flows calling the same callable flow.  One calling flow is on the same integration server as the callable flow, but the other 2 calling flows are on different integration servers.

You can add multiple CallableFlowInvoke or CallableFlowAsyncInvoke nodes to a message flow. You can also include CallableFlowInvoke or CallableFlowAsyncInvoke nodes in your callable flows. Application and endpoint name pairs must be unique on a single integration server. You can have multiple callable flows that have the same application and endpoint names, but they must be in different integration servers. In this case, the Switch server acts as a load balancer.

Deployment

After you develop your message flows, you package the containing applications into BAR files and deploy them to the appropriate locations. If the flow accesses on-premises resources, deploy its BAR file on premises. If the flow will run in the cloud, upload its BAR file to IBM App Connect on IBM Cloud.

When the calling flow receives an input message, the CallableFlowInvoke (or CallableFlowAsyncInvoke) node uses the application name and the Endpoint Name on the CallableInput node to identify and start the callable flow.

For detailed information about how to configure callable message flows, see Splitting the processing of message flows.