Wishbone servers are all about event flow flexibility. In this article we will cover the role of the different built-in flow modules and how to shape the flow of events along with some practical examples.


What are flow modules?

Wishbone module queues can only have one direct connection to another module's queue. It's not possible to connect multiple queues directly to each other.

To setup advanced event routing logic, Wishbone includes the flow module category containing modules which decide the path events follow whilst traveling through the server from one module to the other.

Flow modules do not alter the content of events but merely shovel them between the connected module queues based on the routing logic properties of the flow module itself.

This blog post discusses each of the built-in flow modules including a minimal practical example to illustrate the possibilities 1

Examples

Fanout

The fanout message pattern done by the wishbone.flow.fanout is a well known messaging pattern. Incoming events are simply duplicated to each outgoing queue.

Example use case

A TCP server which accepts, duplicates and forwards the received events to 3 separate TCP servers in different availability zones:


Funnel

The funnel message pattern is the inverse of the fanout pattern and is handled by the wishbone.flow.funnel module. Events of multiple incoming queues are forwarded to 1 outgoing queue.

Example use case

A TCP server which receives JSON as well as MSGPack data and indexes that into Elasticsearch.


Fresh

The wishbone.flow.fresh module shovels incoming events from the inbox queue to the outbox queue without modifying them. If however no data has been submitted to the inbox queue in the last x seconds, a new event with a custom payload is generated and submitted into the timeout queue for another module to process. When the event stream recovers another event with a custom recovery payload is generated and submitted to the timeout queue.

This module is practical to trigger some process one way or the other when no data arrives into the Wishbone server for a configurable amount of time.

Example use case

A server which accepts JSON data over http and validates data against JSON schema 2 prior to forwarding it to RabbitMQ for further processing. We know the incoming stream of data is continuous, if not something is wrong and we want to send an alert event to Pagerduty.

Roundrobin

The wishbone.flow.roundrobin module pretty much does what it says. It takes events from its inbox queue and forwards them in a round robin fashion to the connected modules.

Example use case

A server which collects metrics from Elasticsearch and submits them to a set of Graphite relay nodes in order to spread the load equally over them.

Switch

The wishbone.flow.switch module can reroute the incoming events from one queue to another connected queue by dynamically setting the destination using a lookup value or by submitting a message to the module's switch queue.

Example use case

Temporarily redirect incoming webhook events from one RabbitMQ server to another.

By default incoming events are routed to the rabbitmq-001.az1.company.local RabbitMQ instance.

Switching to the other backend would involve submitting an event with the queue name to the /switch endpoint of the input module:

$  echo backend_az_2|curl -d @- http://localhost:19283/switch

Tippingbucket

The wishbone.flow.tippingbucket module buffers incoming events and flushes the buffered events as a bulk event to the next module.

Example use case

Instead of submitting one metric at a time to Graphite it's much more efficient to submit multiple metrics at once and hereby limit the number of TCP connects.

We extend the roundrobin example to submit 500 metrics at once to each Graphite relay server.

Final words

We have covered the built-in Wishbone flow modules in this article along with some examples clarifying the use case of each of them so it might inspire you to create a setup suited for your specific use case.

Please go ahead and give Wishbone a try and I'd greatly welcome feedback and ideas.

Footnotes:

  1. Some of the examples require external Wishbone modules to be installed.
  2. An external flow module to validate JSON data against a JSON schema https://github.com/smetj/wishbone-flow-jsonvalidate

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