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Case Study - Modularity, Oh My

· 5 min read

Identified Need

The adoption growth of tremor inside Wayfair is such that we have production customers with 3.5 years of experience operating tremor in production with very large, complex tremor-based services - and fresh demands from new domains such as Search - whose requirements for multi-participant transaction orchestration are driving enhancements to qualify of service, guaranteed delivery and circuit breaker capabilities within tremor’s core processing engine.

Some usage patterns are becoming common enough that sharing tremor application logic across teams, across organizations, and across domains is now common. For example - tremor has good support for HTTP based interactions and already interfaces with a number of HTTP-based and REST APIs, and it can expose HTTP-based interfaces to participants and act as a REST-based or HTTP-based service. Many of these services share the same needs around handling HTTP errors and routing errors and alerts.

At the event flow or query level - similar levels of sophistication are emerging in the installed user base.

Required Outcome

Expand tremor’s domain specific languages to support modularity, starting with the scripting language but extending into the query language to maximise reuse of user-defined processing logic.


Provide standard mechanism and syntax to discover, reference and consume functional units of logic that can be shared by all tremor domain specific languages.

A new lexical preprocessing phase has been introduced allowing source to be packaged within a modular set of paths through a TREMOR_PATH environment variable. Since V0.8 of tremor the scripting and query languages both support modules, and share the same module syntax and semantics.

The tremor API was upgraded to supported preprocessed or non-modular source so that no API changes were required to extend the deployment mechanisms to support modularity.

Modular scripting

In a nutshell - this added support for user defined constants and user defined functions.

# File: my_mod.tremor

# Tail recursive implementation of fibonacci
fn fib_(a, b, n) of
case (a, b, n) when n > 0 => recur(b, a + b, n - 1)
default => a

fn fib(n) with
fib_(0, 1, n)

The module can be loaded via the module path and used in other script or query sources:

# File: fib.trickle  
# A streaming fibonacci service*

define script fibber
use my_mod;

create script fib from fibber;

select event from in into fib;

select event from fib into out;

Modules can be file-based, or defined inline in the scripting or query language. A standard set of system modules is provided by tremor out of the box.

Tremor Modules

Modularity in the query language

In the query language windows, scripts and pluggable operators may be defined and shared across queries.

There is an RFC for modular sub-query to allow query sub-graphs to be defined and shared which is being delivered as part of a tremor LFX mentorship.

Modularity in the deployment language

Modular deployments through replacing the YAML configuration syntax with a deployment language will embed the modular query language, which in turn embeds the modular scripting language.

This work is under development and will span multiple releases - but it is being designed in such a way that as clustering capabilities are added to tremor, that clustered or distributed deployments will be modularly composable by end users using the same tooling as the other DSLs within tremor.


The support for modularity is thematic. The scripting language now supports a functional programming paradigm and file-based or nested inline modules. The query language can embed modular scripts using the same module mechanism and definitions in the query language are also modular.

Modular sub-query and the introduction of the deployment language are planned and in progress and will extend modularity to the administration of running tremor nodes.

And, in the fullness of time, clustering will further extend modularity in tremor to maximize operator and tremor developer productivity through efficient reuse and sharing of common tasks.

The first set of connectivity to benefit from modularity is the support for CNCF OpenTelemetry. This is the first set of tremor connectors to be delivered that ships with its own set of modules designed to make designing OpenTelemetry based services in tremor easier.


As tremor grows into new domains, and the algorithm and solution complexity of traditional production domains for tremor increase in sophistication, size, complexity, the subject of modularity has evolved and new demands continue to emerge.

We expect the modularity theme to be long-lived, but its origins derive from production needs. When tremor was developed the user defined logic was small, relatively simple and applications built with tremor were fairly monolithic.

Today, multi-pipeline and medium to large tremor-based applications are common. And adoption of the modular scripting and query language primitives is now driving larger and more sophisticated use cases.

Another dimension of modularity is the ongoing expansion of connectivity in tremor. Modularity here reflects a different production-driven need ( and a few maintainer conveniences ). A plugin development kit is being developed that will allow connectors and other plugins to be developed in separately managed and maintained projects. This in turn allows the core of tremor to be managed independently of connectivity.