Welcome to Solano: Concurrency and Parallelism

Welcome to Welcome to Solano! In this blog series, we’ll examine a few key concepts and features of Solano Labs’ flagship product, Solano CI. The goal of this series is to help orient new users while learning to use the system, and serve as a refresher for current users; we hope you find them handy. Please let us know what you think in the comments or if you have any questions send us a note at support@solanolabs.com.


Solano CI is a powerful and complex system; it is, by design, highly flexible in its configuration and potential scale so that it can support the large variety of use cases that CI/CD systems are often relied upon to perform. However, at its core, Solano CI’s primary function is to help its users produce higher quality software.

One way it does this is by returning the results of automated tests quickly. To do this, Solano CI supports the ability to perform multiple actions at the same time and thereby reduce turnaround time by simply doing more at once. There are two simple ways that Solano CI does this: (1) running multiple build “sessions” at once, or “Concurrency;” and (2) automatically subdividing the tasks performed within a session and distributing that work to “workers” to perform them at the same time, or “Parallelism.”

The number of concurrent sessions a user can perform at once is set by that user’s account plan. Each session occupies what we call a “build slot,” and users can only run as many sessions at once as their plan has build slots. When all build slots are occupied by actively running sessions, subsequent sessions that are started are queued until a running session finishes and a slot becomes available. [Editor’s Note: We’ll discuss Queues in more detail in a subsequent post; we’ll provide a link to that post here once it’s posted.]

Our free trial and metered hourly plans, e.g. the Large plan, by default all have two build slots. At the Pro and Enterprise plan levels, the number is customizable.

As a session goes through the build setup process, Solano CI spins up a number of parallel workers to actually perform tasks, usually tests, the user wants to execute. These parallel workers are Docker-based containers, and can run either all together on a single VM or on an orchestrated set of VMs. When the workers are up, the system will scan the repo’s filesystem for test files based on the user’s configuration settings, collect that set of files, and then subdivide it into discrete batches that are distributed to the workers for execution. This distribution of the batches over time over the duration of the test execution phase of the session is our automatic parallelism.

One important conceptual note: the number of workers used per session is set as part of the plan configuration, like the number of build slots, and are the same for all of a plan’s build slots, regardless of which repo they build, and are not free-ranging. This is often confusing for many new users, as they often think of workers as just a set of units in a resource pool that can be arbitrarily configured on a per-repo basis, and so if a smaller repo is built they want to redirect the superfluous workers towards other larger repos’ builds; this isn’t possible in Solano CI. A helpful analogy for constructing a mental model is that of lanes in a bowling alley: just as a bowling lane uses a predetermined number of pins and all other lanes use that same number, so to build slots have a number of workers they use by default as set by the user’s plan.

The number of workers per build slot is highly diverse by plan, even within the set of metered plans. A list of the workers by metered plan is available on our Product page; the Pro and Enterprise plans’ worker configurations are customizable.

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