Clarity on Stuart Sierra's component system

[Colin Yates]

Hi all,

Am I right in thinking that in order to use https://github.com/stuartsierra/component every consumer of a component must also be a component?

For example, if I have a component DB and I want to use that DB in (defn blob-query [db criteria]...), do I pull the DB out of the system map and call it or am I expected to make a BlobQuery component which offers a (blob-query [criteria]) API?

Initially I thought the "system" was just the stateful components, but after watching https://www.youtube.com/watch?v=13cmHf_kt-Q and reading the doc, particularly the "all or nothing" warnings I think I might have missed something.

Thanks!

[Max Countryman]

Hi Colin,

I'm by no means an expert on Component but I have used it for my last several projects. What I've discovered is where I have query functions such as yours I pass the db component through to their call sites from a component which contains the db as state. For example, I have a component which contains various HTTP routing logic. Some of my routes result in db queries. To facilitate this I pass the db all the way through. It's sometimes a little clunky and there may be better ways of doing this, e.g. maybe all the routes which depended on db state could be partitioned in their own component. 

I hope that's helpful,

Max

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[James Reeves]

On 28 November 2014 at 10:28, Colin Yates <colin...@gmail.com> wrote:

Am I right in thinking that in order to use https://github.com/stuartsierra/component every consumer of a component must also be a component?

Nope.

 

For example, if I have a component DB and I want to use that DB in (defn blob-query [db criteria]...), do I pull the DB out of the system map and call it or am I expected to make a BlobQuery component which offers a (blob-query [criteria]) API?

You only need to make something a component if it has a lifecycle. If you just want to use a database connection, pass the component as an argument to a function.

Look at it another way: do you need to keep blob-query around? Does it have state? If not, then it doesn't need to be a component.

- James

[Colin Yates]

Thanks both. James, that is what I was hoping.

I guess I got a bit misled with the combination of his "Customers"
example in the video, the "all or nothing" warnings and "I do not
intend that application functions should receive the top-level system
as an argument. Rather, functions are defined in terms of components.
" (from the front page on github).

I guess we can just (:interesting-component system) once the system has started?

(Without writing a single line of code though, I can see the
temptation to make stateless consumers Components just for the ease of
use :)).

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[James Reeves]

On 28 November 2014 at 15:20, Colin Yates <colin...@gmail.com> wrote:

I guess I got a bit misled with the combination of his "Customers"
example in the video, the "all or nothing" warnings and "I do not
intend that application functions should receive the top-level system
as an argument. Rather, functions are defined in terms of components.
" (from the front page on github).

I guess we can just (:interesting-component system) once the system has started?

Your system should call your functions, not the other way around.

You can have functions that are not components, but all functions should be called from a component. The direction of the program should flow down from the system to its components, then down into individual functions.

- James

[Colin Yates]

Ah OK. I was confused a little about what makes a component, it isn't
just state it is also about lifecycle.

Imagine a "health-check" which polls some service to see if it is
there, it needs the service and some ohDear notifier component but it
doesn't have any state as such (or at least no state that changes over
time), but it does have a notion of start/stopping. This would be a
good candidate because it has a lifecycle.

Thanks again.

[James Reeves]

On 28 November 2014 at 16:02, Colin Yates <colin...@gmail.com> wrote:

Ah OK. I was confused a little about what makes a component, it isn't
just state it is also about lifecycle.

Imagine a "health-check" which polls some service to see if it is
there, it needs the service and some ohDear notifier component but it
doesn't have any state as such (or at least no state that changes over
time), but it does have a notion of start/stopping. This would be a
good candidate because it has a lifecycle.

Exactly right.

- James

[Daniel Szmulewicz]

You may also want to have a look at the system library. It ships with readymade components, and it contains a full application example (if minimal) to show how things work. 

https://github.com/danielsz/system

[Colin Yates]

Thanks Daniel.

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