I am a long time python developer. I was trying out Go, converting an existing python app to Go. It is modular and works really well for me.
Upon creating the same structure in Go, I seem to land in cyclic import errors, a lot more than I want to. Never had any import problems in python. I never even had to use import aliases. So I may have had some cyclic imports which were not evident in python. I actually find that strange.
Anyways, I am lost, trying to fix these in Go. I have read that interfaces can be used to avoid cyclic dependencies. But I don't understand how. I didn't find any examples on this either. Can somebody help me on this?
The current python application structure is as follows:
/main.py /settings/routes.py contains main routes depends on app1/routes.py, app2/routes.py etc /settings/database.py function like connect() which opens db session /settings/constants.py general constants /apps/app1/views.py url handler functions /apps/app1/models.py app specific database functions depends on settings/database.py /apps/app1/routes.py app specific routes /apps/app2/views.py url handler functions /apps/app2/models.py app specific database functions depends on settings/database.py /apps/app2/routes.py app specific routes
settings/database.py
has generic functions like connect()
which opens a db session. So an app in the apps package calls database.connect()
and a db session is opened.
The same is the case with settings/routes.py
it has functions that allow apps to add their sub-routes to the main route object.
The settings package is more about functions than data/constants. This contains code that is used by apps in the apps package, that would otherwise have to be duplicated in all the apps. So if I need to change the router class, for instance, I just have to change settings/router.py
and the apps will continue to work with no modifications.
A circular dependency occurs when two classes depend on each other. For example, class A needs class B, and class B also needs class A. Circular dependencies can arise in Nest between modules and between providers.
A dependency cycle is a relationship between two or more domains that lead to a situation where a slave domain depends on itself, or a master domain depends on one of its slave domains. The Logical Domains Manager determines whether a dependency cycle exists before adding a dependency.
A cyclic dependency is an indication of a design or modeling problem in your software. Although you can construct your object graph by using property injection, you will ignore the root cause and add another problem: property injection causes Temporal Coupling. Instead, the solution is to look at the design closely.
To avoid the cyclic dependency, we must introduce an interface in a new package say x. This interface will have all the methods that are in struct A and are accessed by struct B.
There're two high-level pieces to this: figuring out which code goes in which package, and tweaking your APIs to reduce the need for packages to take on as many dependencies.
On designing APIs that avoid the need for some imports:
Write config functions for hooking packages up to each other at run time rather than compile time. Instead of routes
importing all the packages that define routes, it can export routes.Register
, which main
(or code in each app) can call. In general, configuration info probably flows through main
or a dedicated package; scattering it around too much can make it hard to manage.
Pass around basic types and interface
values. If you're depending on a package for just a type name, maybe you can avoid that. Maybe some code handling a []Page
can get instead use a []string
of filenames or a []int
of IDs or some more general interface (sql.Rows
) instead.
Consider having 'schema' packages with just pure data types and interfaces, so User
is separate from code that might load users from the database. It doesn't have to depend on much (maybe on anything), so you can include it from anywhere. Ben Johnson gave a lightning talk at GopherCon 2016 suggesting that and organizing packages by dependencies.
On organizing code into packages:
As a rule, split a package up when each piece could be useful on its own. If two pieces of functionality are really intimately related, you don't have to split them into packages at all; you can organize with multiple files or types instead. Big packages can be OK; Go's net/http
is one, for instance.
Break up grab-bag packages (utils
, tools
) by topic or dependency. Otherwise you can end up importing a huge utils
package (and taking on all its dependencies) for one or two pieces of functionality (that wouldn't have so many dependencies if separated out).
Consider pushing reusable code 'down' into lower-level packages untangled from your particular use case. If you have a package page
containing both logic for your content management system and all-purpose HTML-manipulation code, consider moving the HTML stuff "down" to a package html
so you can use it without importing unrelated content management stuff.
Here, I'd rearrange things so the router doesn't need to include the routes: instead, each app package calls a router.Register()
method. This is what the Gorilla web toolkit's mux
package does. Your routes
, database
, and constants
packages sound like low-level pieces that should be imported by your app code and not import it.
Generally, try to build your app in layers. Your higher-layer, use-case-specific app code should import lower-layer, more fundamental tools, and never the other way around. Here are some more thoughts:
Packages are good for separating independently usable bits of functionality from the caller's perspective. For your internal code organization, you can easily shuffle code between source files in the package. The initial namespace for symbols you define in x/foo.go
or x/bar.go
is just package x
, and it's not that hard to split/join files as needed, especially with the help of a utility like goimports
.
The standard library's net/http
is about 7k lines (counting comments/blanks but not tests). Internally, it's split into many smaller files and types. But it's one package, I think 'cause there was no reason users would want, say, just cookie handling on its own. On the other hand, net
and net/url
are separate because they have uses outside HTTP.
It's great if you can push "down" utilities into libraries that are independent and feel like their own polished products, or cleanly layer your application itself (e.g., UI sits atop an API sits atop some core libraries and data models). Likewise "horizontal" separation may help you hold the app in your head (e.g., the UI layer breaks up into user account management, the application core, and administrative tools, or something finer-grained than that). But, the core point is, you're free to split or not as works for you.
Set up APIs to configure behavior at run-time so you don't have to import it at compile time. So, for example, your URL router can expose a Register
method instead of importing appA
, appB
, etc. and reading a var Routes
from each. You could make a myapp/routes
package that imports router
and all your views and calls router.Register
. The fundamental idea is that the router is all-purpose code that needn't import your application's views.
Some ways to put together config APIs:
Pass app behavior via interface
s or func
s: http
can be passed custom implementations of Handler
(of course) but also CookieJar
or File
. text/template
and html/template
can accept functions to be accessible from templates (in a FuncMap
).
Export shortcut functions from your package if appropriate: In http
, callers can either make and separately configure some http.Server
objects, or call http.ListenAndServe(...)
that uses a global Server
. That gives you a nice design--everything's in an object and callers can create multiple Server
s in a process and such--but it also offers a lazy way to configure in the simple single-server case.
If you have to, just duct-tape it: You don't have to limit yourself to super-elegant config systems if you can't fit one to your app: maybe for some stuff a package "myapp/conf"
with a global var Conf map[string]interface{}
is useful. But be aware of downsides to global conf. If you want to write reusable libraries, they can't import myapp/conf
; they need to accept all the info they need in constructors, etc. Globals also risk hard-wiring in an assumption something will always have a single value app-wide when it eventually won't; maybe today you have a single database config or HTTP server config or such, but someday you don't.
Some more specific ways to move code or change definitions to reduce dependency issues:
Separate fundamental tasks from app-dependent ones. One app I work on in another language has a "utils" module mixing general tasks (e.g., formatting datetimes or working with HTML) with app-specific stuff (that depends on the user schema, etc.). But the users package imports the utils, creating a cycle. If I were porting to Go, I'd move the user-dependent utils "up" out of the utils module, maybe to live with the user code or even above it.
Consider breaking up grab-bag packages. Slightly enlarging on the last point: if two pieces of functionality are independent (that is, things still work if you move some code to another package) and unrelated from the user's perspective, they're candidates to be separated into two packages. Sometimes the bundling is harmless, but other times it leads to extra dependencies, or a less generic package name would just make clearer code. So my utils
above might be broken up by topic or dependency (e.g., strutil
, dbutil
, etc.). If you wind up with lots of packages this way, we've got goimports
to help manage them.
Replace import-requiring object types in APIs with basic types and interface
s. Say two entities in your app have a many-to-many relationship like User
s and Group
s. If they live in different packages (a big 'if'), you can't have both u.Groups()
returning a []group.Group
and g.Users()
returning []user.User
because that requires the packages to import each other.
However, you could change one or both of those return, say, a []uint
of IDs or a sql.Rows
or some other interface
you can get to without import
ing a specific object type. Depending on your use case, types like User
and Group
might be so intimately related that it's better just to put them in one package, but if you decide they should be distinct, this is a way.
Thanks for the detailed question and followup.
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