Our app (not yet publicly accessible) is still running on sufia 7.3. (A digital repository framework based on Rails, also known in other versions or other drawings of lines as hydra, samvera, and hyrax).
I had a need to hook into the point after a file has been added to fedora, to do some post-processing at that point.
(Specifically, we are trying to run a riiif instance on another server, without a shared file system (shared FS are expensive and/or tricky on AWS). So, the riiif server needs to copy the original image asset down from fedora. Since our original images are uncompressed TIFFs that average around 100MB, this is somewhat slow, and we want to have the riiif server “pre-load” at least the originals, if not the derivatives it will create. So after a new image is uploaded, we want to ‘ping’ the riiif server with an
info request, causing it to download the original, so it’s there waiting for conversion requests, and at least it won’t have to do that. But it can’t pull down the file until it’s in fedora, so we need to wait until after fedora has it to ping. phew.)
Here are the cooperating objects in Sufia 7.3 that lead to actual ingest in Fedora. As far as I can tell. Much thanks to @jcoyne for giving me some pointers as to where to look to start figuring this out.
Keep in mind that I believe “actor” is just hydra/samvera’s name for a service object involved in handling ‘update a persisted thing’. Don’t get it confused with the concurrency notion of an ‘actor’, it’s just an ordinary fairly simple ruby object (although it can and often does queue up an ActiveJob for further processing).
- Sufia::CreateWithFilesActor does some stuff, but then kicks off a background job,
::AttachFilesToWorkJob, located in Sufia.
- AttachFilesToWork job does some stuff, but then calls out to a CurationConcerns::Actors::FileSetActor#create_content. (we are using curation_concerns 1.7.7 with sufia 7.3) — At least if it was a direct file upload (I think is what this means). If the file was a `CarrierWave::Storage::Fog::File` (not totally sure in what circumstances it would be), it instead kicks off an ImportUrlJob. But we’ll ignore that for now, I think the FileSetActor is the one my codepath is following.
- CurationConcerns::Actors::FileSetActor in #create_content does a bit of stuff, but then calls out to another actor, CurationConcerns::Actors::FileActor.
- CurationConcerns::Actor::FileActor doesn’t do too much but to queue up another background job, ::IngestFileJob (located in CurationConcerns).
- ::IngestFileJob farms out most of it’s work to Hydra::Works::AddFileToFileSet. It also calls out to CurationConcerns::VersioningService and ::CharectorizeJob, but I don’t think those are relevant to what I’m trying to trace through here.
- We are using hydra-works 0.16.0. AddFileToFileSet I believe actually finishes things off synchronously without calling out to anything else related to ‘get this thing into fedora’. Although I don’t really totally understand what the code does, honestly.
- It does call out to Hydra::PCDM::AddTypeToFile, which is confusingly defined in a file called add_type.rb, not add_type_to_file.rb. (I’m curious how that doesn’t break things terribly, but didn’t look into it).
So in summary, we have six fairly cooperating objects involved in following the code path of “how does a file actually get added to fedora”. They go across 3-4 different gems (sufia, curation_concerns, hydra-works, and maybe hydra-pcdm, although that one might not be relevant here). Some of the classes involved inherit from, mix-in, or have references to classes from other gems. The path involves at least two (sometimes more in some paths?) bg jobs — a bg job that queues up another bg job (and maybe more).
That’s just trying to follow the path involved in “get this uploaded file into fedora”, some of those cooperating objects also call out to other cooperating objects (and maybe queue bg jobs?) to do other things, involving a half dozenish additional cooperating objects and maybe one or two more gem dependencies, but I didn’t trace those, this was enough!
I’m not certain how much this changed in hyrax (1.0 or 2.0), at the very least there’d be one fewer gem dependency involved (since Sufia and CurationConcerns were combined into Hyrax). But I kind of ran out of steam for compare and contrast here, although it would be good to prepare for the future with whatever I do.
Oh yeah, what was I trying to do again?
Hook into the point “after the thing has been successfully ingested in fedora” and put some custom code there.
So… I guess… that would be hooking into the
::IngestFileJob (located in CurationConcerns), and doing something after it’s completed. It might be nice to use the ActiveJob#after_perform hook to this. I actually hadn’t known about that callback, haven’t used it before — we’d need to get at least the file_set arg passed into it, which the docs say maybe you can get from the passed-in job.arguments? That’s a weird way to do things in ruby (why aren’t ActiveJob’s instances with their state as ordinary state? I dunno), but okay! Or, of course we could just monkey-patch override-and-call-super on
perform to get a hook.
Or we could maybe hook into Hydra::Works::AddFileToFileSet instead, I think that does the actual work. There’s no callbacks there, so that’d just be monkey-patch-and-call-super on #call, I guess.
This definitely seems a little bit risky, for a couple different reasons.
- There’s at least one place where a potentially different path is followed, if you’re uploading a file that ends up as a CarrierWave::Storage::Fog::File instead of a CarrierWave::SanitizedFile. Maybe there are more I missed? So configuration or behavior changes in the app might cause my hook to be ignored, at least in some cases.
- Forward-compatibility seems unreliable. Will this complicated graph of cooperating instances get refactored? Has it already in future versions of Hyrax? If it gets refactored, will it mean the object I hook into no longer exists (not even with a different namespace/name), or exists but isn’t called in the same way? In some of those failure modes, it might be an entirely silent failure where no error is ever raised, my code I’m trying to insert just never gets called. Which is sad. (Sure, one could try to write a spec for this to warn you… think about how you’d do that. I still am.) Between IngestFileJob and AddFileToFileSet, is one ‘safer’ to hook into than the other? Hard to say. If I did research in hyrax master branch, it might give me some clues.
I guess I’ll probably still do one of these things, or find another way around it. (A colleague suggested there might be an entirely different place to hook into instead, not the ‘actor stack’, but maybe in other code around the controller’s update action).
What are the lessons?
I don’t mean to cast any aspersions on the people who put in a lot of work, very well-intentioned work, conscientious work, to get hydra/samvera/sufia/hyrax where it is, being used by lots of institutions. I don’t mean to say that I could or would have done differently if I had been there when this code was written — I don’t know that I could or would have.
And, unfortunately, I’m not saying I have much idea of what changes to make to this architecture now, in the present environment, with regards to backwards compat, with regards to the fact that I’m still on code one or two major versions (and a name change) behind current development (which makes the local benefit from any work I put into careful upstream PR’s a lot more delayed, for a lot more work; I’m not alone here, there’s a lot of dispersion in what versions of these shared dependencies people are using, which adds a lot of cost to our shared development). I don’t really! My brain is pretty tired after investigating what it’s already doing. Trying to make a shared architecture which is easily customizable like this is hard, no ways around it. (ActiveSupport::Callbacks are trying to do something pretty analogous to the ‘actor stack’, and are one of the most maligned parts of Rails).
But I don’t think that should stop us from some evaluation. Going forward making architecture that works well for us is aided immensely by understanding what has worked out how in what we’ve done before.
If the point of the “Actor stack” was to make it easy/easier to customize code in a safe/reliable way (meaning reasonably forward-compatible)–and I believe it was–I’m not sure it can be considered a success. We gotta start with acknowledging that.
Is it better than what it replaced? I’m not sure, I wasn’t there for what it replaced. It’s probably harder to modify in the shared codebase going forward than the presumably simpler thing it replaced though… I can say I’d personally much rather have just one or two methods, or one ActiveJobs, that I just hackily monkey-patch to do what I want, that if it breaks in a future version will break in a simple way, or one that takes less time and brain to figure out what’s going on anyway. That wouldn’t be a great architecture, but I’d prefer it to what’s there now, I think. Of course, it’s a pendulum, and the grass is always greener, if I had that, I’d probably be wanting something cleaner, and maybe arrive at something like the ‘actor stack’ — but now we’re all here now with what we’ve got, so we can at least consider that this may have gone in some unuseful directions.
What are those unuseful directions? I think, not just in the actor stack, but in many parts of hydra, there’s an ethos that breaking things into many very small single-purpose classes/instances is the way to go, then wiring them all together. Ideally with lots of dependency injection so you can switch in em and out. This reminds me of what people often satirize and demonize in stereotypical maligned Java community architecture, and there’s a reason it’s satirized and demonized. It doesn’t… quite work out.
To pull this off well — especially in shared library/gem codebase, which I think has different considerations than a local bespoke codebase, mainly that API stability is more important because you can’t just search-and-replace all consumers in one codebase when API changes — you’ve got to have fairly stable APIs, which are also consistent and easily comprehensible and semantically reasonable. So you can replace or modify one part, and have some confidence you know what it’s doing, when it will be called, and that it will keep doing this for at least a few months of future versions. To have fairly stable and comfortable APIs, you need to actually design them carefully, and think about developer use cases. How are developers intended to intervene in here to customize? And you’ve got to document those. And those use cases also give you something to evaluate later — did it work for those use cases?
It’s just not borne out by experience that if you make everything into as small single-purpose classes as possible and throw them all together, you’ll get an architecture which is infinitely customizable. You’ve got to think about the big picture. Simplicity matters, but simplicity of the architecture may be more important than simplicity of the individual classes. Simplicity of the API is definitely more important than simplicity of internal non-public implementation.
When in doubt if you’re not sure you’ve got a solid stable comfortable API, fewer cooperating classes with clearly defined interfaces may be preferable to more classes that each only have a few lines. In this regard, rubocop-based development may steer us wrong, too much to the micro-, not enough to the forest.
To do this, you’ve got to be careful, and intentional, and think things through, and consider developer use cases, and maybe go slower and support fewer use cases. Or you wind up with an architecture that not only does not easily support customization, but is very hard to change or improve. Cause there are so many interrelated coupled cooperating parts, and changing any of them requires changes to lots of them, and breaks lots of dependent code in local apps in the process. You can actually make forwards-compatible-safe code harder, not easier.
The ‘actor stack’ is not the only place in hydra/samvera that suffers from some of these challenges, as I think most developers in the stack know. It’s been suggested to me that one reason there’s been a lack of careful, considered, intentional architecture in the stack is because of pressure from institutions and managers to get things done, why are you spending so much time without new features? (I know from personal experience this pressure, despite the best intentions, can be even stronger when working as a project-based contractor, and much of the stack was written by those in that circumstance).
If that’s true, that may be something that has to change. Either a change to those pressures — or resisting them by not doing rearchitectures under those conditions. If you don’t have time to do it carefully, it may be better not to commit the architectural change and new API at all. Hack in what you need in your local app with monkey-patches or other local code instead. Counter-intuitively, this may not actually increase your maintenance burden or decrease your forward-compatibility! Because the wrong architecture or the wrong abstractions can be much more costly than a simple hack, especially when put in a shared codebase. Once a few people have hacked it locally and seen how well it works for their use cases, you have a lot more evidence to abstract the right architecture from.
But it’s still hard! Making a shared codebase that does powerful things, that works out of the box for basic use cases but is still customizable for common use cases, is hard. It’s not just us. I worked last year with spree/solidus, which has an analogous architectural position to hydra/samvera, also based on Rails, but in ecommerce instead of digital repositories. And it suffers from many of the same sorts of problems, even leading to the spree/solidus fork, where the solidus team thought they could do better… and they have… maybe… a little. Heck, the challenges and setbacks of Rails itself can be considered similarly.
Taking account of this challenge may mean scaling back our aspirations a bit, and going slower. It may not be realistic to think you can be all things to all people. It may not be realistic to think you can make something that can be customized safely by experienced developers and by non-developers just writing config files (that last one is a lot harder). Every use case a participant or would-be participant has may not be able to be officially or comfortably supported by the codebase. Use cases and goals have to be identified, lines have to drawn. Which means there has to be a decision-making process for who and how they are drawn, re-drawn, and adjudicated too, whether that’s a single “benevolent dictator” person or institution like many open source projects have (for good or ill), or something else. (And it’s still hard to do that, it’s just that there’s no way around it).
And finally, a particularly touchy evaluation of all for the hydra/samvera project; but the hydra project is 5-7 years old, long enough to evaluate some basic premises. I’m talking about the twin closely related requirements which have been more or less assumed by the community for most of the project’s history:
1) That the stack has to be based on fedora/fcrepo, and
2) that the stack has to be based on native RDF/linked data, or even coupled to RDF/linked data at all.
I believe these were uncontroversial assumptions rather than entirely conscious decisions (edit 13 July, this may not be accurate and is a controversial thing to suggest among some who were around then. See also @barmintor’s response.), but I think it’s time to look back and wonder how well they’ve served us, and I’m not sure it’s well. A flexible powerful out-of-the-box-app shared codebase is hard no matter what, and the RDF/fedora assumptions/requirements have made it a lot harder, with a lot more uncharted territory to traverse, best practices to be invented with little experience to go on, more challenging abstractions, less mature/reliable/performant components to work with.
I think a lot of the challenges and breakdowns of the stack are attributable to those basic requirements — I’m again really not blaming a lack of skill or competence of the developers (and certainly not to lack of good intentions!). Looking at the ‘actor stack’ in particular, it would need to do much simpler things if it was an ordinary ActiveRecord app with paperclip (or better yet shrine), it would be able to lean harder on mature shared-upstream paperclip/shrine to do common file handling operations, it would have a lot less code in it, and less code is always easier to architect and improve than more code. And meanwhile, the actually realized business/institutional/user benefits of these commitments — now after several+ years of work put into it — are still unclear to me. If this is true or becomes consensual, and an evaluation of the fedora/rdf commitments and foundation does not look kindly upon them… where does that leave us, with what options?