Let There Be Databases

Tuesday, May 18, 2021 - Ikey Doherty (BDFL)
  

We haven’t been too great on sharing progress lately, so welcome to an overdue update on timelines, progress, and database related shmexiness.

Quick word on personal circumstances

The last year, as many likely know by now, has been an incredibly stressful one. I’m not talking about the pandemic, although that obviously hasn’t helped. In fact, for the majority of the past 3 years, my family and I have been in a constant vicious cycle in attaining true stability.

You see, we’re Irish Travellers. Having a home that is culturally appropriate, safe, and conducive to our family success has been a long struggle. Without getting bogged down in the politics of it all, the system is rigged against us, making our lives a daily struggle against discrimination.

..But you didn’t click onto this blogpost for a sob story. Suffice to say, a unique stepping-stone opportunity emerged for us which required a radical shift in our own approach. As such we’ve secured a “plot”, on which we can place a mobile home (“static caravan”). To enable this transition to a more stable, discrimination-free life, we’ve liquidated pretty much everything, including our own trailer (“caravan”).

The interim period has required us to stay in holiday camps while we await the delivery of our mobile home. On the 28th of this month, we return to the plot where we’ll get the home ready, including plumbing, electrics, furnishings, etc. A plan is in place to ensure this happens rapidly, with the return of a stable home and work environment anticipated within the first days of June.

As I’ve said before, it’s been a long, multiyear battle, but finally having somewhere to call home that is safe for ourselves and our children, whilst respecting our culture rather than trying to stamp it out, is an extraordinary milestone for us. As such that stability will project through in all aspects of our lives, most importantly for you: my development work.

I’m hopeful that this honest blog section will make it easier for people to grok what we’ve been going through (add upcoming baby #4, a few skirmishes with COVID19, etc) and the high level of commitment I personally have for Serpent OS and Lispy Snake projects.

Every change in our lives right now is designed around longevity and stability, and the fruits of those labours will manifest in less than 2 weeks. Please note this is not some sad attempt at a “bleeding heart, fill me coffers” kind of post, it’s more addressing the growing unrest at slow development, whilst allowing me to be fully honest with the community and not having to hide my cultural identity, as I have done so for too many years within the industry.

Over the past few years I’ve found it utterly depressing that I’ve had to suppress my cultural and ethnic identity in the fear that it might upset others or make me a target (and has done, on many occassions). As my family and I are making a fresh start, it is high time that freshness and honesty was mirrored to my work. So, with a very happy heart, I’ll be investing that freedom and energy into all my projects :)

Sweetness. Tell me bout them databases

Emerging DB design

OK, so you may remember moss-format, our module for reading and writing moss binary archives. It naturally contains much in the way of binary serialisation support, so we’ve extended the format to support “database” files. In reality, they are more like tables binary encoded into a single file, identified by a filepath.

The DB archives are currently stored without compression to ensure 0-copy mmap() access when loading from disk, as a premature optimisation. This may change in future if we find the DB files taking up too much disk space.

So far we’ve implemented a “StateMetaDB”, which stores metadata on every recorded State on the system, and right now I’m in the progress of implementing the “StateEntriesDB”, which is something akin to a binary encoded dpkg selections file with candidate specification reasons.

Next on the list is the LayoutsDB (file manifests) and the CacheDB, for recording refcounts of every cached file in the OS pool.

Integration with Serpent ECS

An interesting trial we’re currently implementing is to hook the DB implementation up to our Entity Component system from the Serpent Engine, in order to provide fast, cache coherent, in memory storage for the DB. It’s implemented using many nice DLang idioms, allowing the full use of std.algorithm APIs:

    auto states()
    {
        import std.algorithm : map;

        auto view = View!ReadOnly(entityManager);
        return view.withComponents!StateMetaArchetype
            .map!((t) => StateDescriptor(t[1].id, t[3].name, t[4].description,
                    t[1].type, t[2].timestamp));
    }
    ...
    
	/* Write the DB back in ascending numerical order */
	db.states
		.array
		.sort!((a, b) => a.id < b.id)
		.each!((s) => writeOne(s));

Tying it all together

Ok, so you can see we need basic DB types for storing the files for each moss archive, plus each cache and state entry. If you look at the ECS code above, it becomes quite easy to imagine how this will impact installation of archives. Our new install code will simply modify the existing state, cache the incoming package, and apply the layout from the DB to disk, before commiting the new DB state.

In essence, our DB work is the current complex target, and installation is a <50 line trick tying it all together.

	/* Psuedocode */
	
	State newState...

	foreach (pkgID; currentState.filter!((s) => s.reason == SelectionReason.Explicit))
	{
		auto fileSet = layoutDB.get(pkgID);
		fileSet.array.sort!((a, b) => a.path < b.path).each!((f) => applyLayout(f));
		/* Record into new state */
		...
	}

I know we’ve gone the long way around creating the package manager, and many times there have been delays over the past year. However, we’re no longer looking at light at the end of the tunnel, we’re actively walking out of it. The remaining minor puzzles will unblock the installation routine, which in time will enable first alpha images of dubiuous quality. However rough our jumping off point, it will be an awesome journey.

Til next time -

Ikey