Why SQLite? Why Now? πŸ‡

Published 2022-08-23

I've been sucked down a sqlite rabbithole and I'm all-in on it.

So why SQLite? And why now?

For me, its about re-architecting how we write code so it can fit the coming world of edge, dstributed and local-first computing.

In one way, sqlite is a great fit for distributed applications given it can be embedded directly into devices at the dge.

In another way, it is an awful choice because it doesn't have a sync protocol nor does it support eventual consistency.

So is it great or is it awful? Let's take a little journey through a few stops to see how all this plays out.

  1. Enabling the relational model for more use cases
  2. Respecting the consistency needs of data
  3. Breaking the speed of light
  4. Turning Edge Architecture Upside Down πŸ™ƒ

Enabling the Relational Model for More Use Cases

The relational model has stood the test of time and proven itself to be one of the best choices you can make for managing the data backing your applications. Maybe you could even say it is an apex predator.

That being said, relational databases are firmly on the CA side of CAP. In the face of a network partition, they sacrafice all availability.

Wouldn't it be great if we could use our relational databases even in cases of prolonged network partitions?

With a few tweaks we can add an eventually consistent layer, atop relational databases, that can stay available in the face of network partitions. This means

This is very appealing for any situation where you want to allow the user to interact with their data without waiting for a response from the server and to be able to sync their data between all devices they use. That this can be built atop existing technologies and existing storage solutions, rather than requiring something new, is an added bonus.

All of these benefits rely on eventual consistency. This begs the question of what data consistency needs your data actually has.

Respecting the Consistency Needs of the Data

respect

The lack of options, aside from stong consistency, in relational databases has locked us into not thinking about what level of data consistency we actually need. As such, in traditional web applications all changes go through a central service and are generally strongly consistent. In this realtional + client-server paradigm, we don't even reach for eventual consistency until we run into performance, availability or scaling problems.

This has blinded us to the fact that large swaths of state do not ever need to go through a central server or need strong consistency.

User registration and bank transfers certainly do need strong consistency -- you don't want two users claiming the same handle or double spending but do:

These all just need to be eventually consistent.

By unlocking eventually consistent options atop the relational model, we give developers the ability to model all of their data in a familiar way and with the consistency options that make sense rather than just those that are at hand.

The pripr lack of options has put us down a path of default strong consistency and centralized client-server architecture that is

This is being revealed in ever more painful detail today as we try to fit the square peg of strong consistency & client-server models into the round hole that is distributed & edge computing in an attempt to overcome the speed of light.

Breaking the Speed of Light

light

For applications that require a server for a large percentage of their interactions (e.g., to ensure strong consistency of writes), the speed of light becomes quite problematic.

Checking our napkin math we can see that

two sequential round trips from NA West to Singapore will get you close to half a second of delay.

In the "strongly consistent, client-server paradigm" (from now on called "the paradigm"), the path of resolving this roughly goes as follows:

  1. Bundle resources (js, css, html, etc.)
  2. Add a CDN
  3. Add application servers in more regions
  4. Do server side rendering
  5. Add read replicas of the database to new regions
  6. Add optimistic writes to the application
  7. Shard the database, putting masters of given replicasets closer to certain users
  8. Put some low-value code into edge functions
  9. ?

This is a lot of work and a lot of moving parts just to shorten the length of and reduce the number of round trips. In other words, a lot of work solving incidental rather than essential complexity. Yeah, the work delivers business value by making your app faster but the work isn't tied to the business's core mission or competency.

My 8th statement might raise some eyebrows. "Low value code." I say this because the example use cases for edge functions certainly aren't very compelling.

Sure you can throw all of your business logic into the edge but that probably won't behave as you expect (to be seen later).

The complexity of scaling "the paradigm" and the lack of compelling edge compute use cases for apps architected under "the paradigm" points to the fact that we've hit the limits of "the paradigm." "The paradigm" isn't ready for and was never made for edge computing.

Turning Edge Architecture Upside Down πŸ™ƒ

As we've seen, the fundamental goal of edge computing is to increase the speed of applications by moving compute & state closer to users. Similar to a CDN but also drastically not similar at all to a CDN. Moving compute and state is just not the same problem as moving static resources and caching responses.

E.g., if you deploy data-reliant application logic to Singapore but leave your database in Ohio you're probably going to have a really bad time. Those Sing. -> Ohio round trips for each DB call will be way worse than the single user -> Ohio round trip that you would have had previously had you kept the application colocated with the database. Static resources and cached data don't have behavior so placing copies of them everywhere only has upside.

Current edge architectures, being beholden to "the paradigm", are an attempt to solve new problems by applying old patterns.

What if we flipped everything on its head?

blue-idea

What if, instead of always assuming that the server is the authortative source, we assumed that the user's local device is the authoritative source of information?

Local-First Paradigm

In this world, all requests for state are immediately resolved on-device. The default deployment surface for compute (logic, code) is also on-device.

In the local-first paradigm, the default consistency mode is eventual consistency. This means that state and compute can naturally exist at the edge and are only brought to the "center" when there is a need for strong consistency.

In other words, the common case is edge computing. The specialized case is centralized computing.

If you want your app to be edge native, you need to switch paradigms.

sqlite is a great fit for all of this.

Misc

The local-first architecutre + embedding a database directly into your app has a number of other benefits on top of those we've already seen --

Simpler Architecture

simpler

When you can treat your local data store as an authoritative source of information, this enables abstracting away a large swath of complexity.

You read and write directly to your local data layer and it is your local data layer that manages:

  1. Syncing changes back to a server or other peers
  2. Receiving updates from a server or peers
  3. Notifying the application that new data is available

Since all reads and writes are local, you're no longer worried about network latency.

Since your local data store is an authoritative source of information, you're not worried about cache invalidation.

Of course there are tradeoffs. The most obvious is data consistency. This only works for data that either never needs to be reconciled with peers or for data that can be eventually consistent / modeled as a CRDT. Others conerns are --

  1. How much data can you store locally?
  2. How do you signal to the user that their local set of data could be incomplete from the perspective of other peers?
  3. How do we bless certain peers (or servers) as authoritative sources of certain sets of information?
  4. What CRDTs are right for which use cases?

These questions are being explored by myself and various working groups:

Lower Cloud Costs

Since compute and storage is moved as much to the user's device as possible, you're no longer footing the bill for every CPU cycle they run and every bit they need to transfer and store.

Data Custody

The user's device is an authoritative set of information. It no longer needs to ship every mutation off to a central service before that mutation is accepted. The user can be given control of what leaves their device and their application(s) will still function.

Transaction Support

oscar

This is an entirely new point of departure from what has been discussed so far. We've mainly dealt with the distribution of state and flipping the model from "default strong consistency, default client-server" to "default eventual consistency, default distributed."

Given that, I'll keep it brief.

In short, applications are plagued by state management problems. Global state, within a program, is always seen as our enemy. More or less as a dumpster fire. But... a database is giant pile of global state that is being read and written by many different applications.

Why do we have so few problems with the global and mutable state that is the database but so many problems with global and/or mutable state that exist in-memory?

It comes down to:

If our programming languages had support for ACID transactions against in-memory data then entire classes of problems related to mutable state (in particular, those of observing partial states) would vanish.

We can, in some ways, use an embedded in-memory sqlite as a hack to gain this power for our application state.

SQL Drawbacks

Another set of ideas for another time but --

One of the key strengths of the relational model is the ability to enter your data from any point. For a blogging app, to find any or all comments you can query the comment table. To find any or all users you can query the user table. The relational model does not rely on access paths.

Compare this with many NoSQL models which often structure their data into trees. Access path suddenly matters. To find any comment you need to know what post it is attached to. To find all comments, you must know that you have to traverse all posts to all comments. If architecture changes and someone changes the branch structure, you have to go update all your application logic to use the new paths.

In this latter way, supposedly schema-less NoSQL has more requirements than SQL. NoSQL requires you to understand how your data is layed out (access path) when retrieving arbitrary data. Relational/SQL does not since all types are flattened / normalized into their own tables.

There are more revelatory observations like this in the original relational databases paper from 1970.

My addition, however, is that once we do have a start node (a comment, post, user, etc.), a Graph query language is a better fit for application queries than SQL.

This is the case since once you do have a start node, the relationships on that start node are properties of the data and the majority of queries made by the application are traversals from that start node. Finding start nodes is what SQL explicitly excels at. (note to self: future post on sharding and resurgence of graph models for sharded sql dbs (and why they're required here) then resurgence, again, of relational for the data warehouse).

Once you do have a post to start from, instead of doing:

select comment.* from post join comment on comment.post_id = post.id where post.id = x and comment.date < cursor.date and comment.id < cursor.id order by date, id desc limit 10

to get it's comments, we traverse like:

post.comments().last(10).after(curosr);

An idea being explored in Aphrodite

Note: traversals / access paths, after having a start node, are a property of the data rather than a property of how we store it. The former being what relational models preserve, the latter being what NoSQL models expose.