[apavlo]

> Wow, the reasons why Redis commands API suck in Andy's video (linked in the post) are the weakest ever.

In my example, the API on a key changes based on its value type. And the same collection can have different value types mixed together. You've recreated the worst parts of IBM IMS from the 1960s. However, the original version of IMS only changed the API when a collection's backing data structure changed. Redis can change it on every key!

We didn't get into the semantics of Redis' MULTI...EXEC, which the documentation mischaracterizes as "Transactions". I'm happy that at least you didn't use BEGIN...COMMIT.

[antirez]

You totally miss that Redis is more like a remote interpreter with a DSL that manipulates data structures stored at global variables (keys): you (hopefully) would never complain about languages having this semantics.

I don't think you understood how Redis collections work. The items are just strings, they can't be mixed like integers or strings together or whatever, nor collections can be nested.

The Redis commands do type checking to ensure the application is performing the right operation.

In your example, GET against a list, does not make sense because:

1. GET is the retrieve-the-key-of-string-type operation.

2. Having GET doing something like LRANGE 0 -1 would have many side effects. Getting for error a huge list and returning a huge data set without any reason, creating latency issues. Also having options for GET to provide ranges (SQL alike query languages horror story). And so forth.

So each "verb" should do a specific action in a given data type. Are you absolutely sure you were exposed enough to the Redis API, how it works, and so forth?

About MULTI/EXEC, when AOF with fsync configured correctly is used, MULTI/EXEC provide some of the transactional guarantees you think when you hear "transaction", but in general the concept refers to the fact that commands inside MULTI/EXEC have an atomic effect from the point of view of an external observer AND point-in-time RDB files (and AOF as well). MULTI / INCR a / INCR a / EXEC will always result in the observer to see either 2, 4, 6, 8, and so forth, and never 3 or 5.

Anyway, I believe you didn't put enough efforts in understanding how really Redis works. Yet you criticized it with weak arguments in front of the most precious good we have: students. This is the sole reason why I wrote my first comment, I believe this to be a wrong teaching approach.

[jsnell]

> 1. GET is the retrieve-the-key-of-string-type operation.

That's a tautological argument. The question isn't what the definition of GET is, but whether the design is good.

> 2. Having GET doing something like LRANGE 0 -1 would have many side effects. Getting for error a huge list and returning a huge data set without any reason, creating latency issues.

If this really were the reason, you'd have separate operations for tiny strings and huge strings. After all, by analogy having GET return a huge string "without any reason" would create latency issues.

But that's not how Redis works, right?

[antirez]

The examples I made are just a subset of the protection that this provides. Similarly you can't LRANGE a set type, and so forth. So this in general makes certain errors evident ASAP (command mismatch with the key type).

This does not meant that Redis would not work having generic LEN, INSERT, RANGE commands. But such commands would end also having type-specific options, that I have the feeling is not very clean. Anyway these are design tastes, but I don't think they dramatically change what Redis is or isn't. The interesting part is the data model, the idea of commands operating on abstract data structures, the memory-disk duality, and so forth. If one wants to analyze Redis, and understand merits and issues, a serious analysis should hardly focus on these kind of small choices.

[josephg]

Eh. What people are really arguing about here is redis’s type system. Redis’s approach has some pros and some cons. I think dismissing redis’s approach out of hand for its choices is too simple a treatment.

Most sql databases (like Postgres) require all types to be declared once, and then they do type checking on mutation. In that sense, sql is like a static language like C. But weirdly, the results returned from a sql query are always dynamically typed values, expressed in a table. Applications reading data from sql will still typically need to know what kind of data they expect back - but they usually do that type checking at runtime.

Redis flips both of those choices. It’s dynamically typed - so it won’t check your mutations. But also, you don’t need schema migrations and all the complexity they bring. And rather than having a single “table” type, redis queries can return scalar values, lists or maps. What kind of return value you get back depends on the query function. (Eg GET vs LRANGE).

If you think of a database as the foundation underneath your house, static typing & type checking is a wonderful way to make that foundation more stable. There’s a reason Postgres is the default, after all. But redis isn’t best used like that. Instead, it’s a Swiss Army knife which is best used in small, specific situations in which a real database would be complex overkill. Stuff like caching, event processing, data processing, task queues, configuration, and on and on. Places where you want some of the advantages of a database (fast, concurrent network-accessible storage) but you don’t want to stress about tables and schema migrations.

If you really hate redis, maybe say the same thing I say about Java when I teach it to my students. “I hate this, and I’ll tell you why. But there are smart people out there who disagree with me.”

If you ask me, I wish sql looked more like redis in some ways. I think it’s quite awkward that every sql query returns exactly one “table”. I’d much rather if queries could return scalar values or even multiple tables, depending on your query.

[gregw2]

Minor nit. Some SQL databases allow you to return multiple tables. IIRC, SQL Server stored procedures can do that. Agreed its not a language feature of SQL.

[CRConrad]

> I’d much rather if queries could return scalar values

Since when can't they?

[josephg]

I mean, they can - but they’re always wrapped up as pseudo-tables.

Not everything is best described as a table, y’know?

[zzzeek]

> You totally miss that Redis is more like a remote interpreter with a DSL that manipulates data structures stored at global variables (keys):

I think he makes the point that these "global variables" are dynamically typed; you can have "listX" and then write a non-list into that same name; statically typed systems would not allow this. He makes the fairly non-controversial point that a statically typed system (SQL, other than that of SQLite) adds a level of type safety that can guard against software bugs.

[thayne]

> you can have "listX" and then write a non-list into that same name; statically typed systems would not allow this

Well, that depends. In most SQL databases there are many cases where supplying the wrong type of value will implicitly convert to the expected type, often in unexpected ways that can result in subtle bugs.

[zzzeek]

PostgreSQL is very very good about really never doing this, and also a scalar vs. list is pretty much a PostgreSQL case since most other relational DBs dont have a native ARRAY type. I think you're mostly thinking of MySQL that has some int/string coercion cases which are to be clear bad, but not as egregious as "any arbitrary type goes right in with no checking whatsoever.

as mentioned, SQLite breaks all these rules and I think SQLite is very wrong on this.

[osigurdson]

>> stored at global variables

This is an interesting (and correct) perspective. Global variables scare us in software but we are ok with it when it comes to application state stored in a db.

[enugu]

There have been proposals to have global state in programming languages which function like databases with the advantages of monitoring/persistence/naming etc, but also retain the modularity of local state.

https://www.scattered-thoughts.net/writing/local-state-is-ha...

https://awelonblue.wordpress.com/2012/10/21/local-state-is-p...

[josephg]

Global variables can definitely be overused, but in the right situations, they’re generally fine. After all, the filesystem is a big global variable too. So is any database. But people don’t complain too much about that.

The strongest argument against global variables is that they don’t show up in the parameter lists of functions. In that way, they’re sort of “spooky action at a distance”. And they encourage functions to be impure. But if this bothers you, you can always pass your database connection as an explicit parameter to any function which interacts with it.

[liontwist]

They aren’t scary. They are useful and you probably use them in many forms (lambdas capturing locals, logging, singletons).

This is yet another reason why single threaded should be the default assumption and multi-threaded require special consideration.

[DrBenCarson]

Just because there are reasons for why Redis sucks doesn’t meant it doesn’t suck

[zbentley]

> We didn't get into the semantics of Redis' MULTI...EXEC, which the documentation mischaracterizes as "Transactions". I'm happy that at least you didn't use BEGIN...COMMIT.

Hmmm, this is a subtler issue than you make it out to be, I think, though I generally agree with you. The quality issues with Redis's technical design here interrelate substantially with user expectations/perceptions/squishier stuff.

The term "transaction" is anchored in most users' minds to a typical RDBMS transactional model assuming: a) some amount of state capture (e.g. snapshot) at the beginning of the transaction and b) "atomicity" of commit being interpreted as "all requested changes are atomically performed (or none are)" rather than "all requested changes are atomically attempted".

Redis has issues with both of those, so I'm sympathetic to your statement that what they call "transactions" is mis-characterized and would be better described as "best-effort command batching".

It's poor naming/branding to call it "transactions", and I don't think it had to be this way: MULTI/EXEC "transactions" should have been deprecated long ago--in favor of Redis scripts and other changes that should have been made in the Redis engine.

First, a defense of scripts: Redis scripts are, to a certain variety of user who wants transaction-esque functionality, not ideal. Those users may be reluctant to engage with a full procedural programming language rather than the database's query language. However, there's substantial overlap between those users and the ones who will be extremely confused by and unhappy with the existing MULTI/EXEC model--they're the folks with the most specific (wrong, in Redis) assumptions of how transactions should work, and suffer the most from them not working that way. Lua scripts, unfamiliar or not, are likely less troublesome in the long run for this cohort. Specifically, requiring users to be explicit about failure behavior of specific commands via call() vs. pcall() would remove one of the worst sharp edges of the MULTI/EXEC world.

Scripts can't answer other transaction-related needs, though. Ideally, I would have preferred that Redis go in the direction of a uniform set of conditions that can be applied to most write commands. There already are conditions in Redis, but they're not uniformly available: SET + NX/XX conditions single-key writes; WATCH semantically/implicitly conditions later EXEC commands with "if version of $key matches the version retrieved by the WATCH statement", etc. If that type of functionality were made explicit and uniformly available to all or most write operations, a further chunk of transaction-related needs could be addressed. When making single commands conditional isn't enough, scripts used to atomically batch-attempt commands could be invoked with parameters used to conditionalize those scripts' internal commands, and so on.

A final simple affordance in support of transaction-ish behavior would be a connection-scoped value type: either a modifier for arbitrary commands to have them operate on an empty database scoped to the connection, or a simple list-like value for connections to "stash" arbitrary data. This wouldn't fundamentally change any semantics, but would, at the cost of some indirection, marginally reduce the need for clients complexity when buffering conditions/commands for later flush via a pseudo-"commit"-script. This is somewhat hair-splitting, though: MULTI/EXEC is already such a connection-scoped buffer, just one that stages commands and not data. My hunch is that a data-only buffer to be consumed by scripts instead of "EXEC" would be an improvement here, but I may well be wrong.

Now, the system that results from these changes is still not as ergonomic/low friction as traditional transactions, and is especially unergonomic when users have to manually capture undo state and decide on rollback semantics during the failure of script execution. As Antirez mentioned in an adjacent comment, AOF can help ensure appropriate conistency in the face of database crashes during script execution, but database level reconciliation--aka "what is the equivalent of 'rollback' for a given script"--is still on the user to work out.

But that's what we're really talking about here, isn't it? That lack of undo (that is: the ability to capture and discard transactional state a la MVCC) is at the root of most of the weird and not-quite-transactional capabilities of Redis in this era.

Antirez is totally right that adding those capabilities would have substantially complicated the Redis engine, and I believe him when he says that made it not worth it to do so. Given that, I'd have vastly preferred a Redis which embraced providing tools that work in spite of/with full acknowledgement of that lack, rather than concealing it/confusing users by mis-branding MULTI/EXEC as "transactions".