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Diving Deep on S3 Consistency (allthingsdistributed.com)
126 points by themarkers 13 days ago | hide | past | favorite | 49 comments

i find this very light on the actual "diving deep" part promised in the title. theres a lot of self congratulatory chest thumping, not a lot of technical detail. Werner of course doesnt owe us any explanation whatsoever. i just dont find this particularly deep.

The higher you go the more your job is marketing

Recent S3 consistency improvements are welcome, but S3 still falls behind Google GCS until they support conditional PUTs.

GCS allows object to be replaced conditionally with `x-goog-if-generation-match` header, which sometimes can be quite useful.

Vogels spoke briefly about why AWS prefers versioned objects instead here: https://queue.acm.org/detail.cfm?id=3434573

BTW, DynamoDB supports conditional PUTs if your data can fit under 400 KiB.

How do versioned objects make conditional puts unnecessary? I see little relation between them, except that you could use the version identifier in the condition.

Because they let AWS offload the hard part to you, which is what AWS does best :)

This doesn't answer the question being asked.

Why would AWS provide a feature that makes additional transaction and storage charges (as well as subsequent reads to see which is the correct version) irrelevant?

S3 (and most AWS services) are extremely price elastic; i.e., the lower you make them cost, the more people use it (a la electricity.) That's why they've done stuff like drop from 50ms billing to 1ms billing, etc.

They could still offer it as a client library feature, just tell the users what kind of r/w amplification and guarantees they can expect, and it's something they could optimize later or not.

You can't reliably implement conditional PUTs in the client, you need a server side mechanism like the `if-match` header.

Conditional PUTs aren't possible with versioned objects, but if you desire immutability, then they do help.

To implement serialisation, (as opposed to using Conditional PUTs) one could implement a ledger on top of versioned buckets with LegalHolds: Basically, the object versions part of the main chain are LegalHolded whilst other versions are reconcilled (rebased) onto main and later deleted. Tricky to implement, for sure, compared to say, maintaining a journal in DynamoDB to track PUTs to maintain serial integrity.

There is a conditional CopyObject though (x-amz-copy-source-if...)

Can cover some of the use cases

Can you explain how this is useful? It seems like the destination is the important thing here not the source.

it can be useful for some coordination problems.

for example imagine that you have N writers and you want only the first of them to write something in a n object.

each writer writes their content into "_foo" and each tries to copy that to "foo", with "x-amz-copy-source-if-match". Only one of them will succeed and "foo" will have one consistent value, which all observers (including other writers) can agree who has won.

Here's what I take away from this post:

> We built automation that can respond rapidly to load concentration and individual server failure. Because the consistency witness tracks minimal state and only in-memory, we are able to replace them quickly without waiting for lengthy state transfers.

So this means that the "system" that contains the witness(es) is a single point of truth and failure (otherwise we would lose consistency again), but because it does not have to store a lot of information, it can be kept in-memory and can be exchanged quickly in case of failure.

Or in other words: minimize the amount of information that is strictly necessary to keep a system consistent and then make that part its own in-memory and quickly failover-able system which is then the bar for the HA component.

Is that what they did?

They've basically bolted on causal consistency.

It's a great change.

Thank you for dropping the "causal consistency" term. I read the wikipedia article.

So, causal consistency in this context means that 1) it does not matter if a write to object A or object B came first, because they are seen as "unrelated". This obviously allows for performance improvements over general "strong consistency" but still offers more guarantees than eventual consistency.

Second, for every "writer" (which would be a 1:1 relationship to the number of S3 objects) an amount of metadata needs to be kept in-memory for such cases where the access to an object _might_ lead to an outdated read or where an "older" write would potentially overwrite a "newer" write.

All that being said, there still is one single instance that, if it goes down, makes the whole system unavailable for the S3 objects it manages until it is replaced. So that means a lower availability compared to a solution that uses eventual consistency (like Cassandra).

Would this be equivalent to having an in-memory SQL database to store the metadata for some other system (such Cassandra) with a quick failover but still single point of failure to enhance the consistency guarantees - just more optimized/customized so that it can work with a huge system like S3?

Anyone else still seeing consistency problems w/S3 & EMR? The latest AWS re:Invent made it sound like this would be fixed but as of yesterday I was still using emrfs to correct S3 consistency problems.

Yeah, yesterday as well

thats... what the post is about..

By the way, Google's GCS had it from the beginning.

Hi, GCS engineer here. GCS offered a lot of consistency from the beginning, but we didn't have strong object listing consistency at the beginning. We got that somewhere around 2017 when we moved object metadata to Spanner. See https://cloud.google.com/blog/products/gcp/how-google-cloud-...

Azure had it from the beginning

Azure doesn't let you write more than 20MB/s to an storage account.

What are you talking about? The default ingress rate is 10 Gbps: https://docs.microsoft.com/en-us/azure/storage/common/scalab...

Elsewhere they state maximum scalability targets of 400 Gbps egress and 80 Gbps ingress: https://docs.microsoft.com/en-us/azure/storage/files/storage...

So it is both available and consistent (but perhaps only in read your own writes way?). What is then with resilence to network partitions, referring to CAP theorm? Did they build super reliable global network, so this is never a real issue?

The consistency level seems to be Causal Consistency, which does include read-your-writes. S3 doesn't provide ACID transactions, so stricter consistency models aren't really needed.

From what I've read, if a network issue occurs which would impair consistency, S3 sacrifices availability. The write would just fail.

But this isn't your 5-node distributed system. Like they mention in the article, the witness system can remove and add nodes very quickly and it's highly redundant. A network issue that would actually cause split-brain or make it difficult to reach consensus would be few and far between.

Can someone elaborate on this Witness system OP talks about?

I'm picturing a replicated, in-memory KV store where the value is some sort of version or timestamp representing the last time the object was modified. Cached reads can verify they are fresh by checking against this version/timestamp, which is acceptable because it's a network+RAM read. Is this somewhat accurate?

I'm picturing the same, but my guess is that it's using a time-synced serializability graph or MVCC in some way.

However, even a "basic" distributed lock system (like a consistently-hashed in-memory DB, sharded across reliable servers) might provide both the scale and single source of truth that's needed. The difficulty arises when one of those servers has a hiccup.

It'd be a delicious irony if it was based on hardware like an old-school mainframe or something like that.

I would have been interested to hear more about the verification techniques and tools they used for this project.

Check out https://cacm.acm.org/magazines/2015/4/184701-how-amazon-web-... ("How Amazon Web Services Uses Formal Methods") and https://d1.awsstatic.com/Security/pdfs/One_Click_Formal_Meth... ("One-Click Formal Methods") for more info.

The folks involved gave a neat talk about the verification techniques and tools they used as part of AWS Pi Week recently: https://www.twitch.tv/videos/951537246?t=1h10m10s

And for those who use minio server, the self hosted s3 storage, that has strong consistency, too.

I have wondered if anyone is using minio at really large scale, or if there any examples of production use at its limits?

Would love a dive (hopefully deep) into IAM, the innards of that must be some impressive wizardry. Surprising there isn't more around about the amazing technical workings of these foundational AWS products.

There are if you work for them....

I'm confused...did you fix the caching issue in S3 or not?

The article seems to explain why there is a caching issue, and that's understandable, but it also reads as if you wanted to fix it. I would think the headliner and bold font if it was actually fixed.

For those curious, the problem is that S3 is "eventually consistent", which is normally not a problem. But consider a scenario where you store a config file on S3, update that config file, and redeploy your app. The way things are today you can (and yes, sometimes do) get a cached version. So now there would be uncertainty of what was actually released. Even worse, some of your redeployed apps could get the new config and others the old config.

Personally, I would be happy if there was simply an extra fee for cache-busting the S3 objects on demand. That would prevent folks from abusing it but also give the option when needed.

Yes, see my December 2020 post at https://aws.amazon.com/blogs/aws/amazon-s3-update-strong-rea... :

"Effective immediately, all S3 GET, PUT, and LIST operations, as well as operations that change object tags, ACLs, or metadata, are now strongly consistent. What you write is what you will read, and the results of a LIST will be an accurate reflection of what’s in the bucket. This applies to all existing and new S3 objects, works in all regions, and is available to you at no extra charge! There’s no impact on performance, you can update an object hundreds of times per second if you’d like, and there are no global dependencies."

Thanks for the link, it made the change being talked about clearer. However, I still don't understand how it was achieved. The explanation in the link appears truncated - lots of talk about the problem, then something about a cache and that's it. Is there an alternate link that talks about the mechanics of the change?

Oh, awesome, I missed that!

It was fixed in December of 2020. Announcement blog post: https://aws.amazon.com/blogs/aws/amazon-s3-update-strong-rea...

This is a general problem in all distributed systems, not just when pulling configuration from S3.

Let's assume you had strong consistency in S3. If your app is distributed (tens, hundreds, or thousands of instances running) then all instances are not going to update at the same time, atomically.

You still need to design flexibility into your app to handle the case where they are not all running the same config (or software) version at the same time.

Thus, once you've built a distributed system that is able to handle a phased rollout of software/config versions (and rollback), then having cache inconsistency in S3 is no big deal.

If you really need atomic updates across a distributed system then you're looking at more expensive solutions, like DynamoDB (which does offer consistent reads), or other distributed caches.

The deeper in your stack you fix the consistency problem, the simpler the rest of your system needs to be. If you use S3 as a canonical store for some use case, that's pretty deep in the stack.

> Thus, once you've built a distributed system that is able to handle a phased rollout of software/config versions (and rollback), then having cache inconsistency in S3 is no big deal.

But this would also mean you can't use S3 as your source of truth for config, which is precisely what a lot of people want to do.

What I need is that when I make a call to a service, it gives back consistent results. Ergo, when the app does do a rolling deploy, it will get the right config on startup, not some random version.

It looks like it does exactly that now, it just wasn't clear from the article.

In that example, do you not see using S3 for that purpose as trying to use the wrong tool for the task at hand. Using AWS SSM parameter store [0] seems to me that it would be a tool designed to fit that purpose nicely.

[0] https://docs.aws.amazon.com/systems-manager/latest/userguide...

Complex config files suck in paramstore. Also, I've used this for mobile app configs that are pulled from s3, so paramstore wouldn't be an option.

It is supposedly fixed.

"After a successful write of a new object, or an overwrite or delete of an existing object, any subsequent read request immediately receives the latest version of the object."


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