Diving Deep on S3 Consistency(allthingsdistributed.com) |
Diving Deep on S3 Consistency(allthingsdistributed.com) |
GCS allows object to be replaced conditionally with `x-goog-if-generation-match` header, which sometimes can be quite useful.
BTW, DynamoDB supports conditional PUTs if your data can fit under 400 KiB.
> 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?
It's a great change.
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an amount of metadata needs to be kept in-memory for such cases where the access to an object _might_ lead to an outdated read or where an "older" write would potentially overwrite a "newer" write.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?
https://aws.amazon.com/about-aws/whats-new/2020/12/amazon-s3...
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.
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?
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.
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.
"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."
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.
> 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.
It looks like it does exactly that now, it just wasn't clear from the article.
[0] https://docs.aws.amazon.com/systems-manager/latest/userguide...
"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."
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.
Elsewhere they state maximum scalability targets of 400 Gbps egress and 80 Gbps ingress: https://docs.microsoft.com/en-us/azure/storage/files/storage...
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.