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Redefining cloud native with the Coalesced Chunk Retrieval Protocol

By Jarrett Keifer, Senior Geospatial Software Engineer, Element 84
The CCRP Logo

In our ongoing series on geospatial raster data formats, Julia Signell and I have been exploring the finer points of array data storage. Throughout our research, we’ve found that chunking – breaking a large dataset down into smaller pieces for individual storage and retrieval – is universally relevant regardless of data format. Chunking, as we’ve seen, is an absolutely necessary strategy for making large datasets usable, but in the cloud era, it has become something tyrannical, making data access efficiency strictly tied to chunk alignment.

Diverging from an access pattern aligning with the dataset’s chunking scheme and compounding inefficiencies will cripple data access at scale. Pretend, for example, we are an official in Chico, a city in the central valley of California near the foothills of the Sierra Nevada, who wants to examine changes in monthly average maximum temperatures in the city over the years 2010 to 2020 to understand possible increases in air conditioner usage. Say we have daily maximum temperature data stored in cloud object storage, chunked into pieces covering the entire state of California for a given day.

Our access pattern is misaligned! We only want to see data for Chico; other areas are not under our jurisdiction, so we have no need to download data for the rest of the state. But we cannot get only the data we want, given the way this dataset is chunked. We want a small part of many large chunks, meaning we have to make many requests for lots of unwanted data, which is inefficient.

The cloud has given us the ability to store petabytes of data cheaply, but we’ve lost our ability to access that data efficiently without considering chunking, access patterns, and how to mitigate misalignment. But this tyranny of the chunk doesn’t have to be permanent. We’ve solved hard problems before; object storage itself was once revolutionary. I believe it is time to solve the next layer: making that stored data actually accessible at the speed of analysis, not the speed of HTTP requests.

To that end, I want to introduce a new idea I’m calling the Coalesced Chunk Retrieval Protocol (CCRP) as a potential new definition of “cloud native” for chunked data. Chunked file formats on local storage once provided an effective abstraction preventing users from having to think about details of data storage and layout, and CCRP aims to get that abstraction back in the cloud, for both raster and tabular data.

But this post isn’t just me offering up an idea wholesale. This is an invitation to the community to collaborate on building cloud native better. This is a chance to work together on making the ecosystem stronger and to reconsider what cloud native really means. If you work with large chunked datasets, I want to hear from you. If you’re building data infrastructure, I want your technical feedback. Or, if you have other ideas entirely, let’s hear them! We don’t have to just accept the status quo; we have the chance to define the future of cloud native geospatial by what we build together today.

The irony of “cloud native”

When we say a data format is “cloud native,” we usually mean the data layout is optimized for access via an object store API, like S3’s. But here’s the thing: S3 was designed for web assets, not scientific data. Cloud object storage is great for storing huge datasets, but it’s terrible for reading many small pieces at once. In the worst case, the object model requires each chunk of a dataset to be read via a separate HTTP request. A data scientist wanting a specific slice of a large dataset might need 10,000 data chunks, leading to 10,000 separate, high-latency HTTP GET requests from their laptop.

This forces data producers to choose between two sub-optimal options: use huge chunks that cannot provide granular data access, or small chunks that force users into a query pattern dominated by time spent requesting data instead of receiving it. As a result, chunk sizes have grown in the cloud, but this means that misaligned access can incur significant overhead, pushing and extracting bytes that aren’t wanted.

Not infrequently, what results is an absurd situation. Savvy users end up having to maintain their own, rechunked mirrors of their most important datasets to get around these inefficiencies. Data producers end up having to duplicate their datasets using multiple chunking schemes to try to address common access patterns, and hope that their users understand when to use which copy. And both sides spend time and energy arguing about what is the “right” way to chunk.

All of this is wasted compute and storage, wasted time and money, and wasted cognitive capacity for users and producers alike.

Enter CCRP

What is CCRP?

CCRP is a protocol that defines a way to make a single API call to fetch multiple data chunks from cloud storage at once, eliminating the network latency that cripples big data analysis. It builds on existing patterns like HTTP and REST, and aims to feel immediately familiar.

With CCRP, a client makes one single batch request specifying all the chunks they need using some combination of dimensional filtering and coordinate slicing (like in xarray when selecting variables from a dataset and slicing arrays based on coordinates), as appropriate. The CCRP API, running inside the cloud next to the object store’s backing on-disk block store, grabs the bytes for all matching chunks from the block store with near-zero latency and streams the combined data back to the client in a single response.

CCRP offloads the responsibility of determining what chunks to fetch from the client. Its job is to resolve a set of chunks that match a client’s query, and return them. The client needs only to map a user query to the dimensions exposed by CCRP, and no longer has to concern itself with chunks and byte ranges.

Think of this like GraphQL, but for chunked datasets in cloud storage.

GraphQL originated from a version of the same problem with REST-style APIs. For example, to get a user, their posts, and their comments, you had to make three separate API calls: /users/1, /users/1/posts, and /users/1/comments.

GraphQL solved this by letting clients send one single query describing everything they need, and they get it all back in one round trip.

CCRP does for data chunks what GraphQL does for web resources. It replaces potentially thousands of slow, chatty requests with a single, efficient one. It offloads chunk byte range calculation from the client side and allows for read coalescing where it would otherwise be impossible. Doing so allows data to be chunked into smaller pieces to facilitate more granular access, mitigating the penalty of misaligned access patterns.

In our original example of wanting temperature data for Chico, California, we saw how a query with a small area and large time range was misaligned to the chunking of the target dataset. To address this misalignment, we could optimize the chunking for our use case by breaking up the chunks into smaller areas and aggregating them across longer periods of time. Thus, we could query for our small area and large time range with fewer requests and less unwanted data.

Except then we’d compromise efficiency for users who want to get temperatures for all of California on only a given day.

To balance both needs, we have to make our chunks small, both spatially and temporally. Small chunks increase access pattern alignment by reducing the amount of unwanted data we have to download for any given request. But small chunks with the current object storage access model force users to make even more HTTP requests to retrieve what they need, which can become untenably inefficient when performing queries at scale. CCRP, by performing read coalescing server-side, allows clients to read a vast range of chunks with a single request.

Why this matters now

Data volumes are exploding. Chunking strategies are getting more complex. The burden on data consumers and producers alike is growing. As we look to the future, the current status quo appears more and more unsustainable. We need to find a way to address the problems before they grow too large.

What makes CCRP different?

Other solutions have proposed similar advantages. Things like OPeNDAP, OGC EDR, and TileDB come to mind, but CCRP is different.

It decouples the logical data model from the physical layout.

The object model is an erroneous abstraction for many large chunked datasets. Modern cloud native formats store datasets across multiple objects. Objects are inherently one-dimensional byte streams; linearization of multidimensional data into objects means that the data layout cannot preserve all possible spatial proximity. Fulfilling logical queries often requires mapping the query to multiple disjoint byte ranges within an object’s one-dimensional byte stream and to byte ranges within multiple objects.

With CCRP, clients can request data in a way that makes sense for their analysis, e.g., “give me this time series”. They don’t have to concern themselves with how a data provider physically organizes data within or across files/objects, e.g., “I want these bytes from this object, and those bytes from that object…”. The API translates requests into an optimal plan for fetching the requested chunks, unencumbered by the erroneous object abstraction in the middle.

It’s a lightweight “byte broker”, not a compute engine.

The API’s only job is to find whole chunks and forward their bytes. It doesn’t decompress, filter, or process the data – it operates on chunks alone. This makes it fast, scalable, and simple.

It should be a native cloud primitive.

This isn’t just a proxy in front of an object store – though the reference implementation will likely be something to this effect. For maximum performance, this should be a parallel, first-class interface into the cloud provider’s internal block store, just like the S3 API is today, to eliminate extra network hops and get around the erroneous object model.

Think about AWS’s recent S3 Express One Zone or their new S3 Tables and [S3 Vector](https://aws.amazon.com/blogs/aws/introducing-amazon-s3-vectors-first-cloud storage-with-native-vector-support-at-scale/) stores. Cloud providers are recognizing that different data patterns need different interfaces. CCRP is proposing the same for chunked raster and tabular data.

But it can be adopted incrementally.

We don’t have to wait for cloud providers to start using CCRP. We can begin with proxy implementations to show the value and build out client-side tooling while the ecosystem evolves toward native cloud support.

As a parallel interface to data in an object store, CCRP is backwards-compatible. Clients with older tooling can continue to interface directly with the objects backing a CCRP dataset.

The goal is an open standard, not a product.

This is a missing piece of global data infrastructure. The goal is to create a compelling specification that all cloud providers can and will implement, making data access better for everyone.

From Chico to California: a detailed example

To illustrate how this solution scales, let’s go back to our hypothetical weather dataset and assume we’ve made the chunks smaller than the whole state of California to better align with our Chico user’s query. Now that the city of Chico has proven its analysis method at the small scale, the state of California wants to repeat the analysis at the larger statewide scale.

How exactly does increasing alignment for a small query affect our ability to query at a larger scale? We can work through a detailed example to better understand what has to happen when querying cloud native data formats through an object store API, how chunk size affects HTTP requests, and see how CCRP allows for more efficient queries.

We need to better define our weather dataset for this example. A realistic weather dataset might be gridded at 0.01° resolution (a 1 km nominal resolution as measured near the equator), with samples every hour. We’re still interested in temperature data, but now we have a larger query covering the whole state of California, roughly the longitude range -125° to -115° and the latitude range 32° to 42° from January 1, 2010, to January 1, 2020, or roughly a 10° x 10° x 3652-day slice.

To request this slice, the first thing we need to do is to map our query range into the dataset indices. In this case, let’s say our dataset grid has the dimension labels x, y, and time, and is indexed in degrees east from the antimeridian (ranging 0-360), degrees north from the south pole (ranging 0-180), and in seconds since an epoch starting midnight January 1, 2000, respectively.

To calculate our x index slice range, we need to determine how many degrees east -125° to -115° is, which we can do by adding both values to 180°, giving us the slice [55:65] (in Python notation). We have to scale this to our grid, though, which has 100 pixels per degree, so we end up with an x index slice of [5_500:6_500]. The y slice is similar, but found by adding our values to 90° and again scaling by 100, so there we get [12_200:13_200]. The time dimension is a little trickier; for brevity, we’ll skip ahead and say that slice is [87_672:175_320] (this is a slice from 87,672 to 175,320 hours since our January 1, 2000 epoch).

Note that these slice calculations happen today under the covers when using Xarray or GDAL or whatever. No matter the tooling, when requesting chunks from S3, the client must take a user query and resolve the matching chunks. Doing this operation is not a requirement added by CCRP – and we’d still leverage existing implementations to handle this operation and to separate users from these low-level concerns – we just show it here to make the example CCRP request below more understandable. With CCRP, clients would no longer have to take the extra steps to resolve the specific chunk byte ranges covered by an index slice and request each of them; responsibility to handle that concern moves from the client to the CCRP API.

If this dataset is divided into 1° x 1° x 1 day chunks in an object store as an unsharded Zarr (read: each chunk is a separate object), then, given our slice ranges above, we end up needing to retrieve 10 x 10 x 3652 = 365,200 chunks, each requiring an individual HTTP request. If the chunks were smaller to accommodate even more granular access, say 0.25° x 0.25° x 6 hours, then we end up having to make (4 x 4 x 4) x (10 x 10 x 3652) = 23,372,800 requests! Assuming a rather optimistic time-to-first-byte of only 15 ms (running compute in region alongside the data, for example), we’d end up with almost 10 hours of time spent in the download process. not transferring any bytes just because we have to make individual requests for each chunk.

Parallelism can definitely help here – if we make ten requests at a time, then theoretically we would only end up waiting an hour for request overhead. Except parallelism just masks the problem: the inefficiency is still there, the server still must spend time servicing every individual request, and parallelism forces a lot of complexity onto the client. Moreover, the inefficiency remains limiting the ability of data providers to use even smaller chunks to further mitigate the effects of query misalignment.

With CCRP, we can make one HTTP request to get all chunks, regardless of the chunk size. Though chunk size does impose one important constraint: because CCRP intentionally omits any means of rechunking, we do have to ensure our query slices align to chunk boundaries. In this case, our query area and time range align to both the aforementioned 1° x 1° x 1 day and 0.25° x 0.25° x 6 hours chunk grids, so we can use the slices we initially calculated to make a CCRP request against either:

GET https://ccrp.example.com/datasets/weather/data
    ?time[gte]=87672
    &time[lt]=175320
    &lon[gte]=5_500
    &lon[lt]=6_500
    &lat[gte]=12_200
    &lat[lt]=13_200
    &variable=temperature

Admittedly, this request is for a whole lot of data, and it is unlikely a user would want to download all this with only one download stream. CCRP, as it is proposed, also has support for cases where users want or need more control over the download process, including progressive downloads to facilitate retries and parallel download streams for large amounts of data, as in this case.

What could this look like with better tooling?

With current tooling, the original S3 request might look like this:

import xarray as xr

# Today - direct S3 access
ds = xr.open_dataset(
    's3://some_bucket/weather',
    engine='zarr',
)
array = ds['temperature'].sel(
    time=slice('2010-01-01', '2020-01-01'),
    lat=slice(122, 132),
    lon=slice(55, 65)
).values  # to realize the values into an array, the
          # tooling has to make a request for every chunk

Xarray and the lower-level tooling hide from the user that accessing the values attribute of the temperature array has to make a separate request for every chunk, as we’ve discussed.

How could this same operation look using CCRP?

# Tomorrow - same interface, just using CCRP
ds = xr.open_dataset(
    'ccrp://example.com/datasets/weather',
    engine='zarr',
)
array = ds['temperature'].sel(
    time=slice('2020-01-01', '2020-02-01'),
    lat=slice(122, 132),
    lon=slice(55, 65)
).values  # the underlying tooling makes only one request
          # to realize the array (or a few for parallelism)

This looks exactly the same, aside from the dataset URL. And that’s the point: CCRP is a low-level abstraction meant to make things easier for users, to plug the leaks in the current object model abstraction. It should be transparent to users.

What happens next

I’ve drafted an initial CCRP specification that defines:

But this is just the beginning. For CCRP to succeed, it needs help from a number of stakeholders.

Questions for users

Questions for data producers

Questions for cloud providers

Other areas for community engagement

An invitation

Again, the tyranny of the chunk doesn’t have to be permanent. CCRP is an effort to fight the tyrrany of the chunk.

Ideally, this work is owned and driven forward by the community, not just me. I’m just trying to kickstart it with some examples of what I’ve been thinking. Everything is on the table; it’s all up for discussion. The spec certainly has gaps, problems, and areas needing refinement. The examples are made up, admittedly not great, and need to be revised with real values for both raster and tabular datasets. Actual proof-of-concept implementations are definitely needed to better identify what works and what doesn’t, and to allow those learnings to feed back into and inform the spec.

So again, if you work with large chunked datasets, I want to hear from you. If you’re building data infrastructure, I want your technical feedback. If you work at a cloud provider, let’s talk about what native implementation would take. Or maybe you think this idea is flawed and can never work: awesome! Perhaps you hate the name: great! I’d love to understand any and all concerns, or, better, to know what you’d propose instead.

So take a look at the documentation and OpenAPI specification on the ccrp.dev site. The FAQs page might answer any questions that were not adequately addressed here. The GitHub repo is a great place to open issues, ask questions, or propose edits via PRs. We also have a short form that we ask anyone interested in getting involved or staying up to date to fill out. With enough interest, we can make this a true community project, form a working group, and even organize a sprint.

The future of cloud native can be whatever we want to make it. I believe the object storage model is merely a local maximum for cloud native. Instead of getting stuck here, I want us to consider how to keep going up. CCRP is my idea to address the problems I’ve been seeing, but I don’t think it is the only answer, nor do I think the object storage model is the only local maximum. I suspect there are others; what’s yours?

Acknowledgements

This is already something of a community project with the number of people I’ve talked to about this and who have graciously given the time and energy to listen and provide feedback, to read and revise, or to generally just nod along and help me realize I needed to do better at communicating this idea. I’m sure I am missing some people, but in no particular order:

Also, thanks to Element 84 for supporting this work to date.

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