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Cache

Summary​

Add cache support for Databend Query so that our users can load hot data from our cache services instead of reading from backend storage services again.

Motivation​

Databend design is based on the separation of storage and computing:

As a result, databend will support different cloud storage services as its backend like AWS s3, azure blob, and google cloud storage. The advantage of this is that we can get very high throughput bandwidth, but the disadvantage is that the latency of our individual requests increases.

Adding a caching layer will improve the overall latency and reduce the number of unnecessary requests, thus reducing costs.

Databend used to have a local disk-based cache at common-cache. This implementation use databend-query's local path to cache blocks.

Guide-level explanation​

After introducing cache, the databend storage layer will be composed as follows:

The query will maintain three OpenDAL operators:

  • Persist Operator: Read and write persist data from/into storage services directly.
  • Cache Operator: Read and write data via transparent caching.
  • Temporary Operator: Read and write temporary data from/into temporary storage which only store data with TTL.

Storage, cache, and temporary services we mentioned here are both OpenDAL-supported services. The difference is:

  • Persist Services: All data will be persisted until users delete them.
  • Cache Services: Backends could have their GC or background auto eviction logic, which means cache services is non-persist, a.k.a, volatile.
  • Temporary Services: Backend will be configured with TTL and timely delete old data.

The query should never try to write persisted data to Cache Operator or Temporary Operator.

For End Users​

Users can specify different cache services for caching or storing temporary data. For example, they can cache data into local fs (the same as current behavior):

[cache]
type = "fs"

[cache.fs]
data_path = "/var/cache/databend/"

Or they can cache data into distributed cache services like opencache:

[cache]
type = "opencache"

[cache.opencache]
endpoints = ["192.168.1.2"]

Temporary data could be shared crossing the whole cluster, so users must use shared storage services like redis or s3 instead of local fs or memory:

[temporary]
type = "s3"

[temporary.s3]
bucket = "temporary-data"
endpoint = "http://127.0.0.1:9900"

For Developers​

As discussed before, developers now have three kinds of Operator to handle different workloads:

  • Persist: Read and write data directly without caching logic, similar to O_DIRECT.
  • Cache: Read and write via transparent caching, similar to the kernel's page cache.
  • Temporary: Read and write temporary data to cache services, similar to Linux's tmpfs, old data could be removed timely.

Reference-level explanation​

Cache Operator​

Cache Operator will be implemented as an OpenDAL Layer:

#[derive(Clone, Default, Debug)]
pub struct CacheLayer {
    // Operator for persist data (via `storage` config)
    persist: Operator,
    // Operator for caching data (via `cache` config)
    cache: Operator,
    // Client for KVApi
    kv: Arc<dyn KVApi>,
}

impl Layer for CacheLayer {}
impl Accessor for CacheLayer {}

databend-query will init the cache layer along with persist operator:

pub fn get_cache_operator(&self) -> Operator {
    self.get_storage_operator().layer(CacheLayer::new())
}

The detailed implementation of CacheLayer will not discuss in this RFC.

Temporary Operator​

The temporary Operator will connect to a shared storage service that shares the same view for the whole cluster. The query can write temporary intermediate results here to share with other nodes.

Most work should be down at the services side. We will not discuss this in this RFC.

Drawbacks​

None.

Rationale and alternatives​

None.

Prior art​

None.

Unresolved questions​

None.

Future possibilities​

OpenCache​

OpenCache is an ongoing work from databend community to build a distributed cache services. OpenDAL will implement native support for OpenCache once its API is stable. After that, users can deploy and use OpenCache as cache storage.