Minimal, idiomatic, stream-based Scala interface for key/value store implementations
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Rolando Manrique
Rolando Manrique Update change for v0.2.2
Latest commit 3adfcfa Oct 5, 2018


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Minimal, idiomatic, stream-based Scala interface for key/value store implementations. It provides abstractions for S3-like key/value store backed by different persistence mechanisms (i.e. S3, FileSystem, sftp, etc).


fs2-blobstore is deployed to maven central, add to build.sbt:

libraryDependencies ++= Seq(
  "com.lendup.fs2-blobstore" %% "core" % "0.1.+",
  "com.lendup.fs2-blobstore" %% "sftp" % "0.1.+",
  "com.lendup.fs2-blobstore" %% "s3" % "0.1.+"

core module has minimal dependencies and only provides FileStore implementation. sftp module provides SftpStore and depends on Jsch client. s3 module provides S3Store and depends on AWS S3 SDK

Store Abstraction

The goal of the Store interface is to have a common representation of key/value functionality (get, put, list, etc) as streams that can be composed, transformed and piped just like any other fs2.Stream or fs2.Sink regardless of the underlying storage mechanism.

This is especially useful for unit testing if you are building a S3 or SFTP backed system as you can provide a filesystem based implementation for tests that is guaranteed to work the same way as you production environment.

The three main activities in a key/value store are modeled like:

def list(path: Path): fs2.Stream[F, Path]
def get(path: Path, chunkSize: Int): fs2.Stream[F, Byte]
def put(path: Path, contentLength: Long): fs2.Sink[F, Byte]  

Note that list and get are modeled as streams since they are reading (potentially) very large amounts of data from storage, while put is represented as a sink of byte so that any stream of bytes can by piped into it to upload data to storage.

Implicit Ops

import blobstore.implicits._

StoreOps and PathOps provide functionality on both Store and Path for commonly performed tasks (i.e. upload/download a file from/to local filesystem, collect all returned paths when listing, composing paths or extracting filename of the path).

Most of these common tasks encapsulate stream manipulation and provide a simpler interface that return the corresponding effect monad. These are also very good examples of how to use blobstore streams and sink in different scenarios.


All store implementations must support and pass the suite of tests in AbstractStoreTest. It is expected that each store implementation (like s3, sftp, file) should contain the Store implementation and at least one test suite that inherits from AbstractStoreTest and overrides store and root attributes:

class MyStoreImplTest extends blobstore.AbstractStoreTest {
  override val store: blobstore.Store[cats.effect.IO] = MyStoreImpl( ... )
  override val root: String = "my_store_impl_tests"

This test suite will guarantee that basic operations are supported properly and consistent with all other Store implementations.

Running Tests:

Tests are set up to run via docker-compose:

docker-compose run --rm sbt "testOnly * -- -l blobstore.IntegrationTest"

This will start a minio (Amazon S3 compatible object storage server) and SFTP containers and run all tests not annotated as @IntegrationTest.

Yes, we understand SftpStoreTest and S3StoreTest are also integration tests because they connect to external services, but we don't mark them as such because we found these containers that allow to run them along unit tests and we want to exercise as much of the store code as possible.

Currently, tests for SftpStore and BoxStore are annotated with @IntegrationTest because: (1) SFTP tests fail to run against sftp container in travis, and (2) we have not found a box docker image. To run BoxStore integration tests locally you need to provide env vars for BOX_TEST_BOX_DEV_TOKEN and BOX_TEST_ROOT_FOLDER_ID.

Run box/sftp tests with:

sbt box/test
docker-compose run --rm sbt sftp/test

Note: this will exercise AbstractStoreTest tests against your account.

Path Abstraction

blobstore.Path is the representation of key in the key/value store. The key representation is based on S3 that has a root (or bucket) and a key string.

When functions in the Store interface that receive a Path should assume that only root and key values are set, there is no guarantee that the other attributes of Path would be filled: size, isDir, lastModified. On the other hand, when a Store implements the list function, it is expected that all 3 fields will be present in the response.

By importing implicit PathOps into the scope you can make use of path composition / and filename function that returns the substring of the path's key after the last path separator.

NOTE: a good improvement to the path abstraction would be to handle OS specific separators when referring to filesystem paths.

Store Implementations

  • FileStore backed by local FileSystem. FileStore is provided as part of core module because it doesn't include any additional dependencies and it is used as the default source store in TransferOps tests. It only requires root path in the local file system:
    import blobstore.Store, blobstore.fs.FileStore 
    import java.nio.file.Paths
    import cats.effect.IO 
    val store: Store[IO] = FileStore[IO](Paths.get("tmp/"))
  • S3Store backed by AWS S3. It requires an authenticated AmazonS3 client:
    import blobstore.Store, blobstore.s3.S3Store
    import cats.effect.IO
    val store: Store[IO] = S3Store[IO](AmazonS3ClientBuilder.standard().build())
  • SftpStore backed by SFTP server with Jsch client. It requires a connected ChannelSftp:
    import blobstore.Store, blobstore.sftp.SftpStore
    import com.jcraft.jsch.{ChannelSftp, JSch}
    import cats.effect.IO
    val jsch = new JSch()
    val session = jsch.getSession("")
    val channel = session.openChannel("sftp").asInstanceOf[ChannelSftp]
    val store: Store[IO] = SftpStore("root/server/path", channel)
  • BoxStore backed by a BoxAPIConnection, which has multiple options for authentication. This requires that you have a Box app set up already. See Box SDK documentation for more details:
    import blobstore.Store,
    val api = new BoxAPIConnection("myDeveloperToken")
    val store: Store[IO] = BoxStore[IO](api, "rootFolderId")