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Version: 2.x

Introduction to ZIO Query

ZIO Query is a library for writing optimized queries to data sources in a high-level compositional style. It can add efficient pipelining, batching, and caching to any data source. ZIO Query helps us dramatically reduce load on data sources and improve performance.

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Some key features of ZIO Query:

  • Batching — ZIO Query detects parts of composite queries that can be executed in parallel without changing the semantics of the query.
  • Pipelining — ZIO Query detects parts of composite queries that can be combined together for fewer individual requests to the data source.
  • Caching — ZIO Query can transparently cache read queries to minimize the cost of fetching the same item repeatedly in the scope of a query.

Compared with Fetch, ZIO Query supports response types that depend on request types, does not require higher-kinded types and implicits, supports ZIO environment and statically typed errors, and has no dependencies except for ZIO.

A ZQuery[R, E, A] is a purely functional description of an effectual query that may contain requests from one or more data sources, requires an environment R, and may fail with an E or succeed with an A.

Requests that can be performed in parallel, as expressed by zipWithPar and combinators derived from it, will automatically be batched. Requests that must be performed sequentially, as expressed by zipWith and combinators derived from it, will automatically be pipelined. This allows for aggressive data source specific optimizations. Requests can also be deduplicated and cached.

This allows for writing queries in a high level, compositional style, with confidence that they will automatically be optimized. For example, consider the following query from a user service.

Assume we have the following database access layer APIs:

def getAllUserIds: ZIO[Any, Nothing, List[Int]] = {
// Get all user IDs e.g. SELECT id FROM users

def getUserNameById(id: Int): ZIO[Any, Nothing, String] = {
// Get user by ID e.g. SELECT name FROM users WHERE id = $id

We can get their corresponding usernames from the database by the following code snippet:

val userNames = for {
ids <- getAllUserIds
names <- ZIO.foreachPar(ids)(getUserNameById)
} yield names

It works, but this is not performant. It is going to query the underlying database N + 1 times, one for getAllUserIds and one for each call to getUserNameById.

In contrast, ZQuery will automatically optimize this to two queries, one for userIds and one for userNames:

lazy val getAllUserIds: ZQuery[Any, Nothing, List[Int]]    = ???
def getUserNameById(id: Int): ZQuery[Any, Nothing, String] = ???

lazy val userQuery: ZQuery[Any, Nothing, List[String]] = for {
userIds <- getAllUserIds
userNames <- ZQuery.foreachPar(userIds)(getUserNameById)
} yield userNames


In order to use this library, we need to add the following line in our build.sbt file:

libraryDependencies += "dev.zio" %% "zio-query" % "0.7.4"


Here is an example of using ZIO Query, which optimizes multiple database queries by batching all of them in one query:

import zio._
import zio.query._

object ZQueryExample extends ZIOAppDefault {
case class GetUserName(id: Int) extends Request[Throwable, String]

lazy val UserDataSource: DataSource.Batched[Any, GetUserName] =
new DataSource.Batched[Any, GetUserName] {
val identifier: String = "UserDataSource"

def run(requests: Chunk[GetUserName])(implicit trace: Trace): ZIO[Any, Nothing, CompletedRequestMap] =
requests.toList match {
case request :: Nil =>
val result: Task[String] = {
// get user by ID e.g. SELECT name FROM users WHERE id = $id
}, _))

case batch: Seq[GetUserName] =>
val result: Task[List[(Int, String)]] = {
// get multiple users at once e.g. SELECT id, name FROM users WHERE id IN ($ids)

CompletedRequestMap.failCause(requests, _),
CompletedRequestMap.fromIterableWith(_)(kv => GetUserName(kv._1), kv => Exit.succeed(kv._2))

def getUserNameById(id: Int): ZQuery[Any, Throwable, String] =

val query: ZQuery[Any, Throwable, List[String]] =
for {
ids <- ZQuery.succeed(1 to 10)
names <- ZQuery.foreachPar(ids)(id => getUserNameById(id)).map(_.toList)
} yield (names)

def run = => Console.printLine(s"Usernames: $usernames"))