TranzactIO

TranzactIO is a ZIO wrapper for some Scala database access libraries (Doobie and Anorm, for now).

If the library comes with an IO monad (like Doobie's ConnectionIO), it lifts it into a ZIO[Connection, E, A]. If the library doesn't have an IO monad to start with (like Anorm), it a ZIO[Connection, E, A] for the role.

When you're done chaining ZIOs and want to execute the transaction, use TranzactIO's Database module to provide a connection for your ZIO. It can also provide a connection in auto-commit mode, without a transaction.

It comes with a very small amount of dependencies: only ZIO and ZIO-interop-Cats are required.

Any constructive criticism, bug report or offer to help is welcome. Just open an issue or a PR.

Why ?

On my applications, I regularly have quite a bunch of business logics around my queries. If I want to run that logic within a transaction, I had to wrap it with Doobie's ConnectionIO. But I'm already using ZIO as my effect monad! I don't want another one... In addition, IO monads on DB libraries (like Doobie's ConnectionIO) misses quite a bit of the operations that ZIO has.

That's where TranzactIO comes from. I wanted a way to use ZIO everywhere, and run the transaction whenever I decided.

Getting started

Sbt setup

TranzactIO is available on the Sonatype Central Repository (see the Nexus badge on top of this README to get the version number). In your build.sbt:

libraryDependencies += "io.github.gaelrenoux" %% "tranzactio" % TranzactIOVersion

In addition, you will need to declare the database access lbrary you are using. For instance with Doobie:

libraryDependencies += "org.tpolecat" %% "doobie-core" % DoobieVersion

Imports

Most of the time, you will need to import two packages. The first is io.github.gaelrenoux.tranzactio._ and contains Tranzactio's generic classes.

The second one is specific to a library, and contains the associated elements. The names of most elements in those packages are the same in each package, for instance the tzio function, or the Connection and Database classes. The package is always named after the library it is used with, e.g.:

• io.github.gaelrenoux.tranzactio.doobie._
• io.github.gaelrenoux.tranzactio.anorm._

Wrapping a query

Just use tzio to wrap your usual query type!

Note that Connection is not Java's java.sql.Connection, it's a TranzactIO library-specific type. DbException is generic (not library-specific), and represents any error in relation with the DB.

Doobie

import zio._
import doobie.implicits._
import io.github.gaelrenoux.tranzactio._
import io.github.gaelrenoux.tranzactio.doobie._

val list: ZIO[Connection, DbException, List[String]] = tzio {
    sql"SELECT name FROM users".query[String].to[List]
}

Anorm

Since Anorm doesn't provide an IO monad (or even a specific query type), tzio will provide the connection you need to run a query. The operation will be wrapped in a ZIO (as a blocking effect).

import zio._
import anorm._
import io.github.gaelrenoux.tranzactio._
import io.github.gaelrenoux.tranzactio.anorm._

val list: ZIO[Connection, DbException, List[String]] = tzio { implicit c =>
    SQL"SELECT name FROM users".as(SqlParser.str(1).*)
}

Running the transaction (or using auto-commit)

The Database module from the same library-specific package provides the method needed to run the queries.

Here are some examples with Doobie. The code for Anorm is identical, except it has a different import: io.github.gaelrenoux.tranzactio.anorm._ instead of io.github.gaelrenoux.tranzactio.doobie._.

import io.github.gaelrenoux.tranzactio._
import io.github.gaelrenoux.tranzactio.doobie._
import zio._
import zio.console.Console

// Let's start with a very simple one. Connection exceptions are transformed into defects.
val zio: ZIO[Connection, String, Long] = ???
val simple: ZIO[Database, String, Long] = Database.transactionOrDie(zio)

// If you have an additional environment, use the ***R method. The environment type must be specified.
val zioEnv: ZIO[Connection with Console, String, Long] = ???
val withEnv: ZIO[Database with Console, String, Long] = Database.transactionOrDieR[Console](zioEnv)

// Do you want to handle connection errors yourself? They will appear on the Left side of the Either.
val withSeparateErrors: ZIO[Database, Either[DbException, String], Long] = Database.transaction(zio)

// Is the only error you are expecting coming from the DB ? Let's handle all of them at the same time.
val zioDbEx: ZIO[Connection, DbException, Long] = ???
val withDbEx: ZIO[Database, DbException, Long] = Database.transactionOrWiden(zioDbEx)

// Or maybe you're just grouping all errors together as exceptions.
val zioEx: ZIO[Connection, java.io.IOException, Long] = ???
val withEx: ZIO[Database, Exception, Long] = Database.transactionOrWiden(zioEx)

// You can also commit even on a failure (only rollbacking on a defect). Useful if you're using the failure channel for short-circuiting!
val commitOnFailure: ZIO[Database, String, Long] = Database.transactionOrDie(zio, commitOnFailure = true)

// And if you're actually not interested in a transaction, you can just auto-commit all queries.
val zioAutoCommit: ZIO[Database, String, Long] = Database.autoCommitOrDie(zio)

Providing the Database

The resulting ZIO requires a Database as an environment, that will be provided through a ZLayer.

The Database object lets you construct a ZLayer which requires a javax.sql.DataSource. Your connection pool implementation (like HikariCP) should provide a DataSource representation. Alternatively (e.g. in a test environment), you can create a DataSource manually. Do you have a use case where you can't get a DataSource? Notify me by creating an issue!

Again, the code for Anorm is identical, except it has a different import: io.github.gaelrenoux.tranzactio.anorm._ instead of io.github.gaelrenoux.tranzactio.doobie._.

import io.github.gaelrenoux.tranzactio.doobie._
import javax.sql.DataSource
import zio._
import zio.blocking.Blocking
import zio.clock.Clock

val dbLayer: ZLayer[Has[DataSource] with Blocking with Clock, Nothing, Database] = Database.fromDatasource

More code samples

Find more in src/main/samples, or look below for some details.

Detailed documentation

Version compatibility

The table below indicates for each version of TranzactIO, the versions of ZIO or libraries it's been built with. Check the backward compatibily information on those libraries to check if your version is supported with a particular version of TranzactIO.

TranzactIO	ZIO	Doobie	Anorm
0.1.0	1.0.0-RC17	0.8.6	-
0.2.0	1.0.0-RC18-2	0.8.6	-
0.3.0	1.0.0-RC18-2	0.8.6	2.6.5
0.4.0	1.0.0-RC19-2	0.9.0	2.6.5
0.5.0	1.0.0-RC20	0.9.0	2.6.5
0.6.0	1.0.0-RC21-1	0.9.0	2.6.5
1.0.0	1.0.0	0.9.0	2.6.7
master	1.0.0	0.9.0	2.6.7

Error definitions

We'll talk a bit about errors in the next sections, so here are two definitions. In TranzactIO, we recognize two categories of errors relating to the DB: query errors, and connection errors:

Query errors happen when you run a specific query. They can be timeouts, SQL syntax errors, constraint errors, etc. When you have a ZIO[Connection, E, A], E is the type for query errors.

Connection errors happen when you manage connections or transactions: opening connections, creating, commiting or rollbacking transactions, etc. They are not linked to a specific query. They are always reported as a DbException.

Database methods

There are two families of operations on the Database class: transaction and autoCommit. I'll only describe transaction here, keep in mind that there's an identical set of operations with autoCommit instead.

When providing the transaction with Database, you have three variants of the transaction method, which will handle unrecovered connection errors.

• With transaction, the resulting error type is an Either: Right wraps a query error, and Left wraps a connection error. This is the most generic method, leaving you to handle all errors how you see fit.
• With transactionOrDie, connection errors are considered as defects, and do not appear in the type signature.
• With transactionOrWiden, the resulting error type will be the closest supertype of the query error type and DbException, and the error in the result may be a query error or a connection error. This is especially useful if your query error type is already DbException or directly Exception, as in the examples above.

val zio: ZIO[Connection, E, A] = ???
val result1: ZIO[Database, Either[DbException, E], A] = Database.transaction(zio)
val result2: ZIO[Database, E, A] = Database.transactionOrDie(zio)
// assuming E extends Exception:
val result3: ZIO[Database, Exception, A] = Database.transactionOrWiden(zio)

In addition, a frequent case is to have an additional environment on your ZIO monad, e.g.: ZIO[ZEnv with Connection, E, A]. To handle this case, all methods mentioned above have an additional variant with a final R.

When using an ***R method, you will need to provide the additional environment type as a type parameter (Scala's compiler is not smart enough to infer it correctly on its own):

val zio: ZIO[ZEnv with Connection, E, A] = ???
val result1: ZIO[Database with ZEnv, Either[DbException, E], A] = Database.transactionR[ZEnv](zio)
val result2: ZIO[Database with ZEnv, E, A] = Database.transactionOrDieR[ZEnv](zio)
// assuming E extends Exception:
val result3: ZIO[Database with ZEnv, Exception, A] = Database.transactionOrWidenR[ZEnv](zio) 

All the transaction methods take an optional argument commitOnFailure (defaults to false). If true, the transaction will be commited on a failure (the E part in ZIO[R, E, A]), and will still be rollbacked on a defect. Obviously, this argument does not exist on the autoCommit methods.

Finally, all those methods take an optional implicit argument of type ErrorStrategies. See Error handling below for details.

Database module configuration

Error handling

TranzactIO has no specific error handling for query errors. Since you, as the developer, have direct access to the ZIO instance representing the query (or aggregation of queries), it's up to you to add timeouts or retries, recover from errors, etc. However, you do not have access to the connection errors, which are hidden in the Database module.

The error handling on connection errors is set up through an ErrorStrategies instance. There are two mechanism to provide it:

• You can pass an ErrorStrategies instance as an implicit parameter when calling the Database methods. If no implicit value is provided, the default is ErrorStrategies.Default, which defers to the next mechanism.
• When declaring the Database layer, you can have an ErrorStrategies instance as an input to the layer.

import io.github.gaelrenoux.tranzactio._
import io.github.gaelrenoux.tranzactio.doobie._
import javax.sql.DataSource
import zio._
import zio.blocking.Blocking
import zio.clock.Clock

implicit val es: ErrorStrategies = ErrorStrategies.RetryForever
Database.transaction(???) // es is passed implicitly

val dbLayerFromDatasource: ZLayer[Has[DataSource] with Has[ErrorStrategiesRef] with Blocking with Clock, Nothing, Database]  =
    Database.fromDatasourceAndErrorStrategies

The ErrorStrategies companinon objects define a few default values. A typical configuration for production would be to start with ErrorStrategies.RetryForever and add timeouts, using the withTimeout and withRetryTimeout methods, using values defined in a configuration layer.

You can also construct manually an ErrorStrategies instance, setting different policies for each action (opening connections, committing, rollbacking, etc.)

If no instance is provided as an implicit and no instance is defined on the layer, the default is ErrorStrategies.Brutal. It is an unforgiving setting, with no retry and 1s timeout on all operations, which makes it great when testing.

What's next ?

Evolve the API

The API is pretty final by now on the main points (like the use of tzio and Database methods).

I've just changed the way the layers are defined, and how the ErrorStrategies are passed. I think it's much more usable now, and it shouldn't change too much any more (maybe some renaming).

Obviously, I'll also follow the new versions of ZIO, so some changes might happen due to changes in ZIO.

More database access libraries

I want to add wrappers around more database access libraries. Anorm was the second one I did, next should probably be Quill (based on the popularity of the project on GitHub), but I'm completely unfamiliar with it.

Slick, however, is a problem. I know it quite well, tried to implement a TranzactIO module for it, and couldn't. Transactions cannot be handled externally using Slick. I don't think it's doable until this ticket is done: slick/slick#1563