spring-retry/README.md

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This project provides declarative retry support for Spring
applications. It is used in Spring Batch, Spring Integration, and
others.
Imperative retry is also supported for explicit usage.

## Quick Start

This section provides a quick introduction to getting started with Spring Retry.
It includes a declarative example and an imperative example.

### Declarative Example

The following example shows how to use Spring Retry in its declarative style:

```java
@Configuration
@EnableRetry
public class Application {

    @Bean
    public Service service() {
        return new Service();
    }

}

@Service
class Service {
    @Retryable(RemoteAccessException.class)
    public void service() {
        // ... do something
    }
    @Recover
    public void recover(RemoteAccessException e) {
       // ... panic
    }
}
```

This example calls the `service` method and, if it fails with a `RemoteAccessException`, retries
(by default, up to three times), and then tries the `recover` method if unsuccessful.
There are various options in the `@Retryable` annotation attributes for including and
excluding exception types, limiting the number of retries, and setting the policy for backoff.

The declarative approach to applying retry handling by using the `@Retryable` annotation shown earlier has an additional
runtime dependency on AOP classes. For details on how to resolve this dependency in your project, see the
['Java Configuration for Retry Proxies'](#javaConfigForRetryProxies) section.

### Imperative Example

The following example shows how to use Spring Retry in its imperative style (available since version 1.3):

```java
RetryTemplate template = RetryTemplate.builder()
				.maxAttempts(3)
				.fixedBackoff(1000)
				.retryOn(RemoteAccessException.class)
				.build();

template.execute(ctx -> {
    // ... do something
});
```

For versions prior to 1.3,
see the examples in the [RetryTemplate](#retryTemplate) section.

## Building

Spring Retry requires Java 1.7 and Maven 3.0.5 (or greater).
To build, run the following Maven command:

```
$ mvn install
```

## Features and API

This section discusses the features of Spring Retry and shows how to use its API.

### <a name="retryTemplate"></a> Using `RetryTemplate`

To make processing more robust and less prone to failure, it sometimes helps to
automatically retry a failed operation, in case it might succeed on a subsequent attempt.
Errors that are susceptible to this kind of treatment are transient in nature. For
example, a remote call to a web service or an RMI service that fails because of a network
glitch or a `DeadLockLoserException` in a database update may resolve itself after a
short wait. To automate the retry of such operations, Spring Retry has the
`RetryOperations` strategy. The `RetryOperations` interface definition follows:

```java
public interface RetryOperations {

    <T> T execute(RetryCallback<T> retryCallback) throws Exception;

    <T> T execute(RetryCallback<T> retryCallback, RecoveryCallback<T> recoveryCallback)
        throws Exception;

    <T> T execute(RetryCallback<T> retryCallback, RetryState retryState)
        throws Exception, ExhaustedRetryException;

    <T> T execute(RetryCallback<T> retryCallback, RecoveryCallback<T> recoveryCallback,
        RetryState retryState) throws Exception;

}
```

The basic callback is a simple interface that lets you insert some business logic to be retried:

```java
public interface RetryCallback<T> {

    T doWithRetry(RetryContext context) throws Throwable;

}
```

The callback is tried, and, if it fails (by throwing an `Exception`), it is retried
until either it is successful or the implementation decides to abort. There are a number
of overloaded `execute` methods in the `RetryOperations` interface, to deal with various
use cases for recovery when all retry attempts are exhausted and to deal with retry state, which
lets clients and implementations store information between calls (more on this later).

The simplest general purpose implementation of `RetryOperations` is `RetryTemplate`.
The following example shows how to use it:

```java
RetryTemplate template = new RetryTemplate();

TimeoutRetryPolicy policy = new TimeoutRetryPolicy();
policy.setTimeout(30000L);

template.setRetryPolicy(policy);

Foo result = template.execute(new RetryCallback<Foo>() {

    public Foo doWithRetry(RetryContext context) {
        // Do stuff that might fail, e.g. webservice operation
        return result;
    }

});
```

In the preceding example, we execute a web service call and return the result to the user.
If that call fails, it is retried until a timeout is reached.

Since version 1.3, fluent configuration of `RetryTemplate` is also available, as follows:

```java
RetryTemplate.builder()
      .maxAttempts(10)
      .exponentialBackoff(100, 2, 10000)
      .retryOn(IOException.class)
      .traversingCauses()
      .build();

RetryTemplate.builder()
      .fixedBackoff(10)
      .withinMillis(3000)
      .build();

RetryTemplate.builder()
      .infiniteRetry()
      .retryOn(IOException.class)
      .uniformRandomBackoff(1000, 3000)
      .build();
```

### Using `RetryContext`

The method parameter for the `RetryCallback` is a `RetryContext`. Many callbacks ignore
the context. However, if necessary, you can use it as an attribute bag to store data for
the duration of the iteration.

A `RetryContext` has a parent context if there is a nested retry in progress in the same
thread. The parent context is occasionally useful for storing data that needs to be shared
between calls to execute.

### Using `RecoveryCallback`

When a retry is exhausted, the `RetryOperations` can pass control to a different
callback: `RecoveryCallback`. To use this feature, clients can pass in the callbacks
together to the same method, as the following example shows:

```
Foo foo = template.execute(new RetryCallback<Foo>() {
    public Foo doWithRetry(RetryContext context) {
        // business logic here
    },
  new RecoveryCallback<Foo>() {
    Foo recover(RetryContext context) throws Exception {
          // recover logic here
    }
});
```

If the business logic does not succeed before the template decides to abort, the client is
given the chance to do some alternate processing through the recovery callback.

## Stateless Retry

In the simplest case, a retry is just a while loop: the `RetryTemplate` can keep trying
until it either succeeds or fails. The `RetryContext` contains some state to determine
whether to retry or abort. However, this state is on the stack, and there is no need to
store it anywhere globally. Consequently, we call this "stateless retry". The distinction
between stateless and stateful retry is contained in the implementation of `RetryPolicy`
(`RetryTemplate` can handle both). In a stateless retry, the callback is always executed
in the same thread as when it failed on retry.

## Stateful Retry

Where the failure has caused a transactional resource to become invalid, there are some
special considerations. This does not apply to a simple remote call, because there is (usually) no
transactional resource, but it does sometimes apply to a database update,
especially when using Hibernate. In this case, it only makes sense to rethrow the
exception that called the failure immediately so that the transaction can roll back and
we can start a new (and valid) one.

In these cases, a stateless retry is not good enough, because the re-throw and roll back
necessarily involve leaving the `RetryOperations.execute()` method and potentially losing
the context that was on the stack. To avoid losing the context, we have to introduce a
storage strategy to lift it off the stack and put it (at a minimum) in heap storage. For
this purpose, Spring Retry provides a storage strategy called `RetryContextCache`, which
you can inject into the `RetryTemplate`. The default implementation of the
`RetryContextCache` is in-memory, using a simple `Map`. It has a strictly enforced maximum
capacity, to avoid memory leaks, but it does not have any advanced cache features (such as
time to live). You should consider injecting a `Map` that has those features if you need
them. For advanced usage with multiple processes in a clustered environment, you might
also consider implementing the `RetryContextCache` with a cluster cache of some sort
(though, even in a clustered environment, this might be overkill).

Part of the responsibility of the `RetryOperations` is to recognize the failed operations
when they come back in a new execution (and usually wrapped in a new transaction). To
facilitate this, Spring Retry provides the `RetryState` abstraction. This works in
conjunction with special `execute` methods in the `RetryOperations`.

The failed operations are recognized by identifying the state across multiple invocations
of the retry. To identify the state, you can provide a `RetryState` object that is
responsible for returning a unique key that identifies the item. The identifier is used as
a key in the `RetryContextCache`.

> *Warning:*
Be very careful with the implementation of `Object.equals()` and `Object.hashCode()` in
the key returned by `RetryState`. The best advice is to use a business key to identify the
items. In the case of a JMS message, you can use the message ID.

When the retry is exhausted, you also have the option to handle the failed item in a
different way, instead of calling the `RetryCallback` (which is now presumed to be likely
to fail). As in the stateless case, this option is provided by the `RecoveryCallback`,
which you can provide by passing it in to the `execute` method of `RetryOperations`.

The decision to retry or not is actually delegated to a regular `RetryPolicy`, so the
usual concerns about limits and timeouts can be injected there (see the [Additional Dependencies](#Additional_Dependencies) section).

## Retry Policies

Inside a `RetryTemplate`, the decision to retry or fail in the `execute` method is
determined by a `RetryPolicy`, which is also a factory for the `RetryContext`. The
`RetryTemplate` is responsible for using the current policy to create a `RetryContext` and
passing that in to the `RetryCallback` at every attempt. After a callback fails, the
`RetryTemplate` has to make a call to the `RetryPolicy` to ask it to update its state
(which is stored in `RetryContext`). It then asks the policy if another attempt can be
made. If another attempt cannot be made (for example, because a limit has been reached or
a timeout has been detected), the policy is also responsible for identifying the
exhausted state -- but not for handling the exception. `RetryTemplate` throws the
original exception, except in the stateful case, when no recovery is available. In that
case, it throws `RetryExhaustedException`. You can also set a flag in the
`RetryTemplate` to have it unconditionally throw the original exception from the
callback (that is, from user code) instead.

> *Tip:*
Failures are inherently either retryable or not -- if the same exception is always going
to be thrown from the business logic, it does not help to retry it. So you should not
retry on all exception types. Rather, try to focus on only those exceptions that you
expect to be retryable. It is not usually harmful to the business logic to retry more
aggressively, but it is wasteful, because, if a failure is deterministic, time is spent
retrying something that you know in advance is fatal.

Spring Retry provides some simple general-purpose implementations of stateless
`RetryPolicy` (for example, a `SimpleRetryPolicy`) and the `TimeoutRetryPolicy` used in
the preceding example.

The `SimpleRetryPolicy` allows a retry on any of a named list of exception types, up to a
fixed number of times. The following example shows how to use it:

```java
// Set the max attempts including the initial attempt before retrying
// and retry on all exceptions (this is the default):
SimpleRetryPolicy policy = new SimpleRetryPolicy(5, Collections.singletonMap(Exception.class, true));

// Use the policy...
RetryTemplate template = new RetryTemplate();
template.setRetryPolicy(policy);
template.execute(new RetryCallback<Foo>() {
    public Foo doWithRetry(RetryContext context) {
        // business logic here
    }
});
```

A more flexible implementation called `ExceptionClassifierRetryPolicy` is also available.
It lets you configure different retry behavior for an arbitrary set of exception types
through the `ExceptionClassifier` abstraction. The policy works by calling on the
classifier to convert an exception into a delegate `RetryPolicy`. For example, one
exception type can be retried more times before failure than another, by mapping it to a
different policy.

You might need to implement your own retry policies for more customized decisions. For
instance, if there is a well-known, solution-specific, classification of exceptions into
retryable and not retryable.

## Backoff Policies

When retrying after a transient failure, it often helps to wait a bit before trying again,
because (usually) the failure is caused by some problem that can be resolved only by
waiting. If a `RetryCallback` fails, the `RetryTemplate` can pause execution according to
the `BackoffPolicy`. The following listing shows the definition of the `BackoffPolicy`
interface:

```java
public interface BackoffPolicy {

    BackOffContext start(RetryContext context);

    void backOff(BackOffContext backOffContext)
        throws BackOffInterruptedException;

}
```

A `BackoffPolicy` is free to implement the backoff in any way it chooses. The policies
provided by Spring Retry all use `Object.wait()`. A common use case is to
back off with an exponentially increasing wait period, to avoid two retries getting into
lock step and both failing (a lesson learned from Ethernet). For this purpose, Spring
Retry provides `ExponentialBackoffPolicy`. Spring Retry also provides randomized versions
of delay policies that are quite useful to avoid resonating between related failures in a
complex system.

## Listeners

It is often useful to be able to receive additional callbacks for cross cutting concerns across a number of different retries. For this purpose, Spring Retry provides the `RetryListener` interface. The `RetryTemplate` lets you register `RetryListener` instances, and they are given callbacks with the `RetryContext` and `Throwable` (where available during the iteration).

The following listing shows the `RetryListener` interface:

```java
public interface RetryListener {

    void open(RetryContext context, RetryCallback<T> callback);

    void onError(RetryContext context, RetryCallback<T> callback, Throwable e);

    void close(RetryContext context, RetryCallback<T> callback, Throwable e);
}
```

The `open` and `close` callbacks come before and after the entire retry in the simplest
case, and `onError` applies to the individual `RetryCallback` calls. The close method
might also receive a `Throwable`. If there has been an error, it is the last one thrown by
the `RetryCallback`.

Note that when there is more than one listener, they are in a list, so there is an order.
In this case, `open` is called in the same order, while `onError` and `close` are called
in reverse order.

### Listeners for Reflective Method Invocations

When dealing with methods that are annotated with `@Retryable` or with Spring AOP intercepted methods, Spring Retry allows a detailed inspection of the method invocation within the `RetryListener` implementation.

Such a scenario could be particularly useful when there is a need to monitor how often a certain method call has been retried and expose it with detailed tagging information (such as class name, method name, or even parameter values in some exotic cases).

The following example registers such a listener:

```java

template.registerListener(new MethodInvocationRetryListenerSupport() {
      @Override
      protected <T, E extends Throwable> void doClose(RetryContext context,
          MethodInvocationRetryCallback<T, E> callback, Throwable throwable) {
        monitoringTags.put(labelTagName, callback.getLabel());
        Method method = callback.getInvocation()
            .getMethod();
        monitoringTags.put(classTagName,
            method.getDeclaringClass().getSimpleName());
        monitoringTags.put(methodTagName, method.getName());

        // register a monitoring counter with appropriate tags
        // ...
      }
    });
```

## Declarative Retry

Sometimes, you want to retry some business processing every time it happens. The classic
example of this is the remote service call. Spring Retry provides an AOP interceptor that
wraps a method call in a `RetryOperations` instance for exactly this purpose. The
`RetryOperationsInterceptor` executes the intercepted method and retries on failure
according to the `RetryPolicy` in the provided `RepeatTemplate`.


### <a name="javaConfigForRetryProxies"></a> Java Configuration for Retry Proxies

You can add the `@EnableRetry` annotation to one of your `@Configuration` classes and use
`@Retryable` on the methods (or on the type level for all methods) that you want to retry.
You can also specify any number of retry listeners. The following example shows how to do
so:

```java
@Configuration
@EnableRetry
public class Application {

    @Bean
    public Service service() {
        return new Service();
    }

    @Bean public RetryListener retryListener1() {
        return new RetryListener() {...}
    }

    @Bean public RetryListener retryListener2() {
        return new RetryListener() {...}
    }

}

@Service
class Service {
    @Retryable(RemoteAccessException.class)
    public service() {
        // ... do something
    }
}
```

You can use the attributes of `@Retryable` to control the `RetryPolicy` and `BackoffPolicy`, as follows:

```java
@Service
class Service {
    @Retryable(maxAttempts=12, backoff=@Backoff(delay=100, maxDelay=500))
    public service() {
        // ... do something
    }
}
```

The preceding example creates a random backoff between 100 and 500 milliseconds and up to
12 attempts. There is also a `stateful` attribute (default: `false`) to control whether
the retry is stateful or not. To use stateful retry, the intercepted method has to have
arguments, since they are used to construct the cache key for the state.

The `@EnableRetry` annotation also looks for beans of type `Sleeper` and other strategies
used in the `RetryTemplate` and interceptors to control the behavior of the retry at runtime.

The `@EnableRetry` annotation creates proxies for `@Retryable` beans, and the proxies
(that is, the bean instances in the application) have the `Retryable` interface added to
them. This is purely a marker interface, but it might be useful for other tools looking to
apply retry advice (they should usually not bother if the bean already implements
`Retryable`).

If you want to take an alternative code path when
the retry is exhausted, you can supply a recovery method. Methods should be declared in the same class as the `@Retryable`
instance and marked `@Recover`. The return type must match the `@Retryable` method. The arguments
for the recovery method can optionally include the exception that was thrown and
(optionally) the arguments passed to the original retryable method (or a partial list of
them as long as none are omitted up to the last one needed). The following example shows how to do so:

```java
@Service
class Service {
    @Retryable(RemoteAccessException.class)
    public void service(String str1, String str2) {
        // ... do something
    }
    @Recover
    public void recover(RemoteAccessException e, String str1, String str2) {
       // ... error handling making use of original args if required
    }
}
```

To resolve conflicts between multiple methods that can be picked for recovery, you can explicitly specify recovery method name.
The following example shows how to do so:

```java
@Service
class Service {
    @Retryable(recover = "service1Recover", value = RemoteAccessException.class)
    public void service1(String str1, String str2) {
        // ... do something
    }

    @Retryable(recover = "service2Recover", value = RemoteAccessException.class)
    public void service2(String str1, String str2) {
        // ... do something
    }

    @Recover
    public void service1Recover(RemoteAccessException e, String str1, String str2) {
        // ... error handling making use of original args if required
    }

    @Recover
    public void service2Recover(RemoteAccessException e, String str1, String str2) {
        // ... error handling making use of original args if required
    }
}
```

Version 1.3.2 and later supports matching a parameterized (generic) return type to detect the correct recovery method:

```java
@Service
class Service {

    @Retryable(RemoteAccessException.class)
    public List<Thing1> service1(String str1, String str2) {
        // ... do something
    }

    @Retryable(RemoteAccessException.class)
    public List<Thing2> service2(String str1, String str2) {
        // ... do something
    }

    @Recover
    public List<Thing1> recover1(RemoteAccessException e, String str1, String str2) {
       // ... error handling for service1
    }

    @Recover
    public List<Thing2> recover2(RemoteAccessException e, String str1, String str2) {
       // ... error handling for service2
    }

}
```

Version 1.2 introduced the ability to use expressions for certain properties. The
following example show how to use expressions this way:

```java

@Retryable(exceptionExpression="message.contains('this can be retried')")
public void service1() {
  ...
}

@Retryable(exceptionExpression="message.contains('this can be retried')")
public void service2() {
  ...
}

@Retryable(exceptionExpression="@exceptionChecker.shouldRetry(#root)",
    maxAttemptsExpression = "#{@integerFiveBean}",
  backoff = @Backoff(delayExpression = "#{1}", maxDelayExpression = "#{5}", multiplierExpression = "#{1.1}"))
public void service3() {
  ...
}
```

Since Spring Retry 1.2.5, for `exceptionExpression`, templated expressions (`#{...}`) are
deprecated in favor of simple expression strings
(`message.contains('this can be retried')`).

Expressions can contain property placeholders, such as `#{${max.delay}}` or
`#{@exceptionChecker.${retry.method}(#root)}`. The following rules apply:

- `exceptionExpression` is evaluated against the thrown exception as the `#root` object.
- `maxAttemptsExpression` and the `@BackOff` expression attributes are evaluated once,
during initialization. There is no root object for the evaluation but they can reference
other beans in the context.

#### <a name="Additional_Dependencies"></a> Additional Dependencies

The declarative approach to applying retry handling by using the `@Retryable` annotation
shown earlier has an additional runtime dependency on AOP classes that need to be declared
in your project. If your application is implemented by using Spring Boot, this dependency
is best resolved by using the Spring Boot starter for AOP. For example, for Gradle, add
the following line to your `build.gradle` file:

```
    runtime('org.springframework.boot:spring-boot-starter-aop')
```

For non-Boot apps, you need to declare a runtime dependency on the latest version of
AspectJ's `aspectjweaver` module. For example, for Gradle, you should add the following
line  to your `build.gradle` file:

```
    runtime('org.aspectj:aspectjweaver:1.8.13')
```

### XML Configuration

The following example of declarative iteration uses Spring AOP to repeat a service call to
a method called `remoteCall`:

```xml
<aop:config>
    <aop:pointcut id="transactional"
        expression="execution(* com..*Service.remoteCall(..))" />
    <aop:advisor pointcut-ref="transactional"
        advice-ref="retryAdvice" order="-1"/>
</aop:config>

<bean id="retryAdvice"
    class="org.springframework.retry.interceptor.RetryOperationsInterceptor"/>
```

For more detail on how to configure AOP interceptors, see the [Spring Framework Documentation](https://docs.spring.io/spring/docs/current/spring-framework-reference/index.html).

The preceding example uses a default `RetryTemplate` inside the interceptor. To change the
policies or listeners, you need only inject an instance of `RetryTemplate` into the
interceptor.

## Contributing

Spring Retry is released under the non-restrictive Apache 2.0 license
and follows a very standard Github development process, using Github
tracker for issues and merging pull requests into the main branch. If you want
to contribute even something trivial, please do not hesitate, but do please
follow the guidelines in the next paragraph.

Before we can accept a non-trivial patch or pull request, we need you
to sign the [contributor's agreement](https://cla.pivotal.io/). Signing
the contributor's agreement does not grant anyone commit rights to the
main repository, but it does mean that we can accept your
contributions, and you will get an author credit if we do.  Active
contributors might be asked to join the core team and be given the
ability to merge pull requests.

## Code of Conduct

This project adheres to the [Contributor Covenant](https://github.com/spring-projects/spring-retry/blob/main/CODE_OF_CONDUCT.adoc).
By participating, you  are expected to uphold this code. Please report unacceptable behavior to
spring-code-of-conduct@pivotal.io.