# Using the Library¶

## Why are Backoff Strategies useful?¶

Because now and again, stuff breaks.

Often, when making function calls, something goes wrong. The internet might glitch. The API we’re calling might timeout. Gremlins might eat your packets. Any number of things can go wrong, and Murphy’s law tells us that they will.

Which is why we need backoff strategies. Basically, a backoff strategy is a technique that we can use to retry failing function calls after a given delay - and keep retrying them until either the function call works, or until we’ve tried so many times that we just give up and handle the error.

## How does this library help?¶

This library provides a simple one-line approach to using backoff strategies in your code.

It provides a simple function (backoff()) and a simple decorator (@apply_backoff()) that let you easily retry problematic functions using five different configurable strategies.

## Installing the Library¶

To install Backoff-Utils, just execute:

\$ pip install backoff-utils


## Importing the Library¶

There are three parts to the library that you should be aware of:

1. The backoff() function, which you can use to to apply a backoff strategy to a given function/method call inside your code.
2. The @apply_backoff() decorator, which you can use to always apply a backoff strategy to one of your function/methods.
3. The Strategies Explained module, which exposes the BackoffStrategy classes that you supply to the function and decorator, telling them how to delay between attempts. The specific strategies are:

All three of these components are importable directly from the backoff_utils package as shown below:

#: Import everything
from backoff_utils import backoff, apply_backoff, strategies

#: Import the backoff() function.
from backoff_utils import backoff

#: Import the @apply_backoff() decorator.
from backoff_utils import apply_backoff

#: Import backoff strategies.
from backoff_utils import strategies


## Using the Backoff Function Call¶

You use the backoff() function when:

• you want to call some other function/method using a backoff strategy, but that function/method is not decorated with @apply_backoff()
• you want to call some other function/method using a backoff strategy, but if that call fails, you want to retry using a different call.

Tip

The function approach is often used when we want to apply a backoff strategy to a function or method called in someone else’s code, like in some imported third-party library.

Since that code won’t be using the @apply_backoff() decorator, if we want to apply a backoff strategy we’ll need to use the backoff() function.

### Basic Usage¶

For example, let’s imagine we have a function:

def some_function(arg1, arg2, kwarg1 = value):
# Function does stuff here


When our code calls some_function(), we want to apply an Exponential backoff strategy. We can do so using:

result = backoff(some_function,
args = ['value1', 'value2'],
kwargs = { 'kwarg1': 'value3' },
max_tries = 3,
max_delay = 30,
strategy = strategies.Exponential)


Let’s breakdown what this does. First, it will try calling:

result = some_function('value1', 'value2', kwarg1 = 'value3')


If this raises an error, it will retry using an Exponential delay. It will continue to retry, until either it has made 3 attempts or 30 seconds have elapsed. If this call is still failing after 3 attempts or 30 seconds, it will raise the last Exception raised by some_function().

Note

The strategy argument can accept either a class that inherits from BackoffStrategy, or it can accept an instance of a class that inherits from BackoffStrategy.

Passing a class will use the default configuration for the backoff strategy, while passing an instance will let you modify that configuration. For example:

my_strategy = strategies.Polynomial(exponent = 3, scale_factor = 0.5)

result = backoff(some_function,
args = ['value1', 'value2'],
kwargs = { 'kwarg1': 'value3' },
max_tries = 3,
max_delay = 30,
strategy = my_strategy)


will call some_function() with a Polynomial strategy using an exponent of 3 and a scale factor of 0.5.

Tip

If you don’t supply a max_tries argument, the backoff strategy will look for a default max in the BACKOFF_DEFAULT_TRIES environment variable. If that environment variable doesn’t exist, it will retry your call three times then fail.

If you don’t supply a max_delay, the backoff strategy look for a default maximum delay in the BACKOFF_DEFAULT_DELAY environment variable. If that environment variable doesn’t exist, it will keep retrying your call until it hits max_tries.

And that’s it!

For more detailed documentation, please see the API Reference for the backoff() function.

### Alternative Fallbacks¶

The backoff() function allows you to fallback to either a different function or a different set of arguments after the first failure.

For example, let’s imagine a situation where we have two functions:

def some_function(arg1, arg2, kwarg1 = None):
# Function does stuff.

def some_alternative_function(arg1, arg2, arg3, arg4):
# Function does stuff.


Now, let’s try to first call some_function(), and if that doesn’t work, we can automatically try calling some_alternative_function() after our delay:

result = backoff(some_function,
args = ['value1', 'value2'],
kwargs = { 'kwarg1': 'value3' },
retry_execute = some_alternative_function,
retry_args = ['value1', 'value2', 'value3', 'something else'],
retry_kwargs = {},
max_tries = 3,
max_delay = 30,
strategy = strategies.Exponential)


Let’s breakdown what this will do. As before, first it will try calling:

result = some_function('value1', 'value2', kwarg1 = 'value3')


When that doesn’t work, it will then try calling:

result = some_alternative_function('value1' ,'value2', 'value3', 'something else')


until either that is successful, or the strategy exceeds the maximum number of tries or the maximum delay. If everything fails, then it will raise the last Exception raised by some_alternative_function().

### Retrying on Specific Errors¶

Not all errors are created equal. For some errors, we know with 100% certainty that retrying a function/method call with the same parameters will produce the exact same error every time. Which means there’s no point to applying a backoff strategy. However, certain errors may be caused by other factors…which means that if we try again, the function/method call might just work.

This is often the cause when a function/method is making a call across a network (like an HTTP request). Such a request might timeout because the API just happened to be over-burdened when the first request was made. If you want a second, maybe your next request will get through.

The backoff() function allows you to only apply the backoff strategy for a defined set of exceptions. If the function/method you’re trying raises an exception that isn’t on the list? Then the call won’t be retried.

Let’s assume we have some_function() as follows:

def some_function(arg1, arg2, kwarg1 = None):
# Function does stuff.


Now, let’s further assume that some_function() will sometimes raise:

If we get NotImplementedError, there’s no point in retrying: The same arguments will always produce the same error. But the other two errors may just be a momentary glitch, and retrying after some delay may work. Here’s how we would do that:

result = backoff(some_function,
args = ['value1', 'value2'],
kwargs = { 'kwarg1': 'value3' },
max_tries = 3,
max_delay = 30,
catch_exceptions = [type(TimeoutError), type(IOError)],
strategy = strategies.Exponential)


Now, when some_function('value1', 'value2', kwarg1 = 'value3') raises a TimeoutError or IOError, the call will be retried up to 3 times or for 30 seconds (whichever comes first). If the call raises any other exception, then the call will fail and bubble that exception up to your code where you’ll need to handle it.

Caution

If catch_exceptions is not None (the default, which will catch all exceptions), then it is very important that the catch_exceptions argument always contain one or more type(Exception()) values. For example:

# GOOD: This will work.
result = backoff(some_function,
args = ['value1', 'value2'],
kwargs = { 'kwarg1': 'value3' },
max_tries = 3,
max_delay = 30,
catch_exceptions = [type(TimeoutError()), type(IOError())],
strategy = strategies.Exponential)

result = backoff(some_function,
args = ['value1', 'value2'],
kwargs = { 'kwarg1': 'value3' },
max_tries = 3,
max_delay = 30,
catch_exceptions = type(TimeoutError()),
strategy = strategies.Exponential)

# BAD: This will not work.
result = backoff(some_function,
args = ['value1', 'value2'],
kwargs = { 'kwarg1': 'value3' },
max_tries = 3,
max_delay = 30,
catch_exceptions = [TimeoutError, IOError],
strategy = strategies.Exponential)

result = backoff(some_function,
args = ['value1', 'value2'],
kwargs = { 'kwarg1': 'value3' },
max_tries = 3,
max_delay = 30,
catch_exceptions = [type(TimeoutError), type(IOError)],
strategy = strategies.Exponential)


### Handling Failures¶

Sometimes, even after retrying stuff, your function/method call will still fail. That’s life. But when that happens, you might want to call some other function or method to do something in response. You can do this by passing that function/method to the backoff() function as the on_failure argument.

For example, let’s imagine we have two functions:

def some_function(arg1, arg2, kwarg1 = None):
# Function does stuff.

def error_handler(*args, **kwargs):
# Function does stuff.


We can have the backoff strategy call error_handler() when it has a final failure - meaning after backoff() has tried and failed multiple times, after it has timed out, or if some_function() raises an exception that is not listed in catch_exceptions.

Here’s how that would look:

result = backoff(some_function,
args = ['value1', 'value2'],
kwargs = { 'kwarg1': 'value3' },
max_tries = 3,
max_delay = 30,
catch_exceptions = [type(TimeoutError()), type(IOError())],
on_failure = error_handler,
strategy = strategies.Exponential)


Tip

If you pass a class that descends from Exception to on_failure, that exception will be raised with the message of the last exception raised by some_function().

Caution

If you are passing a custom function (not an Exception) to on_failure, that custom function must accept three positional arguments:

1. error - the last exception raised
2. message - the message of the last exception raised
3. traceback - the stack trace associated with the last exception raised

If the on_failure function cannot accept those three positional arguments, or if the on_failure function itself fails, then the last exception raised will bubble up.

### Handling Success¶

So we’ve talked a lot about failures here. But sometimes, things work! When the backoff() function is successful, it will always return the value back to where it was called. But sometimes, you want to fire a success handler before that value is returned. You can do this by passing a handler function to the backoff() function’s on_success argument.

Let’s imagine we have the following:

def some_function(arg1, arg2, kwarg1 = None):
# Function does stuff.

def success_handler(value_on_success):
# Function does stuff.

result = backoff(some_function,
args = ['value1', 'value2'],
kwargs = { 'kwarg1': 'value3' },
max_tries = 3,
max_delay = 30,
catch_exceptions = [type(TimeoutError()), type(IOError())],
on_success = success_handler,
strategy = strategies.Exponential)

# some more stuff happens here


Now, when some_function() is successful, before result is returned to your code, the backoff() function will call:

success_handler(result)


When success_handler() returns control, the backoff() function will return result and your code can continue.

Caution

It is very important that your on_success function always accept a single result value. This will always be the value returned by function/method you were trying to call using a backoff strategy.

Tip

A common pattern is to make your on_success function an asynchronous function. This can help parallelize your code to some extent, which means your code isn’t waiting for your on_success handler to complete before continuing.

## Using the Decorator Approach¶

You use the @apply_backoff() decorator when you want to always apply a particular backoff strategy to one of your functions or methods.

### Basic Usage¶

For example, let’s imagine we have a function:

def some_function(arg1, arg2, kwarg1 = value):
# Function does stuff here

result = some_function('value1', 'value2', kwarg1 = 'value3')


Whenever your code calls some_function(), we want to apply an Exponential backoff strategy for a maximum of 5 tries provided they don’t take longer than 30 seconds. Here’s how we would do that:

@apply_backoff(strategies.Exponential, max_tries = 5, max_delay = 30)
def some_function(arg1, arg2, kwarg1 = value):
# Function does stuff here

result = some_function('value1', 'value2', kwarg1 = 'value3')


That’s it! Now, whenever you call some_function(), the decorator will look for an error, and if it catches one, will retry the call after an exponential delay. It will keep retrying until it has tried five times, or until 30 seconds have passed - whichever is first.

Note

Just as when using the function call approach, you can pass the BackoffStrategy, the number of max_tries, and max_delay to the @apply_backoff decorator.

For more detailed documentation, please see the API Reference for the @apply_backoff() decorator.

### Alternative Fallbacks¶

Caution

The @apply_backoff() decorator does not support alternative fallbacks. If you want to use alternative fallbacks, then we suggest using the function approach.

### Retrying on Specific Errors¶

When using the @apply_backoff() decorator, you can retry on specific errors by passing those error types to the decorator’s catch_exceptions argument.

This works the same as the catch_exceptions argument when using the function call approach.

### Handling Failures¶

When using the @apply_backoff() decorator, you can fire an on_failure handler by passing an on_failure argument just as you can for the function call approach.

### Handling Success¶

When using the @apply_backoff() decorator, you can fire an on_success handler by passing an on_success argument just as you can for the function call approach.

## Stacking / Nesting / Chaining Strategies¶

Let’s imagine that the function/method you want to call will raise two different errors, and you want to apply a different backoff strategy for each error. Using the library, that’s fairly straightforward.

For example, let’s imagine we have a function:

def some_function(arg1, arg2, kwarg1 = None):
# Function does stuff.


which sometimes raises a TimeoutError and sometimes an IOError.

Let’s further assume that if it raises a TimeoutError, we want to apply an Exponential strategy up to five times, but for an IOError we want to apply a Linear strategy up to 3 times.

### Using the Function Approach¶

Here’s how we could do that using the function approach:

def backoff_for_timeout():
return backoff(some_function,
args = ['value1', 'value2'],
kwargs = { 'kwarg1': 'value3' },
max_tries = 5,
catch_exceptions = [type(TimeoutError())],
strategy = strategies.Exponential)

result = backoff(backoff_for_timeout,
max_tries = 3,
catch_exceptions = [type(IOError())],
strategy = strategies.Linear)


First, your code will call the backoff() function for backoff_for_timeout(). It will be looking to catch any IOError that backoff_for_timeout() raises. When it catches one, it will retry up to three times using the Linear strategy.

When the backoff() function calls backoff_for_timeout(), that function will in turn call another backoff() function for some_function(). It will be looking to catch any TimeoutError exceptions that some_function() raises. When it catches one, it will retry up to five times using the Exponential strategy.

At this point, if some_function() raises an IOError, however, it will bubble up to the first backoff() function, which will catch and handle it.

### Using the Decorator Approach¶

Here’s how we could do it using the @apply_backoff decorator:

@apply_backoff(strategies.Linear, max_tries = 3, catch_exceptions = type(IOError))
@apply_backoff(strategies.Exponential, max_tries = 5, catch_exceptions = type(TimeoutError))
def some_function(arg1, arg2, kwarg1 = None):
# Function does stuff.

result = some_function('value1', 'value2', kwarg1 = 'value3')


Now, when your code calls some_function(), it will first try to catch any TimeoutError raised by some_function(). If it catches one, it will retry some_function() up to 5 times using an Exponential strategy.

If some_function() raises anything other than a TimeoutError, that error will bubble up to the next decorator you’ve applied. That decorator looks for a IOError. If it catches one, it will retry up to 3 times using a Linear strategy.