defmodule Tesla.Middleware.Retry do
@moduledoc """
Retry using exponential backoff and full jitter.
By defaults, this middleware only retries in the case of connection errors (`nxdomain`, `connrefused`, etc).
Application error checking for retry can be customized through `:should_retry` option.
## Backoff algorithm
The backoff algorithm optimizes for tight bounds on completing a request successfully.
It does this by first calculating an exponential backoff factor based on the
number of retries that have been performed. It then multiplies this factor against
the base delay. The total maximum delay is found by taking the minimum of either
the calculated delay or the maximum delay specified. This creates an upper bound
on the maximum delay we can see.
In order to find the actual delay value we apply additive noise which is proportional
to the current desired delay. This ensures that the actual delay is kept within
the expected order of magnitude, while still having some randomness, which ensures
that our retried requests don't "harmonize" making it harder for the downstream service to heal.
## Examples
```elixir
defmodule MyClient do
def client do
Tesla.client([
{Tesla.Middleware.Retry,
delay: 500,
max_retries: 10,
max_delay: 4_000,
should_retry: fn
{:ok, %{status: status}}, _env, _context when status in [400, 500] -> true
{:ok, _reason}, _env, _context -> false
{:error, _reason}, %Tesla.Env{method: :post}, _context -> false
{:error, _reason}, %Tesla.Env{method: :put}, %{retries: 2} -> false
{:error, _reason}, _env, _context -> true
end
}
])
end
end
```
## Options
- `:delay` - The base delay in milliseconds (positive integer, defaults to 50)
- `:max_retries` - maximum number of retries (non-negative integer, defaults to 5)
- `:max_delay` - maximum delay in milliseconds (positive integer, defaults to 5000)
- `:should_retry` - function with an arity of 1 or 3 used to determine if the request should
be retried the first argument is the result, the second is the env and the third is
the context: options + `:retries` (defaults to a match on `{:error, _reason}`)
- `:jitter_factor` - additive noise proportionality constant
(float between 0 and 1, defaults to 0.2)
"""
@behaviour Tesla.Middleware
@defaults [
delay: 50,
max_retries: 5,
max_delay: 5_000,
jitter_factor: 0.2
]
@impl Tesla.Middleware
def call(env, next, opts) do
opts = opts || []
context = %{
retries: 0,
delay: integer_opt!(opts, :delay, 1),
max_retries: integer_opt!(opts, :max_retries, 0),
max_delay: integer_opt!(opts, :max_delay, 1),
should_retry: should_retry_opt!(opts),
jitter_factor: float_opt!(opts, :jitter_factor, 0, 1)
}
retry(env, next, context)
end
# If we have max retries set to 0 don't retry
defp retry(env, next, %{max_retries: 0}), do: Tesla.run(env, next)
# If we're on our last retry then just run and don't handle the error
defp retry(env, next, %{max_retries: max, retries: max} = context) do
env
|> put_retry_count_opt(context)
|> Tesla.run(next)
end
# Otherwise we retry if we get a retriable error
defp retry(env, next, context) do
res =
env
|> put_retry_count_opt(context)
|> Tesla.run(next)
{:arity, should_retry_arity} = :erlang.fun_info(context.should_retry, :arity)
cond do
should_retry_arity == 1 and context.should_retry.(res) ->
do_retry(env, next, context)
should_retry_arity == 3 and context.should_retry.(res, env, context) ->
do_retry(env, next, context)
true ->
res
end
end
defp do_retry(env, next, context) do
backoff(context.max_delay, context.delay, context.retries, context.jitter_factor)
context = update_in(context, [:retries], &(&1 + 1))
retry(env, next, context)
end
# Exponential backoff with jitter
defp backoff(cap, base, attempt, jitter_factor) do
factor = Bitwise.bsl(1, attempt)
max_sleep = min(cap, base * factor)
# This ensures that the delay's order of magnitude is kept intact, while still having some jitter.
# Generates a value x where 1 - jitter_factor <= x <= 1
jitter = 1 - jitter_factor * :rand.uniform()
# The actual delay is in the range max_sleep * (1 - jitter_factor) <= delay <= max_sleep
delay = trunc(max_sleep * jitter)
:timer.sleep(delay)
end
defp put_retry_count_opt(env, %{retries: 0} = _context) do
env
end
defp put_retry_count_opt(env, context) do
opts = Keyword.put(env.opts, :retry_count, context.retries)
%{env | opts: opts}
end
defp integer_opt!(opts, key, min) do
case Keyword.fetch(opts, key) do
{:ok, value} when is_integer(value) and value >= min -> value
{:ok, invalid} -> invalid_integer(key, invalid, min)
:error -> @defaults[key]
end
end
defp float_opt!(opts, key, min, max) do
case Keyword.fetch(opts, key) do
{:ok, value} when is_float(value) and value >= min and value <= max -> value
{:ok, invalid} -> invalid_float(key, invalid, min, max)
:error -> @defaults[key]
end
end
defp should_retry_opt!(opts) do
case Keyword.get(opts, :should_retry, &match?({:error, _}, &1)) do
should_retry_fun when is_function(should_retry_fun, 1) ->
should_retry_fun
should_retry_fun when is_function(should_retry_fun, 3) ->
should_retry_fun
value ->
invalid_should_retry_fun(value)
end
end
defp invalid_integer(key, value, min) do
raise(ArgumentError, "expected :#{key} to be an integer >= #{min}, got #{inspect(value)}")
end
defp invalid_float(key, value, min, max) do
raise(
ArgumentError,
"expected :#{key} to be a float >= #{min} and <= #{max}, got #{inspect(value)}"
)
end
defp invalid_should_retry_fun(value) do
raise(
ArgumentError,
"expected :should_retry to be a function with arity of 1 or 3, got #{inspect(value)}"
)
end
end