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diff --git a/lib/tesla/middleware/retry.ex b/lib/tesla/middleware/retry.ex
index eeb8d81..e6fc744 100644
--- a/lib/tesla/middleware/retry.ex
+++ b/lib/tesla/middleware/retry.ex
@@ -1,60 +1,98 @@
defmodule Tesla.Middleware.Retry do
@behaviour Tesla.Middleware
@moduledoc """
- Retry the HTTP call in case of connection error by default (`nxdomain`, `connrefused` etc).
- Application error checking for retry can be customized through `:should_retry` option by
+ Retry using exponential backoff and full jitter. 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 by
providing a function in returning a boolean.
+ ## 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 take a random number between 0 and
+ the maximum delay based on a uniform distribution. This randomness ensures that
+ our retried requests don't "harmonize" making it harder for the downstream
+ service to heal.
+
### Example
```
defmodule MyClient do
use Tesla
plug Tesla.Middleware.Retry,
delay: 500,
max_retries: 10,
+ max_delay: 4_000,
should_retry: fn
{:ok, %{status: status}} when status in [400, 500] -> true
{:ok, _} -> false
{:error, _} -> true
end
end
```
### Options
- - `:delay` - number of milliseconds to wait before retrying (defaults to 1000)
+ - `:delay` - The base delay in milliseconds (defaults to 50)
- `:max_retries` - maximum number of retries (defaults to 5)
+ - `:max_delay` - maximum delay in milliseconds (defaults to 5000)
- `:should_retry` - function to determine if request should be retried
"""
@defaults [
- delay: 1000,
- max_retries: 5
+ delay: 50,
+ max_retries: 5,
+ max_delay: 5_000
]
@doc false
def call(env, next, opts) do
opts = opts || []
- delay = Keyword.get(opts, :delay, @defaults[:delay])
- max_retries = Keyword.get(opts, :max_retries, @defaults[:max_retries])
- should_retry = Keyword.get(opts, :should_retry, &match?({:error, _}, &1))
- retry(env, next, delay, max_retries, should_retry)
+ context = %{
+ retries: 0,
+ delay: Keyword.get(opts, :delay, @defaults[:delay]),
+ max_retries: Keyword.get(opts, :max_retries, @defaults[:max_retries]),
+ max_delay: Keyword.get(opts, :max_delay, @defaults[:max_delay]),
+ should_retry: Keyword.get(opts, :should_retry, &match?({:error, _}, &1))
+ }
+
+ retry(env, next, context)
end
- defp retry(env, next, _delay, retries, _should_retry) when retries <= 1 do
+ # 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}) do
Tesla.run(env, next)
end
- defp retry(env, next, delay, retries, should_retry) do
+ # Otherwise we retry if we get a retriable error
+ defp retry(env, next, context) do
res = Tesla.run(env, next)
- if should_retry.(res) do
- :timer.sleep(delay)
- retry(env, next, delay, retries - 1, should_retry)
+ if context.should_retry.(res) do
+ backoff(context.max_delay, context.delay, context.retries)
+ context = update_in(context, [:retries], &(&1 + 1))
+ retry(env, next, context)
else
res
end
end
+
+ # Exponential backoff with jitter
+ defp backoff(cap, base, attempt) do
+ factor = :math.pow(2, attempt)
+ max_sleep = trunc(min(cap, base * factor))
+ delay = :rand.uniform(max_sleep)
+
+ :timer.sleep(delay)
+ end
end

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