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Backoff

Backoff is an Erlang library to deal with exponential backoffs and timers to be used within OTP processes when dealing with cyclical events, such as reconnections, or generally retrying things.

Compiling

rebar3 compile

Running Tests

Tests are implemented as a basic PropEr property-based test suite. Running them requires getting PropEr for the project. The following command line does everything needed:

$ rebar3 as test proper

Modes of Operation

Backoff can be used in 3 main ways:

  1. a simple way to calculate exponential backoffs
  2. calculating exponential backoffs with caps and state tracking
  3. using it to fire timeout events

Simple Backoffs

Simple backoffs work by calling the functions increment/1-2. The function with one argument will grow in an unbounded manner:

1> backoff:increment(1).
2
2> backoff:increment(backoff:increment(1)).
4
3> backoff:increment(backoff:increment(backoff:increment(1))).
8

The version with 2 arguments specifies a ceiling to the value:

4> backoff:increment(backoff:increment(backoff:increment(2))).
16
5> backoff:increment(backoff:increment(backoff:increment(2)), 10).
10

Simple Backoffs with jitter

Jitter based incremental backoffs increase the back off period for each retry attempt using a randomization function that grows exponentially. They work by calling the functions rand_increment/1-2. The function with one argument will grow in an unbounded manner:

1> backoff:rand_increment(1).
3
2> backoff:rand_increment(backoff:rand_increment(1)).
7
3> backoff:rand_increment(backoff:rand_increment(backoff:rand_increment(1))).
19
4> backoff:rand_increment(backoff:rand_increment(backoff:rand_increment(1))).
14
5> backoff:rand_increment(backoff:rand_increment(backoff:rand_increment(1))).
17

The version with 2 arguments specifies a ceiling to the value. If the delay is close to the ceiling the new delay will also be close to the ceiling and may be less than the previous delay.

6> backoff:rand_increment(backoff:rand_increment(backoff:rand_increment(2))).
21
7> backoff:rand_increment(backoff:rand_increment(backoff:rand_increment(2)), 10).
10

State Backoffs

State backoffs keep track of the current value, the initial value, and the maximal value for you. A backoff of that kind is initialized by calling init(Start,Max) and returns an opaque data type to be used with get/1 (fetches the current timer value), fail/1 (increments the value), and succeed/1 (resets the value):

6> B0 = backoff:init(2, 10).
...
7> {_, B1} = backoff:fail(B0).
{4, ...}
8> backoff:get(B1).
4
9> {_, B2} = backoff:fail(B1).
{8, ...}
10> {_, B3} = backoff:fail(B2).
{10, ...}
11> {_, _} = backoff:fail(B3).
{10, ...}

And here we've hit the cap with the failures. Now to succeed again:

12> {_, B4} = backoff:succeed(B3).
{2, ...}
13> backoff:get(B4).
2

That way, backoffs carry all their relevant state.

If what you want are unbound exponential backoffs, you can initiate them with:

14> backoff:init(Start, 'infinity').

And still use them as usual. The increments will have no upper limit.

State Backoffs with jitter

You can enable a jitter based incremental backoff by calling type/2 that swaps the state of the backoff:

1> B0 = backoff:init(2, 30).
{backoff,2,30,2,normal,undefined,undefined}
2> B1 = backoff:type(B0, jitter).
{backoff,2,30,2,jitter,undefined,undefined}
3> {_, B2} = backoff:fail(B1).
{7, ...}
4> {_, B3} = backoff:fail(B2).
{12, ...}

Calling type/2 with argument normal will swap the backoff state back to its default behavior:

5> B4 = backoff:type(B3, normal).
{backoff,2,30,12,normal,undefined,undefined}
6> {_, B5} = backoff:fail(B4).
{24, ...}

Timeout Events

A very common usage for exponential backoffs are with timer events, to be used when driving reconnections or retries to certain sources. Most implementations of this will call erlang:start_timer(Delay, Dest, Message) to do this, and re-use the same values all the time.

Given we want Backoff to carry us the whole way there, additional arguments can be given to the init function to deal with such state and fire events whenever necessary. We first initialize the backoff with init(Start, Max, Dest, Message):

1> B = backoff:init(5000, 20000, self(), hello_world).
...

Then by entering:

2> backoff:fire(B). timer:sleep(2500), flush(). timer:sleep(3000), flush().

and pressing enter, the following sequence of events will unfold:

3> backoff:fire(B). timer:sleep(2500), flush(). timer:sleep(3000), flush().
#Ref<0.0.0.719>
4> timer:sleep(2500), flush(). timer:sleep(3000), flush().
ok
5> timer:sleep(3000), flush().
Shell got {timeout,#Ref<0.0.0.719>,hello_world}
ok

Showing that backoff:fire/1 generates a new timer, and returns the timer reference. This reference can be manipulated with erlang:cancel_timer(Ref) and erlang:read_timer(Ref).

The shell then sleeps (2000 ms), receives nothing, then sleeps some more (3000 ms) and finally receives the timeout event as a regular Erlang timeout message.

Do note that Backoff will not track the timer references given there can be enough use cases with multiple timers, event cancellation, and plenty of other things that can happen with them. Backoff makes it easy to fire them for the right interval, but it is not a wrapper around Erlang timers for all operations.

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