Retry Delay & Retry Strategy

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By default, a message will be retried three times then lost forever. Well... in a few minutes... I'll show you how you can avoid even those messages from being lost.

Anyways... the process... just works! And it's even cooler than it looks at first. It's a bit hard to see - especially because there's a sleep in our handler - but this message was sent for retry #3 at the 13 second timestamp and it was finally handled again down at the 17 second timestamp - a 4 second delay. That delay was not caused by our worker just being busy until then: it was 100% intentional.

Check it out: I'll hit Ctrl+C to stop the worker and then run:

php bin/console config:dump framework messenger

This should give us a big tree of "example" configuration that you can put under the framework messenger config key. I love this command: it's a great way to find options that you maybe didn't know existed.

Cool! Look closely at the transports key - it lists an "example" transport below with all the possible config options. One of them is retry_strategy where we can control the maximum number of retries and the delay that should happen between those retries.

This delay number is smarter than it looks: it works together with the "multiplier" to create an exponentially growing delay. With these settings, the first retry will delay one second, the second 2 seconds and the third 4 seconds.

This is important because, if a message fails due to some temporary issue - like connecting to a third-party server - you might not want to try again immediately. In fact, you might choose to set these to way higher values so that it retries maybe 1 minute or even a day later.

Let's also try a similar command:

php bin/console debug:config framework messenger

Instead of showing example config, this tells us what our current configuration is, including any default values: our async transport has a retry_strategy, which is defaulting to 3 max retries with a 1000 millisecond delay and a multiplier of 2.

Configuring the Delay

Let's make this a bit more interesting. In the handler, let's make it always fail by adding || true.

... lines 1 - 13
class AddPonkaToImageHandler implements MessageHandlerInterface, LoggerAwareInterface
... lines 16 - 30
public function __invoke(AddPonkaToImage $addPonkaToImage)
... lines 33 - 46
if (rand(0, 10) < 7 || true) {
throw new \Exception('I failed randomly!!!!');
... lines 50 - 56

Now, under messenger, let's play with the retry config. Wait... but the async transport is set to a string... are we allowed to include config options under that? No! Well, yes, sort of. As soon as you need to configure a transport beyond just the connection details, you'll need to drop this string onto the next line and assign it to a dsn key. Now we can add retry_strategy, and let's set the delay to 2 seconds instead of 1.

... lines 3 - 5
delay: 2000
... lines 12 - 20

Oh, and I also want to mention this service key. If you want to completely control the retry config - maybe even having different retry logic per message - you can create a service that implements RetryStrategyInterface and put its service id - usually its class name - right here.

Anyways, let's see what happens with the longer delay: restart the worker process:

php bin/console messenger:consume -vv

This time, upload just one photo so we can watch it fail over and over again. And... yep! It fails and sends for retry #1... then fails again and sends for retry #2. But check out that delay! 09 to 11 - 2 seconds - then 11 to 15 - a 4 second delay. And... if... we... are... super... patient... yea! Retry #3 starts a full 8 seconds later. Then it's "rejected" - removed from the queue - and lost forever. Tragic!

Retries are great... but I don't like that last part: when the message is eventually lost forever. Change the delay to 500 - it'll make this easier to test.

... lines 3 - 5
delay: 500
... lines 12 - 20

Next, let's talk about a special concept called the "failure transport": a better alternative than allowing failed messages to simply... disappear.

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This tutorial is built with Symfony 4.3, but will work well on Symfony 4.4 or 5.

What PHP libraries does this tutorial use?

// composer.json
    "require": {
        "php": "^7.1.3",
        "ext-ctype": "*",
        "ext-iconv": "*",
        "intervention/image": "^2.4", // 2.4.2
        "league/flysystem-bundle": "^1.0", // 1.1.0
        "sensio/framework-extra-bundle": "^5.3", // v5.3.1
        "symfony/console": "4.3.*", // v4.3.2
        "symfony/dotenv": "4.3.*", // v4.3.2
        "symfony/flex": "^1.1", // v1.4.4
        "symfony/framework-bundle": "4.3.*", // v4.3.2
        "symfony/messenger": "4.3.*", // v4.3.4
        "symfony/orm-pack": "^1.0", // v1.0.6
        "symfony/serializer-pack": "^1.0", // v1.0.2
        "symfony/validator": "4.3.*", // v4.3.2
        "symfony/webpack-encore-bundle": "^1.5", // v1.6.2
        "symfony/yaml": "4.3.*" // v4.3.2
    "require-dev": {
        "symfony/debug-pack": "^1.0", // v1.0.7
        "symfony/maker-bundle": "^1.0", // v1.12.0
        "symfony/test-pack": "^1.0" // v1.0.6