package forwarder

This package is responsible for sending payloads to the backend. Payloads can come from different sources in different formats, the forwarder will not inspect them.

The forwarder can receive multiple domains with a list of API keys for each of them. Every payload will be sent to every domain/API keys couple, this became a Transaction. Transactions will be retried on error. The newest transactions will be retried first. Transactions are consumed by Workers asynchronously.

Usage

KeysPerDomains := map[string][]string{
	"http://api.datadog.com": {"my_secret_key_1", "my_secret_key_2"},
	"http://debug.api.com":   {"secret_api"},
}

forwarder := forwarder.NewForwarder(KeysPerDomains)
forwarder.NumberOfWorkers = 1 // default: config.Datadog.GetInt("forwarder_num_workers")
forwarder.Start()

// ...

payload1 := []byte("some payload")
payload2 := []byte("another payload")
forwarder.SubmitSeries(Payloads{&payload1, &payload2}, ...)

// ...

forwarder.Stop()

Configuration

There are several settings that influence the behavior of the forwarder.

Exponential backoff and circuit breaker settings

• forwarder_backoff_factor - This controls the overlap between consecutive retry interval ranges. When set to 2, there is a guarantee that there will be no overlap. The overlap will asymptotically approach 50% the higher the value is set. Values less then 2 are verboten as there will be range gaps. Default: 2
• forwarder_backoff_base - This controls the rate of exponential growth. Also, you can calculate the start of the very first retry interval range by evaluating the following expression: forwarder_backoff_base / forwarder_backoff_factor * 2. Default: 2
• forwarder_backoff_max - This is the maximum number of seconds to wait for a retry. Default: 64
• forwarder_recovery_interval - This controls how many retry interval ranges to step down for an endpoint upon success. Default: 2
• forwarder_recovery_reset - Whether or not a successful request should completely clear an endpoint's error count. Default: false

Internal

The forwarder is composed of multiple parts:

DefaultForwarder

DefaultForwarder it the default implementation of the Forwarder interface (and the only one for now). This class is in charge of receiving payloads, creating the HTTP transactions and distributing them among every domainForwarder.

domainForwarder

The agent can be configured to send the same payload to multiple destinations. Each destination (or domain) can be configured with 1 or more API keys. Every payload will be sent to each domain/API key pair.

A domainForwarder is in charge of sending payloads to one domain. This avoids slowing down every domain when one is down/slow. Each domainForwarder will have a number of dedicated Worker to process Transaction. We process new transactions first and then (when the workers have time) we retry the erroneous ones (newest transactions are retried first).

We start dropping transactions (oldest first) when the sum of all the payload sizes is bigger than forwarder_retry_queue_payloads_max_size (see the agent configuration).

Disclaimer: using multiple API keys with the Datadog backend will multiply your billing ! Most customers will only use one API key.

Worker

A Worker processes transactions coming from 2 queues: HighPrio and LowPrio. New transactions are sent to the HighPrio queue and the ones to retry are sent to LowPrio. A Worker is dedicated to on domain (ie: domainForwarder).

blockedEndpoints (or exponential backoff)

When a transaction fails to be sent to a backend we blacklist that particular endpoints for some time to avoid flooding an unavailable endpoint (the transactions will be retried later). A blacklist is specific to one endpoint on one domain (ie: "http(s):///"). The blacklist time will grow, up to a maximum, has more and more errors are encountered for that endpoint and is gradually cleared when a transaction is successful. The blacklist is shared by all workers.

Transaction

A HTTPTransaction contains every information about a payload and how/where to send it. On failure a transaction will be retried later (see blockedEndpoints).