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Tracker instances must be initialized with an emitter. This section will go into more depth about the Emitter class and its subclasses.

The basic Emitter class‚Äč

At its most basic, the Emitter class only needs a collector URI:

from snowplow_tracker import Emitter

e = Emitter(endpoint="")

This is the signature of the constructor for the base Emitter class:

def __init__(
endpoint: str,
protocol: Literal["http", "https"] = "https",
port: Optional[int] = None,
method: Literal["get", "post"] = "post",
batch_size: Optional[int] = None,
on_success: Optional[Callable[[PayloadDictList], None]] = None,
on_failure: Optional[Callable[[int, PayloadDictList], None]] = None,
byte_limit: Optional[int] = None,
request_timeout: Optional[Union[float, Tuple[float, float]]] = None,
max_retry_delay_seconds: int = 60,
buffer_capacity: Optional[int] = None,
custom_retry_codes: Dict[int, bool] = {},
event_store: Optional[EventStore] = None,
session: Optional[requests.Session] = None,
)-> None:
endpointThe collector URIYesString
protocolRequest protocol: http or httpsNoStringhttps
portThe port to connect toNoPositive integerNone
methodThe method to use: ‚Äúget‚ÄĚ or ‚Äúpost‚ÄĚNoStringpost
batch_sizeNumber of events to store before flushingNoPositive integer10
on_successCallback executed when a flush is successfulNoFunction taking 1 argumentNone
on_failureCallback executed when a flush is unsuccessfulNoFunction taking 2 argumentsNone
byte_limitNumber of bytes to store before flushingNoPositive integerNone
request_timeoutTimeout for HTTP requestsNoPositive integer or tuple of 2 integersNone
max_retry_delay_secondsThe maximum time between attempts to send failed events to the collectorNoint60
buffer_capacityThe maximum capacity of the event bufferNointNone
custom_retry_codesCustom retry rules for HTTP status codes received in emit responses from the CollectorNodictNone
event_storeStores the event buffer and buffer capacityNoEventStoreNone
sessionPersist parameters across requests by using a session objectNorequests.SessionNone

If no event_store is provided, an InMemoryEventStore will be initialized with a buffer_capacity of 10,000

See the API docs for more information.


protocol defaults to "https" but also supports "http".


When the emitter receives an event, it adds it to a buffer. When the queue is full, all events in the queue get sent to the collector. The batch_size argument allows you to customize the queue size. By default, it is 1 for GET requests and 10 for POST requests. (So in the case of GET requests, each event is fired as soon as the emitter receives it.) If the emitter is configured to send POST requests, then instead of sending one for every event in the buffer, it will send a single request containing all those events in JSON format.


byte_limit is similar to batch_size, but instead of counting events - it takes into account only the amount of bytes to be sent over the network. Warning: this limit is approximate with error < 1%.


on_success is an optional callback that will execute whenever the queue is flushed successfully, that is, whenever every request sent has status code 200. It will be passed one argument: the number of events that were successfully sent.


on_failure is similar, but executes when the flush is not wholly successful. It will be passed two arguments: the number of events that were successfully sent, and an array of unsent events.

An example:

def success(num):
print(str(num) + " events sent successfully!")

def new_success(arr):
for event_dict in arr:

def failure(num, arr):
print(str(num) + " events sent successfully!")
print("These events were not sent successfully:")
for event_dict in arr:

e = Emitter(endpoint="", buffer_size=3, on_success=new_success, on_failure=failure)

t = Tracker(namespace="snowplow_tracker", emitter=e)


Timeout for HTTP requests. Can be set either as single float value which applies to both "connect" AND "read" timeout, or as tuple with two float values which specify the "connect" and "read" timeouts separately.


The maximum time between attempts to send failed events to the collector.


The maximum capacity of the event buffer. When the buffer is full new events are lost.


Custom retry rules for HTTP status codes received in emit responses from the Collector. By default, retry will not occur for status codes 400, 401, 403, 410 or 422. This can be overridden here by parsing a dictionary of status codes and booleans.


The event store is used to store an event queue with events scheduled to be sent. Events are added to the event store when they are tracked and removed when they are successfully emitted or when emitting fails without any scheduled retries. The default is an InMemoryEventStore object with a buffer_capacity of 10,000 events.


The session object can be parsed into the emitter to use the requests.Session API. This allows users to persist parameters across requests, as well as pool connections to increase efficiency under heavy usage. If no session is parsed, the requests API is used.

What happens if an event fails to send?‚Äč

After trying to send a batch of events the collector will return an http status code. A 2xx code is always considered successful. If a failure code is returned (anything other than 2xx, with certain exceptions, see below), the events (as PayloadDictList objects) are returned to the buffer. They will be retried in future sending attempts.

To prevent unnecessary requests being made while the collector is unavailable, an exponential backoff is added to all subsequent event sending attempts. This resets after a request is successful. The default maximum backoff time between attempts is 1 minute but this can be configured by setting max_retry_delay_seconds.

The status codes 400 Bad Request, 401 Unauthorised, 403 Forbidden, 410 Gone, or 422 Unprocessable Entity are the exceptions: they are not retried by default. Payloads in requests receiving these responses are not returned to the buffer for retry. They are just deleted.

Configure which codes to retry on or not using the EmitterConfiguration when creating your tracker. This method takes a dictionary of status codes and booleans (True for retry and False for not retry).

# by default 401 isn't retried, but 500 is
custom_retry_codes = {500: False, 401: True}
emitter_config = EmitterConfiguration(custom_retry_codes=custom_retry_codes)


Configuring how events are buffered‚Äč

As events are collected, the are stored in a buffer until there are enough to send. By default, tracked events are stored in the InMemoryEventStore. This is an implemenation of the EventStore protocol and stores payloads in a List object and is cleared once the buffer capacity is reached.

The default buffer capacity is 10,000 events. This is the number of events that can be stored. When the buffer is full, new tracked payloads are dropped, so choosing the right capacity is important. You can set the buffer capacity through the EmitterConfiguration object, for example:

emitter_config = EmitterConfiguration(buffer_capacity=25,000)


The emitter will store 25,000 events before starting to lose data.

The AsyncEmitter class‚Äč

from snowplow_tracker import AsyncEmitter

e = AsyncEmitter(endpoint="", thread_count=10)

The AsyncEmitter class works just like the Emitter class, which is its parent class. It has one advantage, though: HTTP(S) requests are sent asynchronously, so the Tracker won't be blocked while the Emitter waits for a response. For this reason, the AsyncEmitter is recommended over the base Emitter class.

The AsyncEmitter uses a fixed-size thread pool to perform network I/O. By default, this pool contains only one thread, but you can configure the number of threads in the constructor using the thread_count argument, which is the only specific to AsyncEmitter argument.

Here is a complete example with all constructor parameters set:

from snowplow_tracker import AsyncEmitter

e = AsyncEmitter(
protocol = "https",
max_retry_delay_seconds = 60,
buffer_capacity = 5000,
event_store = None,

Manual flushing‚Äč

You can flush the emitter manually using the flush method of the Tracker instance which is sending events to the emitter. This is a blocking call which synchronously sends all events in the emitter's buffer.


You can alternatively perform an asynchronous flush, which tells the tracker to send all buffered events but doesn't wait for the sending to complete:


If you are using the AsyncEmitter, you shouldn't perform a synchronous flush inside an on_success or on_failure callback function as this can cause a deadlock.

Multiple emitters‚Äč

You can configure a tracker instance to send events to multiple emitters by passing the Tracker constructor function an array of emitters instead of a single emitter, or by using the addEmitter method:

from snowplow_tracker import Subject, Tracker, AsyncEmitter

e1 = AsyncEmitter(endpoint="", method="get")
e2 = AsyncEmitter(endpoint="", method="post")

tracker = Tracker(namespace="snowplow_tracker", emitters=[e1, e2])

e3 = AsyncEmitter(endpoint="", method="post")


Custom emitters‚Äč

You can create your own custom emitter class, either from scratch or by subclassing one of the existing classes. The only requirement for compatibility is that is must have an input method which accepts a Python dictionary of name-value pairs.

Setting flush timer‚Äč

You can flush your emitter based on some time interval:

e1 = AsyncEmitter(endpoint="", method="post")
e1.set_flush_timer(5) # flush each 5 seconds

Automatic flush can also be cancelled:

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