# Distributed & Parallel Execution¶

The topology of a Flow defines how information is distributed between all Executors.

Each Executor has predecessors and successors. An Executor receives requests from all of its predecessors and sends them to all of its successors.

Additionally, an Executor can be split into Shards and Replicas. This enables horizontal scaling of Executors. A shard is typically used for partitioning data (like a big index) and Replicas are used to increase throughput and availability.

## Define predecessors via needs¶

To define predecessors of an Executor, use the needs parameter. If needs is not provided, the previously added Executor is set as the default:

from jina import Flow

f = (Flow()
.needs(['p1', 'p2', 'p3'], name='r1'))


p1, p2, p3 now subscribe to Gateway and conduct their work in parallel. The last .needs() blocks all Executors until they finish their work.

.needs() is syntax sugar and roughly equal to:

.add(needs=['p1', 'p2', 'p3'])


.needs_all() is syntax sugar and roughly equal to:

.add(needs=[all_orphan_executors_so_far])


“Orphan” Executors have no connected Executors to their outputs. The above code snippet can be also written as:

from jina import Flow

f = (Flow()
.needs_all())


### Merging results¶

In the previous Flow, each of the Executors p1, p2 and p3 work in parallel and generate one set of results. If you have an Executor that depends on all of them like r1, you might need to have all 3 results combined into one DocumentArray in the next steps. Let’s suppose the following concrete example:

• The Documents you sent are images containing some text.

• p1 fills the embedding attribute of Documents using an encoder model.

• p2 segments further the image into smaller chunks of images.

• p3 extracts the text and adds them as chunks.

We need to end up with Documents that contain all this new data. Well, by default, if you have a pod that needs all 3 Executors (p1, p2 and p3), this happens with our default Reduce logic.

If you don’t want to use the default Reduce logic, you can implement a custom reducing logic in an Executor and specify it with uses_before:

import itertools
from jina import Executor, requests, Flow, DocumentArray
from typing import List

class CustomReducerExecutor(Executor):
@requests
def reduce(self, docs_matrix: List['DocumentArray'] = [], **kwargs):
# simply chain all DocumentArrays
return DocumentArray(itertools.chain.from_iterable(docs_matrix))

f = (Flow()
.needs(['p1', 'p2', 'p3'], name='r1', uses_before=CustomReducerExecutor))


Note

If there is no pod that needs all 3 pods p1, p2 and p3, there will be no reduce logic. If you want to add a pod that needs all of them but you don’t want to have any reducing, use uses_before='BaseExecutor'.

## Scale Executors by using Replicas¶

Replicas can be used to create multiple copies of the same Executor. Each request in the Flow is then passed to only one replica (instance) of your Executor. This can be useful for a couple of challenges like performance and availability:

• If you have slow Executors (like some Encoders) you may want to scale up the number of instances of this particular Executor so that you can process multiple requests in parallel

• Executors might need to be taken offline from time to time (updates, failures, etc.), but you may want your Flow to be able to process requests all the time. In this case Replicas can be used as well so that any Replica of an Executor can be taken offline as long as there is still one running Replica online. Using this technique it is possible to create a High availability setup for your Flow.

from jina import Flow

f = (Flow()


The above Flow will create a topology with three Replicas of Executor slow_encoder. The Gateway will send every request to exactly one of the three instances. Then the replica will send its result to fast_indexer.

### Avoiding Bottlenecks on Executors with replicas¶

In this section, we will showcase how we can avoid having bottlenecks on slow Executors by using replicas.

Suppose we have the following Executor:

import time
from jina import Executor, requests

class MyExecutor(Executor):

@requests(on='/slow')
def foo(self, **kwargs):
time.sleep(5)

@requests(on='/fast')
def bar(self, **kwargs):
pass


We will simulate parallel requests to both endpoints of the Executor with these utility functions:

from jina.logging.profile import TimeContext
from jina import Client
import multiprocessing

def make_request(endpoint):
with TimeContext(f'calling {endpoint} roundtrip'):
c = Client(protocol='grpc', port=12345)
c.post(endpoint)

def simulate(flow):
with flow:
mp = []

# make multiple requests to both endpoints, in parallel
for endpoint in ['/slow', '/fast', '/slow', '/fast']:
p = multiprocessing.Process(target=make_request, args=(endpoint,))
p.start()
mp.append(p)

#
for p in mp:
p.join()


If we simply create a Flow with only 1 instance of MyExecutor, requests to the slow endpoint will make a bottleneck. However, creating replicas will unblock the Flow:

from jina import Flow

with TimeContext('calling scaled flow'):
simulate(scaled_f)  # will take around 5 seconds

calling scaled flow takes 6 seconds (6.11s)

from jina import Flow

with TimeContext('calling normal flow'):
simulate(f)  # will take around 10 seconds

calling normal flow takes 11 seconds (11.75s)


Therefore, by using replicas, the Flow enjoys a non-blocking behavior and we can avoid bottlenecks.

Important

By default, polling='ANY'. If polling is set to ALL, this will not be valid anymore: All instances of the Executor will receive each request and there will be no performance benefits.

## Partition data by using Shards¶

Shards can be used to partition data (like an Index) into several parts. This enables the distribution of data across multiple machines. This is helpful in two situations:

• When the full data does not fit on one machine

• When the latency of a single request becomes too large.

Then splitting the load across two or more machines yields better results.

For Shards, you can define which shard (instance) will receive the request from its predecessor. This behaviour is called polling. By default polling is set to ANY, which means only one shard will receive a request. If polling is to ALL it means that all Shards will receive a request.

When you shard your index, the request handling usually differs between index and search requests:

• Index (and update, delete) will just be handled by a single shard => polling='any'

• Search requests are handled by all Shards => polling='all'

For indexing, you only want a single shard to receive a request, because this is sufficient to add it to the index. For searching, you probably need to send the search request to all Shards, because the requested data could be on any shard.

from jina import Flow



### Merging search results¶

Each shard of a search Flow returns one set of results for each query Document. By default, these different results will be combined into one DocumentArray using our default Reduce logic.

You can customize the reduce logic and specify a different Reducer Executor with parameter uses_after. For example, you can use the pre-built MatchMerger or define your merger.

Example

A search Flow has 10 Shards for an Indexer. Each shard returns the top 20 results. After the merger there will be 200 results per query Document.

from jina import Flow

from jina import Flow
search_flow = Flow().add(name='ExecutorWithShards', shards=3, polling='all', uses_after='jinahub://MatchMerger')


Warning

The default Reduce logic does not keep the matches sorted when combining them. If you wish to have sorted matches, you should add an Executor to sort the matches.

## Combining Replicas & Shards¶

Replicas and Shards can also be combined, which is necessary for Flows with high scalability needs. In this case Jina will shard the Executor and create Replicas for each shard.

from jina import Flow

f = (Flow()


This Flow has a single Executor with 2 Shards and 3 Replicas, which means it gets split into 2 Shards with 3 Replicas each. In total this Flow has 2*3=6 workers and could be distributed to six different machines if necessary.

## Replicas vs Shards¶

The next table shows the difference between shards and replicas.

Replicas Shards
Create multiple copies of an executor
Partition data into several parts
Request handled by one of the executors ✅, if polling = 'any'
Request handled by all of the executors ✅, if polling = 'all'

Think of using replicas when you have slow Executors and you want to be able to process multiple requests in parallel. Also replicas provide high availability in case some executors are taken down (for updates, failures, etc)

On the other hand, when your data is too large to fit in one machine or if the latency of a request is too large shards is your best option since it allows you to split your data across multiple machines.

Warning

Sometimes you’ll also encouter parallel, this is equivalent to shards and is only kept for backwards compatibility.

## Reducing¶

Reducing is a mechanism that applies by default to DocumentArrays resulted by instances of Executors running in parallel in a Flow. It combines the multiple DocumentArrays into one DocumentArray when you have one of these 2 cases:

• Your Flow contains 2 or more shards of the same Executor with polling='all'

• Your Flow contains 2 or more different Executors running in parallel and you have another Executor that needs 2 or more of them. Reducing is necessary if your Flow needs the combined DocumentArray in later steps.

Reduction consists in reducing all DocumentArrays sequentially using DocumentArray.reduce. The resulting DocumentArray contains Documents of all DocumentArrays.

If a Document exists in many DocumentArrays, data properties are merged.

Matches and chunks of a Document belonging to many DocumentArrays are also reduced in the same way. Other non-data properties are ignored.

Warning

If a data property is set on many Documents with the same ID, only one value is kept in the final Document. This value can be belong to any of these Documents. This also means, if you make the same request, with such conflicting data properties, you can end up with undeterministic results. When you use shards or parallel branches, you should always make sure that all data properties of the same Documents either do not conflict or have the same value.