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Update fission architecture doc (#1356)

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Ta-Ching Chen
2019-10-22 19:46:57 +08:00
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# A high-level view of the internals of Fission.
A high level view of the internals of Fission. ## How it works
How it works
============
Fission is a FaaS -- users create functions (source level), register Fission is a FaaS -- users create functions (source level), register
them with fission using a CLI, and associate functions with triggers. them with Fission using a CLI, and associate functions with triggers.
Fission wraps those functions into a service, and runs them on Fission wraps those functions into a service and runs them on
Kubernetes on demand. Kubernetes on demand.
Here's an overview of the services that make up fission. Here's an overview of the services that make up Fission.
Components ## Components
==========
Language-neutral components: Core Components:
* Controller
* Executor
* Environment Container (language-specific)
* Router
* Builder Manager
* Storage Service
* controller Optional Components:
* poolmgr * Logger
* router * Kubewatcher
* kubewatcher * Message Queue Trigger
* Timer
Language-specific components: Third-party components:
* InfluxDB: To store function logs.
* Prometheus: For metric collection and canary deployment.
* NATS Streaming: For message queue trigger. (Kafka, Azure are not included in charts deployment.)
* Environment container(s) ## Core Components
### Controller
Controller The controller contains CRUD APIs for functions, triggers, environments,
---------- Kubernetes event watches, etc. This is the component that the client talks to.
The controller contains CRUD APIs for functions, http triggers, All fission resources are stored in kubernetes CRDs. It needs to be able to
environments, Kubernetes event watches. This is the component that talk to kubernetes API service.
the client talks to.
This is the only stateful component. It needs to be configured with a ### Executor
URL to an etcd cluster and a path to a persistent volume. The volume
is used to store the functions' source code. Etcd is used as the DB.
[Work to extend to other storage backends is planned, see issue #83.] The executor has two simple APIs; the router calls both these endpoints.
Pool Manager * GetFunctionService takes function metadata and dispatches the corresponding executor
------------ type to get the address of a service/pod and returns it to the router.
* TapService lets executor know a service/pod is being used; if it's not
called for a few minutes the pod(s) backing the service are killed.
Poolmgr manages pools of generic containers and function containers. It now supports two different executor types:
It has a simple API; both these endpoints are called by the router. * PoolManager
* NewDeploy
* GetFunctionService takes function metadata and returns the address These two executor types have different strategies to launch, specialize, and manage pod(s).
of a service. You should choose one of the executor types wisely based on the scenario.
* TapService lets poolmgr know a service is being used; if it's not
called for a few minutes the pod(s) backing the service are killed.
Poolmgr watches the controller API and eagerly creates generic pools #### PoolManager
for environments. It uses Kubernetes deployments to do that. The
environment container runs in a pod with the 'fetcher' container.
Fetcher is a very simple utility that downloads a URL sent to it and
saves it at a configured location.
GetFunctionService "specializes" a pod. The implementation chooses a PoolManager manages pools of generic containers and function containers.
pod from the pool, relabels it to "orphan" the pod from the
deployment, invokes fetcher to copy the function into the pod, and
hits the the specialize endpoint on the environment container. This
causes the function to be loaded. The pod is now specific to that
function.
This function pod is cached; it's cleaned up if it's unused for a few PoolManager watches the environment CRD changes and eagerly creates generic pools
minutes. for environments. It uses Kubernetes deployments to do that. The
environment container runs in a pod with the 'fetcher' container.
Fetcher is a straightforward utility that downloads a URL sent to it
and saves it at a configured location (shared volume).
Router The implementation chooses a generic pod from the pool, relabels it to
------ "orphan". The pod from the deployment invokes fetcher to copy the function
into the pod and hits the specialize endpoint on the environment container.
This causes the function to be loaded. The pod is now specific to that
function. This function pod is cached; it's cleaned up if it's unused for a few minutes.
The router forwards HTTP requests to function pods. If there's no PoolManager selects a generic pod from the warm pool, specializes it,
running service for a function, it requests one from poolmgr, while and recycles the pod if there are no further requests to the function after a few minutes.
holding on to the request; when the function's service is ready it It makes PoolManager suitable for functions that are short-living
forwards the request. and requires a short cold start time [1].
The router is stateless and can be scaled up if needed, according to However, PoolManager only selects one pod per function, which is not
suitable for serving massive traffic. In such cases, you should consider
using NewDeploy as executor type of function.
[1] The cold start time depends on the package size of the function. If it's
a snippet of code, the cold start time usually is less then 100ms.
#### NewDeploy
NewDeploy creates deployment, service, and HPA for functions in order to handle
massive traffic.
NewDeploy watches the function CRD changes and creates a Kubernetes deployment,
service, and HPA for a function. NewDeploy will scale the replicas of a function
deployment to the minimum feasible scale setting, if the minimum scale setting of
a function is greater than 0. The 'fetcher' inside the pod uses a URL in the
JSON payload, which is attached as a parameter to start fetcher, to download the
function package instead of waiting for calls from NewDeploy.
When a function experiences a traffic spike, the service helps to distribute the requests to
pods belonging to the function for better workload distribution and lower latency. Also,
the HPA scales the replicas of the deployment based on the conditions set by the user.
This approach though increases the cold time of a function, but also makes NewDeploy
suitable for functions designed to serve massive traffic.
### Environment Container
Environment containers run user-defined functions and are language-specific.
Each environment container must contain an HTTP server and a loader for functions.
The pool manager deploys the environment container into a pod with fetcher
(fetcher is a simple utility that can fetch an HTTP URL to a file at a
configured location). This pod forms a "generic pod" because it can
be loaded with any function in that coding language.
When the pool manager needs to create a service for a function, it calls
fetcher to fetch the function. Fetcher downloads the function into a
volume shared between fetcher and this environment container. Poolmgr
then requests the container to load the function.
### Router
The router forwards HTTP requests to function pods. If there's no
running service for a function, it requests one from executor, while
holding on to the request; the router will forward the request to
the pod once the function service is ready.
The router is the only stateless component and can be scaled up if needed, according to
load. load.
Kubewatcher ### Builder Manager
-----------
The builder manager watches the package & environments CRD changes and manages
the builds of function source code. Once an environment that contains a builder
image is created, the builder manager will then creates the Kubernetes service
and deployment under the fission-builder namespace to start the environment
builder. And once a package that contains a source archive is created, the
builder manager talks to the environment builder to build the function's source
archive into a deploy archive for function deployment.
After the build, the builder manager asks Builder to upload the deploy archive to the
Storage Service once the build succeeded, and updates the package status attached with build logs.
### Storage Service
The storage service is the home for all archives of packages with sizes larger than 256KB.
The Builder pulls the source archive from the storage service and uploads deploy archive to it.
The fetcher inside the function pod also pulls the deploy archive for function specialization.
## Optional Components
### Logger
Logger is deployed as DaemonSet to help to forward function logs to a centralized
database service for log persistence. Currently, only InfluxDB is supported to store logs.
Following is a diagram describe how log service works:
1. Logger watches pod changes and creates a symlink to the container log if the pod runs on the same node.
2. Fluentd reads logs from symlink and pipes them to InfluxDB
3. `fission function logs ...` retrieve event logs from InfluxDB with optional log filter
4. Logger removes the symlink if the pod no longer exists.
### Kubewatcher
Kubewatcher watches the Kubernetes API and invokes functions Kubewatcher watches the Kubernetes API and invokes functions
associated with watches, sending the watch event to the function. associated with watches, sending the watch event to the function.
The controller keeps track of user's requested watches and associated The controller keeps track of the user's requested watches and associated
functions. Kubewatcher watches the API based on these requests; when functions. Kubewatcher watches the API based on these requests; when
a watch event occurs, it serializes the object and calls the function a watch event occurs, it serializes the object and calls the function
via the router. via the router.
While a few simple retries are done, there isn't yet a reliable While a few simple retries are done, there isn't yet a reliable
message bus between Kubewatcher and the function. Work for this is message bus between Kubewatcher and the function. Work for this is
tracked in issue #64. tracked in issue #64.
Message Queue Trigger ### Message Queue Trigger
---------------------
A message queue trigger binds a message queue topic to a function: A message queue trigger binds a message queue topic to a function:
events from that topic cause the function to be invoked with the Events from that topic cause the function to be invoked with the
message as the body of the request. The trigger may also contain a message as the body of the request. The trigger may also contain a
response topic: if specified, the function's output is sent to this response topic: if specified, the function's output is sent to this
response. response.
@@ -108,37 +189,12 @@ Here's a diagram of the components:
![Message queue trigger Diagram](https://user-images.githubusercontent.com/202578/27012344-9457cb24-4f00-11e7-8d6b-926ff01637b3.jpg) ![Message queue trigger Diagram](https://user-images.githubusercontent.com/202578/27012344-9457cb24-4f00-11e7-8d6b-926ff01637b3.jpg)
Environment Container ### Timer
---------------------
Environment containers run user-defined functions. Environment The timer works like kubernetes CronJob but instead of creating a pod to do the task,
containers are language specific. Each environment container must it sends a request to router to invoke the function. It's suitable for the background tasks that
contain an HTTP server and a loader for functions. need to executor periodically.
Poolmgr deploys the environment container into a pod with fetcher The timer works like a Kubernetes CronJob, but instead of creating a
(fetcher is a simple utility that can fetch an HTTP url to a file at a pod to do the task, it sends a request to the router to invoke the
configured location). This pod forms a "generic pod", because it can function. It is suitable for background tasks that need to execute periodically.
be loaded with any function in that language.
When poolmgr needs to create a service for a function, it calls
fetcher to fetch the function. Fetcher downloads the function into a
volume shared between fetcher and this environment container. Poolmgr
then requests the container to load the function.
Logger
------
Logger helps to forward function logs to centralized db service for log
persistence. Currently only influxdb is supported to store logs.
Following is a diagram describe how log service works:
![Logger Diagram](https://cloud.githubusercontent.com/assets/202578/23100399/b0e3ea00-f6ba-11e6-8f2f-6588cfef2e84.png)
1. Pool manager chooses a pod from the pool to execute user function
2. Pool manager makes a HTTP POST to logger helper, once helper receives
the request it creates a symlink to container log for fluentd.
3. Fluentd reads log from symlink and pipes to influxdb
4. `fission function logs ...` retrieve event logs from influxdb with
optional log filter
5. Pool manager removes function pod from pool
6. Pool manager asks logger helper to stop piping logs, logger removes symlink.