Selenium 简明教程

Selenium Grid - Architecture

Selenium Grid 被开发为一系列组件，每个组件执行自己的任务，以便 Selenium Grid 作为一个整体工作。各个组件的工作听起来可能有点复杂。最新版本的 Selenium Grid 4 是在不利用旧版本 Selenium Grid 的代码库的情况下开发的。

Selenium Grid is developed as a group of components with each component performing its own tasks so that the Selenium Grid works as a unit. The working of individual components may sound a bit complicated. The latest version of Selenium Grid 4, is developed without leveraging the code base of the older versions of the Selenium Grid.

Components of Selenium Grid

Selenium Grid 的主要组件如下：

The key components of Selenium Grid are listed below −

  1. Event Bus − A Event Bus is used to convey messages which may be obtained asynchronously among the other components.

  2. Distributor − A Distributor keeps a track of every accessible resource in the Selenium Grid which can be utilized to start a session(as called the slots) and take up new inward requests and give it a slot.

  3. Node − A Node is used to execute a webdriver session. Every session is provided a slot and a Node may or may not have multiple slots.

  4. Session Map − A Session Map maintains a relation between the session id and the location of the Node where the session is running.

  5. New Session Queue − A New Session Queue stores the list of incoming sessions which do not have a Node given by the Distributor.

  6. Router − A Router is as if the face of the Selenium Grid and exposed to the web. A Router directs the incoming requests either to the New Session Queue or to the Node where a session is going on.

  7. Slot − A slot is a location where a session can execute. A slot should have some minimum features which a new session request should match so that the Distributor can dispatch that request to a Node having a slot.

  8. Grid Model − A Grid Model describes the way the Distributor keeps a watch on the performance of the Selenium Grid.

Communication Methods Among the Components

Selenium Grid 中组件之间的通信方法如下所示 -

The communication methods among the components in the Selenium Grid are listed below −

  1. Synchronous JSON over HTTP requests.

  2. Asynchronous events dispatched to the Event Bus.

在执行任务同步，或错失响应会很困难的情况下，会执行同步通信。然而，在仅进行信息广播或错失响应无所谓的情况下，将异步通信发送到事件总线。

In the scenarios where the task being done is synchronous, or if misplacing a response would be difficult, a synchronous communication is done. However, in the scenarios where only broadcasting information is being carried out, or if misplacing a response is fine, an asynchronous communication is sent to the Event Bus.

Dependencies Among the Selenium Grid Components

任何 Selenium Grid 组件都可以按任何顺序启动，但理想情况下，组件按以下顺序启动 -

Any Selenium Grid component can begin in any order, however ideally the components start in the below sequences −

  1. The Session Map and the Event Bus begin at the first. They are independent of each other and can work simultaneously.

  2. Next, the Session Queue takes part.

  3. Next, the Distributor can come into picture. It will connect with the Session Queue at an interval and poll for the jobs. The polling can be done either in an interval or by an event.

  4. At this point, the Router can be kicked off. The New Session request is sent to the Session Queue, and the Distributor will try to search for a slot to execute the session.

  5. Next, the Node can be started. Once Node registration is done, Selenium Grid can work with traffic.

Event Bus

Distributor

Node

Router

Session Map

Session Queue

Event Bus

No

Distributor

Yes

No

Yes

Yes

Node

Yes

No

Router

Yes

No

Yes

Session Map

No

Session Queue

Yes

No

上表显示了组件之间的依赖关系，其中“是”表示组件之间存在同步依赖关系。

The above table shows the dependencies among the components, where Yes reflects there is a presence of synchronous dependencies between the components.

How is Node Registration Done in Selenium Grid?

节点注册的整个过程是轻量级的。当一个节点变为活动时，它应定期发布“心跳”事件。这表明了节点状态。

The entire process of Node registration is a light weight process. When a Node becomes active, it should release a ‘heart beat’ event on an interval. This gives an idea of the status of the Node.

分配器会监听“心跳”事件。当它接收到“心跳”事件时，便会尝试 GET 节点的 /status 端点。该信息用于配置 Selenium Grid。分配器执行的此检查遵循有条不紊的方法。但是，即使节点已开始工作，节点也应该总是分发“心跳”事件。这么做是为了可以在没有任何 Selenium Grid 状态存储库的情况下恢复分配器。

The Distributor listens to the ‘heart beat’ events. As it receives the ‘heart beat’ events, it tries to GET the /status endpoint of the Node. This information is used to configure the Selenium Grid. This check performed by the Distributor is done in a methodical manner. However, the Node should always dispatch the ‘heart beat’ events even after it has begun working. This is done so that the Distributor can be resumed without a consistent store of the status of the Selenium Grid.

Fields in a Node Status Object in Selenium Grid

节点状态接收为具有如下字段的 JSON 对象 -

The Node Status is received as a JSON object with the below fields −

Name

Type

Description

availability

string

Can be one of three string values - up, draining, and down. The most common one is draining which says that no new session should be directed to the Node, and that once the previous session on it is done, the Node shall terminate or restart.

externalUrl

string

The Url that every component in the Selenium Grid should contact.

lastSessionCreated

integer

The epoch timestamp of the last session of the Node. The Distributor tries to direct new sessions to the Node that is not in use for the longest duration if all the other parameters are similar.

maxSessionCount

integer

This integer value suggests the maximum number of sessions that should be running in parallel on the Node prior being considered as full.

nodeId

string

A UUID used to identify this object of the Node.

osInfo

object

An object having either of the three fields - arch, name, and version. These are used by GridUI and GraphQL queries.

slots

array

Slot objects array

version

string

The Node version

在以上所有字段中设置值是一个好的做法。

It is a good practice to set values in all the above fields.

Slot Objects in Selenium Grid

Slot 对象指向节点内的某个插槽。某个会话可以在插槽内运行。Slot 对象中的字段如下所列：

A Slot Object points to one slot inside the Node. A particular session can run within a slot. The fields in the Slot Object are listed below −

Name

Type

Description

id

string

A UUID is used to identify a slot.

lastStarted

string

This indicates the time when the last session had begun in ISO-8601 format.

stereotype

object

The minimum group of capabilities that a slot should have. For example: {"browserName": "chrome"}

sessionObject

object

Indicates the Session object.

Session Objects in Selenium Grid

会话对象用于引用插槽中的当前会话。会话对象中的字段如下所示：

The Session Objects are used to refer to a current session inside a slot. The fields in the Session Object are listed below −

Name

Type

Description

capabilities

object

The correct features given by the session.

startTime

string

This indicates the beginning time of the session in the ISO-8601 format.

stereotype

object

The minimum group of capabilities that a slot should have. For example: {"browserName": "chrome"}

uri

string

The Node uri to interact with the session.

这结束了我们对 Selenium Grid 架构教程的全面讲解。我们从描述 Selenium Grid 的关键组件、这些组件之间的通信方法和依赖关系、如何进行节点注册、Selenium Grid 中的节点状态对象、插槽对象和会话对象中的字段开始。

This concludes our comprehensive take on the tutorial on Selenium Grid - Architecture. We’ve started with describing what are the key components of Selenium Grid, what are the communication methods, and dependencies among these components, how Node registration is done, what are the fields in the Node Status Objects, Slot Objects, and Session Objects in Selenium Grid.

这使你能够深入了解 Selenium Grid 架构。明智的做法是继续练习你学到的知识，并探索与 Selenium 相关的其他知识，以加深你的理解并拓展你的视野。

This equips you with in-depth knowledge of the Selenium Grid Architecture. It is wise to keep practicing what you’ve learned and exploring others relevant to Selenium to deepen your understanding and expand your horizons.