8 figures | analyze Eureka's first synchronization registry

The registry is particularly important for the registry , All functions are expanded around this registry . Services such as A To access the service B, You have to know the service B Of IP Address and port number . As shown in the figure below , The traditional way is to serve A I see. Service B After the address of , send out HTTP Request to the corresponding API Address .

That service A and service B The information is actually placed in the registry of the Registration Center , The registration center manages the registration of all services 、 Offline . service A and service B Want to get registration information , Unified access to the registry , Get the registry , I know the of other services IP Address And port number .

Last time , Let's talk about a Eureka Client Successfully registered to Eureka Server after ,Eureka Server Will save the registry information in a ConcurrentHashMap in .

that Client How to get other customer registration information ？

Two 、 Get registration information for the first time

First of all, let's think about , service B Sent the registration request to the Registration Center , That service A You have to get the registry , service A There must have been no registry information at the beginning , Then you must get the registration center to pull all the registration forms .（ Service here A Also known as Eureka client ）

service A For the registry , Namely to see somebody for the first time , service A Want to put all Save a copy of the registration information in your own place , Convenient for follow-up API call .

Next, let's analyze how the client obtains the full registry from the perspective of source code .

The client sends a request to get

Client When initializing , It will start from Eureka Get the full registry from the registry ：

The first time to obtain registration information is to use it in DiscoveryClient Obtained during initialization . We can find the following judgment from the source code ：

if (clientConfig.shouldFetchRegistry() && !fetchRegistry(false)) {
            fetchRegistryFromBackup();
        }

The meaning of this code is shown in the figure ：

First, it depends on whether it is configured shouldFetchRegistry, If the , It will call fetchRegistry Method to get the registry .

Because it's new client, So there must be no registration information , So local variables applications = null. Then judge whether it is necessary to obtain the full registry according to several conditions , If one of the conditions is met, the full amount of ：

• Conditions for a ： Whether to force full access . Transmitted false, You don't need a full amount .
• Condition 2 ： Whether the registry information is empty .application == null, It's empty , Need full access to .
• Condition 3 ： Get registered client Whether the number of is equal to 0. Yes , Need full access to .

Because of satisfaction applications=null, So you need to get the full amount .

Method for obtaining full registration information ：

getAndStoreFullRegistry()

In this, you will send the following HTTP Request calling jersey Of restful Interface ：

getApplications()

then Eureka Server Deal with this http The requested class is here ：ApplicationsResource Class getContainers Method . This method will be used to get Server Over there .

3、 ... and 、Server Side of the registry cache

Server The client will put the registry into the cache , Reading the registry is actually read from the cache .

There are two levels of cache , A read-only cache  readOnlyCacheMap  And read-write cache  readOnlyCacheMap.

As shown in the figure below ：

The read logic of the cache is as follows ：

• Jersey Servlet Handle HTTP request .

• First, the default will start from read-only Find it in the cache .

• If not , Again from Reading and writing Find it in the cache .

• If you find it, update it read-only cache , And return the found cache .

• If you can't find it yet , Return empty .

Leave a few questions , Put it in the cache architecture article ：

（1） How did the two-level cache data come from ？

（2） How cache data is updated ？

（3） How the cache expires ？

then ,Eureka Client After getting the registry information , It will be saved locally localRegionApps variable . such Client There will be a copy of Server The registry information of .

localRegionApps.set(this.filterAndShuffle(apps));

Four 、 summary

Registry, whether for Client still Server Come on , It's all very important ：

• about Server End to speak , In order to better provide the service of querying the registry , Multi level cache is used to cache registry information .
• about Client End to speak , When the registry is obtained for the first time, it will grab the registry in full , Have their own local .